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*Pages 893*
*Year 2015*

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- Michael J. Moran
- Howard N. Shapiro
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- Margaret B. Bailey

*Table of contents : CoverTitle PageCopyright PagePrefaceAcknowledgmentsContents1 Getting Started: Introductory Concepts and Definitions 1.1 Using Thermodynamics 1.2 Defining Systems 1.2.1 Closed Systems 1.2.2 Control Volumes 1.2.3 Selecting the System Boundary 1.3 Describing Systems and Their Behavior 1.3.1 Macroscopic and Microscopic Views of Thermodynamics 1.3.2 Property, State, and Process 1.3.3 Extensive and Intensive Properties 1.3.4 Equilibrium 1.4 Measuring Mass, Length, Time, and Force 1.4.1 SI Units 1.4.2 English Engineering Units 1.5 Specific Volume 1.6 Pressure 1.6.1 Pressure Measurement 1.6.2 Buoyancy 1.6.3 Pressure Units 1.7 Temperature 1.7.1 Thermometers 1.7.2 Kelvin Temperature Scale 1.7.3 Celsius Scale 1.8 Engineering Design and Analysis 1.8.1 Design 1.8.2 Analysis 1.9 Methodology for Solving Thermodynamics Problems Chapter Summary and Study Guide2 Energy and the First Law of Thermodynamics 2.1 Reviewing Mechanical Concepts of Energy 2.1.1 Work and Kinetic Energy 2.1.2 Potential Energy 2.1.3 Units for Energy 2.1.4 Conservation of Energy in Mechanics 2.1.5 Closing Comment 2.2 Broadening Our Understanding of Work 2.2.1 Sign Convention and Notation 2.2.2 Power 2.2.3 Modeling Expansion or Compression Work 2.2.4 Expansion or Compression Work in Actual Processes 2.2.5 Expansion or Compression Work in Quasiequilibrium Processes 2.2.6 Further Examples of Work 2.2.7 Further Examples of Work in Quasiequilibrium Processes 2.2.8 Generalized Forces and Displacements 2.3 Broadening Our Understanding of Energy 2.4 Energy Transfer by Heat 2.4.1 Sign Convention, Notation, and Heat Transfer Rate 2.4.2 Heat Transfer Modes 2.4.3 Closing Comments 2.5 Energy Accounting: Energy Balance for Closed Systems 2.5.1 Important Aspects of the Energy Balance 2.5.2 Using the Energy Balance: Processes of Closed Systems 2.5.3 Using the Energy Rate Balance: Steady-State Operation 2.5.4 Using the Energy Rate Balance: Transient Operation 2.6 Energy Analysis of Cycles 2.6.1 Cycle Energy Balance 2.6.2 Power Cycles 2.6.3 Refrigeration and Heat Pump Cycles 2.7 Energy Storage 2.7.1 Overview 2.7.2 Storage Technologies Chapter Summary and Study Guide3 Evaluating Properties 3.1 Getting Started 3.1.1 Phase and Pure Substance 3.1.2 Fixing the State Evaluating Properties: General Considerations 3.2 p–?–T Relation 3.2.1 p–?–T Surface 3.2.2 Projections of the p–?–T Surface 3.3 Studying Phase Change 3.4 Retrieving Thermodynamic Properties 3.5 Evaluating Pressure, Specific Volume, and Temperature 3.5.1 Vapor and Liquid Tables 3.5.2 Saturation Tables 3.6 Evaluating Specific Internal Energy and Enthalpy 3.6.1 Introducing Enthalpy 3.6.2 Retrieving u and h Data 3.6.3 Reference States and Reference Values 3.7 Evaluating Properties Using Computer Software 3.8 Applying the Energy Balance Using Property Tables and Software 3.8.1 Using Property Tables 3.8.2 Using Software 3.9 Introducing Specific Heats C? and Cp 3.10 Evaluating Properties of Liquids and Solids 3.10.1 Approximations for Liquids Using Saturated Liquid Data 3.10.2 Incompressible Substance Model 3.11 Generalized Compressibility Chart 3.11.1 Universal Gas Constant, R 3.11.2 Compressibility Factor, Z 3.11.3 Generalized Compressibility Data, Z Chart 3.11.4 Equations of State Evaluating Properties Using the Ideal Gas Model 3.12 Introducing the Ideal Gas Model 3.12.1 Ideal Gas Equation of State 3.12.2 Ideal Gas Model 3.12.3 Microscopic Interpretation 3.13 Internal Energy, Enthalpy, and Specific Heats of Ideal Gases 3.13.1 ∆u, ∆h, c?,, and cp Relations 3.13.2 Using Specific Heat Functions 3.14 Applying the Energy Balance Using Ideal Gas Tables, Constant Specific Heats, and Software 3.14.1 Using Ideal Gas Tables 3.14.2 Using Constant Specific Heats 3.14.3 Using Computer Software 3.15 Polytropic Process Relations Chapter Summary and Study Guide4 Control Volume Analysis Using Energy 4.1 Conservation of Mass for a Control Volume 4.1.1 Developing the Mass Rate Balance 4.1.2 Evaluating the Mass Flow Rate 4.2 Forms of the Mass Rate Balance 4.2.1 One-Dimensional Flow Form of the Mass Rate Balance 4.2.2 Steady-State Form of the Mass Rate Balance 4.2.3 Integral Form of the Mass Rate Balance 4.3 Applications of the Mass Rate Balance 4.3.1 Steady-State Application 4.3.2 Time-Dependent (Transient) Application 4.4 Conservation of Energy for a Control Volume 4.4.1 Developing the Energy Rate Balance for a Control Volume 4.4.2 Evaluating Work for a Control Volume 4.4.3 One-Dimensional Flow Form of the Control Volume Energy Rate Balance 4.4.4 Integral Form of the Control Volume Energy Rate Balance 4.5 Analyzing Control Volumes at Steady State 4.5.1 Steady-State Forms of the Mass and Energy Rate Balances 4.5.2 Modeling Considerations for Control Volumes at Steady State 4.6 Nozzles and Diffusers 4.6.1 Nozzle and Diffuser Modeling Considerations 4.6.2 Application to a Steam Nozzle 4.7 Turbines 4.7.1 Steam and Gas Turbine Modeling Considerations 4.7.2 Application to a Steam Turbine 4.8 Compressors and Pumps 4.8.1 Compressor and Pump Modeling Considerations 4.8.2 Applications to an Air Compressor and a Pump System 4.8.3 Pumped-Hydro and Compressed-Air Energy Storage 4.9 Heat Exchangers 4.9.1 Heat Exchanger Modeling Considerations 4.9.2 Applications to a Power Plant Condenser and Computer Cooling 4.10 Throttling Devices 4.10.1 Throttling Device Modeling Considerations 4.10.2 Using a Throttling Calorimeter to Determine Quality 4.11 System Integration 4.12 Transient Analysis 4.12.1 The Mass Balance in Transient Analysis 4.12.2 The Energy Balance in Transient Analysis 4.12.3 Transient Analysis Applications Chapter Summary and Study Guide5 The Second Law of Thermodynamics 5.1 Introducing the Second Law 5.1.1 Motivating the Second Law 5.1.2 Opportunities for Developing Work 5.1.3 Aspects of the Second Law 5.2 Statements of the Second Law 5.2.1 Clausius Statement of the Second Law 5.2.2 Kelvin–Planck Statement of the Second Law 5.2.3 Entropy Statement of the Second Law 5.2.4 Second Law Summary 5.3 Irreversible and Reversible Processes 5.3.1 Irreversible Processes 5.3.2 Demonstrating Irreversibility 5.3.3 Reversible Processes 5.3.4 Internally Reversible Processes 5.4 Interpreting the Kelvin–Planck Statement 5.5 Applying the Second Law to Thermodynamic Cycles 5.6 Second Law Aspects of Power Cycles Interacting with Two Reservoirs 5.6.1 Limit on Thermal Efficiency 5.6.2 Corollaries of the Second Law for Power Cycles 5.7 Second Law Aspects of Refrigeration and Heat Pump Cycles Interacting with Two Reservoirs 5.7.1 Limits on Coefficients of Performance 5.7.2 Corollaries of the Second Law for Refrigeration and Heat Pump Cycles 5.8 The Kelvin and International Temperature Scales 5.8.1 The Kelvin Scale 5.8.2 The Gas Thermometer 5.8.3 International Temperature Scale 5.9 Maximum Performance Measures for Cycles Operating between Two Reservoirs 5.9.1 Power Cycles 5.9.2 Refrigeration and Heat Pump Cycles 5.10 Carnot Cycle 5.10.1 Carnot Power Cycle 5.10.2 Carnot Refrigeration and Heat Pump Cycles 5.10.3 Carnot Cycle Summary 5.11 Clausius Inequality Chapter Summary and Study Guide6 Using Entropy 6.1 Entropy—A System Property 6.1.1 Defining Entropy Change 6.1.2 Evaluating Entropy 6.1.3 Entropy and Probability 6.2 Retrieving Entropy Data 6.2.1 Vapor Data 6.2.2 Saturation Data 6.2.3 Liquid Data 6.2.4 Computer Retrieval 6.2.5 Using Graphical Entropy Data 6.3 Introducing the T dS Equations 6.4 Entropy Change of an Incompressible Substance 6.5 Entropy Change of an Ideal Gas 6.5.1 Using Ideal Gas Tables 6.5.2 Assuming Constant Specific Heats 6.5.3 Computer Retrieval 6.6 Entropy Change in Internally Reversible Processes of Closed Systems 6.6.1 Area Representation of Heat Transfer 6.6.2 Carnot Cycle Application 6.6.3 Work and Heat Transfer in an Internally Reversible Process of Water 6.7 Entropy Balance for Closed Systems 6.7.1 Interpreting the Closed System Entropy Balance 6.7.2 Evaluating Entropy Production and Transfer 6.7.3 Applications of the Closed System Entropy Balance 6.7.4 Closed System Entropy Rate Balance 6.8 Directionality of Processes 6.8.1 Increase of Entropy Principle 6.8.2 Statistical Interpretation of Entropy 6.9 Entropy Rate Balance for Control Volumes 6.10 Rate Balances for Control Volumes at Steady State 6.10.1 One-Inlet, One-Exit Control Volumes at Steady State 6.10.2 Applications of the Rate Balances to Control Volumes at Steady State 6.11 Isentropic Processes 6.11.1 General Considerations 6.11.2 Using the Ideal Gas Model 6.11.3 Illustrations: Isentropic Processes of Air 6.12 Isentropic Efficiencies of Turbines, Nozzles, Compressors, and Pumps 6.12.1 Isentropic Turbine Efficiency 6.12.2 Isentropic Nozzle Efficiency 6.12.3 Isentropic Compressor and Pump Efficiencies 6.13 Heat Transfer and Work in Internally Reversible, Steady-State Flow Processes 6.13.1 Heat Transfer 6.13.2 Work 6.13.3 Work in Polytropic Processes Chapter Summary and Study Guide7 Exergy Analysis 7.1 Introducing Exergy 7.2 Conceptualizing Exergy 7.2.1 Environment and Dead State 7.2.2 Defining Exergy 7.3 Exergy of a System 7.3.1 Exergy Aspects 7.3.2 Specific Exergy 7.3.3 Exergy Change 7.4 Closed System Exergy Balance 7.4.1 Introducing the Closed System Exergy Balance 7.4.2 Closed System Exergy Rate Balance 7.4.3 Exergy Destruction and Loss 7.4.4 Exergy Accounting 7.5 Exergy Rate Balance for Control Volumes at Steady State 7.5.1 Comparing Energy and Exergy for Control Volumes at Steady State 7.5.2 Evaluating Exergy Destruction in Control Volumes at Steady State 7.5.3 Exergy Accounting in Control Volumes at Steady State 7.6 Exergetic (Second Law) Efficiency 7.6.1 Matching End Use to Source 7.6.2 Exergetic Efficiencies of Common Components 7.6.3 Using Exergetic Efficiencies 7.7 Thermoeconomics 7.7.1 Costing 7.7.2 Using Exergy in Design 7.7.3 Exergy Costing of a Cogeneration System Chapter Summary and Study Guide8 Vapor Power Systems Introducing Power Generation Considering Vapor Power Systems 8.1 Introducing Vapor Power Plants 8.2 The Rankine Cycle 8.2.1 Modeling the Rankine Cycle 8.2.2 Ideal Rankine Cycle 8.2.3 Effects of Boiler and Condenser Pressures on the Rankine Cycle 8.2.4 Principal Irreversibilities and Losses 8.3 Improving Performance—Superheat, Reheat, and Supercritical 8.4 Improving Performance—Regenerative Vapor Power Cycle 8.4.1 Open Feedwater Heaters 8.4.2 Closed Feedwater Heaters 8.4.3 Multiple Feedwater Heaters 8.5 Other Vapor Power Cycle Aspects 8.5.1 Working Fluids 8.5.2 Cogeneration 8.5.3 Carbon Capture and Storage 8.6 Case Study: Exergy Accounting of a Vapor Power Plant Chapter Summary and Study Guide9 Gas Power Systems Considering Internal Combustion Engines 9.1 Introducing Engine Terminology 9.2 Air-Standard Otto Cycle 9.3 Air-Standard Diesel Cycle 9.4 Air-Standard Dual Cycle Considering Gas Turbine Power Plants 9.5 Modeling Gas Turbine Power Plants 9.6 Air-Standard Brayton Cycle 9.6.1 Evaluating Principal Work and Heat Transfers 9.6.2 Ideal Air-Standard Brayton Cycle 9.6.3 Considering Gas Turbine Irreversibilities and Losses 9.7 Regenerative Gas Turbines 9.8 Regenerative Gas Turbines with Reheat and Intercooling 9.8.1 Gas Turbines with Reheat 9.8.2 Compression with Intercooling 9.8.3 Reheat and Intercooling 9.8.4 Ericsson and Stirling Cycles 9.9 Gas Turbine–Based Combined Cycles 9.9.1 Combined Gas Turbine–Vapor Power Cycle 9.9.2 Cogeneration 9.10 Integrated Gasification Combined-Cycle Power Plants 9.11 Gas Turbines for Aircraft Propulsion Considering Compressible Flow through Nozzles and Diffusers 9.12 Compressible Flow Preliminaries 9.12.1 Momentum Equation for Steady One-Dimensional Flow 9.12.2 Velocity of Sound and Mach Number 9.12.3 Determining Stagnation State Properties 9.13 Analyzing One-Dimensional Steady Flow in Nozzles and Diffusers 9.13.1 Exploring the Effects of Area Change in Subsonic and Supersonic Flows 9.13.2 Effects of Back Pressure on Mass Flow Rate 9.13.3 Flow Across a Normal Shock 9.14 Flow in Nozzles and Diffusers of Ideal Gases with Constant Specific Heats 9.14.1 Isentropic Flow Functions 9.14.2 Normal Shock Functions Chapter Summary and Study Guide10 Refrigeration and Heat Pump Systems 10.1 Vapor Refrigeration Systems 10.1.1 Carnot Refrigeration Cycle 10.1.2 Departures from the Carnot Cycle 10.2 Analyzing Vapor-Compression Refrigeration Systems 10.2.1 Evaluating Principal Work and Heat Transfers 10.2.2 Performance of Ideal Vapor-Compression Systems 10.2.3 Performance of Actual Vapor-Compression Systems 10.2.4 The p–h Diagram 10.3 Selecting Refrigerants 10.4 Other Vapor-Compression Applications 10.4.1 Cold Storage 10.4.2 Cascade Cycles 10.4.3 Multistage Compression with Intercooling 10.5 Absorption Refrigeration 10.6 Heat Pump Systems 10.6.1 Carnot Heat Pump Cycle 10.6.2 Vapor-Compression Heat Pumps 10.7 Gas Refrigeration Systems 10.7.1 Brayton Refrigeration Cycle 10.7.2 Additional Gas Refrigeration Applications 10.7.3 Automotive Air Conditioning Using Carbon Dioxide Chapter Summary and Study Guide11 Thermodynamic Relations 11.1 Using Equations of State 11.1.1 Getting Started 11.1.2 Two-Constant Equations of State 11.1.3 Multiconstant Equations of State 11.2 Important Mathematical Relations 11.3 Developing Property Relations 11.3.1 Principal Exact Differentials 11.3.2 Property Relations from Exact Differentials 11.3.3 Fundamental Thermodynamic Functions 11.4 Evaluating Changes in Entropy, Internal Energy, and Enthalpy 11.4.1 Considering Phase Change 11.4.2 Considering Single-Phase Regions 11.5 Other Thermodynamic Relations 11.5.1 Volume Expansivity, Isothermal and Isentropic Compressibility 11.5.2 Relations Involving Specific Heats 11.5.3 Joule–Thomson Coefficient 11.6 Constructing Tables of Thermodynamic Properties 11.6.1 Developing Tables by Integration Using p–?–T and Specific Heat Data 11.6.2 Developing Tables by Differentiating a Fundamental Thermodynamic Function 11.7 Generalized Charts for Enthalpy and Entropy 11.8 p–?–T Relations for Gas Mixtures 11.9 Analyzing Multicomponent Systems 11.9.1 Partial Molal Properties 11.9.2 Chemical Potential 11.9.3 Fundamental Thermodynamic Functions for Multicomponent Systems 11.9.4 Fugacity 11.9.5 Ideal Solution 11.9.6 Chemical Potential for Ideal Solutions Chapter Summary and Study Guide12 Ideal Gas Mixture and Psychrometric Applications Ideal Gas Mixtures: General Considerations 12.1 Describing Mixture Composition 12.2 Relating p, V, and T for Ideal Gas Mixtures 12.3 Evaluating U, H, S, and Specific Heats 12.3.1 Evaluating U and H 12.3.2 Evaluating c? and cp 12.3.3 Evaluating S 12.3.4 Working on a Mass Basis 12.4 Analyzing Systems Involving Mixtures 12.4.1 Mixture Processes at Constant Composition 12.4.2 Mixing of Ideal Gases Psychrometric Applications 12.5 Introducing Psychrometric Principles 12.5.1 Moist Air 12.5.2 Humidity Ratio, Relative Humidity, Mixture Enthalpy, and Mixture Entropy 12.5.3 Modeling Moist Air in Equilibrium with Liquid Water 12.5.4 Evaluating the Dew Point Temperature 12.5.5 Evaluating Humidity Ratio Using the Adiabatic-Saturation Temperature 12.6 Psychrometers: Measuring the Wet-Bulb and Dry-Bulb Temperatures 12.7 Psychrometric Charts 12.8 Analyzing Air-Conditioning Processes 12.8.1 Applying Mass and Energy Balances to Air-Conditioning Systems 12.8.2 Conditioning Moist Air at Constant Composition 12.8.3 Dehumidification 12.8.4 Humidification 12.8.5 Evaporative Cooling 12.8.6 Adiabatic Mixing of Two Moist Air Streams 12.9 Cooling Towers Chapter Summary and Study Guide13 Reacting Mixtures and Combustion Combustion Fundamentals 13.1 Introducing Combustion 13.1.1 Fuels 13.1.2 Modeling Combustion Air 13.1.3 Determining Products of Combustion 13.1.4 Energy and Entropy Balances for Reacting Systems 13.2 Conservation of Energy—Reacting Systems 13.2.1 Evaluating Enthalpy for Reacting Systems 13.2.2 Energy Balances for Reacting Systems 13.2.3 Enthalpy of Combustion and Heating Values 13.3 Determining the Adiabatic Flame Temperature 13.3.1 Using Table Data 13.3.2 Using Computer Software 13.3.3 Closing Comments 13.4 Fuel Cells 13.4.1 Proton Exchange Membrane Fuel Cell 13.4.2 Solid Oxide Fuel Cell 13.5 Absolute Entropy and the Third Law of Thermodynamics 13.5.1 Evaluating Entropy for Reacting Systems 13.5.2 Entropy Balances for Reacting Systems 13.5.3 Evaluating Gibbs Function for Reacting Systems Chemical Exergy 13.6 Conceptualizing Chemical Exergy 13.6.1 Working Equations for Chemical Exergy 13.6.2 Evaluating Chemical Exergy for Several Cases 13.6.3 Closing Comments 13.7 Standard Chemical Exergy 13.7.1 Standard Chemical Exergy of a Hydrocarbon: CaHb 13.7.2 Standard Chemical Exergy of Other Substances 13.8 Applying Total Exergy 13.8.1 Calculating Total Exergy 13.8.2 Calculating Exergetic Efficiencies of Reacting Systems Chapter Summary and Study Guide14 Chemical and Phase Equilibrium Equilibrium Fundamentals 14.1 Introducing Equilibrium Criteria 14.1.1 Chemical Potential and Equilibrium 14.1.2 Evaluating Chemical Potentials Chemical Equilibrium 14.2 Equation of Reaction Equilibrium 14.2.1 Introductory Case 14.2.2 General Case 14.3 Calculating Equilibrium Compositions 14.3.1 Equilibrium Constant for Ideal Gas Mixtures 14.3.2 Illustrations of the Calculation of Equilibrium Compositions for Reacting Ideal Gas Mixtures 14.3.3 Equilibrium Constant for Mixtures and Solutions 14.4 Further Examples of the Use of the Equilibrium Constant 14.4.1 Determining Equilibrium Flame Temperature 14.4.2 Van’t Hoff Equation 14.4.3 Ionization 14.4.4 Simultaneous Reactions Phase Equilibrium 14.5 Equilibrium between Two Phases of a Pure Substance 14.6 Equilibrium of Multicomponent, Multiphase Systems 14.6.1 Chemical Potential and Phase Equilibrium 14.6.2 Gibbs Phase Rule Chapter Summary and Study GuideAppendix Tables, Figures, and ChartsIndex to Tables in SI UnitsIndex to Figures and ChartsIndexEULA*

M o r an | Sh ap ir o | Boet t ner | Bai l e y

Principles of Engineering Thermodynamics Eighth Edition EXCLUSIVE CONTENT

SI Ver s i o n

How to Use This Book Effectively This book is organized by chapters and sections within chapters. For a listing of contents, see pp. xi–xviii. Fundamental concepts and associated equations within each section lay the foundation for applications of engineering thermodynamics provided in solved examples, end-of-chapter problems and exercises, and accompanying discussions. Boxed material within sections of the book allows you to explore selected topics in greater depth, as in the boxed discussion of properties and nonproperties on p. 8. Contemporary issues related to thermodynamics are introduced throughout the text with three unique features: ENERGY & ENVIRONMENT discussions explore issues related to energy resource use and the environment, as in the discussion of hybrid vehicles on p. 32. BIOCONNECTIONS discussions tie topics to applications in bioengineering and biomedicine, as in the discussion of control volumes of living things and their organs on p. 5. Horizons link subject matter to emerging technologies and thought-provoking issues, as in the discussion of nanotechnology on p. 13. Other core features of this book that facilitate your study and contribute to your understanding include:

Examples c Numerous annotated solved examples are provided that feature the solution methodology presented in Sec. 1.9 and illustrated in Example 1.1. We encourage you to study these examples, including the accompanying comments.

c Each solved example concludes with a list of the Skills Developed in solving the example and a QuickQuiz that allows an immediate check of understanding.

c Less formal examples are given throughout the text. They open with

c FOR EXAMPLE

and close with b b b b b. These examples

also should be studied.

Exercises c Each chapter has a set of discussion questions under the heading

c EXERCISES: THINGS ENGINEERS THINK ABOUT that may be done on an individual or small-group basis. They allow you to gain a deeper understanding of the text material, think critically, and test yourself.

c A large number of end-of-chapter problems also are provided under the heading

c PROBLEMS: DEVELOPING ENGINEERING SKILLS . The problems are sequenced to coordinate with the subject matter and are listed in increasing order of difficulty. The problems are also classified under headings to expedite the process of selecting review problems to solve. Answers to selected problems are provided on the student companion website that accompanies this book.

c Because one purpose of this book is to help you prepare to use thermodynamics in engineering practice, design considerations related to thermodynamics are included. Every chapter has a set of problems under the heading c DESIGN & OPEN ENDED PROBLEMS: EXPLORING ENGINEERING PRACTICE that provide opportunities for practicing creativity, formulating and solving design and open-ended problems, using the Internet and library resources to find relevant information, making engineering judgments, and developing communications skills. See, for example, problem 1.10D on p. 29.

Further Study Aids c Each chapter opens with an introduction giving the engineering context, stating the chapter objective, and listing the learning outcomes.

c Each chapter concludes with a

c CHAPTER SUMMARY AND STUDY GUIDE

that provides a point of departure to study for

examinations.

c For easy reference, each chapter also concludes with lists of

c KEY ENGINEERING CONCEPTS

and

c KEY EQUATIONS

.

c Important terms are listed in the margins and coordinated with the text material at those locations. c Important equations are set off by a color screen, as for Eq. 1.8. c TAKE NOTE... in the margin provides just-in-time information that illuminates the current discussion, as on p. 6, or refines our problem-solving methodology, as on p. 10 and p. 20.

c A

in the margin identifies an animation that reinforces the text presentation at that point. Animations can be viewed by going to the student companion website for this book. See TAKE NOTE... on p. 6 for further detail about accessing animations.

c

in the margin denotes end-of-chapter problems where the use of appropriate computer software is recommended.

c For quick reference, conversion factors and important constants are provided on the next page. c A list of symbols is provided on the inside back cover.

Conversion Factors Mass and Density

Pressure

1 1 1 1 1 1

1 Pa

kg g/cm3 g/cm3 lb lb/ft3 lb/ft3

5 5 5 5 5 5

2.2046 lb 103 kg/m3 62.428 lb/ft3 0.4536 kg 0.016018 g/cm3 16.018 kg/m3

1 1 1 1 1

5 5 5 5 5 5 5

bar atm lbf/in.2 lbf/in.2 atm

1 N/m2 1.4504 3 1024 lbf/in.2 105 N/m2 1.01325 bar 6894.8 Pa 144 lbf/ft2 14.696 lbf/in.2

Length

Energy and Specific Energy

1 1 1 1

1 1 1 1 1 1 1 1 1

cm m in. ft

5 5 5 5

0.3937 in. 3.2808 ft 2.54 cm 0.3048 m

Velocity 1 km/h 5 0.62137 mile/h 1 mile/h 5 1.6093 km/h

Volume 1 1 1 1 1 1 1 1

cm3 m3 L L in.3 ft3 gal gal

5 5 5 5 5 5 5 5

0.061024 in.3 35.315 ft3 1023 m3 0.0353 ft3 16.387 cm3 0.028317 m3 0.13368 ft3 3.7854 3 1023 m3

Force 1 1 1 1

N N lbf lbf

5 5 5 5

1 kg ? m/s2 0.22481 lbf 32.174 lb ? ft/s2 4.4482 N

J kJ kJ kJ/kg ft ? lbf Btu Btu Btu/lb kcal

5 5 5 5 5 5 5 5 5

1 N ? m 5 0.73756 ft ? lbf 737.56 ft ? lbf 0.9478 Btu 0.42992 Btu/lb 1.35582 J 778.17 ft ? lbf 1.0551 kJ 2.326 kJ/kg 4.1868 kJ

Energy Transfer Rate 1W 1 kW 1 Btu/h 1 hp 1 hp 1 hp

5 5 5 5 5 5

1 J/s 5 3.413 Btu/h 1.341 hp 0.293 W 2545 Btu/h 550 ft ? lbf/s 0.7457 kW

Specific Heat 1 kJ/kg ? K 5 0.238846 Btu/lb ? 8R 1 kcal/kg ? K 5 1 Btu/lb ? 8R 1 Btu/h ? 8R 5 4.1868 kJ/kg ? K

Others 1 ton of refrigeration 5 200 Btu/min 5 211 kJ/min 1 volt 5 1 watt per ampere

Constants Universal Gas Constant

Standard Atmospheric Pressure

8.314 kJ/kmol ? K R 5 • 1545 ft ? lbf/lbmol ? °R 1.986 Btu/lbmol ? °R

1.01325 bar 1 atm 5 • 14.696 lbf/in.2 760 mm Hg 5 29.92 in. Hg

Standard Acceleration of Gravity 9.80665 m/s2 g5 e 32.174 ft/s2

Temperature Relations T(°R) 5 1.8 T(K) T(°C) 5 T(K) 2 273.15 T(°F) 5 T(°R) 2 459.67

How to Use This Book Effectively This book is organized by chapters and sections within chapters. For a listing of contents, see pp. xi–xviii. Fundamental concepts and associated equations within each section lay the foundation for applications of engineering thermodynamics provided in solved examples, end-of-chapter problems and exercises, and accompanying discussions. Boxed material within sections of the book allows you to explore selected topics in greater depth, as in the boxed discussion of properties and nonproperties on p. 8. Contemporary issues related to thermodynamics are introduced throughout the text with three unique features: ENERGY & ENVIRONMENT discussions explore issues related to energy resource use and the environment, as in the discussion of hybrid vehicles on p. 32. BIOCONNECTIONS discussions tie topics to applications in bioengineering and biomedicine, as in the discussion of control volumes of living things and their organs on p. 5. Horizons link subject matter to emerging technologies and thought-provoking issues, as in the discussion of nanotechnology on p. 13. Other core features of this book that facilitate your study and contribute to your understanding include:

Examples c Numerous annotated solved examples are provided that feature the solution methodology presented in Sec. 1.9 and illustrated in Example 1.1. We encourage you to study these examples, including the accompanying comments.

c Each solved example concludes with a list of the Skills Developed in solving the example and a QuickQuiz that allows an immediate check of understanding.

c Less formal examples are given throughout the text. They open with

c FOR EXAMPLE

and close with b b b b b. These examples

also should be studied.

Exercises c Each chapter has a set of discussion questions under the heading

c EXERCISES: THINGS ENGINEERS THINK ABOUT that may be done on an individual or small-group basis. They allow you to gain a deeper understanding of the text material, think critically, and test yourself.

c A large number of end-of-chapter problems also are provided under the heading

c PROBLEMS: DEVELOPING ENGINEERING SKILLS . The problems are sequenced to coordinate with the subject matter and are listed in increasing order of difficulty. The problems are also classified under headings to expedite the process of selecting review problems to solve. Answers to selected problems are provided on the student companion website that accompanies this book.

c Because one purpose of this book is to help you prepare to use thermodynamics in engineering practice, design considerations related to thermodynamics are included. Every chapter has a set of problems under the heading c DESIGN & OPEN ENDED PROBLEMS: EXPLORING ENGINEERING PRACTICE that provide opportunities for practicing creativity, formulating and solving design and open-ended problems, using the Internet and library resources to find relevant information, making engineering judgments, and developing communications skills. See, for example, problem 1.10D on p. 29.

Further Study Aids c Each chapter opens with an introduction giving the engineering context, stating the chapter objective, and listing the learning outcomes.

c Each chapter concludes with a

c CHAPTER SUMMARY AND STUDY GUIDE

that provides a point of departure to study for

examinations.

c For easy reference, each chapter also concludes with lists of

c KEY ENGINEERING CONCEPTS

and

c KEY EQUATIONS

.

c Important terms are listed in the margins and coordinated with the text material at those locations. c Important equations are set off by a color screen, as for Eq. 1.8. c TAKE NOTE... in the margin provides just-in-time information that illuminates the current discussion, as on p. 6, or refines our problem-solving methodology, as on p. 10 and p. 20.

c A

in the margin identifies an animation that reinforces the text presentation at that point. Animations can be viewed by going to the student companion website for this book. See TAKE NOTE... on p. 6 for further detail about accessing animations.

c For quick reference, conversion factors and important constants are provided on the next page. c A list of symbols is provided on the inside back cover.

Conversion Factors Mass and Density

Pressure

1 1 1 1 1 1

1 Pa

kg g/cm3 g/cm3 lb lb/ft3 lb/ft3

5 5 5 5 5 5

2.2046 lb 103 kg/m3 62.428 lb/ft3 0.4536 kg 0.016018 g/cm3 16.018 kg/m3

Length 1 1 1 1

cm m in. ft

5 5 5 5

0.3937 in. 3.2808 ft 2.54 cm 0.3048 m

Velocity 1 km/h 5 0.62137 mile/h 1 mile/h 5 1.6093 km/h

Volume 1 1 1 1 1 1 1 1

cm3 m3 L L in.3 ft3 gal gal

5 5 5 5 5 5 5 5

0.061024 in.3 35.315 ft3 1023 m3 0.0353 ft3 16.387 cm3 0.028317 m3 0.13368 ft3 3.7854 3 1023 m3

Force 1 1 1 1

N N lbf lbf

5 5 5 5

1 kg ? m/s2 0.22481 lbf 32.174 lb ? ft/s2 4.4482 N

1 1 1 1 1

5 5 5 5 5 5 5

bar atm lbf/in.2 lbf/in.2 atm

1 N/m2 1.4504 3 1024 lbf/in.2 105 N/m2 1.01325 bar 6894.8 Pa 144 lbf/ft2 14.696 lbf/in.2

Energy and Specific Energy 1 1 1 1 1 1 1 1 1

J kJ kJ kJ/kg ft ? lbf Btu Btu Btu/lb kcal

5 5 5 5 5 5 5 5 5

1 N ? m 5 0.73756 ft ? lbf 737.56 ft ? lbf 0.9478 Btu 0.42992 Btu/lb 1.35582 J 778.17 ft ? lbf 1.0551 kJ 2.326 kJ/kg 4.1868 kJ

Energy Transfer Rate 1W 1 kW 1 Btu/h 1 hp 1 hp 1 hp

5 5 5 5 5 5

1 J/s 5 3.413 Btu/h 1.341 hp 0.293 W 2545 Btu/h 550 ft ? lbf/s 0.7457 kW

Specific Heat 1 kJ/kg ? K 5 0.238846 Btu/lb ? 8R 1 kcal/kg ? K 5 1 Btu/lb ? 8R 1 Btu/h ? 8R 5 4.1868 kJ/kg ? K

Others 1 ton of refrigeration 5 200 Btu/min 5 211 kJ/min 1 volt 5 1 watt per ampere

Constants Universal Gas Constant

Standard Atmospheric Pressure

8.314 kJ/kmol ? K R 5 • 1545 ft ? lbf/lbmol ? °R 1.986 Btu/lbmol ? °R

1.01325 bar 1 atm 5 • 14.696 lbf/in.2 760 mm Hg 5 29.92 in. Hg

Standard Acceleration of Gravity

Temperature Relations

g5 e

9.80665 m/s 32.174 ft/s2

2

T(°R) 5 1.8 T(K) T(°C) 5 T(K) 2 273.15 T(°F) 5 T(°R) 2 459.67

PRINCIPLES OF ENGINEERING THERMODYNAMICS S I Ve r s i o n EIGHTH EDITION

MICHAEL J. MORAN The Ohio State University

HOWARD N. SHAPIRO Wayne State University

DAISIE D. BOETTNER Colonel, U.S. Army

MARGARET B. BAILEY Rochester Institute of Technology

Copyright © 2012, 2015 John Wiley & Sons Singapore Pte. Ltd. Cover photo from © Janaka Dharmasena/Shutterstock Founded in 1807, John Wiley & Sons, Inc. has been a valued source of knowledge and understanding for more than 200 years, helping people around the world meet their needs and fulfill their aspirations. Our company is built on a foundation of principles that include responsibility to the communities we serve and where we live and work. In 2008, we launched a Corporate Citizenship Initiative, a global effort to address the environmental, social, economic, and ethical challenges we face in our business. Among the issues we are addressing are carbon impact, paper specifications and procurement, ethical conduct within our business and among our vendors, and community and charitable support. For more information, please visit our website: www.wiley.com/go/citizenship. All rights reserved. This book is authorized for sale in Australia, Europe, Asia, Africa and the Middle East only and may not be exported. The content is materially different than products for other markets including the authorized U.S. counterpart of this title. Exportation of this book to another region without the Publisher’s authorization may be illegal and a violation of the Publisher’s rights. The Publisher may take legal action to enforce its rights. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, website www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, website http://www.wiley.com/go/permissions. ISBN: 978-1-118-96088-2 Printed in Asia 10 9 8 7 6 5 4 3 2 1

Preface A Textbook for the 21st Century In the twenty-first century, engineering thermodynamics plays a central role in developing improved ways to provide and use energy, while mitigating the serious human health and environmental consequences accompanying energy—including air and water pollution and global climate change. Applications in bioengineering, biomedical systems, and nanotechnology also continue to emerge. This book provides the tools needed by specialists working in all such fields. For non-specialists, this book provides background for making decisions about technology related to thermodynamics—on the job and as informed citizens. Engineers in the twenty-first century need a solid set of analytical and problem-solving skills as the foundation for tackling important societal issues relating to engineering thermodynamics. The eighth edition develops these skills and significantly expands our coverage of their applications to provide • current context for the study of thermodynamic principles. • relevant background to make the subject meaningful for meeting the challenges of the decades ahead. • significant material related to existing technologies in light of new challenges. In the eighth edition, we build on the core features that have made the text the global leader in engineering thermodynamics education. (The present discussion of core features centers on new aspects; see the Preface to the seventh edition for more.) We are known for our clear and concise explanations grounded in the fundamentals, pioneering pedagogy for effective learning, and relevant, up-to-date applications. Through the creativity and experience of our newly expanded author team, and based on excellent feedback from instructors and students, we continue to enhance what has become the leading text in the field.

New in the Eighth Edition In a major departure from previous editions of this book and all other texts intended for the same student population, we have introduced animations that strengthen students’ understanding of basic phenomena and applications. The eighth edition also features a crisp new interior design aimed at helping students • better understand and apply the subject matter, and

• fully appreciate the relevance of the topics to engineering practice and to society. This edition also provides, inside the front cover under the heading How to Use This Book Effectively , an updated roadmap to core features of this text that make it so effective for student learning. To fully understand all of the many features we have built into the book, be sure to see this important element. In this edition, several enhancements to improve student learning have been introduced or upgraded: • New animations are offered at key subject matter locations to improve student learning. When viewing the animations, students will develop deeper understanding by visualizing key processes and phenomena. • Special text elements feature important illustrations of engineering thermodynamics applied to our environment, society, and world: • New ENERGY & ENVIRONMENT presentations explore topics related to energy resource use and environmental issues in engineering. • Updated BIOCONNECTIONS discussions tie textbook topics to contemporary applications in biomedicine and bioengineering. • Additional Horizons features have been included that link subject matter to thoughtprovoking 21st century issues and emerging technologies. Suggestions for additional reading and sources for topical content presented in these elements can be provided on request. • End-of-chapter problems in each of the three modes, conceptual, skill building, and design, have been extensively revised and hundreds of new problems added. • New and revised class-tested material contributes to student learning and instructor effectiveness: • Significant new content explores how thermodynamics contributes to meeting the challenges of the 21st century. • Key aspects of fundamentals and applications within the text have been enhanced. • In response to instructor and student needs, classtested changes that contribute to a more just-intime presentation have been introduced:

vii

viii

Preface • TAKE NOTE... entries in the margins are expanded throughout the textbook to improve student learning. For example, see p. 10. • Boxed material allows students and instructors to explore topics in greater depth. For example, see p. 8. • New margin terms at many locations aid in navigating subject matter.

s

• Student Companion Site: Helps students learn the subject matter with resources including s

animations—new in this edition.

s

answers to selected problems.

s

s

Supplements s

The following supplements are available with the text: • Outstanding Instructor and Student companion websites (visit www.wiley.com/college/moran) that greatly enhance teaching and learning: • Instructor Companion Site: Assists instructors in delivering an effective course with resources including s

s

animations—new in this edition. chapter-by-chapter summary of Special Features, including j

the subject of each solved example,

the topics of all ENERGY & ENVIRONMENT, BIOCONNECTIONS, and Horizons features, j the themes of the accompanying c DESIGN j

chapter learning outcomes.

chapter summary information, including Key Terms and Key Equations. chapter learning outcomes. chapter-by-chapter summary of Special Features as listed in the Instructor Companion Site.

• Interactive Thermodynamic: IT software is available as a stand-alone product or with the textbook. IT is a highly valuable learning tool that allows students to develop engineering models, perform “what-if” analyses, and examine principles in more detail to enhance their learning. Brief tutorials of IT are included within the text, and the use of IT is illustrated within selected solved examples. • WileyPLUS is an online set of instructional, practice, and course management resources, including the full text, for students and instructors. Visit www.wiley.com/college/moran or contact your local Wiley representative for information on the above-mentioned supplements.

& OPEN ENDED PROBLEMS

Ways to Meet Different Course Needs s

s

s

s

a complete solution manual that is easy to navigate. solutions to computer-based problems for use with both IT: Interactive Thermodynamics as well as EES: Engineering Equation Solver. image galleries with text images available in various helpful electronic formats. chapter summary information, including Key Terms and Key Equations.

In recognition of the evolving nature of engineering curricula, and in particular of the diverse ways engineering thermodynamics is presented, the text is structured to meet a variety of course needs. The following table illustrates several possible uses of the textbook assuming a semester basis (3 credits). Courses could be taught using this textbook to engineering students with appropriate background beginning in their second year of study.

Preface

Type of course

Intended audience

Chapter coverage

Nonmajors

• Principles. Chaps. 1–6. • Applications. Selected topics from Chaps. 8–10 (omit compressible flow in Chap. 9).

Majors

• Principles. Chaps. 1–6. • Applications. Same as above plus selected topics from Chaps. 12 and 13.

Majors

• First course. Chaps. 1–7. (Chap. 7 may be deferred to second course or omitted.) • Second course. Selected topics from Chaps. 8–14 to meet particular course needs.

Surveys

Two-course sequences

ix

Acknowledgments We thank the many users of our previous editions, located at hundreds of universities and colleges in the United States, Canada, and worldwide, who continue to contribute to the development of our text through their comments and constructive criticism. The following colleagues have assisted in the development of this edition. We greatly appreciate their contributions: John Abbitt, University of Florida Ralph Aldredge, University of California, Davis Leticia Anaya, University of North Texas Kendrick Aung, Lamar University Justin Barone, Virginia Polytechnic Institute and State University William Bathie, Iowa State University Cory Berkland, The University of Kansas Leonard Berkowitz, California State Polytechnic University, Pomona Eugene F. Brown, Virginia Polytechnic Institute and State University David L. Ernst, Texas Tech University Sebastien Feve, Iowa State University Timothy Fox, California State University, Northridge Nick Glumac, University of Illinois at UrbanaChampaign Tahereh S. Hall, Virginia Polytechnic Institute and State University Daniel W. Hoch, University of North Carolina– Charlotte Timothy J. Jacobs, Texas A&M University Fazal B. Kauser, California State Polytechnic University, Pomona MinJun Kim, Drexel University Joseph F. Kmec, Purdue University Feng C. Lai, University of Oklahoma Kevin Lyons, North Carolina State University Pedro Mago, Mississippi State University Raj M. Manglik, University of Cincinnati Thuan Nguyen, California State Polytechnic University, Pomona John Pfotenhauer, University of Wisconsin–Madison

x

Paul Puzinauskas, University of Alabama Muhammad Mustafizur Rahman, University of South Florida Jacques C. Richard, Texas A&M University Charles Ritz, California State Polytechnic University, Pomona Francisco Ruiz, Illinois Institute of Technology Iskender Sahin, Western Michigan University Will Schreiber, University of Alabama Enrico Sciubba, University of Rome (Italy) Tien-Mo Shih, University of Maryland Larry Sobel, Raytheon Missile Systems Thomas Twardowski, Widener University V. Ismet Ugursal, Dalhousie University, Nova Scotia Angela Violi, University of Michigan K. Max Zhang, Cornell University The views expressed in this text are those of the authors and do not necessarily reflect those of individual contributors listed, the Ohio State University, Wayne State University, Rochester Institute of Technology, the United States Military Academy, the Department of the Army, or the Department of Defense. We also acknowledge the efforts of many individuals in the John Wiley and Sons, Inc., organization who have contributed their talents and energy to this edition. We applaud their professionalism and commitment. We continue to be extremely gratified by the reception this book has enjoyed over the years. With this edition we have made the text more effective for teaching the subject of engineering thermodynamics and have greatly enhanced the relevance of the subject matter for students who will shape the 21st century. As always, we welcome your comments, criticisms, and suggestions. Michael J. Moran [email protected] Howard N. Shapiro [email protected] Daisie D. Boettner [email protected] Margaret B. Bailey [email protected]

Contents 2.1.1 Work and Kinetic Energy

1 Getting Started: Introductory

2.1.2 Potential Energy

Concepts and Definitions 1

2.1.3

1.1 Using Thermodynamics 2 1.2 Defining Systems 2

2.1.5 Closing Comment

34

35

2.2 Broadening Our Understanding of Work 35

4 Control Volumes 4

1.2.3 Selecting the System Boundary

2.2.1 Sign Convention and Notation

5

2.2.2 Power

1.3 Describing Systems and Their Behavior 6

6

1.3.2 Property, State, and Process

36

37

2.2.3 Modeling Expansion or Compression

1.3.1 Macroscopic and Microscopic Views

of Thermodynamics

33 Units for Energy 34

2.1.4 Conservation of Energy in Mechanics

1.2.1 Closed Systems 1.2.2

31

1.3.4 Equilibrium

7

8 1.4 Measuring Mass, Length, Time, and Force 9 1.4.1 SI Units 9 1.4.2 English Engineering Units 10 1.5 Specific Volume 11 1.6 Pressure 12 1.6.1 Pressure Measurement 13 1.6.2 Buoyancy 14 1.6.3 Pressure Units 15 1.7 Temperature 16 1.7.1 Thermometers 17 1.7.2 Kelvin Temperature Scale 18 1.7.3 Celsius Scale 19 1.8 Engineering Design and Analysis 20 1.8.1 Design 20 1.8.2 Analysis 21 1.9 Methodology for Solving Thermodynamics Problems 22 Chapter Summary and Study Guide 24

2 Energy and the First Law of Thermodynamics 30 2.1 Reviewing Mechanical Concepts of Energy 31

38

Processes

7

1.3.3 Extensive and Intensive Properties

Work

2.2.4 Expansion or Compression Work in Actual

39

2.2.5 Expansion or Compression Work in

39 43

Quasiequilibrium Processes 2.2.6 Further Examples of Work

2.2.7 Further Examples of Work in

Quasiequilibrium Processes

44

2.2.8 Generalized Forces and Displacements

45

2.3 Broadening Our Understanding of Energy 46 2.4 Energy Transfer by Heat 47 2.4.1 Sign Convention, Notation, and

47 Heat Transfer Modes 48 Closing Comments 50 Heat Transfer Rate

2.4.2 2.4.3

2.5 Energy Accounting: Energy Balance for Closed Systems 51 2.5.1 Important Aspects of the Energy

Balance

53

2.5.2 Using the Energy Balance: Processes

of Closed Systems

55

2.5.3 Using the Energy Rate Balance:

Steady-State Operation

58

2.5.4 Using the Energy Rate Balance:

Transient Operation

61

2.6 Energy Analysis of Cycles 63 2.6.1 Cycle Energy Balance 63 2.6.2 Power Cycles 64 2.6.3 Refrigeration and Heat Pump Cycles

65

2.7 Energy Storage 67

xi

xii

Contents 2.7.1 Overview

3.11.1 Universal Gas Constant, R

67

2.7.2 Storage Technologies

3.11.2 Compressibility Factor, Z

67

3.11.3 Generalized Compressibility Data,

Chapter Summary and Study Guide 68

Z Chart

110

3.11.4 Equations of State

3

Evaluating Properties 78

3.1.2 Fixing the State

79

79

3.12.1 Ideal Gas Equation of State

Evaluating Properties: General Considerations 80 3.2 p–𝜐–T Relation 80 3.2.1 p–𝜐–T Surface 81

3.12.2 Ideal Gas Model

83

3.3 Studying Phase Change 84 3.4 Retrieving Thermodynamic Properties 87 3.5 Evaluating Pressure, Specific Volume, and Temperature 87 3.5.1 Vapor and Liquid Tables 87 3.5.2 Saturation Tables 90 3.6 Evaluating Specific Internal Energy and Enthalpy 93 3.6.1 Introducing Enthalpy 93 3.6.2 Retrieving u and h Data 94 3.6.3 Reference States and Reference

95 3.7 Evaluating Properties Using Computer Software 96 3.8 Applying the Energy Balance Using Property Tables and Software 97 3.8.1 Using Property Tables 99 3.8.2 Using Software 102 3.9 Introducing Specific Heats c𝜐 and cp 104 3.10 Evaluating Properties of Liquids and Solids 105 Values

3.10.1 Approximations for Liquids Using

3.11 Generalized Compressibility Chart 109

117

3.13 Internal Energy, Enthalpy, and Specific Heats of Ideal Gases 117 3.13.1 Du, Dh, c𝜐, and cp Relations 117 3.13.2 Using Specific Heat Functions 119 3.14 Applying the Energy Balance Using Ideal Gas Tables, Constant Specific Heats, and Software 120 3.14.1 Using Ideal Gas Tables 120 3.14.2 Using Constant Specific Heats 122 3.14.3 Using Computer Software 125 3.15 Polytropic Process Relations 128

Chapter Summary and Study Guide 131

4 Control Volume Analysis Using Energy 142 4.1 Conservation of Mass for a Control Volume 143 4.1.1 Developing the Mass Rate

Balance

143

4.1.2 Evaluating the Mass Flow

144 4.2 Forms of the Mass Rate Balance 145 Rate

4.2.1 One-Dimensional Flow Form of the Mass

Rate Balance

145

4.2.2 Steady-State Form of the Mass Rate

Balance

146

4.2.3 Integral Form of the Mass Rate

105

3.10.2 Incompressible Substance Model

114

115

3.12.3 Microscopic Interpretation

3.2.2 Projections of the p–𝜐–T Surface

Saturated Liquid Data

113

Evaluating Properties Using the Ideal Gas Model 114 3.12 Introducing the Ideal Gas Model 114

3.1 Getting Started 79 3.1.1 Phase and Pure Substance

109 109

146 4.3 Applications of the Mass Rate Balance 147 4.3.1 Steady-State Application 147 Balance

106

Contents 4.3.2 Time-Dependent (Transient)

Application

xiii

4.12.2 The Energy Balance in Transient

148

Analysis 179 4.12.3 Transient Analysis Applications 180

4.4 Conservation of Energy for a Control Volume 151

Chapter Summary and Study Guide 188

4.4.1 Developing the Energy Rate Balance for

a Control Volume

151

4.4.2 Evaluating Work for a Control Volume

152

4.4.3 One-Dimensional Flow Form of the Control

Volume Energy Rate Balance

152

4.4.4 Integral Form of the Control Volume

Energy Rate Balance

153

4.5 Analyzing Control Volumes at Steady State 154

155

Second Law 206 Law 208

157 157

4.7 Turbines 159 4.7.1 Steam and Gas Turbine Modeling

Considerations 161 4.7.2 Application to a Steam Turbine 161

4.8 Compressors and Pumps 163 4.8.1 Compressor and Pump Modeling

163

4.8.2 Applications to an Air Compressor and a

Pump System

163

4.8.3 Pumped-Hydro and Compressed-Air

Energy Storage 167 4.9 Heat Exchangers 168 4.9.1 Heat Exchanger Modeling

Considerations

169

4.9.2 Applications to a Power Plant Condenser

and Computer Cooling 169

4.10 Throttling Devices 173 4.10.1 Throttling Device Modeling

Considerations 173 4.10.2 Using a Throttling Calorimeter to

Determine Quality 174

4.11 System Integration 175 4.12 Transient Analysis 178 4.12.1 The Mass Balance in Transient

Analysis 178

Law 206

5.2.3 Entropy Statement of the Second

4.6.1 Nozzle and Diffuser Modeling

Considerations

5.2 Statements of the Second Law 206

5.2.2 Kelvin–Planck Statement of the

4.6 Nozzles and Diffusers 156

4.6.2 Application to a Steam Nozzle

5.1.1 Motivating the Second Law 203 5.1.2 Opportunities for Developing

5.2.1 Clausius Statement of the Second

4.5.2 Modeling Considerations for Control

Considerations

5.1 Introducing the Second Law 203

Work 205

154

Volumes at Steady State

The Second Law of Thermodynamics 202

5.1.3 Aspects of the Second Law 205

4.5.1 Steady-State Forms of the Mass and

Energy Rate Balances

5

5.2.4 Second Law Summary 209

5.3 Irreversible and Reversible Processes 209 5.3.1 Irreversible Processes 209 5.3.2 Demonstrating Irreversibility 211 5.3.3 Reversible Processes 212 5.3.4 Internally Reversible Processes 213

5.4 Interpreting the Kelvin–Planck Statement 214 5.5 Applying the Second Law to Thermodynamic Cycles 215 5.6 Second Law Aspects of Power Cycles Interacting with Two Reservoirs 216 5.6.1 Limit on Thermal Efficiency 216 5.6.2 Corollaries of the Second Law for Power

Cycles

216

5.7 Second Law Aspects of Refrigeration and Heat Pump Cycles Interacting with Two Reservoirs 218 5.7.1 Limits on Coefficients of

Performance

218

5.7.2 Corollaries of the Second Law for

Refrigeration and Heat Pump Cycles 219

xiv

Contents

5.8 The Kelvin and International Temperature Scales 220 5.8.1 The Kelvin Scale 220 5.8.2 The Gas Thermometer 222 5.8.3 International Temperature Scale

223 5.9 Maximum Performance Measures for Cycles Operating between Two Reservoirs 223 5.9.1 Power Cycles 224 5.9.2 Refrigeration and Heat Pump

229

231

5.10.3 Carnot Cycle Summary

231

5.11 Clausius Inequality 231

Chapter Summary and Study Guide 234

6

Using Entropy 243

6.1 Entropy—A System Property 244 6.1.1 Defining Entropy Change 244 6.1.2 Evaluating Entropy 245 6.1.3 Entropy and Probability 245 6.2 Retrieving Entropy Data 245 6.2.1 Vapor Data

246

6.2.2 Saturation Data 6.2.3 6.2.4

6.7 Entropy Balance for Closed Systems 257 6.7.1 Interpreting the Closed System Entropy

258

Balance

6.7.2 Evaluating Entropy Production and

Transfer

259

6.7.3 Applications of the Closed System

Entropy Balance

259

262

Balance

5.10.2 Carnot Refrigeration and Heat Pump

Cycles

255

Reversible Process of Water

6.7.4 Closed System Entropy Rate

226 5.10 Carnot Cycle 229 Cycles

5.10.1 Carnot Power Cycle

6.6.3 Work and Heat Transfer in an Internally

246 Liquid Data 246 Computer Retrieval 247

6.2.5 Using Graphical Entropy Data

247 6.3 Introducing the T dS Equations 248 6.4 Entropy Change of an Incompressible Substance 250 6.5 Entropy Change of an Ideal Gas 251 6.5.1 Using Ideal Gas Tables 251 6.5.2 Assuming Constant Specific Heats 253 6.5.3 Computer Retrieval 253 6.6 Entropy Change in Internally Reversible Processes of Closed Systems 254 6.6.1 Area Representation of Heat Transfer 254 6.6.2 Carnot Cycle Application 254

6.8 Directionality of Processes 264 6.8.1 Increase of Entropy Principle 264 6.8.2 Statistical Interpretation

of Entropy

267

6.9 Entropy Rate Balance for Control Volumes 269 6.10 Rate Balances for Control Volumes at Steady State 270 6.10.1 One-Inlet, One-Exit Control Volumes

at Steady State

270

6.10.2 Applications of the Rate Balances to

Control Volumes at Steady State 271

6.11 Isentropic Processes 277 6.11.1 General Considerations

278 278

6.11.2 Using the Ideal Gas Model

6.11.3 Illustrations: Isentropic Processes

of Air

280

6.12 Isentropic Efficiencies of Turbines, Nozzles, Compressors, and Pumps 284 6.12.1 Isentropic Turbine Efficiency 284 6.12.2 Isentropic Nozzle Efficiency 287 6.12.3 Isentropic Compressor and Pump

Efficiencies

289

6.13 Heat Transfer and Work in Internally Reversible, Steady-State Flow Processes 291 6.13.1 Heat Transfer 291 6.13.2 Work 292 6.13.3 Work in Polytropic Processes 293

Chapter Summary and Study Guide 295

Contents

7 Exergy Analysis

8.2.3 Effects of Boiler and Condenser

309

Pressures on the Rankine Cycle

7.1 Introducing Exergy 310 7.2 Conceptualizing Exergy 311 7.2.1 Environment and Dead State 312 7.2.2 Defining Exergy 312

7.3 Exergy of a System 312 7.3.1 Exergy Aspects 315 7.3.2 Specific Exergy 316 7.3.3 Exergy Change 318

7.4 Closed System Exergy Balance 318 7.4.1 Introducing the Closed System Exergy

Balance

319

7.4.2 Closed System Exergy Rate

Balance 323 7.4.3 Exergy Destruction and Loss 324 7.4.4 Exergy Accounting 326

7.5 Exergy Rate Balance for Control Volumes at Steady State 327 7.5.1 Comparing Energy and Exergy for

Control Volumes at Steady State 330 7.5.2 Evaluating Exergy Destruction in Control

Volumes at Steady State 330 7.5.3 Exergy Accounting in Control Volumes at

Steady State 335

7.6 Exergetic (Second Law) Efficiency 339

8.3 Improving Performance—Superheat, Reheat, and Supercritical 389 8.4 Improving Performance—Regenerative Vapor Power Cycle 395 8.4.1 Open Feedwater Heaters 395 8.4.2 Closed Feedwater Heaters 400 8.4.3 Multiple Feedwater Heaters 401 8.5 Other Vapor Power Cycle Aspects 405 8.5.1 Working Fluids 8.5.2

405 Cogeneration 407

8.5.3 Carbon Capture and Storage

Chapter Summary and Study Guide 417

9 Gas Power Systems

427

Considering Internal Combustion Engines 428 9.1 Introducing Engine Terminology 428 9.2 Air-Standard Otto Cycle 431

7.6.2 Exergetic Efficiencies of Common

9.4 Air-Standard Dual Cycle 440

7.6.3 Using Exergetic Efficiencies 344

7.7 Thermoeconomics 345 7.7.1 Costing 345 7.7.2 Using Exergy in Design 346 7.7.3 Exergy Costing of a Cogeneration

348 Chapter Summary and Study Guide 353 System

8 Vapor Power Systems

Considering Gas Turbine Power Plants 443 9.5 Modeling Gas Turbine Power Plants 443 9.6 Air-Standard Brayton Cycle 445 9.6.1 Evaluating Principal Work and Heat

Transfers

445

9.6.2 Ideal Air-Standard Brayton Cycle

Introducing Power Generation 368 Considering Vapor Power Systems 372 8.1 Introducing Vapor Power Plants 372 8.2 The Rankine Cycle 375 8.2.1 Modeling the Rankine Cycle 376 8.2.2 Ideal Rankine Cycle 379

446

9.6.3 Considering Gas Turbine Irreversibilities

and Losses

367

407

8.6 Case Study: Exergy Accounting of a Vapor Power Plant 410

9.3 Air-Standard Diesel Cycle 436

342

383 385

8.2.4 Principal Irreversibilities and Losses

7.6.1 Matching End Use to Source 340

Components

xv

452

9.7 Regenerative Gas Turbines 455 9.8 Regenerative Gas Turbines with Reheat and Intercooling 459 9.8.1 Gas Turbines with Reheat 460 9.8.2 Compression with Intercooling 462 9.8.3 Reheat and Intercooling 466 9.8.4 Ericsson and Stirling Cycles 469

xvi

Contents

9.9 Gas Turbine–Based Combined Cycles 471

10.2.3 Performance of Actual Vapor-

Compression Systems

9.9.1 Combined Gas Turbine–Vapor Power

Cycle

471

9.9.2 Cogeneration

478 9.10 Integrated Gasification CombinedCycle Power Plants 478 9.11 Gas Turbines for Aircraft Propulsion 480 Considering Compressible Flow through Nozzles and Diffusers 484 9.12 Compressible Flow Preliminaries 485 9.12.1 Momentum Equation for Steady

One-Dimensional Flow

485

9.12.3 Determining Stagnation State

489 9.13 Analyzing One-Dimensional Steady Flow in Nozzles and Diffusers 489 Properties

9.13.1 Exploring the Effects of Area Change

in Subsonic and Supersonic Flows 489

492 494

9.14 Flow in Nozzles and Diffusers of Ideal Gases with Constant Specific Heats 495 9.14.1 Isentropic Flow Functions 496 9.14.2 Normal Shock Functions 499

Chapter Summary and Study Guide 503

10 Refrigeration and Heat Pump Systems 516

Intercooling

532

10.5 Absorption Refrigeration 533 10.6 Heat Pump Systems 535 10.6.1 Carnot Heat Pump Cycle

535

535

10.7 Gas Refrigeration Systems 539 10.7.1 Brayton Refrigeration Cycle 539 10.7.2 Additional Gas Refrigeration

Applications

543

10.7.3 Automotive Air Conditioning Using

Carbon Dioxide

544

11 Thermodynamic Relations

554

11.1 Using Equations of State 555 11.1.1 Getting Started 555 11.1.2 Two-Constant Equations of State

556 560 11.2 Important Mathematical Relations 561 11.3 Developing Property Relations 564 11.3.1 Principal Exact Differentials 565 11.1.3 Multiconstant Equations of State

11.3.2 Property Relations from Exact

Differentials

565

11.3.3 Fundamental Thermodynamic

10.1 Vapor Refrigeration Systems 517 10.1.1 Carnot Refrigeration Cycle 517 10.1.2 Departures from the Carnot Cycle 518 10.2 Analyzing Vapor-Compression Refrigeration Systems 519 10.2.1 Evaluating Principal Work and Heat

Transfers

10.4.3 Multistage Compression with

Chapter Summary and Study Guide 546

9.13.2 Effects of Back Pressure on Mass 9.13.3 Flow Across a Normal Shock

527

10.3 Selecting Refrigerants 527 10.4 Other Vapor-Compression Applications 530 10.4.1 Cold Storage 530 10.4.2 Cascade Cycles 531

Pumps

486

Flow Rate

523

10.6.2 Vapor-Compression Heat

9.12.2 Velocity of Sound and Mach

Number

10.2.4 The p –h Diagram

519

10.2.2 Performance of Ideal Vapor-

Compression Systems

520

Functions

570

11.4 Evaluating Changes in Entropy, Internal Energy, and Enthalpy 571 11.4.1 Considering Phase Change 571 11.4.2 Considering Single-Phase

Regions

574

11.5 Other Thermodynamic Relations 579 11.5.1 Volume Expansivity, Isothermal and

Isentropic Compressibility

580

Contents 11.5.2 Relations Involving Specific

Heats

581

11.5.3 Joule–Thomson Coefficient

584 11.6 Constructing Tables of Thermodynamic Properties 586 11.6.1 Developing Tables by Integration

Using p–𝜐–T and Specific Heat Data 587 11.6.2 Developing Tables by Differentiating

a Fundamental Thermodynamic Function 588

11.7 Generalized Charts for Enthalpy and Entropy 591 11.8 p–𝝊–T Relations for Gas Mixtures 598 11.9 Analyzing Multicomponent Systems 602 11.9.1 Partial Molal Properties 603 11.9.2 Chemical Potential 605 11.9.3 Fundamental Thermodynamic Functions

for Multicomponent Systems 11.9.4 Fugacity

606

608

11.9.5 Ideal Solution

11.9.6 Chemical Potential for Ideal

612 Chapter Summary and Study Guide 613 Solutions

12.5.2 Humidity Ratio, Relative Humidity, Mixture

Enthalpy, and Mixture Entropy

Psychrometric Applications 625 Ideal Gas Mixtures: General Considerations 626 12.1 Describing Mixture Composition 626 12.2 Relating p, V, and T for Ideal Gas Mixtures 630 12.3 Evaluating U, H, S, and Specific Heats 631 12.3.1 Evaluating U and H 631 12.3.2 Evaluating c𝜐 and cp 632 12.3.3 Evaluating S 632 12.3.4 Working on a Mass Basis 633 12.4 Analyzing Systems Involving Mixtures 634 12.4.1 Mixture Processes at Constant

634

12.4.2 Mixing of Ideal Gases

648

12.5.3 Modeling Moist Air in Equilibrium with

650

Liquid Water

12.5.4 Evaluating the Dew Point Temperature

651 12.5.5 Evaluating Humidity Ratio Using the

657 12.6 Psychrometers: Measuring the Wet-Bulb and Dry-Bulb Temperatures 658 12.7 Psychrometric Charts 660 12.8 Analyzing Air-Conditioning Processes 661 Adiabatic-Saturation Temperature

12.8.1 Applying Mass and Energy Balances

to Air-Conditioning Systems

663

12.8.3 Dehumidification 12.8.4 12.8.5

666 Humidification 670 Evaporative Cooling 672

12.8.6 Adiabatic Mixing of Two Moist Air

675

12.9 Cooling Towers 678

Chapter Summary and Study Guide 681

13 Reacting Mixtures and Combustion 693 Combustion Fundamentals 694 13.1 Introducing Combustion 694 13.1.1 Fuels 694 13.1.2 Modeling Combustion Air 695 13.1.3 Determining Products of Combustion 13.1.4 Energy and Entropy Balances for

702 13.2 Conservation of Energy—Reacting Systems 703 Reacting Systems

13.2.1 Evaluating Enthalpy for Reacting

Systems

703

13.2.2 Energy Balances for Reacting

641

661

12.8.2 Conditioning Moist Air at Constant

Streams

12 Ideal Gas Mixture and

Composition

Psychrometric Applications 647 12.5 Introducing Psychrometric Principles 647 12.5.1 Moist Air 647

Composition

611

xvii

Systems

705

698

xviii

Contents 13.2.3 Enthalpy of Combustion and Heating

Values

713

13.4 Fuel Cells 720 13.4.1 Proton Exchange Membrane

722

13.4.2 Solid Oxide Fuel Cell

724

13.5 Absolute Entropy and the Third Law of Thermodynamics 724 13.5.1 Evaluating Entropy for Reacting

Systems

725

13.5.2 Entropy Balances for Reacting

Systems

726

13.5.3 Evaluating Gibbs Function for React-

ing Systems

731

13.6.1 Working Equations for Chemical

735

14.2.1 Introductory Case

764 14.2.2 General Case 765 14.3 Calculating Equilibrium Compositions 766 14.3.1 Equilibrium Constant for Ideal Gas

766

Mixtures

14.3.2 Illustrations of the Calculation of

Equilibrium Compositions for Reacting Ideal Gas Mixtures 769 14.3.3 Equilibrium Constant for Mixtures and

774 14.4 Further Examples of the Use of the Equilibrium Constant 776 Solutions

776

14.4.2 Van’t Hoff Equation 14.4.3 Ionization

780

781

14.4.4 Simultaneous Reactions

13.6.2 Evaluating Chemical Exergy for

Several Cases

735

13.6.3 Closing Comments

737

13.7 Standard Chemical Exergy 737 13.7.1 Standard Chemical Exergy of a

Hydrocarbon: CaHb

761

Chemical Equilibrium 764 14.2 Equation of Reaction Equilibrium 764

Temperature

13.6 Conceptualizing Chemical Exergy 733

738

13.7.2 Standard Chemical Exergy of Other

Substances

14.1.2 Evaluating Chemical Potentials

14.4.1 Determining Equilibrium Flame

Chemical Exergy 732

Exergy

760

Equilibrium

13.3 Determining the Adiabatic Flame Temperature 716 13.3.1 Using Table Data 717 13.3.2 Using Computer Software 717 13.3.3 Closing Comments 720

Fuel Cell

14.1.1 Chemical Potential and

741

Phase Equilibrium 785 14.5 Equilibrium between Two Phases of a Pure Substance 785 14.6 Equilibrium of Multicomponent, Multiphase Systems 787 14.6.1 Chemical Potential and Phase

Equilibrium

787

14.6.2 Gibbs Phase Rule

13.8 Applying Total Exergy 742 13.8.1 Calculating Total Exergy

782

742

790 Chapter Summary and Study Guide

791

13.8.2 Calculating Exergetic Efficiencies

745 Chapter Summary and Study Guide 748 of Reacting Systems

14 Chemical and Phase Equilibrium 758 Equilibrium Fundamentals 759 14.1 Introducing Equilibrium Criteria 759

Appendix Tables, Figures, and Charts 799 Index to Tables in SI Units 799 Index to Figures and Charts 847 Index 859 Answers to Selected Problems: Visit the student companion site at www.wiley.com/ college/moran.

1 Getting Started Introductory Concepts and Definitions ENGINEERING CONTEXT Although aspects of thermodynamics have been studied since ancient times, the formal study of thermodynamics began in the early nineteenth century through consideration of the capacity of hot objects to produce work. Today the scope is much larger. Thermodynamics now provides essential concepts and methods for addressing critical twenty-first-century issues, such as using fossil fuels more effectively, fostering renewable energy technologies, and developing more fuel-efficient means of transportation. Also critical are the related issues of greenhouse gas emissions and air and water pollution. Thermodynamics is both a branch of science and an engineering specialty. The scientist is normally interested in gaining a fundamental understanding of the physical and chemical behavior of fixed quantities of matter at rest and uses the principles of thermodynamics to relate the properties of matter. Engineers are generally interested in studying systems and how they interact with their surroundings. To facilitate this, thermodynamics has been extended to the study of systems through which matter flows, including bioengineering and biomedical systems. The objective of this chapter is to introduce you to some of the fundamental concepts and definitions that are used in our study of engineering thermodynamics. In most instances this introduction is brief, and further elaboration is provided in subsequent chapters.

LEARNING OUTCOMES When you complete your study of this chapter, you will be able to... c

demonstrate understanding of several fundamental concepts used throughout the book . . . including closed system, control volume, boundary and surroundings, property, state, process, the distinction between extensive and intensive properties, and equilibrium.

c

apply SI Engineering units, including units for specific volume, pressure, and temperature.

c

work with the Kelvin and Celsius temperature scales.

c

apply the problem-solving methodology used in this book.

1

2

Chapter 1 Getting Started

1.1 Using Thermodynamics Engineers use principles drawn from thermodynamics and other engineering sciences, including fluid mechanics and heat and mass transfer, to analyze and design things intended to meet human needs. Throughout the twentieth century, engineering applications of thermodynamics helped pave the way for significant improvements in our quality of life with advances in major areas such as surface transportation, air travel, space flight, electricity generation and transmission, building heating and cooling, and improved medical practices. The wide realm of these applications is suggested by Table 1.1. In the twenty-first century, engineers will create the technology needed to achieve a sustainable future. Thermodynamics will continue to advance human well-being by addressing looming societal challenges owing to declining supplies of energy resources: oil, natural gas, coal, and fissionable material; effects of global climate change; and burgeoning population. Life is expected to change in several important respects by mid-century. In the area of power use, for example, electricity will play an even greater role than today. Table 1.2 provides predictions of other changes experts say will be observed. If this vision of mid-century life is correct, it will be necessary to evolve quickly from our present energy posture. As was the case in the twentieth century, thermodynamics will contribute significantly to meeting the challenges of the twenty-first century, including using fossil fuels more effectively, advancing renewable energy technologies, and developing more energy-efficient transportation systems, buildings, and industrial practices. Thermodynamics also will play a role in mitigating global climate change, air pollution, and water pollution. Applications will be observed in bioengineering, biomedical systems, and the deployment of nanotechnology. This book provides the tools needed by specialists working in all such fields. For nonspecialists, the book provides background for making decisions about technology related to thermodynamics—on the job, as informed citizens, and as government leaders and policy makers.

1.2 Defining Systems

system

surroundings boundary

The key initial step in any engineering analysis is to describe precisely what is being studied. In mechanics, if the motion of a body is to be determined, normally the first step is to define a free body and identify all the forces exerted on it by other bodies. Newton’s second law of motion is then applied. In thermodynamics the term system is used to identify the subject of the analysis. Once the system is defined and the relevant interactions with other systems are identified, one or more physical laws or relations are applied. The system is whatever we want to study. It may be as simple as a free body or as complex as an entire chemical refinery. We may want to study a quantity of matter contained within a closed, rigid-walled tank, or we may want to consider something such as a pipeline through which natural gas flows. The composition of the matter inside the system may be fixed or may be changing through chemical or nuclear reactions. The shape or volume of the system being analyzed is not necessarily constant, as when a gas in a cylinder is compressed by a piston or a balloon is inflated. Everything external to the system is considered to be part of the system’s surroundings. The system is distinguished from its surroundings by a specified boundary, which may be at rest or in motion. You will see that the interactions between a system and its surroundings, which take place across the boundary, play an important part in engineering thermodynamics. Two basic kinds of systems are distinguished in this book. These are referred to, respectively, as closed systems and control volumes. A closed system refers to a fixed quantity of matter, whereas a control volume is a region of space through which mass may flow. The term control mass is sometimes used in place of closed system, and the term open system is used interchangeably with control volume. When the terms control mass and control volume are used, the system boundary is often referred to as a control surface.

1.2 Defining Systems

3

TABLE 1.1

Selected Areas of Application of Engineering Thermodynamics Aircraft and rocket propulsion Alternative energy systems Fuel cells Geothermal systems Magnetohydrodynamic (MHD) converters Ocean thermal, wave, and tidal power generation Solar-activated heating, cooling, and power generation Thermoelectric and thermionic devices Wind turbines Automobile engines Bioengineering applications Biomedical applications Combustion systems Compressors, pumps Cooling of electronic equipment Cryogenic systems, gas separation, and liquefaction Fossil and nuclear-fueled power stations Heating, ventilating, and air-conditioning systems Absorption refrigeration and heat pumps Vapor-compression refrigeration and heat pumps Steam and gas turbines Power production Propulsion

Solar-cell arrays

Surfaces with thermal control coatings International Space Station

control coatings

International Space Station

Steam generator

Combustion gas cleanup

Coal

Air Steam

Turbine Generator

Electric power

Cooling tower

Condenser

Ash Condensate Refrigerator

Stack

Cooling water Vehicle engine

Electrical power plant Trachea

Lung Fuel in Combustor Compressor Air in

Turbine Hot gases out Heart

Turbojet engine

Biomedical applications

4

Chapter 1 Getting Started TABLE 1.2

Predictions of Life in 2050 At home c Homes are constructed better to reduce heating and cooling needs. c Homes have systems for electronically monitoring and regulating energy use. c Appliances and heating and air-conditioning systems are more energy-efficient. c Use of solar energy for space and water heating is common. c More food is produced locally. Transportation c Plug-in hybrid vehicles and all-electric vehicles dominate. c Hybrid vehicles mainly use biofuels. c Use of public transportation within and between cities is common. c An expanded passenger railway system is widely used. Lifestyle c Efficient energy-use practices are utilized throughout society. c Recycling is widely practiced, including recycling of water. c Distance learning is common at most educational levels. c Telecommuting and teleconferencing are the norm. c The Internet is predominately used for consumer and business commerce. Power generation c Electricity plays a greater role throughout society. c Wind, solar, and other renewable technologies contribute a significant share of the nation’s electricity needs. c A mix of conventional fossil-fueled and nuclear power plants provide a smaller, but still significant, share of the nation’s electricity needs. c A smart and secure national power transmission grid is in place.

1.2.1 Closed Systems closed system

isolated system

Gas

Boundary

A closed system is defined when a particular quantity of matter is under study. A closed system always contains the same matter. There can be no transfer of mass across its boundary. A special type of closed system that does not interact in any way with its surroundings is called an isolated system. Figure 1.1 shows a gas in a piston–cylinder assembly. When the valves are closed, we can consider the gas to be a closed system. The boundary lies just inside the piston and cylinder walls, as shown by the dashed lines on the figure. Since the portion of the boundary between the gas and the piston moves with the piston, the system volume varies. No mass would cross this or any other part of the boundary. If combustion occurs, the composition of the system changes as the initial combustible mixture becomes products of combustion.

1.2.2 Control Volumes

Fig. 1.1 Closed system: A gas in a piston–cylinder assembly. control volume

In subsequent sections of this book, we perform thermodynamic analyses of devices such as turbines and pumps through which mass flows. These analyses can be conducted in principle by studying a particular quantity of matter, a closed system, as it passes through the device. In most cases it is simpler to think instead in terms of a given region of space through which mass flows. With this approach, a region within a prescribed boundary is studied. The region is called a control volume. Mass may cross the boundary of a control volume. A diagram of an engine is shown in Fig. 1.2a. The dashed line defines a control volume that surrounds the engine. Observe that air, fuel, and exhaust gases cross the boundary. A schematic such as in Fig. 1.2b often suffices for engineering analysis.

1.2 Defining Systems Air in

Drive shaft

5

Air in

Exhaust gas out Fuel in

Fuel in

Drive shaft Exhaust gas out Boundary (control surface)

Boundary (control surface)

(a)

(b)

Fig. 1.2 Example of a control volume (open system). An automobile engine.

BIOCONNECTIONS Living things and their organs can be studied as control volumes. For the pet shown in Fig. 1.3a, air, food, and drink essential to sustain life and for activity enter across the boundary, and waste products exit. A schematic such as Fig. 1.3b can suffice for biological analysis. Particular organs, such as the heart, also can be studied as control volumes. As shown in Fig. 1.4, plants can be studied from a control volume viewpoint. Intercepted solar radiation is used in the production of essential chemical substances within plants by photosynthesis. During photosynthesis, plants take in carbon dioxide from the atmosphere and discharge oxygen to the atmosphere. Plants also draw in water and nutrients through their roots.

1.2.3 Selecting the System Boundary The system boundary should be delineated carefully before proceeding with any thermodynamic analysis. However, the same physical phenomena often can be analyzed in terms of alternative choices of the system, boundary, and surroundings. The choice of a particular boundary defining a particular system depends heavily on the convenience it allows in the subsequent analysis.

CO2, other gases

Ingestion (food, drink)

Air

Air Gut

Ingestion (food, drink)

Boundary (control surface)

Lungs CO2

Boundary (control surface)

O2 CO2

Circulatory system Kidneys

Excretion (waste products)

Body tissues

O2

Photosynthesis (leaf)

Heart Excretion (undigested food)

(a)

Solar radiation

CO2, other gases

Excretion (urine) (b)

Fig. 1.3 Example of a control volume (open system) in biology.

H2O, minerals

Fig. 1.4 Example of a control volume (open system) in botany.

6

Chapter 1 Getting Started Air

TAKE NOTE...

Animations reinforce many of the text presentations. You can view these animations by going to the student companion site for this book. Animations are keyed to specific content by an icon in the margin. The first of these icons appears directly below. In this example, the label System_Types refers to the text content while A.1–Tabs a,b&c refers to the particular animation (A.1) and the tabs (Tabs a,b&c) of the animation recommended for viewing now to enhance your understanding.

A

System_Types A.1 – Tabs a, b & c

Tank Air compressor

Fig. 1.5 Air compressor and storage tank.

–

+

In general, the choice of system boundary is governed by two considerations: (1) what is known about a possible system, particularly at its boundaries, and (2) the objective of the analysis. Figure 1.5 shows a sketch of an air compressor connected to a storage tank. The system boundary shown on the figure encloses the compressor, tank, and all of the piping. This boundary might be selected if the electrical power input is known, and the objective of the analysis is to determine how long the compressor must operate for the pressure in the tank to rise to a specified value. Since mass crosses the boundary, the system would be a control volume. A control volume enclosing only the compressor might be chosen if the condition of the air entering and exiting the compressor is known and the objective is to determine the electric power input. b b b b b

1.3 Describing Systems and Their Behavior Engineers are interested in studying systems and how they interact with their surroundings. In this section, we introduce several terms and concepts used to describe systems and how they behave.

1.3.1 Macroscopic and Microscopic Views of Thermodynamics Systems can be studied from a macroscopic or a microscopic point of view. The macroscopic approach to thermodynamics is concerned with the gross or overall behavior. This is sometimes called classical thermodynamics. No model of the structure of matter at the molecular, atomic, and subatomic levels is directly used in classical thermodynamics. Although the behavior of systems is affected by molecular structure, classical thermodynamics allows important aspects of system behavior to be evaluated from observations of the overall system. The microscopic approach to thermodynamics, known as statistical thermodynamics, is concerned directly with the structure of matter. The objective of statistical thermodynamics is to characterize by statistical means the average behavior of the particles making up a system of interest and relate this information to the observed macroscopic behavior of the system. For applications involving lasers, plasmas, high-speed gas flows, chemical kinetics, very low temperatures (cryogenics), and others, the methods of statistical thermodynamics are essential. The microscopic approach is used in this text to interpret internal energy in Chap. 2 and entropy in Chap 6. Moreover, as

1.3 Describing Systems and Their Behavior

7

noted in Chap. 3, the microscopic approach is instrumental in developing certain data, for example, ideal gas specific heats. For a wide range of engineering applications, classical thermodynamics not only provides a considerably more direct approach for analysis and design but also requires far fewer mathematical complications. For these reasons the macroscopic viewpoint is the one adopted in this book. Finally, relativity effects are not significant for the systems under consideration in this book.

1.3.2 Property, State, and Process To describe a system and predict its behavior requires knowledge of its properties and how those properties are related. A property is a macroscopic characteristic of a system such as mass, volume, energy, pressure, and temperature to which a numerical value can be assigned at a given time without knowledge of the previous behavior (history) of the system. The word state refers to the condition of a system as described by its properties. Since there are normally relations among the properties of a system, the state often can be specified by providing the values of a subset of the properties. All other properties can be determined in terms of these few. When any of the properties of a system change, the state changes and the system is said to have undergone a process. A process is a transformation from one state to another. However, if a system exhibits the same values of its properties at two different times, it is in the same state at these times. A system is said to be at steady state if none of its properties change with time. Many properties are considered during the course of our study of engineering thermodynamics. Thermodynamics also deals with quantities that are not properties, such as mass flow rates and energy transfers by work and heat. Additional examples of quantities that are not properties are provided in subsequent chapters. For a way to distinguish properties from nonproperties, see the box on p. 8.

property

state

process steady state

Prop_State_Process A.2 – Tab a

A

1.3.3 Extensive and Intensive Properties Thermodynamic properties can be placed in two general classes: extensive and intensive. A property is called extensive if its value for an overall system is the sum of its values for the parts into which the system is divided. Mass, volume, energy, and several other properties introduced later are extensive. Extensive properties depend on the size or extent of a system. The extensive properties of a system can change with time, and many thermodynamic analyses consist mainly of carefully accounting for changes in extensive properties, such as mass and energy, as a system interacts with its surroundings. Intensive properties are not additive in the sense previously considered. Their values are independent of the size or extent of a system and may vary from place to place within the system at any moment. Thus, intensive properties may be functions of both position and time, whereas extensive properties can vary only with time. Specific volume (Sec. 1.5), pressure, and temperature are important intensive properties; several other intensive properties are introduced in subsequent chapters. to illustrate the difference between extensive and intensive properties, consider an amount of matter that is uniform in temperature, and imagine that it is composed of several parts, as illustrated in Fig. 1.6. The mass of the whole is the sum of the masses of the parts, and the overall volume is the sum of the volumes of the parts. However, the temperature of the whole is not the sum of the temperatures of the parts; it is the same for each part. Mass and volume are extensive, but temperature is intensive. b b b b b

extensive property

intensive property

Ext_Int_Properties A.3 – Tab a

A

8

Chapter 1 Getting Started

Fig. 1.6 Figure used to discuss the extensive and intensive property concepts.

(a)

(b)

Distinguishing Properties from Nonproperties At a given state each property has a definite value that can be assigned without knowledge of how the system arrived at that state. Therefore, the change in value of a property as the system is altered from one state to another is determined solely by the two end states and is independent of the particular way the change of state occurred. That is, the change is independent of the details of the process. Conversely, if the value of a quantity is independent of the process between two states, then that quantity is the change in a property. This provides a test for determining whether a quantity is a property: A quantity is a property if, and only if, its change in value between two states is independent of the process. It follows that if the value of a particular quantity depends on the details of the process, and not solely on the end states, that quantity cannot be a property.

1.3.4 Equilibrium equilibrium

equilibrium state

Classical thermodynamics places primary emphasis on equilibrium states and changes from one equilibrium state to another. Thus, the concept of equilibrium is fundamental. In mechanics, equilibrium means a condition of balance maintained by an equality of opposing forces. In thermodynamics, the concept is more far-reaching, including not only a balance of forces but also a balance of other influences. Each kind of influence refers to a particular aspect of thermodynamic, or complete, equilibrium. Accordingly, several types of equilibrium must exist individually to fulfill the condition of complete equilibrium; among these are mechanical, thermal, phase, and chemical equilibrium. Criteria for these four types of equilibrium are considered in subsequent discussions. For the present, we may think of testing to see if a system is in thermodynamic equilibrium by the following procedure: Isolate the system from its surroundings and watch for changes in its observable properties. If there are no changes, we conclude that the system was in equilibrium at the moment it was isolated. The system can be said to be at an equilibrium state. When a system is isolated, it does not interact with its surroundings; however, its state can change as a consequence of spontaneous events occurring internally as its intensive properties, such as temperature and pressure, tend toward uniform values. When all such changes cease, the system is in equilibrium. At equilibrium, temperature is uniform throughout the system. Also, pressure can be regarded as uniform throughout as long as the effect of gravity is not significant; otherwise a pressure variation can exist, as in a vertical column of liquid. There is no requirement that a system undergoing a process be in equilibrium during the process. Some or all of the intervening states may be nonequilibrium states. For many such processes we are limited to knowing the state before the process occurs and the state after the process is completed.

1.4 Measuring Mass, Length, Time, and Force

1.4 Measuring Mass, Length, Time, and Force When engineering calculations are performed, it is necessary to be concerned with the units of the physical quantities involved. A unit is any specified amount of a quantity by comparison with which any other quantity of the same kind is measured. For example, meters, centimeters, and kilometers are all units of length. Seconds, minutes, and hours are alternative time units. Because physical quantities are related by definitions and laws, a relatively small number of physical quantities suffice to conceive of and measure all others. These are called primary dimensions. The others are measured in terms of the primary dimensions and are called secondary. For example, if length and time were regarded as primary, velocity and area would be secondary. A set of primary dimensions that suffice for applications in mechanics are mass, length, and time. Additional primary dimensions are required when additional physical phenomena come under consideration. Temperature is included for thermodynamics, and electric current is introduced for applications involving electricity. Once a set of primary dimensions is adopted, a base unit for each primary dimension is specified. Units for all other quantities are then derived in terms of the base units. Let us illustrate these ideas by considering briefly two systems of units: SI units and English Engineering units.

base unit

1.4.1 SI Units In the present discussion we consider the system of units called SI that takes mass, length, and time as primary dimensions and regards force as secondary. SI is the abbreviation for Système International d’Unités (International System of Units), which is the legally accepted system in most countries. The conventions of the SI are published and controlled by an international treaty organization. The SI base units for mass, length, and time are listed in Table 1.3 and discussed in the following paragraphs. The SI base unit for temperature is the kelvin, K. The SI base unit of mass is the kilogram, kg. It is equal to the mass of a particular cylinder of platinum–iridium alloy kept by the International Bureau of Weights and Measures near Paris. The mass standard for the United States is maintained by the National Institute of Standards and Technology. The kilogram is the only base unit still defined relative to a fabricated object. The SI base unit of length is the meter (metre), m, defined as the length of the path traveled by light in a vacuum during a specified time interval. The base unit of time is the second, s. The second is defined as the duration of 9,192,631,770 cycles of the radiation associated with a specified transition of the cesium atom.

TABLE 1.3

Units for Mass, Length, Time, and Force SI Quantity

Unit

mass length time force

kilogram meter second newton (5 1 kg · m/s2)

Symbol

kg m s N

SI base units

9

10

Chapter 1 Getting Started The SI unit of force, called the newton, is a secondary unit, defined in terms of the base units for mass, length, and time. Newton’s second law of motion states that the net force acting on a body is proportional to the product of the mass and the acceleration, written F r ma. The newton is defined so that the proportionality constant in the expression is equal to unity. That is, Newton’s second law is expressed as the equality F 5 ma

(1.1)

The newton, N, is the force required to accelerate a mass of 1 kilogram at the rate of 1 meter per second per second. With Eq. 1.1 1 N 5 (1 kg)(1 m/s2) 5 1 kg ? m/s2

TAKE NOTE...

Observe that in the calculation of force in newtons, the unit conversion factor is set off by a pair of vertical lines. This device is used throughout the text to identify unit conversions.

to illustrate the use of the SI units introduced thus far, let us determine the weight in newtons of an object whose mass is 1000 kg, at a place on the earth’s surface where the acceleration due to gravity equals a standard value defined as 9.80665 m/s2. Recalling that the weight of an object refers to the force of gravity, and is calculated using the mass of the object, m, and the local acceleration of gravity, g, with Eq. 1.1 we get F 5 mg 5 (1000 kg)(9.80665 m/s2) 5 9806.65 kg ? m/s2 This force can be expressed in terms of the newton by using Eq. 1.2 as a unit conversion factor. That is, F 5 a9806.65

kg ? m s

2

b`

1N ` 5 9806.65 N 1 kg ? m/s2

b b b b b

Since weight is calculated in terms of the mass and the local acceleration due to gravity, the weight of an object can vary because of the variation of the acceleration of gravity with location, but its mass remains constant.

TABLE 1.4

SI Unit Prefixes Factor

Prefix

Symbol

12

tera giga mega kilo hecto centi milli micro nano pico

T G M k h c m 𝜇 n p

10 109 106 103 102 1022 1023 1026 1029 10212

(1.2)

if the object considered previously were on the surface of a planet at a point where the acceleration of gravity is one-tenth of the value used in the above calculation, the mass would remain the same but the weight would be onetenth of the calculated value. b b b b b SI units for other physical quantities are also derived in terms of the SI base units. Some of the derived units occur so frequently that they are given special names and symbols, such as the newton. SI units for quantities pertinent to thermodynamics are given as they are introduced in the text. Since it is frequently necessary to work with extremely large or small values when using the SI unit system, a set of standard prefixes is provided in Table 1.4 to simplify matters. For example, km denotes kilometer, that is, 103 m.

1.4.2 English Engineering Units

English base units

Although SI units are the worldwide standard, at the present time many segments of the engineering community in the United States regularly use other units. A large portion of America’s stock of tools and industrial machines and much valuable engineering data utilize units other than SI units. For many years to come, engineers in the United States will have to be conversant with a variety of units. In this section we consider a system of units that is commonly used in the United States, called the English Engineering system. The English base units for mass, length, and time are listed in Table 1.3 and discussed in the following paragraphs. English units for other quantities pertinent to thermodynamics are given as they are introduced in the text.

1.5 Specific Volume

11

The base unit for length is the foot, ft, defined in terms of the meter as 1 ft 5 0.3048 m

(1.3)

The inch, in., is defined in terms of the foot as 12 in. 5 1 ft One inch equals 2.54 cm. Although units such as the minute and the hour are often used in engineering, it is convenient to select the second as the English Engineering base unit for time. The English Engineering base unit of mass is the pound mass, lb, defined in terms of the kilogram as 1 lb 5 0.45359237 kg

(1.4)

The symbol lbm also may be used to denote the pound mass. Once base units have been specified for mass, length, and time in the English Engineering system of units, a force unit can be defined, as for the newton, using Newton’s second law written as Eq. 1.1. From this viewpoint, the English unit of force, the pound force, lbf, is the force required to accelerate one pound mass at 32.1740 ft/s2, which is the standard acceleration of gravity. Substituting values into Eq. 1.1 1 lbf 5 (1 lb)(32.1740 ft/s2) 5 32.1740 lb ? ft/s2

(1.5)

With this approach force is regarded as secondary. The pound force, lbf, is not equal to the pound mass, lb, introduced previously. Force and mass are fundamentally different, as are their units. The double use of the word “pound” can be confusing, however, and care must be taken to avoid error. to show the use of these units in a single calculation, let us determine the weight of an object whose mass is 1000 lb at a location where the local acceleration of gravity is 32.0 ft/s2. By inserting values into Eq. 1.1 and using Eq. 1.5 as a unit conversion factor, we get ft 1 lbf F 5 mg 5 (1000 lb)a32.0 2 b ` ` 5 994.59 lbf s 32.1740 lb ? ft/s2 This calculation illustrates that the pound force is a unit of force distinct from the pound mass, a unit of mass. b b b b b

1.5 Specific Volume Three measurable intensive properties that are particularly important in engineering thermodynamics are specific volume, pressure, and temperature. Specific volume is considered in this section. Pressure and temperature are considered in Secs. 1.6 and 1.7, respectively. From the macroscopic perspective, the description of matter is simplified by considering it to be distributed continuously throughout a region. The correctness of this idealization, known as the continuum hypothesis, is inferred from the fact that for an extremely large class of phenomena of engineering interest the resulting description of the behavior of matter is in agreement with measured data. When substances can be treated as continua, it is possible to speak of their intensive thermodynamic properties “at a point.” Thus, at any instant the density 𝜌 at a point is defined as m 𝜌 5 lim a b VSV¿ V

(1.6)

Ext_Int_Properties A.3 – Tabs b & c

A

12

Chapter 1 Getting Started where V9 is the smallest volume for which a definite value of the ratio exists. The volume V9 contains enough particles for statistical averages to be significant. It is the smallest volume for which the matter can be considered a continuum and is normally small enough that it can be considered a “point.” With density defined by Eq. 1.6, density can be described mathematically as a continuous function of position and time. The density, or local mass per unit volume, is an intensive property that may vary from point to point within a system. Thus, the mass associated with a particular volume V is determined in principle by integration m5

# 𝜌dV

(1.7)

V

specific volume

molar basis

and not simply as the product of density and volume. The specific volume 𝜐 is defined as the reciprocal of the density, 𝜐 5 1Y𝜌. It is the volume per unit mass. Like density, specific volume is an intensive property and may vary from point to point. SI units for density and specific volume are kg/m3 and m3/kg, respectively. However, they are also often expressed, respectively, as g/cm3 and cm3/g. In certain applications it is convenient to express properties such as specific volume on a molar basis rather than on a mass basis. A mole is an amount of a given substance numerically equal to its molecular weight. In this book we express the amount of substance on a molar basis in terms of the kilomole (kmol). In each case we use n5

m M

(1.8)

The number of kilomoles of a substance, n, is obtained by dividing the mass, m, in kilograms by the molecular weight, M, in kg/kmol. When m is in grams, Eq. 1.8 gives n in gram moles, or mol for short. Recall from chemistry that the number of molecules in a gram mole, called Avogadro’s number, is 6.022 3 1023. Appendix Tables A-1 and A-1E provide molecular weights for several substances. To signal that a property is on a molar basis, a bar is used over its symbol. Thus, 𝜐 signifies the volume per kmol. In this text, the unit used for 𝜐 is m3/kmol. With Eq. 1.8, the relationship between 𝜐 and 𝜐 is 𝜐 5 M𝜐

(1.9)

where M is the molecular weight in kg/kmol.

1.6 Pressure

pressure

Next, we introduce the concept of pressure from the continuum viewpoint. Let us begin by considering a small area A passing through a point in a fluid at rest. The fluid on one side of the area exerts a compressive force on it that is normal to the area, Fnormal. An equal but oppositely directed force is exerted on the area by the fluid on the other side. For a fluid at rest, no other forces than these act on the area. The pressure p at the specified point is defined as the limit p 5 lim a ASA¿

A

Ext_Int_Properties A.3 – Tab d

Fnormal b A

(1.10)

where A9 is the area at the “point” in the same limiting sense as used in the definition of density.

1.6 Pressure

13

Big Hopes for Nanotechnology Nanoscience is the study of molecules and molecular structures, called nanostructures, having one or more dimensions less than about 100 nanometers. One nanometer is one billionth of a meter: 1 nm 5 1029 m. To grasp this level of smallness, a stack of 10 hydrogen atoms would have a height of 1 nm, while a human hair has a diameter about 50,000 nm. Nanotechnology is the engineering of nanostructures into useful products. At the nanotechnology scale, behavior may differ from our macroscopic expectations. For example, the averaging used to assign property values at a point in the

continuum model may no longer apply owing to the interactions among the atoms under consideration. Also at these scales, the nature of physical phenomena such as current flow may depend explicitly on the physical size of devices. After many years of fruitful research, nanotechnology is now poised to provide new products with a broad range of uses, including implantable chemotherapy devices, biosensors for glucose detection in diabetics, novel electronic devices, new energy conversion technologies, and “smart materials,” as, for example, fabrics that allow water vapor to escape while keeping liquid water out.

If the area A9 were given new orientations by rotating it around the given point, and the pressure were determined for each new orientation, it would be found that the pressure at the point is the same in all directions as long as the fluid is at rest. This is a consequence of the equilibrium of forces acting on an element of volume surrounding the point. However, the pressure can vary from point to point within a fluid at rest; examples are the variation of atmospheric pressure with elevation and the pressure variation with depth in oceans, lakes, and other bodies of water. Consider next a fluid in motion. In this case the force exerted on an area passing through a point in the fluid may be resolved into three mutually perpendicular components: one normal to the area and two in the plane of the area. When expressed on a unit area basis, the component normal to the area is called the normal stress, and the two components in the plane of the area are termed shear stresses. The magnitudes of the stresses generally vary with the orientation of the area. The state of stress in a fluid in motion is a topic that is normally treated thoroughly in fluid mechanics. The deviation of a normal stress from the pressure, the normal stress that would exist were the fluid at rest, is typically very small. In this book we assume that the normal stress at a point is equal to the pressure at that point. This assumption yields results of acceptable accuracy for the applications considered. Also, the term pressure, unless stated otherwise, refers to absolute pressure: pressure with respect to the zero pressure of a complete vacuum.

absolute pressure patm Gas at pressure p

L

Tank a

b

Manometer liquid

1.6.1 Pressure Measurement

Fig. 1.7 Manometer.

Manometers and barometers measure pressure in terms of the length of a column of liquid such as mercury, water, or oil. The manometer shown in Fig. 1.7 has one end open to the atmosphere and the other attached to a tank containing a gas at a uniform pressure. Since pressures at equal elevations in a continuous mass of a liquid or gas at rest are equal, the pressures at points a and b of Fig. 1.7 are equal. Applying an elementary force balance, the gas pressure is p 5 patm 1 𝜌gL

Mercury vapor, pvapor

L patm

(1.11)

where patm is the local atmospheric pressure, 𝜌 is the density of the manometer liquid, g is the acceleration of gravity, and L is the difference in the liquid levels. The barometer shown in Fig. 1.8 is formed by a closed tube filled with liquid mercury and a small amount of mercury vapor inverted in an open container of liquid mercury. Since the pressures at points a and b are equal, a force balance gives the

b Mercury, ρm

Fig. 1.8 Barometer.

a

14

Chapter 1 Getting Started

Elliptical metal Bourdon tube

Pointer

atmospheric pressure as patm 5 pvapor 1 𝜌mgL

Pinion gear Support Linkage

where 𝜌m is the density of liquid mercury. Because the pressure of the mercury vapor is much less than that of the atmosphere, Eq. 1.12 can be approximated closely as patm 5 𝜌mgL. For short columns of liquid, 𝜌 and g in Eqs. 1.11 and 1.12 may be taken as constant. Pressures measured with manometers and barometers are frequently expressed in terms of the length L in millimeters of mercury (mmHg), inches of mercury (inHg), inches of water (inH2O), and so on. a barometer reads 750 mmHg. If 𝜌m 5 13.59 g/cm3 and g 5 9.81 m/s , the atmospheric pressure, in N/m2, is calculated as follows: 2

Gas at pressure p

Fig. 1.9 Pressure measurement by a Bourdon tube gage.

(1.12)

patm 5 𝜌mgL 5 c a13.59

1 kg 102 cm 3 1N m 1m `` `d ` b ` ` d c9.81 2 d c (750 mmHg) ` 3 ` 3 3 cm 10 g 1 m s 10 mm 1 kg ? m/s2

5 105 N/m2

g

b b b b b

A Bourdon tube gage is shown in Fig. 1.9. The figure shows a curved tube having an elliptical cross section with one end attached to the pressure to be measured and the other end connected to a pointer by a mechanism. When fluid under pressure fills the tube, the elliptical section tends to become circular, and the tube straightens. This motion is transmitted by the mechanism to the pointer. By calibrating the deflection of the pointer for known pressures, a graduated scale can be determined from which any applied pressure can be read in suitable units. Because of its construction, the Bourdon tube measures the pressure relative to the pressure of the surroundings existing at the instrument. Accordingly, the dial reads zero when the inside and outside of the tube are at the same pressure. Pressure can be measured by other means as well. An important class of sensors utilize the piezoelectric effect: A charge is generated within certain solid materials when they are deformed. This mechanical input/electrical output provides the basis for pressure measurement as well as displacement and force measurements. Another important type of sensor employs a diaphragm that deflects when a force is applied, altering an inductance, resistance, or capacitance. Fig. 1.10 Pressure sensor with automatic data Figure 1.10 shows a piezoelectric pressure sensor together with an acquisition. automatic data acquisition system.

1.6.2 Buoyancy buoyant force

When a body is completely, or partially, submerged in a liquid, the resultant pressure force acting on the body is called the buoyant force. Since pressure increases with depth from the liquid surface, pressure forces acting from below are greater than pressure forces acting from above; thus the buoyant force acts vertically upward. The buoyant force has a magnitude equal to the weight of the displaced liquid (Archimedes’ principle). applying Eq. 1.11 to the submerged rectangular block shown in Fig. 1.11, the magnitude of the net force of pressure acting upward, the buoyant

1.6 Pressure

15

patm

force, is F 5 A(p2 2 p1) 5 A(patm 1 𝜌gL2) 2 A(patm 1 𝜌gL1) 5 𝜌gA(L2 2 L1) 5 𝜌gV

Liquid with density ρ

where V is the volume of the block and 𝜌 is the density of the surrounding liquid. Thus, the magnitude of the buoyant force acting on the block is equal to the weight of the displaced liquid. b b b b b

p1A

L1 L2

Block

Area = A

1.6.3 Pressure Units

p2A

The SI unit of pressure and stress is the pascal: 1 pascal 5 1 N/m2

Fig. 1.11 Evaluation of buoyant force for a submerged body.

However, multiples of the pascal: the kPa, the bar, and the MPa are frequently used: 1 kPa 5 103 N/m2 1 bar 5 105 N/m2 1 MPa 5 106 N/m2 Although atmospheric pressure varies with location on the earth, a standard reference value can be defined and used to express other pressures.

1 standard atmosphere (atm) 5 e

1.01325 3 105 N/m2 760 mmHg 5 29.92 inHg

(1.13)

Since 1 bar (105 N/m2) closely equals one standard atmosphere, it is a convenient pressure unit despite not being a standard SI unit. When working in SI, the bar, MPa, and kPa are all used in this text. Although absolute pressures must be used in thermodynamic relations, pressuremeasuring devices often indicate the difference between the absolute pressure of a system and the absolute pressure of the atmosphere existing outside the measuring device. The magnitude of the difference is called a gage pressure or a vacuum pressure. The term gage pressure is applied when the pressure of the system is greater than the local atmospheric pressure, patm: p(gage) 5 p(absolute) 2 patm(absolute)

(1.14)

When the local atmospheric pressure is greater than the pressure of the system, the term vacuum pressure is used: p(vacuum) 5 patm(absolute) 2 p(absolute)

gage pressure vacuum pressure

(1.15)

Engineers frequently use the letters a and g to distinguish between absolute and gage pressures. The relationship among the various ways of expressing pressure measurements is shown in Fig. 1.12.

TAKE NOTE...

In this book, the term pres es-sure refers to absolute pressure unless indicated otherwise.

16

Chapter 1 Getting Started

p (gage)

Absolute pressure that is greater than the local atmospheric pressure

Atmospheric pressure

p (vacuum)

p (absolute)

Absolute pressure that is less than the local atmospheric pressure

patm (absolute)

p (absolute)

Zero pressure

Zero pressure

Fig. 1.12 Relationships among the absolute, atmospheric, gage, and vacuum pressures.

BIOCONNECTIONS One in three Americans is said to have high blood pressure. Since this can lead to heart disease, strokes, and other serious medical complications, medical practitioners recommend regular blood pressure checks for everyone. Blood pressure measurement aims to determine the maximum pressure (systolic pressure) in an artery when the heart is pumping blood and the minimum pressure (diastolic pressure) when the heart is resting, each pressure expressed in millimeters of mercury, mmHg. The systolic and diastolic pressures of healthy persons should be less than about 120 mmHg and 80 mmHg, respectively. The standard blood pressure measurement apparatus in use for decades involving an inflatable cuff, mercury manometer, and stethoscope is gradually being replaced because of concerns over mercury toxicity and in response to special requirements, including monitoring during clinical exercise and during anesthesia. Also, for home use and selfmonitoring, many patients prefer easy-to-use automated devices that provide digital displays of blood pressure data. This has prompted biomedical engineers to rethink blood pressure measurement and develop new mercury-free and stethoscope-free approaches. One of these uses a highly sensitive pressure transducer to detect pressure oscillations within an inflated cuff placed around the patient’s arm. The monitor’s software uses these data to calculate the systolic and diastolic pressures, which are displayed digitally.

1.7 Temperature In this section the intensive property temperature is considered along with means for measuring it. A concept of temperature, like our concept of force, originates with our sense perceptions. Temperature is rooted in the notion of the “hotness” or “coldness” of objects. We use our sense of touch to distinguish hot objects from cold objects and to arrange objects in their order of “hotness,” deciding that 1 is hotter than 2, 2 hotter

1.7 Temperature than 3, and so on. But however sensitive human touch may be, we are unable to gauge this quality precisely. A definition of temperature in terms of concepts that are independently defined or accepted as primitive is difficult to give. However, it is possible to arrive at an objective understanding of equality of temperature by using the fact that when the temperature of an object changes, other properties also change. To illustrate this, consider two copper blocks, and suppose that our senses tell us that one is warmer than the other. If the blocks were brought into contact and isolated from their surroundings, they would interact in a way that can be described as a thermal (heat) interaction. During this interaction, it would be observed that the volume of the warmer block decreases somewhat with time, while the volume of the colder block increases with time. Eventually, no further changes in volume would be observed, and the blocks would feel equally warm. Similarly, we would be able to observe that the electrical resistance of the warmer block decreases with time, and that of the colder block increases with time; eventually the electrical resistances would become constant also. When all changes in such observable properties cease, the interaction is at an end. The two blocks are then in thermal equilibrium. Considerations such as these lead us to infer that the blocks have a physical property that determines whether they will be in thermal equilibrium. This property is called temperature, and we postulate that when the two blocks are in thermal equilibrium, their temperatures are equal. It is a matter of experience that when two objects are in thermal equilibrium with a third object, they are in thermal equilibrium with one another. This statement, which is sometimes called the zeroth law of thermodynamics, is tacitly assumed in every measurement of temperature. Thus, if we want to know if two objects are at the same temperature, it is not necessary to bring them into contact and see whether their observable properties change with time, as described previously. It is necessary only to see if they are individually in thermal equilibrium with a third object. The third object is usually a thermometer.

Ext_Int_Properties A.3 – Tab e

17

A

thermal (heat) interaction

thermal equilibrium

temperature

zeroth law of thermodynamics

1.7.1 Thermometers Any object with at least one measurable property that changes as its temperature changes can be used as a thermometer. Such a property is called a thermometric property. The particular substance that exhibits changes in the thermometric property is known as a thermometric substance. A familiar device for temperature measurement is the liquid-in-glass thermometer pictured in Fig. 1.13a, which consists of a glass capillary tube connected to a bulb filled with a liquid such as alcohol and sealed at the other end. The space above the liquid is occupied by the vapor of the liquid or an inert gas. As temperature increases, the liquid expands in volume and rises in the capillary. The length L of the liquid in the capillary depends on the temperature. Accordingly, the liquid is the thermometric substance and L is the thermometric property. Although this type of thermometer is commonly used for ordinary temperature measurements, it is not well suited for applications where extreme accuracy is required. More accurate sensors known as thermocouples are based on the principle that when two dissimilar metals are joined, an electromotive force (emf) that is primarily a function of temperature will exist in a circuit. In certain thermocouples, one thermocouple wire is platinum of a specified purity and the other is an alloy of platinum and rhodium. Thermocouples also utilize copper and constantan (an alloy of copper and nickel), iron and constantan, as well as several other pairs of materials. Electricalresistance sensors are another important class of temperature measurement devices. These sensors are based on the fact that the electrical resistance of various materials changes in a predictable manner with temperature. The materials used for this purpose are normally conductors (such as platinum, nickel, or copper) or semiconductors.

thermometric property

18

Chapter 1 Getting Started

L

Liquid (a)

(b)

(c)

Fig. 1.13 Thermometers. (a) Liquid-in-glass. (b) Electrical-resistance (c) Infraredsensing ear thermometer. Devices using conductors are known as resistance temperature detectors. Semiconductor types are called thermistors. A battery-powered electrical-resistance thermometer commonly used today is shown in Fig. 1.13b. A variety of instruments measure temperature by sensing radiation, such as the ear thermometer shown in Fig. 1.13c. They are known by terms such as radiation thermometers and optical pyrometers. This type of thermometer differs from those previously considered because it is not required to come in contact with the object whose temperature is to be determined, an advantage when dealing with moving objects or objects at extremely high temperatures.

ENERGY & ENVIRONMENT The mercury-in-glass fever thermometers, once found in nearly every medicine cabinet, are a thing of the past. The American Academy of Pediatrics has designated mercury as too toxic to be present in the home. Families are turning to safer alternatives and disposing of mercury thermometers. Proper disposal is an issue, experts say. The safe disposal of millions of obsolete mercury-filled thermometers has emerged in its own right as an environmental issue. For proper disposal, thermometers must be taken to hazardous-waste collection stations rather than simply thrown in the trash where they can be easily broken, releasing mercury. Loose fragments of broken thermometers and anything that contacted mercury should be transported in closed containers to appropriate disposal sites. The present generation of liquid-in-glass fever thermometers for home use contains patented liquid mixtures that are nontoxic, safe alternatives to mercury. Other types of thermometers also are used in the home, including battery-powered electrical-resistance thermometers.

1.7.2 Kelvin Temperature Scale Empirical means of measuring temperature such as considered in Sec. 1.7.1 have inherent limitations. the tendency of the liquid in a liquid-in-glass thermometer to freeze at low temperatures imposes a lower limit on the range of temperatures that can

1.7 Temperature be measured. At high temperatures liquids vaporize, and therefore these temperatures also cannot be determined by a liquid-in-glass thermometer. Accordingly, several different thermometers might be required to cover a wide temperature interval. b b b b b In view of the limitations of empirical means for measuring temperature, it is desirable to have a procedure for assigning temperature values that does not depend on the properties of any particular substance or class of substances. Such a scale is called a thermodynamic temperature scale. The Kelvin scale is an absolute thermodynamic temperature scale that provides a continuous definition of temperature, valid over all ranges of temperature. The unit of temperature on the Kelvin scale is the kelvin (K). The kelvin is the SI base unit for temperature. To develop the Kelvin scale, it is necessary to use the conservation of energy principle and the second law of thermodynamics; therefore, further discussion is deferred to Sec. 5.8 after these principles have been introduced. However, we note here that the Kelvin scale has a zero of 0 K, and lower temperatures than this are not defined. In thermodynamic relationships, temperature is always in terms of the Kelvin scale unless specifically stated otherwise. Still, the Celsius scale considered next is commonly encountered.

Kelvin scale

1.7.3 Celsius Scale

373.15

0.01 0.00 –273.15

Absolute zero

0.00

Kelvin

Ice point

Celsius

Triple point of water

273.16

Steam point

°C

273.15

K

100.0

The relationship of the Kelvin and Celsius scales is shown in Fig. 1.14 together with values for temperature at three fixed points: the triple point, ice point, and steam point. By international agreement, temperature scales are defined by the numerical value assigned to the easily reproducible triple point of water: the state of equilibrium between steam, ice, and liquid water (Sec. 3.2). As a matter of convenience, the temperature at this standard fixed point is defined as 273.16 kelvins, abbreviated as 273.16 K. This makes the temperature interval from the ice point 1 (273.15 K) to the steam point 2 equal to 100 K and thus in agreement over the interval with the Celsius scale, which assigns 100 Celsius degrees to it.

Fig. 1.14 Comparison of temperature scales. 1

The state of equilibrium between ice and air-saturated water at a pressure of 1 atm. The state of equilibrium between steam and liquid water at a pressure of 1 atm.

2

triple point

19

20

Chapter 1 Getting Started

Celsius scale

TAKE NOTE...

When making engineering calculations, it’s usually okay to round off the last number in Eq. 1.16. This is frequently done in this book.

The Celsius temperature scale uses the unit degree Celsius (°C), which has the same magnitude as the kelvin. Thus, temperature differences are identical on both scales. However, the zero point on the Celsius scale is shifted to 273.15 K, as shown by the following relationship between the Celsius temperature and the Kelvin temperature: T(°C) 5 T(K) 2 273.15

(1.16)

From this it can be concluded that on the Celsius scale the triple point of water is 0.01°C and that 0 K corresponds to −273.15°C. These values are shown on Fig. 1.14.

BIOCONNECTIONS Cryobiology, the science of life at low temperatures, comprises the study of biological materials and systems (proteins, cells, tissues, and organs) at temperatures ranging from the cryogenic (below about 120 K) to the hypothermic (low body temperature). Applications include freeze-drying pharmaceuticals, cryosurgery for removing unhealthy tissue, study of cold-adaptation of animals and plants, and long-term storage of cells and tissues (called cryopreservation). Cryobiology has challenging engineering aspects owing to the need for refrigerators capable of achieving the low temperatures required by researchers. Freezers to support research requiring cryogenic temperatures in the low-gravity environment of the International Space Station, shown in Table 1.1, are illustrative. Such freezers must be extremely compact and miserly in power use. Further, they must pose no hazards. On-board research requiring a freezer might include the growth of near-perfect protein crystals, important for understanding the structure and function of proteins and ultimately in the design of new drugs.

1.8 Engineering Design and Analysis The word engineer traces its roots to the Latin ingeniare, relating to invention. Today invention remains a key engineering function having many aspects ranging from developing new devices to addressing complex social issues using technology. In pursuit of many such activities, engineers are called upon to design and analyze things intended to meet human needs. Design and analysis are considered in this section.

1.8.1 Design

design constraints

Engineering design is a decision-making process in which principles drawn from engineering and other fields such as economics and statistics are applied, usually iteratively, to devise a system, system component, or process. Fundamental elements of design include the establishment of objectives, synthesis, analysis, construction, testing, and evaluation. Designs typically are subject to a variety of constraints related to economics, safety, environmental impact, and so on. Design projects usually originate from the recognition of a need or an opportunity that is only partially understood initially. Thus, before seeking solutions, it

1.8 Engineering Design and Analysis is important to define the design objectives. Early steps in engineering design include pinning down quantitative performance specifications and identifying alternative workable designs that meet the specifications. Among the workable designs are generally one or more that are “best” according to some criteria: lowest cost, highest efficiency, smallest size, lightest weight, etc. Other important factors in the selection of a final design include reliability, manufacturability, maintainability, and marketplace considerations. Accordingly, a compromise must be sought among competing criteria, and there may be alternative design solutions that are feasible.3

1.8.2 Analysis Design requires synthesis: selecting and putting together components to form a coordinated whole. However, as each individual component can vary in size, performance, cost, and so on, it is generally necessary to subject each to considerable study or analysis before a final selection can be made.

a proposed design for a fire-protection system might entail an overhead piping network together with numerous sprinkler heads. Once an overall configuration has been determined, detailed engineering analysis is necessary to specify the number and type of the spray heads, the piping material, and the pipe diameters of the various branches of the network. The analysis also must aim to ensure all components form a smoothly working whole while meeting relevant cost constraints and applicable codes and standards. b b b b b Engineers frequently do analysis, whether explicitly as part of a design process or for some other purpose. Analyses involving systems of the kind considered in this book use, directly or indirectly, one or more of three basic laws. These laws, which are independent of the particular substance or substances under consideration, are 1. the conservation of mass principle 2. the conservation of energy principle 3. the second law of thermodynamics In addition, relationships among the properties of the particular substance or substances considered are usually necessary (Chaps. 3, 6, 11–14). Newton’s second law of motion (Chaps. 1, 2, 9), relations such as Fourier’s conduction model (Chap. 2), and principles of engineering economics (Chap. 7) also may play a part. The first steps in a thermodynamic analysis are definition of the system and identification of the relevant interactions with the surroundings. Attention then turns to the pertinent physical laws and relationships that allow the behavior of the system to be described in terms of an engineering model. The objective in modeling is to obtain a simplified representation of system behavior that is sufficiently faithful for the purpose of the analysis, even if many aspects exhibited by the actual system are ignored. For example, idealizations often used in mechanics to simplify an analysis and arrive at a manageable model include the assumptions of point masses, frictionless pulleys, 3

For further discussion, see A. Bejan, G. Tsatsaronis, and M. J. Moran, Thermal Design and Optimization, John Wiley & Sons, New York, 1996, Chap. 1.

engineering model

21

22

Chapter 1 Getting Started and rigid beams. Satisfactory modeling takes experience and is a part of the art of engineering. Engineering analysis is most effective when it is done systematically. This is considered next.

1.9 Methodology for Solving Thermodynamics Problems A major goal of this textbook is to help you learn how to solve engineering problems that involve thermodynamic principles. To this end, numerous solved examples and endof-chapter problems are provided. It is extremely important for you to study the examples and solve problems, for mastery of the fundamentals comes only through practice. To maximize the results of your efforts, it is necessary to develop a systematic approach. You must think carefully about your solutions and avoid the temptation of starting problems in the middle by selecting some seemingly appropriate equation, substituting in numbers, and quickly “punching up” a result on your calculator. Such a haphazard problemsolving approach can lead to difficulties as problems become more complicated. Accordingly, it is strongly recommended that problem solutions be organized using the five steps in the box below, which are employed in the solved examples of this text.

cccc

➊ Known: State briefly in your own words what is known. This requires that you read the problem carefully and think about it. ➋ Find: State concisely in your own words what is to be determined. ➌ Schematic and Given Data: Draw a sketch of the system to be considered. Decide whether a closed system or control volume is appropriate for the analysis, and then carefully identify the boundary. Label the diagram with relevant information from the problem statement. Record all property values you are given or anticipate may be required for subsequent calculations. Sketch appropriate property diagrams (see Sec. 3.2), locating key state points and indicating, if possible, the processes executed by the system. The importance of good sketches of the system and property diagrams cannot be overemphasized. They are often instrumental in enabling you to think clearly about the problem. ➍ Engineering Model: To form a record of how you model the problem, list all simplifying assumptions and idealizations made to reduce it to one that is manageable. Sometimes this information also can be noted on the sketches of the previous step. The development of an appropriate model is a key aspect of successful problem solving. ➎ Analysis: Using your assumptions and idealizations, reduce the appropriate governing equations and relationships to forms that will produce the desired results. It is advisable to work with equations as long as possible before substituting numerical data. When the equations are reduced to final forms, consider them to determine what additional data may be required. Identify the tables, charts, or property equations that provide the required values. Additional property diagram sketches may be helpful at this point to clarify states and processes. When all equations and data are in hand, substitute numerical values into the equations. Carefully check that a consistent and appropriate set of units is being employed. Then perform the needed calculations. Finally, consider whether the magnitudes of the numerical values are reasonable and the algebraic signs associated with the numerical values are correct.

1.9 Methodology for Solving Thermodynamics Problems

23

The problem solution format used in this text is intended to guide your thinking, not substitute for it. Accordingly, you are cautioned to avoid the rote application of these five steps, for this alone would provide few benefits. Indeed, as a particular solution evolves you may have to return to an earlier step and revise it in light of a better understanding of the problem. For example, it might be necessary to add or delete an assumption, revise a sketch, determine additional property data, and so on. The solved examples provided in the book are frequently annotated with various comments intended to assist learning, including commenting on what was learned, identifying key aspects of the solution, and discussing how better results might be obtained by relaxing certain assumptions. In some of the earlier examples and end-of-chapter problems, the solution format may seem unnecessary or unwieldy. However, as the problems become more complicated you will see that it reduces errors, saves time, and provides a deeper understanding of the problem at hand. The example to follow illustrates the use of this solution methodology together with important system concepts introduced previously, including identification of interactions occurring at the boundary. cccc

EXAMPLE 1.1 c

Using the Solution Methodology and System Concepts A wind turbine–electric generator is mounted atop a tower. As wind blows steadily across the turbine blades, electricity is generated. The electrical output of the generator is fed to a storage battery. (a) Considering only the wind turbine–electric generator as the system, identify locations on the system boundary where the system interacts with the surroundings. Describe changes occurring within the system with time. (b) Repeat for a system that includes only the storage battery. SOLUTION Known: A wind turbine–electric generator provides electricity to a storage battery. Find: For a system consisting of (a) the wind turbine–electric generator and (b) the storage battery, identify locations where the system interacts with its surroundings, and describe changes occurring within the system with time. Schematic and Given Data: Engineering Model:

Part (a)

1. In part (a), the system is the control volume shown by

the dashed line on the figure. Air flow

Turbine–generator

2. In part (b), the system is the closed system shown by the

dashed line on the figure. 3. The wind is steady.

Electric current flow

Part (b) Storage battery

Thermal interaction

Fig. E1.1 Analysis: (a) In this case, the wind turbine is studied as a control volume with air flowing across the boundary. Another principal interaction between the system and surroundings is the electric current passing through the wires. From the macroscopic perspective, such an interaction is not considered a mass transfer, however. With a

24

Chapter 1 Getting Started steady wind, the turbine–generator is likely to reach steady-state operation, where the rotational speed of the blades is constant and a steady electric current is generated. ➊ (b) In this case, the battery is studied as a closed system. The principal interaction between the system and its surroundings is the electric current passing into the battery through the wires. As noted in part (a), this interaction is not considered a mass transfer. As the battery is charged and chemical reactions occur within it, the temperature of the battery surface may become somewhat elevated and a thermal interaction might occur between the battery and its surroundings. This interaction is likely to be of secondary importance. Also, as the battery is charged, the state within changes with time. The battery is not at steady state. ➊ Using terms familiar from a previous physics course, the system of part (a) involves the conversion of kinetic energy to electricity, whereas the system of part (b) involves energy storage within the battery.

✓ Skills Developed Ability to… ❑ apply the problem-solving

methodology used in this book. ❑ define a control volume and identify interactions on its boundary. ❑ define a closed system and identify interactions on its boundary. ❑ distinguish steady-state operation from nonsteady operation.

May an overall system consisting of the turbine-generator and battery be considered as operating at steady state? Explain. Ans. No. A system is at steady state only if none of its properties change with time.

c CHAPTER SUMMARY AND STUDY GUIDE In this chapter, we have introduced some of the fundamental concepts and definitions used in the study of thermodynamics. The principles of thermodynamics are applied by engineers to analyze and design a wide variety of devices intended to meet human needs. An important aspect of thermodynamic analysis is to identify systems and to describe system behavior in terms of properties and processes. Three important properties discussed in this chapter are specific volume, pressure, and temperature. In thermodynamics, we consider systems at equilibrium states and systems undergoing processes (changes of state). We study processes during which the intervening states are not equilibrium states and processes during which the departure from equilibrium is negligible. In this chapter, we have introduced SI units for mass, length, time, force, and temperature. Chapter 1 concludes with discussions of how thermodynamics is used in engineering design and how to solve thermodynamics problems systematically.

This book has several features that facilitate study and contribute to understanding. For an overview, see How To Use This Book Effectively inside the front cover of the book. The following checklist provides a study guide for this chapter. When your study of the text and the end-of-chapter exercises has been completed you should be able to c write out the meanings of the terms listed in the margin

c c c c

throughout the chapter and understand each of the related concepts. The subset of key concepts listed below is particularly important in subsequent chapters. use SI units for mass, length, time, force, and temperature and apply appropriately Newton’s second law and Eq. 1.16. work on a molar basis using Eq. 1.8. identify an appropriate system boundary and describe the interactions between the system and its surroundings. apply the methodology for problem solving discussed in Sec. 1.9.

c KEY ENGINEERING CONCEPTS system, p. 2 surroundings, p. 2 boundary, p. 2 closed system, p. 4 control volume, p. 4 property, p. 7

state, p. 7 process, p. 7 extensive property, p. 7 intensive property, p. 7 equilibrium, p. 8 specific volume, p. 12

pressure, p. 12 temperature, p. 17 Kelvin scale, p. 19

Problems: Developing Engineering Skills

25

c KEY EQUATIONS n 5 m/M

(1.8) p. 12

Relation between amounts of matter on a mass basis, m, and on a molar basis, n.

T(8C) 5 T(K) 2 273.15

(1.16) p. 20

Relation between the Celsius and Kelvin temperatures.

c EXERCISES: THINGS ENGINEERS THINK ABOUT 1. Consider a leaf blower driven by a gasoline engine as the system. Would it be best analyzed as a closed system or a control volume? Are there any environmental impacts associated with such a leaf blower? Repeat if it were an electricallydriven leaf blower. 2. What components would be required for a high school laboratory project aimed at determining the electricity generated by a hamster running on its exercise wheel? 3. Based on the macroscopic view, a quantity of air at 100 kPa, 20°C is in equilibrium. Yet the atoms and molecules of the air are in constant motion. How do you reconcile this apparent contradiction? 4. Air at 1 atm, 20°C in a closed tank adheres to the continuum hypothesis. Yet when sufficient air has been drawn from the tank, the hypothesis no longer applies to the remaining air. Why?

7. You may have used the mass unit kg in previous engineering or physics courses. What is the relation between the kg and gram mass? Is the kg a convenient mass unit? 8. Laura takes an elevator from the tenth floor of her office building to the lobby. Should she expect the air pressure on the two levels to differ much? 9. Are the systolic and diastolic pressures reported in blood pressure measurements absolute, gage, or vacuum pressures? 10. A data sheet indicates that the pressure at the inlet to a pump is 215 kPa. What does the negative sign denote? 11. When buildings have large exhaust fans, exterior doors can be difficult to open due to a pressure difference between the inside and outside. Do you think you could open a 1.2 m by 2.1 m door if the inside pressure were 0.28 kPa (vacuum)?

5. What is a nanotube?

12. How do dermatologists remove pre-cancerous skin blemishes cryosurgically?

6. Can the value of an intensive property such as pressure or temperature be uniform with position throughout a system? Be constant with time? Both?

13. When the instrument panel of a car provides the outside air temperature, where is the temperature sensor located?

c PROBLEMS: DEVELOPING ENGINEERING SKILLS Working with Force and Mass 1.1 An object has a mass of 4.5 kg. Determine its weight, in lbf, at a location where the acceleration of gravity is 9.81 m/s2. 1.2 An object weighs 25 kN at a location where the acceleration of gravity is 9.8 m/s2. Determine its mass, in kg. 1.3 If Superman has a mass of 90 kg on his birth planet Krypton, where the acceleration of gravity is 20 m/s2, determine (a) his weight on Krypton, in N, and (b) his mass, in kg, and weight, in N, on Earth where g 5 9.81 m/s2. 1.4 In severe head-on automobile accidents, a deceleration of 60 g’s or more (1 g 5 9.81 m/s2) often results in a fatality. What force, in N, acts on a child whose mass is 22.7 kg, when subjected to a deceleration of 60 g’s? 1.5 When an object of mass 5 kg is suspended from a spring, the spring is observed to stretch by 8 cm. The deflection of the spring is related linearly to the weight of the suspended mass. What is the proportionality constant, in newtons per cm, if g 5 9.81 m/s2?

1.6 A spring compresses in length by 3 mm, for every 4.5 N of applied force. Determine the deflection, in inches, of the spring caused by the weight of an object whose mass is 6.8 kg. The local acceleration of gravity is g 5 9.8 m/s2. 1.7 A simple instrument for measuring the acceleration of gravity employs a linear spring from which a mass is suspended. At a location on Earth where the acceleration of gravity is 9.81 m/s2, the spring extends 7.391 mm. If the spring extends 2.946 mm when the instrument is on Mars, what is the Martian acceleration of gravity? How much would the spring extend on the moon, where g 5 1.668 m/s2? 1.8 Estimate the magnitude of the force, in N, exerted by a seat belt on a 100 kg driver during a frontal collision that decelerates a car from 1.5 km/h to rest in 0.1 s. Express the car’s deceleration in multiples of the standard acceleration of gravity, or gs. 1.9 An object whose mass is 2 kg is subjected to an applied upward force. The only other force acting on the object is the force of gravity. The net acceleration of the object is upward with a magnitude of 5 m/s2. The acceleration of

26

Chapter 1 Getting Started

gravity is 9.81 m/s2. Determine the magnitude of the applied upward force, in N. 1.10 An object whose mass is 16 kg is subjected to an applied upward force of 66.7 N. The only other force acting on the object is the force of gravity. Determine the net acceleration of the object, in m/s2, assuming the acceleration of gravity is constant, g 5 9.81 m/s2. Is the net acceleration upward or downward? 1.11 If the variation of the acceleration of gravity, in m/s2, with elevation z, in m, above sea level is g 5 9.81 2 (3.3 3 1026) z, determine the percent change in weight of an airliner landing from a cruising altitude of 10 km on a runway at sea level. 1.12 The storage tank of a water tower is nearly spherical in shape with a radius of 9 m. If the density of the water is 1000 kg/m3, what is the mass of water stored in the tower, in lb, when the tank is full? What is the weight, in N, of the water if the local acceleration of gravity is 9.8 m/s2? 1.13 As shown in Fig. P1.13, a cylinder of compacted scrap metal measuring 2 m in length and 0.5 m in diameter is suspended from a spring scale at a location where the acceleration of gravity is 9.78 m/s2. If the scrap metal density, in kg/m3, varies with position z, in m, according to 𝜌 5 7800 2 360(z/L)2, determine the reading of the scale, in N.

(b) Plot the amount of CO2 that has leaked from the cylinder, in kg, versus the specific volume of the CO2 remaining in the cylinder. Consider 𝜐 ranging up to 1.0 m3/kg. 1.16 Go to http://www.weather.gov for weather data at three locations of your choice. At each location, express the local atmospheric pressure in bar and atmospheres. 1.17 A closed system consisting of 5 kg of a gas undergoes a process during which the relationship between pressure and specific volume is p𝜐1.3 5 constant. The process begins with p1 5 1 bar, 𝜐1 5 0.2 m3/kg and ends with p2 5 0.25 bar. Determine the final volume, in m3, and plot the process on a graph of pressure versus specific volume. 1.18 A closed system consisting of 2 kg of a gas undergoes a process during which the relation between pressure and volume is pV n 5 constant. The process begins with p1 5 150 kPa, V1 5 0.3 m3 and ends with p2 5 450 kPa, V2 5 0.137 m3. Determine (a) the value of n and (b) the specific volume at states 1 and 2, each in m3/kg. (c) Sketch the process on pressure– volume coordinates. 1.19 A system consists of nitrogen (N2) in a piston–cylinder assembly, initially at p1 5 140 kPa, and occupying a volume of 0.068 m3. The nitrogen is compressed to p2 5 690 kPa and a final volume of 0.041 m3. During the process, the relation between pressure and volume is linear. Determine the pressure, in kPa, at an intermediate state where the volume is 0.057 m3, and sketch the process on a graph of pressure versus volume. 1.20 A gas initially at p1 5 1 bar and occupying a volume of 1 liter is compressed within a piston–cylinder assembly to a final pressure p2 5 4 bar. (a) If the relationship between pressure and volume during the compression is pV 5 constant, determine the volume, in liters, at a pressure of 3 bar. Also plot the overall process on a graph of pressure versus volume. (b) Repeat for a linear pressure–volume relationship between the same end states.

L=2m z

D = 0.5 m

Fig. P1.13 Using Specific Volume and Pressure 1.14 A closed system consists of 0.2 kmol of ammonia occupying a volume of 3 m3. Determine (a) the weight of the system, in N, and (b) the specific volume, in m3/kmol and m3/kg. Let g 5 9.81 m/s2. 1.15 Fifteen kg of carbon dioxide (CO2) gas is fed to a cylinder having a volume of 20 m3 and initially containing 15 kg of CO2 at a pressure of 10 bar. Later a pinhole develops and the gas slowly leaks from the cylinder. (a) Determine the specific volume, in m3/kg, of the CO2 in the cylinder initially. Repeat for the CO2 in the cylinder after the 15 kg has been added.

1.21 For a process taking place in a closed system containing gas, the volume and pressure relationship is pV1.4 5 constant. The process starts with initial conditions, p1 5 1.5 bar, V1 5 0.03 m3 and ends with final volume, V2 5 0.05 m3. Determine the final pressure p2 in bar. 1.22 As shown in Fig. 1.7, a manometer is attached to a tank of gas in which the pressure is 104.0 kPa. The manometer liquid is mercury, with a density of 13.59 g/cm3. If g 5 9.81 m/s2 and the atmospheric pressure is 101.33 kPa, calculate (a) the difference in mercury levels in the manometer, in cm. (b) the gage pressure of the gas, in kPa. 1.23 A vacuum gage at the intake duct to a fan gives a reading of 105 mm of manometer fluid. The surrounding atmospheric pressure is 100 kPa. Determine the absolute pressure inside the duct, in kPa. The density of the manometer fluid is and the acceleration of gravity is 9.81 m/s2. 1.24 The absolute pressure inside a tank is 0.4 bar, and the surrounding atmospheric pressure is 98 kPa. What reading would a Bourdon gage mounted in the tank wall give, in kPa? Is this a gage or vacuum reading?

Problems: Developing Engineering Skills 1.25 An open storage tank is placed at the top of a building. The tank contains water up to a depth of 1.5 m. Calculate the pressure at the bottom of the tank. It is given that atmospheric pressure is 101.3 kPa and density of water is 1000 kg/m3. 1.26 Water flows through a Venturi meter, as shown in Fig. P1.26. The pressure of the water in the pipe supports columns of water that differ in height by 200 mm. Determine the difference in pressure between points a and b, in kPa. Does the pressure increase or decrease in the direction of flow? The atmospheric pressure is 101.3 kPa, the specific volume of water is 0.001 m3/kg, and the acceleration of gravity is g 5 9.8 m/s2.

27

1.28 Refrigerant 22 vapor enters the compressor of a refrigeration system at an absolute pressure of 140 kPa. A pressure gage at the compressor exit indicates a pressure of 1930 kPa (gage). The atmospheric pressure is 101.3 kPa. Determine the change in absolute pressure from inlet to exit, in kPa, and the ratio of exit to inlet pressure. 1.29 Air contained within a vertical piston–cylinder assembly is shown in Fig. P1.29. On its top, the 10-kg piston is attached to a spring and exposed to an atmospheric pressure of 1 bar. Initially, the bottom of the piston is at x 5 0, and the spring exerts a negligible force on the piston. The valve is opened and air enters the cylinder from the supply line, causing the volume of the air within the cylinder to increase by 3.9 3 1024 m3. The force exerted by the spring as the air expands within the cylinder varies linearly with x according to Fspring 5 kx

patm = 101.3 kPa g = 9.8 m/s2

where k 5 10,000 N/m. The piston face area is 7.8 3 1023 m2. Ignoring friction between the piston and the cylinder wall, determine the pressure of the air within the cylinder, in bar, when the piston is in its initial position. Repeat when the piston is in its final position. The local acceleration of gravity is 9.81 m/s2.

L = 200 mm

Water v = 0.001 m3/kg

patm

b

a

Fig. P1.26 1.27 As shown in Fig. P1.27, an underwater exploration vehicle submerges to a depth of 500 m. If the atmospheric pressure at the surface is 101.3 kPa, the water density is 1000 kg/m3, and g 5 9.8 m/s2, determine the pressure on the vehicle, in atm. patm = 101.3 kPa g = 9.8 m/s2

500 m

Air supply line

x=0 Air

Valve

Fig. P1.29 1.30 Determine the total force, in kN, on the bottom of a 100 3 50 m swimming pool. The depth of the pool varies linearly along its length from 1 m to 4 m. Also, determine the pressure on the floor at the center of the pool, in kPa. The atmospheric pressure is 0.98 bar, the density of the water is 998.2 kg/m3, and the local acceleration of gravity is 9.8 m/s2. 1.31 The pressure from water mains located at street level may be insufficient for delivering water to the upper floors of tall buildings. In such a case, water may be pumped up to a tank that feeds water to the building by gravity. For an open storage tank atop a 90 m tall building, determine the pressure, in kPa, at the bottom of the tank when filled to a depth of 4 m. The density of water is 1000 kg/m3, g 5 9.8 m/s2, and the local atmospheric pressure is 101.3 kPa. 1.32 The variation of pressure within the biosphere affects not only living things but also systems such as aircraft and undersea exploration vehicles.

Fig. P1.27

(a) Plot the variation of atmospheric pressure, in atm, versus elevation z above sea level, in km, ranging from 0 to 10 km.

28

Chapter 1 Getting Started

Assume that the specific volume of the atmosphere, in m3/kg, varies with the local pressure p, in kPa, according to 𝜐 5 72.435/p. (b) Plot the variation of pressure, in atm, versus depth z below sea level, in km, ranging from 0 to 2 km. Assume that the specific volume of seawater is constant, 𝜐 5 0.956 3 1023 m3/kg. In each case, g 5 9.81 m/s2 and the pressure at sea level is 1 atm. 1.33 One thousand kg of natural gas at 100 bar and 255 K is stored in a tank. If the pressure, p, specific volume, 𝜐, and temperature, T, of the gas are related by the following expression p 5 [(5.18 3 1023)Ty(𝜐 2 0.002668)] 2 (8.91 3 1023)y𝜐2

Exploring Temperature 1.35 Convert the following temperatures from °C to K: (a) 21°C, (b) 240°C, (c) 500°C, (d) 0°C, (e) 100°C, (f) 2273.15°C. 1.36 Convert the following temperatures from K to °C: (a) 293.15 K, (b) 233.15 K, (c) 533.15 K, (d) 255.4 K, (e) 373.15 K, (f) 0 K. 1.37 As shown in Fig. P1.37, a small-diameter water pipe passes through the 150 mm thick exterior wall of a dwelling. Assuming that temperature varies linearly with position x through the wall from 20°C to 27°C, would the water in the pipe freeze? Explain.

T = 20°C

T= Pipe

where 𝜐 is in m3/kg, T is in K, and p is in bar, determine the volume of the tank, in m3. Also, plot pressure versus specific volume for the isotherms T 5 250, 500, and 1000 K.

75 mm

3

1.34 A 2.2 m tank contains water vapor at 10340 kPa and 633 K. If the pressure, p, specific volume, 𝜐, and temperature, T, of water vapor are related by the expression p 5 [(7183.04)Ty(𝜐 2 0.0169)] 2 (25.02 3 103)y𝜐2 where 𝜐 is in m3/kg, T is in K, and p is in kPa, determine the mass of water in the tank. Also, plot pressure versus specific volume for the isotherms T 5 667, 778, and 889 K.

7°C

150 mm x

Fig. P1.37 1.38 Calculate the value of temperature x such that x°F is equal to x°C. Also convert the unit of x into Kelvin.

c DESIGN & OPEN ENDED PROBLEMS: EXPLORING ENGINEERING PRACTICE 1.1D The issue of global warming is receiving considerable attention these days. Write a technical report including at least three references on the subject of global warming. Explain what is meant by the term global warming and discuss objectively the scientific evidence that is cited as the basis for the argument that global warming is occurring. 1.2D Barometers and liquid-in-glass thermometers have customarily used mercury, which is now recognized as a biohazard. Investigate medical complications of mercury exposure. Write a report including at least three references.

1.5D Magnetic resonance imaging (MRI) employs a strong magnetic field to produce detailed pictures of internal organs and tissues. As shown in Fig. P1.5D, the patient reclines on a table that slides into the cylindrical opening where the field is created. Considering a MRI scanner as a system, identify locations on the system boundary where the system interacts with its surroundings. Also describe events occurring within the system and the measures taken for patient comfort and safety. Write a report including at least three references. MRI scanner

1.3D List several aspects of engineering economics relevant to design. What are the important contributors to cost that should be considered in engineering design? Discuss what is meant by annualized costs. Present your findings in a memorandum. 1.4D Determine the respective contributions to the electric power provided to customers by the electric utility serving your locale attributable to coal, natural gas, oil, biomass, nuclear power, hydropower, wind power, and solar power. Summarize your findings in a pie chart. For each contribution providing 1%, or more, of the total, identify associated air emissions, solid waste produced, including radioactive waste, and wildlife impacts. Write a report including at least three references.

Fig. P1.5D 1.6D The sphygmomanometer, commonly used to measure blood pressure, is shown in Fig. P1.6D. During testing, the cuff is placed around the patient’s arm and fully inflated by repeated squeezing of the inflation bulb. Then, as the cuff

Design & Open Ended Problems: Exploring Engineering Practice pressure is gradually reduced, arterial sounds known as Korotkoff sounds are monitored with a stethoscope. Using these sounds as cues, the systolic and diastolic pressures can be identified. These pressures are reported in terms of the mercury column length, in mmHg. Investigate the physical basis for the Korotkoff sounds, their role in identifying the systolic and diastolic pressures, and why these pressures are significant in medical practice. Write a report including at least three references. patm

Cuff pressure, pc

Mercury column

L pc

Inflation bulb Air

Pressure release valve

One-way valve

Fig. P1.6D 1.7D Design a low-cost, compact, lightweight, handheld, humanpowered air pump capable of directing a stream of air for cleaning computer keyboards, circuit boards, and hard-to-reach locations in electronic devices. The pump cannot use electricity, including batteries, nor employ any chemical propellants. All materials must be recyclable. Owing to existing patent protections, the pump must be a distinct alternative to the familiar tube and plunger bicycle pump and to existing products aimed at accomplishing the specified computer and electronic cleaning tasks. 1.8D In Bangladesh, unsafe levels of arsenic, which is a tasteless, odorless, and colorless poison, are present in underground

29

wells providing drinking water to millions of people living in rural areas. The task is to identify affordable, easy-to-use treatment technologies for removing arsenic from their drinking water. Technologies considered should include, but not be limited to, applications of smart materials and other nanotechnology approaches. Write a report including at least three references. 1.9D Conduct a term-length design project in the realm of bioengineering done on either an independent or a smallgroup basis. The project might involve a device or technique for minimally invasive surgery, an implantable drug-delivery device, a biosensor, artificial blood, or something of special interest to you or your design group. Take several days to research your project idea and then prepare a brief written proposal, including several references, that provides a general statement of the core concept plus a list of objectives. During the project, observe good design practices such as discussed in Sec. 1.3 of Thermal Design and Optimization, John Wiley & Sons Inc., New York, 1996, by A. Bejan, G. Tsatsaronis, and M.J. Moran. Provide a well-documented final report, including several references. 1.10D Conduct a term-length design project involving the International Space Station pictured in Table 1.1, done on either an independent or a small-group basis. The project might involve an experiment that is best conducted in a lowgravity environment, a device for the comfort or use of the astronauts, or something of special interest to you or your design group. Take several days to research your project idea and then prepare a brief written proposal, including several references, that provides a general statement of the core concept plus a list of objectives. During the project, observe good design practices such as discussed in Sec. 1.3 of Thermal Design and Optimization, John Wiley & Sons Inc., New York, 1996, by A. Bejan, G. Tsatsaronis, and M. J. Moran. Provide a well-documented final report, including several references.

1.6

2 Energy and the First Law of Thermodynamics ENGINEERING CONTEXT Energy is a fundamental concept of thermodynamics and one of the most significant aspects of engineering analysis. In this chapter we discuss energy and develop equations for applying the principle of conservation of energy. The current presentation is limited to closed systems. In Chap. 4 the discussion is extended to control volumes. Energy is a familiar notion, and you already know a great deal about it. In the present chapter several important aspects of the energy concept are developed. Some of these you have encountered before. A basic idea is that energy can be stored within systems in various forms. Energy also can be converted from one form to another and transferred between systems. For closed systems, energy can be transferred by work and heat transfer. The total amount of energy is conserved in all conversions and transfers. The objective of this chapter is to organize these ideas about energy into forms suitable for engineering analysis. The presentation begins with a review of energy concepts from mechanics. The thermodynamic concept of energy is then introduced as an extension of the concept of energy in mechanics.

LEARNING OUTCOMES When you complete your study of this chapter, you will be able to…

30

c

demonstrate understanding of key concepts related to energy and the first law of thermodynamics . . . including internal, kinetic, and potential energy, work and power, heat transfer and heat transfer modes, heat transfer rate, power cycle, refrigeration cycle, and heat pump cycle.

c

apply closed-system energy balances, appropriately modeling the case at hand, and correctly observing sign conventions for work and heat transfer.

c

conduct energy analyses of systems undergoing thermodynamic cycles, evaluating as appropriate thermal efficiencies of power cycles and coefficients of performance of refrigeration and heat pump cycles.

2.1 Reviewing Mechanical Concepts of Energy

31

2.1 Reviewing Mechanical Concepts of Energy Building on the contributions of Galileo and others, Newton formulated a general description of the motions of objects under the influence of applied forces. Newton’s laws of motion, which provide the basis for classical mechanics, led to the concepts of work, kinetic energy, and potential energy, and these led eventually to a broadened concept of energy. The present discussion begins with an application of Newton’s second law of motion.

2.1.1 Work and Kinetic Energy The curved line in Fig. 2.1 represents the path of a body of mass m (a closed system) moving relative to the x–y coordinate frame shown. The velocity of the center of mass of the body is denoted by V. The body is acted on by a resultant force F, which may vary in magnitude from location to location along the path. The resultant force is resolved into a component Fs along the path and a component Fn normal to the path. The effect of the component Fs is to change the magnitude of the velocity, whereas the effect of the component Fn is to change the direction of the velocity. As shown in Fig. 2.1, s is the instantaneous position of the body measured along the path from some fixed point denoted by 0. Since the magnitude of F can vary from location to location along the path, the magnitudes of Fs and Fn are, in general, functions of s. Let us consider the body as it moves from s 5 s1, where the magnitude of its velocity is V1, to s 5 s2, where its velocity is V2. Assume for the present discussion that the only interaction between the body and its surroundings involves the force F. By Newton’s second law of motion, the magnitude of the component Fs is related to the change in the magnitude of V by dV dt

(2.1)

dV ds dV 5 mV ds dt ds

(2.2)

Fs 5 m Using the chain rule, this can be written as Fs 5 m

where V 5 ds/dt. Rearranging Eq. 2.2 and integrating from s1 to s2 gives

#

V2

mV dV 5

V1

y

s

0

FS ds

(2.3)

S1

Fs ds

#

S2

Path

V F

Body Fn

Fig. 2.1 Forces acting on a moving x

system.

TAKE NOTE...

Boldface symbols denote vectors. Vector magnitudes are shown in lightface type.

32

Chapter 2 Energy and the First Law of Thermodynamics The integral on the left of Eq. 2.3 is evaluated as follows

#

V2

V1

kinetic energy

TAKE NOTE...

The symbol D always means “final value minus initial value.”

mV dV 5

V2 1 1 mV2 d 5 m(V222V21) 2 2 V

(2.4)

1

The quantity 12 mV2 is the kinetic energy, KE, of the body. Kinetic energy is a scalar quantity. The change in kinetic energy, DKE, of the body is 1 DKE 5 KE2 2 KE1 5 m(V22 2 V21) 2

(2.5)

The integral on the right of Eq. 2.3 is the work of the force Fs as the body moves from s1 to s2 along the path. Work is also a scalar quantity. With Eq. 2.4, Eq. 2.3 becomes s2 1 m(V22 2 V21) 5 F ? ds 2 S1

#

(2.6)

where the expression for work has been written in terms of the scalar product (dot product) of the force vector F and the displacement vector ds. Equation 2.6 states that the work of the resultant force on the body equals the change in its kinetic energy. When the body is accelerated by the resultant force, the work done on the body can be considered a transfer of energy to the body, where it is stored as kinetic energy. Kinetic energy can be assigned a value knowing only the mass of the body and the magnitude of its instantaneous velocity relative to a specified coordinate frame, without regard for how this velocity was attained. Hence, kinetic energy is a property of the body. Since kinetic energy is associated with the body as a whole, it is an extensive property.

ENERGY & ENVIRONMENT Did you ever wonder what happens to the kinetic energy when you step on the brakes of your moving car? Automotive engineers have, and the result is the hybrid vehicle combining regenerative braking, batteries, an electric motor, and a conventional engine. When brakes are applied in a hybrid vehicle, some of the vehicle’s kinetic energy is harvested and stored on board electrically for use when needed. Through regenerative braking and other innovative features, hybrids get much better mileage than comparably sized conventional vehicles. Hybrid vehicle technology is quickly evolving. Today’s hybrids use electricity to supplement conventional engine power, while future plug-in hybrids will use the power of a smaller engine to supplement electricity. The hybrids we now see on the road have enough battery power on board for acceleration to about 32 km/h and after that assist the engine when necessary. This improves fuel mileage, but the batteries are recharged by the engine—they are never plugged in. Plug-in hybrids achieve even better fuel economy. Instead of relying on the engine to recharge batteries, most recharging will be received from an electrical outlet while the car is idle—overnight, for example. This will allow cars to get the energy they need mainly from the electrical grid, not the fuel pump. Widespread deployment of plug-ins awaits development of a new generation of batteries and ultra-capacitors (see Sec. 2.7). Better fuel economy not only allows our society to be less reliant on oil to meet transportation needs but also reduces release of CO2 into the atmosphere from vehicles. Each gallon of gasoline burned by a car’s engine produces about 9 kg of CO2. A conventional vehicle produces several tons of CO2 annually; fuel-thrifty hybrids produce much less. Still, since hybrids use electricity from the grid, we will have to make a greater effort to reduce power plant emissions by including more wind power, solar power, and other renewables in the national mix.

2.1 Reviewing Mechanical Concepts of Energy

2.1.2

33

Potential Energy

Equation 2.6 is a principal result of the previous section. Derived from Newton’s second law, the equation gives a relationship between two defined concepts: kinetic energy and work. In this section it is used as a point of departure to extend the concept of energy. To begin, refer to Fig. 2.2, which shows a body of mass m that moves vertically from an elevation z1 to an elevation z2 relative to the surface of the earth. Two forces are shown acting on the system: a downward force due to gravity with magnitude mg and a vertical force with magnitude R representing the resultant of all other forces acting on the system. The work of each force acting on the body shown in Fig. 2.2 can be determined by using the definition previously given. The total work is the algebraic sum of these individual values. In accordance with Eq. 2.6, the total work equals the change in kinetic energy. That is z2 z2 1 R dz 2 mg dz m(V22 2 V21) 5 2 z1 z1

#

#

R z2

z

mg

z1

Earth’s surface

(2.7)

Fig. 2.2 Illustration used to introduce the potential energy A minus sign is introduced before the second term on the right because the concept. gravitational force is directed downward and z is taken as positive upward. The first integral on the right of Eq. 2.7 represents the work done by the force R on the body as it moves vertically from z1 to z2. The second integral can be evaluated as follows: z2

#z mg dz 5 mg(z 2 z ) 2

(2.8)

1

1

where the acceleration of gravity has been assumed to be constant with elevation. By incorporating Eq. 2.8 into Eq. 2.7 and rearranging z2 1 R dz m(V22 2 V21) 1 mg(z2 2 z1) 5 2 z1

#

(2.9)

The quantity mgz is the gravitational potential energy, PE. The change in gravitational potential energy, DPE, is DPE 5 PE2 2 PE1 5 mg(z2 2 z1)

(2.10)

Potential energy is associated with the force of gravity and is therefore an attribute of a system consisting of the body and the earth together. However, evaluating the force of gravity as mg enables the gravitational potential energy to be determined for a specified value of g knowing only the mass of the body and its elevation. With this view, potential energy is regarded as an extensive property of the body. Throughout this book it is assumed that elevation differences are small enough that the gravitational force can be considered constant. The concept of gravitational potential energy can be formulated to account for the variation of the gravitational force with elevation, however. To assign a value to the kinetic energy or the potential energy of a system, it is necessary to assume a datum and specify a value for the quantity at the datum. Values of kinetic and potential energy are then determined relative to this arbitrary choice of datum and reference value. However, since only changes in kinetic and potential energy between two states are required, these arbitrary reference specifications cancel.

gravitational potential energy

34

Chapter 2 Energy and the First Law of Thermodynamics

2.1.3 Units for Energy Work has units of force times distance. The units of kinetic energy and potential energy are the same as for work. In SI, the energy unit is the newton-meter, N ? m, called the joule, J. In this book it is convenient to use the kilojoule, kJ. When a system undergoes a process where there are changes in kinetic and potential energy, special care is required to obtain a consistent set of units.

to illustrate the proper use of units in the calculation of such terms, consider a system having a mass of 1 kg whose velocity increases from 15 m/s to 30 m/s while its elevation decreases by 10 m at a location where g 5 9.7 m/s2. Then DKE 5

5

1 m(V22 2 V21) 2 1 m 2 m 2 1N 1 kJ ` (1 kg) c a30 b 2 a15 b d ` ` ` s s 2 1 kg ? m/s 2 10 3 N ? m

5 0.34 kJ DPE 5 mg(z2 2 z1) 5 (1 kg)a9.7

m 1N 1 kJ ` b(210 m) ` ` ` s2 1 kg ? m/s 2 10 3 N ? m

5 2 0.10 kJ

2.1.4 Conservation of Energy in Mechanics Equation 2.9 states that the total work of all forces acting on the body from the surroundings, with the exception of the gravitational force, equals the sum of the changes in the kinetic and potential energies of the body. When the resultant force causes the elevation to be increased, the body to be accelerated, or both, the work done by the force can be considered a transfer of energy to the body, where it is stored as gravitational potential energy and/or kinetic energy. The notion that energy is conserved underlies this interpretation. The interpretation of Eq. 2.9 as an expression of the conservation of energy principle can be reinforced by considering the special case of a body on which the only force acting is that due to gravity, for then the right side of the equation vanishes and the equation reduces to 1 m(V22 2 V21) 1 mg(z2 2 z1) 5 0 2 z

mg

(2.11)

or 1 1 mV22 1 mgz2 5 mV21 1 mgz1 2 2

Under these conditions, the sum of the kinetic and gravitational potential energies remains constant. Equation 2.11 also illustrates that energy can be converted from one form to another: For an object falling under the influence of gravity only, the potential energy would decrease as the kinetic energy increases by an equal amount.

2.2 Broadening our Understanding of Work

2.1.5 Closing Comment The presentation thus far has centered on systems for which applied forces affect only their overall velocity and position. However, systems of engineering interest normally interact with their surroundings in more complicated ways, with changes in other properties as well. To analyze such systems, the concepts of kinetic and potential energy alone do not suffice, nor does the rudimentary conservation of energy principle introduced in this section. In thermodynamics the concept of energy is broadened to account for other observed changes, and the principle of conservation of energy is extended to include a wide variety of ways in which systems interact with their surroundings. The basis for such generalizations is experimental evidence. These extensions of the concept of energy are developed in the remainder of the chapter, beginning in the next section with a fuller discussion of work.

2.2 Broadening Our Understanding of Work The work W done by, or on, a system evaluated in terms of macroscopically observable forces and displacements is W5

#

s2 F ? ds

(2.12)

S1

This relationship is important in thermodynamics, and is used later in the present section to evaluate the work done in the compression or expansion of gas (or liquid), the extension of a solid bar, and the stretching of a liquid film. However, thermodynamics also deals with phenomena not included within the scope of mechanics, so it is necessary to adopt a broader interpretation of work, as follows. A particular interaction is categorized as a work interaction if it satisfies the following criterion, which can be considered the thermodynamic definition of work: Work is done by a system on its surroundings if the sole effect on everything external to the system could have been the raising of a weight. Notice that the raising of a weight is, in effect, a force acting through a distance, so the concept of work in thermodynamics is a natural extension of the concept of work in mechanics. However, the test of whether a work interaction has taken place is not that the elevation of a weight has actually taken place, or that a force has actually acted through a distance, but that the sole effect could have been an increase in the elevation of a weight. consider Fig. 2.3 showing two systems labeled A and B. In system A, a gas is stirred by a paddle wheel: the paddle wheel does work on the gas. In principle, the work could be evaluated in terms of the forces and motions at the boundary between the paddle wheel and the gas. Such an evaluation of work is consistent with Eq. 2.12, where work is the product of force and displacement. By contrast, consider system B, which includes only the battery. At the boundary of system B, forces and motions are not evident. Rather, there is an electric current i driven by an electrical potential difference existing across the terminals a and b. That this type of interaction at the boundary can be classified as work follows from the thermodynamic definition of work given previously: We can imagine the current is supplied to a hypothetical electric motor that lifts a weight in the surroundings. b b b b b

work

35

36

Chapter 2 Energy and the First Law of Thermodynamics

Paddle wheel

System A i

Gas a

b

System B

Battery

Fig. 2.3 Two examples of work.

Work is a means for transferring energy. Accordingly, the term work does not refer to what is being transferred between systems or to what is stored within systems. Energy is transferred and stored when work is done.

2.2.1 Sign Convention and Notation Engineering thermodynamics is frequently concerned with devices such as internal combustion engines and turbines whose purpose is to do work. Hence, in contrast to the approach generally taken in mechanics, it is often convenient to consider such work as positive. That is, W . 0: work done by the system W , 0: work done on the system sign convention for work

work is not a property

This sign convention is used throughout the book. In certain instances, however, it is convenient to regard the work done on the system to be positive, as has been done in the discussion of Sec. 2.1. To reduce the possibility of misunderstanding in any such case, the direction of energy transfer is shown by an arrow on a sketch of the system, and work is regarded as positive in the direction of the arrow. To evaluate the integral in Eq. 2.12, it is necessary to know how the force varies with the displacement. This brings out an important idea about work: The value of W depends on the details of the interactions taking place between the system and surroundings during a process and not just the initial and final states of the system. It follows that work is not a property of the system or the surroundings. In addition, the limits on the integral of Eq. 2.12 mean “from state 1 to state 2” and cannot be interpreted as the values of work at these states. The notion of work at a state has no meaning, so the value of this integral should never be indicated as W2 2 W1.

Nanoscale Machines on the Move Engineers working in the field of nanotechnology, the engineering of molecular-sized devices, look forward to the time when practical nanoscale machines can be fabricated that are capable of movement, sensing and responding to stimuli such as light and sound, delivering medication within the body, performing computations, and numerous other functions that promote human well-being. For inspiration, engineers study biological nanoscale machines in living things that perform functions such as creating and

repairing cells, circulating oxygen, and digesting food. These studies have yielded positive results. Molecules mimicking the function of mechanical devices have been fabricated, including gears, rotors, ratchets, brakes, switches, and abacuslike structures. A particular success is the development of molecular motors that convert light to rotary or linear motion. Although devices produced thus far are rudimentary, they do demonstrate the feasibility of constructing nanomachines, researchers say.

2.2 Broadening our Understanding of Work The differential of work, 𝛿W, is said to be inexact because, in general, the following integral cannot be evaluated without specifying the details of the process: 2

# 𝛿W 5 W 1

On the other hand, the differential of a property is said to be exact because the change in a property between two particular states depends in no way on the details of the process linking the two states. For example, the change in volume between two states can be determined by integrating the differential dV, without regard for the details of the process, as follows:

#

v2

dV 5 V2 2 V1

v1

where V1 is the volume at state 1 and V2 is the volume at state 2. The differential of every property is exact. Exact differentials are written, as above, using the symbol d. To stress the difference between exact and inexact differentials, the differential of work is written as 𝛿W. The symbol 𝛿 is also used to identify other inexact differentials encountered later.

2.2.2 Power Many thermodynamic analyses are concerned with the time rate at which energy transfer occurs. The rate of energy transfer by work is called power and is denoted ? by W. When a work interaction involves a macroscopically observable force, the rate of energy transfer by work is equal to the product of the force and the velocity at the point of application of the force ?

W5F?V

power

(2.13)

?

A dot appearing over a symbol, as in W, is used throughout this book to indicate a time rate. In principle, Eq. 2.13 can be integrated from time t1 to time ts to get the total work done during the time interval W5

#

t2

. W dt 5

t1

t2

# F ? V dt

(2.14)

t1

?

The same sign convention applies for W as for W. Since power is a time rate of doing work, it can be expressed in terms of any units for energy and time. In SI, the unit for power is J/s, called the watt. In this book the kilowatt, kW, is generally used.

to illustrate the use of Eq. 2.13, let us evaluate the power required for a bicyclist traveling at 20 miles per hour to overcome the drag force imposed by the surrounding air. This aerodynamic drag force is given by Fd 5 12 CdA𝜌V 2 where Cd is a constant called the drag coefficient, A is the frontal area of the bicycle and rider, and 𝜌 is the air density. By Eq. 2.13 the required power is Fd ? V or . W 5 (12 CdA𝜌V2)V 5 12Cd A𝜌V3

units for power

37

38

Chapter 2 Energy and the First Law of Thermodynamics Using typical values Cd 5 0.88, A 5 0.362 m2, and 𝜌 5 1.2 kg/m3, together with V 5 20 mi/h 5 29.33 ft/s, and also converting units to horsepower, the power required is ?

1 (0.88)(0.362 m2)(1.2 kg/m3)(8.94 m/s) 2 5 136.6 W b b b b b

W5

Drag can be reduced by streamlining the shape of a moving object and using the strategy known as drafting (see box).

Drafting Drafting occurs when two or more moving vehicles or individuals align closely to reduce the overall effect of drag. Drafting is seen in competitive events such as auto racing, bicycle racing, speed-skating, and running. Studies show that air flow over a single vehicle or individual in motion is characterized by a high-pressure region in front and a low-pressure region behind. The difference between these pressures creates a force, called drag, impeding motion. During drafting, as seen in the sketch below, a second vehicle or individual is closely aligned with another, and air flows over the pair nearly as if they were a single entity, thereby altering the pressure between them and reducing the drag each experiences. While race-car drivers use drafting to increase speed, non–motor sport competitors usually aim to reduce demands on their bodies while maintaining the same speed.

2.2.3

Modeling Expansion or Compression Work

There are many ways in which work can be done by or on a system. The remainder of this section is devoted to considering several examples, beginning with the important case of the work done when the volume of a quantity of a gas (or liquid) changes by expansion or compression. Let us evaluate the work done by the closed system shown in Fig. 2.4 consisting of a gas (or liquid) contained in a piston–cylinder assembly as the gas expands. During the process the gas pressure exerts a normal force on the piston. Let p denote the pressure acting at the interface between the gas and the piston. The force exerted System boundary Area = A

Average pressure at the piston face = p

F = pA Gas or liquid x

x1

x2

Fig. 2.4 Expansion or compression of a gas or liquid.

2.2 Broadening our Understanding of Work

39

by the gas on the piston is simply the product pA, where A is the area of the piston face. The work done by the system as the piston is displaced a distance dx is 𝛿W 5 pA dx

(2.15)

The product A dx in Eq. 2.15 equals the change in volume of the system, dV. Thus, the work expression can be written as 𝛿W 5 p dV

(2.16)

Since dV is positive when volume increases, the work at the moving boundary is positive when the gas expands. For a compression, dV is negative, and so is work found from Eq. 2.16. These signs are in agreement with the previously stated sign convention for work. For a change in volume from V1 to V2, the work is obtained by integrating Eq. 2.16 W5

#

V2

p dV

(2.17)

V1

Although Eq. 2.17 is derived for the case of a gas (or liquid) in a piston–cylinder assembly, it is applicable to systems of any shape provided the pressure is uniform with position over the moving boundary.

2.2.4 Expansion or Compression Work in Actual Processes There is no requirement that a system undergoing a process be in equilibrium during the process. Some or all of the intervening states may be nonequilibrium states. For many such processes we are limited to knowing the state before the process occurs and the state after the process is completed. Typically, at a nonequilibrium state intensive properties vary with position at a given time. Also, at a specified position intensive properties may vary with time, sometimes chaotically. In certain cases, spatial and temporal variations in properties such as temperature, pressure, and velocity can be measured, or obtained by solving appropriate governing equations, which are generally differential equations. To perform the integral of Eq. 2.17 requires a relationship between the gas pressure at the moving boundary and the system volume. However, due to nonequilibrium effects during an actual expansion or compression process, this relationp ship may be difficult, or even impossible, to obtain. In the cylinder of an automobile engine, for example, combustion and other nonequilibrium Measured data Curve fit effects give rise to nonuniformities throughout the cylinder. Accordingly, if a pressure transducer were mounted on the cylinder head, the recorded output might provide only an approximation for the pressure at the piston face required by Eq. 2.17. Moreover, even when the measured pressure is essentially equal to that at the piston face, scatter might exist in the pressure–volume data, as illustrated in Fig. 2.5. Still, performing the integral of Eq. 2.17 based on a curve fitted to the data could give a plausible estimate of the work. We will see later that in some cases where lack of the required pressure–volume relationship keeps us from evaluating the work from Eq. 2.17, the work can be determined alternatively from an Fig. 2.5 Pressure at the piston face energy balance (Sec. 2.5). versus cylinder volume.

2.2.5 Expansion or Compression Work in Quasiequilibrium Processes Processes are sometime modeled as an idealized type of process called a quasi- quasiequilibrium process equilibrium (or quasistatic) process. A quasiequilibrium process is one in which the

V

40

Chapter 2 Energy and the First Law of Thermodynamics Incremental masses removed during an expansion of the gas or liquid

Gas or liquid Boundary

Fig. 2.6 Illustration of a quasiequilibrium expansion or compression.

departure from thermodynamic equilibrium is at most infinitesimal. All states through which the system passes in a quasiequilibrium process may be considered equilibrium states. Because nonequilibrium effects are inevitably present during actual processes, systems of engineering interest can at best approach, but never realize, a quasiequilibrium process. Still the quasiequilibrium process plays a role in our study of engineering thermodynamics. For details, see the box. To consider how a gas (or liquid) might be expanded or compressed in a quasiequilibrium fashion, refer to Fig. 2.6, which shows a system consisting of a gas initially at an equilibrium state. As shown in the figure, the gas pressure is maintained uniform throughout by a number of small masses resting on the freely moving piston. Imagine that one of the masses is removed, allowing the piston to move upward as the gas expands slightly. During such an expansion the state of the gas would depart only slightly from equilibrium. The system would eventually come to a new equilibrium state, where the pressure and all other intensive properties would again be uniform in value. Moreover, were the mass replaced, the gas would be restored to its initial state, while again the departure from equilibrium would be slight. If several of the masses were removed one after another, the gas would pass through a sequence of equilibrium states without ever being far from equilibrium. In the limit as the increments of mass are made vanishingly small, the gas would undergo a quasiequilibrium expansion process. A quasiequilibrium compression can be visualized with similar considerations.

Using the Quasiequilibrium Process Concept Our interest in the quasiequilibrium process concept stems mainly from two considerations: c

Simple thermodynamic models giving at least qualitative information about the behavior of actual systems of interest often can be developed using the quasiequilibrium process concept. This is akin to the use of idealizations such as the point mass or the frictionless pulley in mechanics for the purpose of simplifying an analysis.

c

The quasiequilibrium process concept is instrumental in deducing relationships that exist among the properties of systems at equilibrium (Chaps. 3, 6, and 11).

Equation 2.17 can be applied to evaluate the work in quasiequilibrium expansion or compression processes. For such idealized processes the pressure p in the equation is the pressure of the entire quantity of gas (or liquid) undergoing the process, and not just the pressure at the moving boundary. The relationship between the pressure and volume may be graphical or analytical. Let us first consider a graphical relationship. A graphical relationship is shown in the pressure–volume diagram ( p–V diagram) of Fig. 2.7. Initially, the piston face is at position x1, and the gas pressure is p1; at the conclusion of a quasiequilibrium expansion process the piston face is at position x2, and the pressure is reduced to p2. At each intervening piston position, the uniform pressure throughout the gas is shown as a point on the diagram. The curve, or path, connecting states 1 and 2 on the diagram represents the equilibrium states through which the system has passed during the process. The work done by the gas on the piston during the expansion is given by ep dV, which can be interpreted as the area under the curve of pressure versus volume. Thus, the shaded area on Fig. 2.7 is equal to the work for the process. Had the gas been compressed from 2 to 1 along the same path on the p–V diagram, the magnitude of the work would be the same, but the sign would be negative, indicating that for the compression the energy transfer was from the piston to the gas. The area interpretation of work in a quasiequilibrium expansion or compression process allows a simple demonstration of the idea that work depends on the process.

2.2 Broadening our Understanding of Work

41

1

p1

Path

Pressure

δ W = p dV p 1 2

p2

Area = 2 ∫1 p dV V1

dV

V2

Volume

A

Gas or liquid

x

B

Area = work for process A x1

x2

Fig. 2.7 Work of a quasiequilibrium expansion or compression process.

2

V

Fig. 2.8 Illustration that work depends on the process.

This can be brought out by referring to Fig. 2.8. Suppose the gas in a piston–cylinder assembly goes from an initial equilibrium state 1 to a final equilibrium state 2 along two different paths, labeled A and B on Fig. 2.8. Since the area beneath each path represents the work for that process, the work depends on the details of the process as defined by the particular curve and not just on the end states. Using the test for a property given in Sec. 1.3.3, we can conclude again (Sec. 2.2.1) that work is not a property. The value of work depends on the nature of the process between the end states. The relation between pressure and volume, or pressure and specific volume, also can be described analytically. A quasiequilibrium process described by pV n 5 constant, or pyn 5 constant, where n is a constant, is called a polytropic process. Additional analytical forms for the pressure–volume relationship also may be considered. The example to follow illustrates the application of Eq. 2.17 when the relationship between pressure and volume during an expansion is described analytically as pV n 5 constant.

c c c cEXAMPLE

Comp_Work A.4 – All Tabs

A

Exp_Work A.5 – All Tabs

polytropic process

2.1 c

Evaluating Expansion Work A gas in a piston–cylinder assembly undergoes an expansion process for which the relationship between pressure and volume is given by pV n 5 constant The initial pressure is 3 bar, the initial volume is 0.1 m3, and the final volume is 0.2 m3. Determine the work for the process, in kJ, if (a) n 5 1.5, (b) n 5 1.0, and (c) n 5 0. SOLUTION Known: A gas in a piston–cylinder assembly undergoes an expansion for which pV n 5 constant. Find: Evaluate the work if (a) n 5 1.5, (b) n 5 1.0, (c) n 5 0.

42

Chapter 2 Energy and the First Law of Thermodynamics Schematic and Given Data: The given p–V relationship and the given data for pressure and volume can be used to construct the accompanying pressure–volume diagram of the process. Engineering Model:

p (bar)

3.0

➊

1

1. The gas is a closed system.

2c p1 = 3.0 bar V1 = 0.1 m3

Gas 2.0

pVn

= constant

2b 1.0

Area = work for part a 0.1

2a

V2 = 0.2 m3

2. The moving boundary is the

only work mode. 3. The expansion is a polytropic

process.

➋

0.2

Fig. E2.1

V (m3)

Analysis: The required values for the work are obtained by integration of Eq. 2.17 using the given pressure– volume relation. (a) Introducing the relationship p 5 constant/V n into Eq. 2.17 and performing the integration

W5

#

V2

p dV 5

V1

#

V2

V1

constant dV n V

(constant)V12n 2 (constant)V12n 2 1 12n n n The constant in this expression can be evaluated at either end state: constant 5 p1V 1 5 p2V 2 . The work expression then becomes 5

n

12n

(p2V 2 )V2

W5

n

12n

2 (p1V 1 )V1 12n

5

p2 V 2 2 p1 V 1 12n

(a)

This expression is valid for all values of n except n 5 1.0. The case n 5 1.0 is taken up in part (b). n n To evaluate W, the pressure at state 2 is required. This can be found by using p1V 1 5 p2V 2 , which on rearrangement yields p2 5 p1 a

V1 n 0.1 1.5 b 5 (3 bar)a b 5 1.06 bar V2 0.2

Accordingly, ➌

W5a

(1.06 bar)(0.2 m3) 2 (3)(0.1) 105 N/m2 1 kJ ` b` ` ` 3 1 2 1.5 1 bar 10 N ? m

5 1 17.6 kJ (b) For n 5 1.0, the pressure–volume relationship is pV 5 constant or p 5 constant/V. The work is

W 5 constant

#

V2

V1

V2 V2 dV 5 (p1V1) ln 5 (constant) ln V V1 V1

(b)

Substituting values W 5 (3 bar)(0.1 m3) `

0.2 105 N/m2 1 kJ ` ln a b 5 1 20.79 kJ ` ` 3 1 bar 0.1 10 N ? m

(c) For n 5 0, the pressure–volume relation reduces to p 5 constant, and the integral becomes W 5 p(V2 2 V1), which is a special case of the expression found in part (a). Substituting values and converting units as above, W 5 130 kJ. ➍

2.2 Broadening our Understanding of Work ➊ In each case, the work for the process can be interpreted as the area under the curve representing the process on the accompanying p–V diagram. Note that the relative areas are in agreement with the numerical results.

43

✓ Skills Developed Ability to… ❑ apply the problem-solving

➋ The assumption of a polytropic process is significant. If the given pressure– volume relationship were obtained as a fit to experimental pressure–volume data, the value of e p d V would provide a plausible estimate of the work only when the measured pressure is essentially equal to that exerted at the piston face.

methodology. ❑ define a closed system and

identify interactions on its boundary. ❑ evaluate work using Eq. 2.17. ❑ apply the pressure–volume relation pV n 5 constant.

➌ Observe the use of unit conversion factors here and in part (b).

➍ In each of the cases considered, it is not necessary to identify the gas (or liquid) contained within the piston–cylinder assembly. The calculated values for W are determined by the process path and the end states. However, if it is desired to evaluate a property such as temperature, both the nature and amount of the substance must be provided because appropriate relations among the properties of the particular substance would then be required. Evaluate the work, in kJ, for a two-step process consisting of an expansion with n 5 1.0 from p1 5 3 bar, V1 5 0.1 m3 to V 5 0.15 m3, followed by an expansion with n 5 0 from V 5 0.15 m3 to V2 5 0.2 m3. Ans. 22.16 kJ.

2.2.6 Further Examples of Work To broaden our understanding of the work concept, we now briefly consider several other examples of work.

Extension of a Solid Bar Consider a system consisting of a solid bar under tension, as shown in Fig. 2.9. The bar is fixed at x 5 0, and a force F is applied at the other end. Let the force be represented as F 5 𝜎A, where A is the cross-sectional area of the bar and s the normal stress acting at the end of the bar. The work done as the end of the bar moves a distance dx is given by 𝛿W 5 2𝜎A dx. The minus sign is required because work is done on the bar when dx is positive. The work for a change in length from x1 to x2 is found by integration W52

#

Area = A

F

x x1

x2

𝜎A dx

x2

(2.18)

x1

Fig. 2.9 Elongation of a solid bar.

Equation 2.18 for a solid is the counterpart of Eq. 2.17 for a gas undergoing an expansion or compression.

Rigid wire frame Surface of film

Stretching of a Liquid Film Figure 2.10 shows a system consisting of a liquid film suspended on a wire frame. The two surfaces of the film support the thin liquid layer inside by the effect of surface tension, owing to microscopic forces between molecules near the liquid–air interfaces. These forces give rise to a macroscopically measurable force perpendicular to any line in the surface. The force per unit length across such a line is the surface tension. Denoting the surface tension acting at the movable wire by 𝜏, the force F indicated on the figure can be expressed as F 5 2l𝜏, where the factor 2 is introduced because two film surfaces act at the wire. If the movable wire is displaced by dx, the work is given by 𝛿W 5 22l𝜏 dx. The minus sign is required because work is done on the system when dx is positive. Corresponding to a displacement dx is a change in the total area of the surfaces in contact with the wire

Movable wire F

l dx x

Fig. 2.10 Stretching of a liquid film.

44

Chapter 2 Energy and the First Law of Thermodynamics of dA 5 2l dx, so the expression for work can be written alternatively as 𝛿W 5 2𝜏 dA. The work for an increase in surface area from A1 to A2 is found by integrating this expression: W52

#

A2

𝜏 dA

(2.19)

A1

Power Transmitted by a Shaft +

W˙ shaft

Motor

–

,ω

– + System boundary

A rotating shaft is a commonly encountered machine element. Consider a shaft rotating with angular velocity 𝜔 and exerting a torque 7 on its surroundings. Let the torque be expressed in terms of a tangential force Ft and radius R: 7 5 Ft R. The velocity at the point of application of the force is V 5 R𝜔, where 𝜔 is in radians per unit time. Using these relations with Eq. 2.13, we obtain an expression for the power transmitted from the shaft to the surroundings . W 5 FtV 5 (7/R)(R𝜔) 5 7𝜔 (2.20) A related case involving a gas stirred by a paddle wheel is considered in the discussion of Fig. 2.3.

i a

Ᏹ

b

Electrolytic cell

Fig. 2.11 Electrolytic cell used to discuss electric power.

Electric Power Shown in Fig. 2.11 is a system consisting of an electrolytic cell. The cell is connected to an external circuit through which an electric current, i, is flowing. The current is driven by the electrical potential difference % existing across the terminals labeled a and b. That this type of interaction can be classed as work is considered in the discussion of Fig. 2.3. The rate of energy transfer by work, or the power, is . W 5 2%i (2.21) Since the current i equals dZ/dt, the work can be expressed in differential form as 𝛿W 5 2% dZ

TAKE NOTE...

When power is evaluated in terms of the watt, and the unit of current is the ampere (an SI base unit), the unit of electric potential is the volt, defined as 1 watt per ampere.

(2.22)

where dZ is the amount of electrical charge that flows into the system. The minus signs appearing in Eqs. 2.21 and 2.22 are required to be in accord with our previously stated sign convention for work.

Work due to Polarization or Magnetization Let us next refer briefly to the types of work that can be done on systems residing in electric or magnetic fields, known as the work of polarization and magnetization, respectively. From the microscopic viewpoint, electrical dipoles within dielectrics resist turning, so work is done when they are aligned by an electric field. Similarly, magnetic dipoles resist turning, so work is done on certain other materials when their magnetization is changed. Polarization and magnetization give rise to macroscopically detectable changes in the total dipole moment as the particles making up the material are given new alignments. In these cases the work is associated with forces imposed on the overall system by fields in the surroundings. Forces acting on the material in the system interior are called body forces. For such forces the appropriate displacement in evaluating work is the displacement of the matter on which the body force acts.

2.2.7 Further Examples of Work in Quasiequilibrium Processes Systems other than a gas or liquid in a piston–cylinder assembly also can be envisioned as undergoing processes in a quasiequilibrium fashion. To apply the quasiequilibrium process concept in any such case, it is necessary to conceive of an ideal situation in

2.2 Broadening our Understanding of Work which the external forces acting on the system can be varied so slightly that the resulting imbalance is infinitesimal. As a consequence, the system undergoes a process without ever departing significantly from thermodynamic equilibrium. The extension of a solid bar and the stretching of a liquid surface can readily be envisioned to occur in a quasiequilibrium manner by direct analogy to the piston– cylinder case. For the bar in Fig. 2.9 the external force can be applied in such a way that it differs only slightly from the opposing force within. The normal stress is then essentially uniform throughout and can be determined as a function of the instantaneous length: 𝜎 5 𝜎(x). Similarly, for the liquid film shown in Fig. 2.10 the external force can be applied to the movable wire in such a way that the force differs only slightly from the opposing force within the film. During such a process, the surface tension is essentially uniform throughout the film and is functionally related to the instantaneous area: 𝜏 5 𝜏(A). In each of these cases, once the required functional relationship is known, the work can be evaluated using Eq. 2.18 or 2.19, respectively, in terms of properties of the system as a whole as it passes through equilibrium states. Other systems also can be imagined as undergoing quasiequilibrium processes. For example, it is possible to envision an electrolytic cell being charged or discharged in a quasiequilibrium manner by adjusting the potential difference across the terminals to be slightly greater, or slightly less, than an ideal potential called the cell electromotive force (emf). The energy transfer by work for passage of a differential quantity of charge to the cell, dZ, is given by the relation 𝛿W 5 2% dZ

(2.23)

In this equation % denotes the cell emf, an intensive property of the cell, and not just the potential difference across the terminals as in Eq. 2.22. Consider next a dielectric material residing in a uniform electric field. The energy transferred by work from the field when the polarization is increased slightly is 𝛿W 5 2E ? d(VP)

(2.24)

where the vector E is the electric field strength within the system, the vector P is the electric dipole moment per unit volume, and V is the volume of the system. A similar equation for energy transfer by work from a uniform magnetic field when the magnetization is increased slightly is 𝛿W 5 2𝜇0H ? d(V M)

(2.25)

where the vector H is the magnetic field strength within the system, the vector M is the magnetic dipole moment per unit volume, and 𝜇0 is a constant, the permeability of free space. The minus signs appearing in the last three equations are in accord with our previously stated sign convention for work: W takes on a negative value when the energy transfer is into the system.

2.2.8 Generalized Forces and Displacements The similarity between the expressions for work in the quasiequilibrium processes considered thus far should be noted. In each case, the work expression is written in the form of an intensive property and the differential of an extensive property. This is brought out by the following expression, which allows for one or more of these work modes to be involved in a process: 𝛿W 5 p dV 2 𝜎d(Ax) 2 𝜏 dA 2 % dZ 2 E ? d(VP) 2 𝜇0H ? d(VM) 1 ? ? ? (2.26) where the last three dots represent other products of an intensive property and the differential of a related extensive property that account for work. Because of the notion of work being a product of force and displacement, the intensive property in these relations is sometimes referred to as a “generalized” force and the extensive

45

TAKE NOTE...

Some readers may elect to defer Secs. 2.2.7 and 2.2.8 and proceed directly to Sec. 2.3 where the energy concept is broadened.

46

Chapter 2 Energy and the First Law of Thermodynamics F

property as a “generalized” displacement, even though the quantities making up the work expressions may not bring to mind actual forces and displacements. Owing to the underlying quasiequilibrium restriction, Eq. 2.26 does not represent every type of work of practical interest. An example is provided by a paddle wheel that stirs a gas or liquid taken as the system. Whenever any shearing action takes place, the system necessarily passes through nonequilibrium states. To appreciate more fully the implications of the quasiequilibrium process concept requires consideration of the second law of thermodynamics, so this concept is discussed again in Chap. 5 after the second law has been introduced. i

2.3 Broadening Our Understanding of Energy Battery

Paddle wheel

Gas

internal energy

A

Total_Energy A.6 – Tab a

The objective in this section is to use our deeper understanding of work developed in Sec. 2.2 to broaden our understanding of the energy of a system. In particular, we consider the total energy of a system, which includes kinetic energy, gravitational potential energy, and other forms of energy. The examples to follow illustrate some of these forms of energy. Many other examples could be provided that enlarge on the same idea. When work is done to compress a spring, energy is stored within the spring. When a battery is charged, the energy stored within it is increased. And when a gas (or liquid) initially at an equilibrium state in a closed, insulated vessel is stirred vigorously and allowed to come to a final equilibrium state, the energy of the gas is increased in the process. In keeping with the discussion of work in Sec. 2.2, we can also think of other ways in which work done on systems increases energy stored within those systems— work related to magnetization, for example. In each of these examples the change in system energy cannot be attributed to changes in the system’s overall kinetic or gravitational potential energy as given by Eqs. 2.5 and 2.10, respectively. The change in energy can be accounted for in terms of internal energy, as considered next. In engineering thermodynamics the change in the total energy of a system is considered to be made up of three macroscopic contributions. One is the change in kinetic energy, associated with the motion of the system as a whole relative to an external coordinate frame. Another is the change in gravitational potential energy, associated with the position of the system as a whole in the earth’s gravitational field. All other energy changes are lumped together in the internal energy of the system. Like kinetic energy and gravitational potential energy, internal energy is an extensive property of the system, as is the total energy. Internal energy is represented by the symbol U, and the change in internal energy in a process is U2 2 U1. The specific internal energy is symbolized by u or u, respectively, depending on whether it is expressed on a unit mass or per mole basis. The change in the total energy of a system is E2 2 E1 5 (U2 2 U1) 1 (KE2 2 KE1) 1 (PE2 2 PE1)

(2.27a)

or DE 5 DU 1 DKE 1 DPE

(2.27b)

All quantities in Eq. 2.27 are expressed in terms of the energy units previously introduced. The identification of internal energy as a macroscopic form of energy is a significant step in the present development, for it sets the concept of energy in thermodynamics apart from that of mechanics. In Chap. 3 we will learn how to evaluate changes

2.4 Energy Transfer by Heat in internal energy for practically important cases involving gases, liquids, and solids by using empirical data. To further our understanding of internal energy, consider a system we will often encounter in subsequent sections of the book, a system consisting of a gas contained in a tank. Let us develop a microscopic interpretation of internal energy by thinking of the energy attributed to the motions and configurations of the individual molecules, atoms, and subatomic particles making up the matter in the system. Gas molecules move about, encountering other molecules or the walls of the container. Part of the internal energy of the gas is the translational kinetic energy of the molecules. Other contributions to the internal energy include the kinetic energy due to rotation of the molecules relative to their centers of mass and the kinetic energy associated with vibrational motions within the molecules. In addition, energy is stored in the chemical bonds between the atoms that make up the molecules. Energy storage on the atomic level includes energy associated with electron orbital states, nuclear spin, and binding forces in the nucleus. In dense gases, liquids, and solids, intermolecular forces play an important role in affecting the internal energy.

47

microscopic interpretation of internal energy for a gas

2.4 Energy Transfer by Heat Thus far, we have considered quantitatively only those interactions between a system and its surroundings that can be classed as work. However, closed systems also can interact with their surroundings in a way that cannot be categorized as work.

Gas

when a gas in a rigid container interacts with a hot plate, the energy of the gas is increased even though no work is done. b b b b b This type of interaction is called an energy transfer by heat. On the basis of experiment, beginning with the work of Joule in the early part of the nineteenth century, we know that energy transfers by heat are induced only as a result of a temperature difference between the system and its surroundings and occur only in the direction of decreasing temperature. Because the underlying concept is so important in thermodynamics, this section is devoted to a further consideration of energy transfer by heat.

Hot plate

energy transfer by heat

2.4.1 Sign Convention, Notation, and Heat Transfer Rate The symbol Q denotes an amount of energy transferred across the boundary of a system in a heat interaction with the system’s surroundings. Heat transfer into a system is taken to be positive, and heat transfer from a system is taken as negative. Q . 0: heat transfer to the system Q , 0: heat transfer from the system This sign convention is used throughout the book. However, as was indicated for work, it is sometimes convenient to show the direction of energy transfer by an arrow on a sketch of the system. Then the heat transfer is regarded as positive in the direction of the arrow. The sign convention for heat transfer is just the reverse of the one adopted for work, where a positive value for W signifies an energy transfer from the system to the surroundings. These signs for heat and work are a legacy from engineers and scientists who were concerned mainly with steam engines and other devices that develop a work output from an energy input by heat transfer. For such applications, it was convenient to regard both the work developed and the energy input by heat transfer as positive quantities.

sign convention for heat transfer

HT_Modes A.7 – Tab a

A

48

Chapter 2 Energy and the First Law of Thermodynamics

heat is not a property

The value of a heat transfer depends on the details of a process and not just the end states. Thus, like work, heat is not a property, and its differential is written as 𝛿Q. The amount of energy transfer by heat for a process is given by the integral 2

Q5

# 𝛿Q

(2.28)

1

heat transfer

where the limits mean “from state 1 to state 2” and do not refer to the values of heat at those states. As for work, the notion of “heat” at a state has no meaning, and the integral should never be evaluated as Q2 2 Q1. ? The net rate of heat transfer is denoted by Q. In principle, the amount of energy transfer by heat during a period of time can be found by integrating from time t1 to time t2 t2

Q5

# Q dt ?

(2.29)

t1

To perform the integration, it is necessary to know how the rate of heat transfer varies with time. In some cases it is convenient to use the heat flux, q? , which is the heat transfer ? rate per unit of system surface area. The net rate of heat transfer, Q, is related to the ? heat flux q by the integral . Q 5

# q. dA

(2.30)

A

adiabatic

where A represents the area on the boundary of the system where heat transfer occurs. ? The units for heat transfer Q and heat transfer rate Q are the same as those introduced ? previously for W and W, respectively. The unit for the heat flux is the heat transfer rate per unit area: kW/m2. The word adiabatic means without heat transfer. Thus, if a system undergoes a process involving no heat transfer with its surroundings, that process is called an adiabatic process.

BIOCONNECTIONS Medical researchers have found that by gradually increasing the temperature of cancerous tissue to 41–458C the effectiveness of chemotherapy and radiation therapy is enhanced for some patients. Different approaches can be used, including raising the temperature of the entire body with heating devices and, more selectively, by beaming microwaves or ultrasound onto the tumor or affected organ. Speculation about why a temperature increase may be beneficial varies. Some say it helps chemotherapy penetrate certain tumors more readily by dilating blood vessels. Others think it helps radiation therapy by increasing the amount of oxygen in tumor cells, making them more receptive to radiation. Researchers report that further study is needed before the efficacy of this approach is established and the mechanisms whereby it achieves positive results are known.

2.4.2 Heat Transfer Modes Methods based on experiment are available for evaluating energy transfer by heat. These methods recognize two basic transfer mechanisms: conduction and thermal radiation. In addition, empirical relationships are available for evaluating energy transfer involving a combined mode called convection. A brief description of each of these is given next. A detailed consideration is left to a course in engineering heat transfer, where these topics are studied in depth.

2.4 Energy Transfer by Heat

49

Conduction Energy transfer by conduction can take place in solids, liquids, and gases. Conduction can be thought of as the transfer of energy from the more energetic particles of a substance to adjacent particles that are less energetic due to interactions between particles. The time rate of energy transfer by conduction is quantified macroscopically by Fourier’s law. As an elementary application, consider Fig. 2.12 showing a plane wall of thickness L at steady state, where the temperature T(x) varies linearly with position x. By Fourier’s law, the rate of heat transfer across any plane normal to the ? x direction, Qx, is proportional to the wall area, A, and the temperature gradient in the x direction, dT/dx: dT (2.31) dx where the proportionality constant k is a property called the thermal conductivity. The minus sign is a consequence of energy transfer in the direction of decreasing temperature.

. Qx

T1

L

?

Qx 5 2𝜅A

in the case of Fig. 2.12 the temperature varies linearly; thus, the temperature gradient is

T2 Area, A x

Fig. 2.12 Illustration of Fourier’s conduction law. Fourier’s law

dT T 2 T1 5 2 (, 0) dx L and the rate of heat transfer in the x direction is then Qx 5 2𝜅A c ?

T2 2 T1 d L

b b b b b

Values of thermal conductivity are given in Table A-19 for common materials. Substances with large values of thermal conductivity such as copper are good conductors, and those with small conductivities (cork and polystyrene foam) are good insulators.

HT_Modes A.7 – Tab b

A

Radiation Thermal radiation is emitted by matter as a result of changes in the electronic configurations of the atoms or molecules within it. The energy is transported by electromagnetic waves (or photons). Unlike conduction, thermal radiation requires no intervening medium to propagate and can even take place in a vacuum. Solid surfaces, gases, and liquids all emit, absorb, and transmit thermal radiation to varying degrees. The rate ? at which energy is emitted, Qe, from a surface of area A is quantified macroscopically by a modified form of the Stefan–Boltzmann law ?

Qe 5 𝜀𝜎AT b4

Stefan–Boltzmann law

(2.32)

which shows that thermal radiation is associated with the fourth power of the absolute temperature of the surface, Tb. The emissivity, 𝜀, is a property of the surface that indicates how effectively the surface radiates (0 # 𝜀 # 1.0), and 𝜎 is the Stefan– Boltzmann constant: 𝜎 5 5.67 3 1028 W/m2 ? K4 In general, the net rate of energy transfer by thermal radiation between two surfaces involves relationships among the properties of the surfaces, their orientations with respect to each other, the extent to which the intervening medium scatters, emits, and absorbs thermal radiation, and other factors. A special case that occurs frequently is radiation exchange between a surface at temperature Tb and a much larger surrounding surface at Ts, as shown in Fig. 2.13. The net rate of radiant exchange between the smaller surface, whose area is A and emissivity is 𝜀, and the larger surroundings is ?

Qe 5 𝜀𝜎A[T b4 2 T s4]

(2.33)

HT_Modes A.7 – Tab d

A

50

Chapter 2 Energy and the First Law of Thermodynamics

Cooling air flow Tf < Tb

. Qc

Surrounding surface at Ts . Qe

A Surface of emissivity ε, area A, and temperature Tb

Transistor

Tb

Wire leads Circuit board

Fig. 2.14 Illustration of Newton’s law of cooling.

Fig. 2.13 Net radiation exchange.

Convection Energy transfer between a solid surface at a temperature Tb and an adjacent gas or liquid at another temperature Tf plays a prominent role in the performance of many devices of practical interest. This is commonly referred to as convection. As an illustration, consider Fig. 2.14, where Tb . Tf . In this case, energy is transferred in the direction indicated by the arrow due to the combined effects of conduction within the air and the bulk motion of the air. The rate of energy transfer from the surface to the air can be quantified by the following empirical expression: ?

Qc 5 hA(Tb 2 Tf) Newton’s law of cooling

A

HT_Modes A.7 – Tab c

(2.34)

known as Newton’s law of cooling. In Eq. 2.34, A is the surface area and the proportionality factor h is called the heat transfer coefficient. In subsequent applications of Eq. 2.34, a minus sign may be introduced on the right side to conform to the sign convention for heat transfer introduced in Sec. 2.4.1. The heat transfer coefficient is not a thermodynamic property. It is an empirical parameter that incorporates into the heat transfer relationship the nature of the flow pattern near the surface, the fluid properties, and the geometry. When fans or pumps cause the fluid to move, the value of the heat transfer coefficient is generally greater than when relatively slow buoyancy-induced motions occur. These two general categories are called forced and free (or natural) convection, respectively. Table 2.1 provides typical values of the convection heat transfer coefficient for forced and free convection.

2.4.3 Closing Comments The first step in a thermodynamic analysis is to define the system. It is only after the system boundary has been specified that possible heat interactions with the surroundings are considered, for these are always evaluated at the system boundary.

TABLE 2.1

Typical Values of the Convection Heat Transfer Coefficient Applications

Free convection Gases Liquids Forced convection Gases Liquids

h (W/m2 ? K)

2–25 50–1,000 25–250 50–20,000

2.5 Energy Accounting: Energy Balance for Closed Systems In ordinary conversation, the term heat is often used when the word energy would be more correct thermodynamically. For example, one might hear, “Please close the door or ‘heat’ will be lost.” In thermodynamics, heat refers only to a particular means whereby energy is transferred. It does not refer to what is being transferred between systems or to what is stored within systems. Energy is transferred and stored, not heat. Sometimes the heat transfer of energy to, or from, a system can be neglected. This might occur for several reasons related to the mechanisms for heat transfer discussed above. One might be that the materials surrounding the system are good insulators, or heat transfer might not be significant because there is a small temperature difference between the system and its surroundings. A third reason is that there might not be enough surface area to allow significant heat transfer to occur. When heat transfer is neglected, it is because one or more of these considerations apply. In the discussions to follow, the value of Q is provided or it is an unknown in the analysis. When Q is provided, it can be assumed that the value has been determined by the methods introduced above. When Q is the unknown, its value is usually found by using the energy balance, discussed next.

2.5 Energy Accounting: Energy Balance for Closed Systems As our previous discussions indicate, the only ways the energy of a closed system can be changed are through transfer of energy by work or by heat. Further, based on the experiments of Joule and others, a fundamental aspect of the energy concept is that energy is conserved; we call this the first law of thermodynamics. For further discussion of the first law, see the box.

Joule’s Experiments and the First Law In classic experiments conducted in the early part of the nineteenth century, Joule studied processes by which a closed system can be taken from one equilibrium state to another. In particular, he considered processes that involve work interactions but no heat interactions between the system and its surroundings. Any such process is an adiabatic process, in keeping with the discussion of Sec. 2.4.1. Based on his experiments Joule deduced that the value of the net work is the same for all adiabatic processes between two equilibrium states. In other words, the value of the net work done by or on a closed system undergoing an adiabatic process between two given states depends solely on the end states and not on the details of the adiabatic process. If the net work is the same for all adiabatic processes of a closed system between a given pair of end states, it follows from the definition of property (Sec. 1.3) that the net work for such a process is the change in some property of the system. This property is called energy. Following Joule’s reasoning, the change in energy between the two states is defined by E2 2 E1 5 2Wad

(a)

where the symbol E denotes the energy of a system and Wad represents the net work for any adiabatic process between the two states. The minus sign before the work

first law of thermodynamics

51

52

Chapter 2 Energy and the First Law of Thermodynamics

term is in accord with the previously stated sign convention for work. Finally, note that since any arbitrary value E1 can be assigned to the energy of a system at a given state 1, no particular significance can be attached to the value of the energy at state 1 or at any other state. Only changes in the energy of a system have significance. The foregoing discussion is based on experimental evidence beginning with the experiments of Joule. Because of inevitable experimental uncertainties, it is not possible to prove by measurements that the net work is exactly the same for all adiabatic processes between the same end states. However, the preponderance of experimental findings supports this conclusion, so it is adopted as a fundamental principle that the work actually is the same. This principle is an alternative formulation of the first law, and has been used by subsequent scientists and engineers as a springboard for developing the conservation of energy concept and the energy balance as we know them today.

Summarizing Energy Concepts All energy aspects introduced in this book thus far are summarized in words as follows: change in the amount net amount of energy net amount of energy of energy contained transferred in across transferred out across ≥ within a system ¥ 5 ≥the system boundary by¥ 2 ≥the system boundary ¥ during some time heat transfer during by work during the interval the time interval time interval

energy balance

This word statement is just an accounting balance for energy, an energy balance. It requires that in any process of a closed system the energy of the system increases or decreases by an amount equal to the net amount of energy transferred across its boundary. The phrase net amount used in the word statement of the energy balance must be carefully interpreted, for there may be heat or work transfers of energy at many different places on the boundary of a system. At some locations the energy transfers may be into the system, whereas at others they are out of the system. The two terms on the right side account for the net results of all the energy transfers by heat and work, respectively, taking place during the time interval under consideration. The energy balance can be expressed in symbols as E2 2 E1 5 Q 2 W

(2.35a)

When introducing Eq. 2.27, an alternative form is DKE 1 DPE 1 DU 5 Q 2 W

A

Energy_Bal_Closed _Sys A.8 – All Tabs

(2.35b)

which shows that an energy transfer across the system boundary results in a change in one or more of the macroscopic energy forms: kinetic energy, gravitational potential energy, and internal energy. All previous references to energy as a conserved quantity are included as special cases of Eqs. 2.35. Note that the algebraic signs before the heat and work terms of Eqs. 2.35 are different. This follows from the sign conventions previously adopted. A minus sign appears before W because energy transfer by work from the system to the surroundings is taken to be positive. A plus sign appears before Q because it is regarded to be positive when the heat transfer of energy is into the system from the surroundings.

2.5 Energy Accounting: Energy Balance for Closed Systems BIOCONNECTIONS The energy required by animals to sustain life is derived from oxidation of ingested food. We often speak of food being burned in the body. This is an appropriate expression because experiments show that when food is burned with oxygen, approximately the same energy is released as when the food is oxidized in the body. Such an experimental device is the well-insulated, constant-volume calorimeter shown in Fig. 2.15. Thermometer

Access port – +

O2

Igniter Stirrer

Sample Water bath Insulation

Fig. 2.15 Constant-volume calorimeter.

A carefully weighed food sample is placed in the chamber of the calorimeter together with oxygen (O2). The entire chamber is submerged in the calorimeter’s water bath. The chamber contents are then electrically ignited, fully oxidizing the food sample. The energy released during the reaction within the chamber results in an increase in calorimeter temperature. Using the measured temperature rise, the energy released can be calculated from an energy balance for the calorimeter as the system. This is reported as the calorie value of the food sample, usually in terms of kilocalorie (kcal), which is the “Calorie” seen on food labels.

2.5.1 Important Aspects of the Energy Balance Various special forms of the energy balance can be written. For example, the energy balance in differential form is dE 5 𝛿Q 2 𝛿W

(2.36)

where dE is the differential of energy, a property. Since Q and W are not properties, their differentials are written as 𝛿Q and 𝛿W, respectively. The instantaneous time rate form of the energy balance is dE ? ? 5Q2W dt The rate form of the energy balance expressed in words is time rate of change net rate at which net rate at which of the energy energy is being energy is being contained within transferred in 5 2 ≥ ¥ ≥ ¥ ≥ transferred out ¥ the system at by heat transfer by work at time t at time t time t

(2.37)

time rate form of the energy balance

53

54

Chapter 2 Energy and the First Law of Thermodynamics Since the time rate of change of energy is given by dE d KE d PE dU 5 1 1 dt dt dt dt Equation 2.37 can be expressed alternatively as # # d KE d PE dU 1 1 5Q2W dt dt dt

(2.38)

Equations 2.35 through 2.38 provide alternative forms of the energy balance that are convenient starting points when applying the principle of conservation of energy to closed systems. In Chap. 4 the conservation of energy principle is expressed in forms suitable for the analysis of control volumes. When applying the energy balance in any of its forms, it is important to be careful about signs and units and to distinguish carefully between rates and amounts. In addition, it is important to recognize that the location of the system boundary can be relevant in determining whether a particular energy transfer is regarded as heat or work. consider Fig. 2.16, in which three alternative systems are shown that include a quantity of a gas (or liquid) in a rigid, well-insulated container. In Fig. 2.16a, the gas itself is the system. As current flows through the copper plate, there is an energy transfer from the copper plate to the gas. Since this energy transfer occurs as a result of the temperature difference between the plate and the gas, it is classified as a heat transfer. Next, refer to Fig. 2.16b, where the boundary is drawn to include the copper plate. It follows from the thermodynamic definition of work that the energy transfer that occurs as current crosses the boundary of this system must be regarded as work. Finally, in Fig. 2.16c, the boundary is located so that no energy is transferred across it by heat or work. b b b b b

Copper plate Gas or liquid Q W=0

System boundary

Rotating shaft

+

Gas W or liquid

– Electric generator

Insulation

System boundary Mass

(a)

–

Q=0

(b)

+

Gas or liquid

System boundary

+

–

Q = 0, W = 0 (c)

Fig. 2.16 Alternative choices for system boundaries.

2.5 Energy Accounting: Energy Balance for Closed Systems

55

Closing Comments Thus far, we have been careful to emphasize that the quantities symbolized by W and Q in the foregoing equations account for transfers of energy and not transfers of work and heat, respectively. The terms work and heat denote different means whereby energy is transferred and not what is transferred. However, to achieve economy of expression in subsequent discussions, W and Q are often referred to simply as work and heat transfer, respectively. This less formal manner of speaking is commonly used in engineering practice. The five solved examples provided in Secs. 2.5.2–2.5.4 bring out important ideas about energy and the energy balance. They should be studied carefully, and similar approaches should be used when solving the end-of-chapter problems. In this text, most applications of the energy balance will not involve significant kinetic or potential energy changes. Thus, to expedite the solutions of many subsequent examples and end-of-chapter problems, we indicate in the problem statement that such changes can be neglected. If this is not made explicit in a problem statement, you should decide on the basis of the problem at hand how best to handle the kinetic and potential energy terms of the energy balance.

2.5.2 Using the Energy Balance: Processes of Closed Systems The next two examples illustrate the use of the energy balance for processes of closed systems. In these examples, internal energy data are provided. In Chap. 3, we learn how to obtain internal energy and other thermodynamic property data using tables, graphs, equations, and computer software. cccc

EXAMPLE 2.2 c

Cooling a Gas in a Piston–Cylinder Four-tenths kilogram of a certain gas is contained within a piston–cylinder assembly. The gas undergoes a process for which the pressure–volume relationship is pV1.5 5 constant The initial pressure is 3 bar, the initial volume is 0.1 m3, and the final volume is 0.2 m3. The change in specific internal energy of the gas in the process is u2 2 u1 5 255 kJ/kg. There are no significant changes in kinetic or potential energy. Determine the net heat transfer for the process, in kJ. SOLUTION Known: A gas within a piston–cylinder assembly undergoes an expansion process for which the pressure–volume relation and the change in specific internal energy are specified. Find: Determine the net heat transfer for the process. Schematic and Given Data: p

1

u2 – u1 = –55 kJ/kg pV1.5 = constant

➊ Area = work

Gas pV1.5 = constant

2

V

Engineering Model: 1. The gas is a closed system. 2. The process is described by

pV1.5 5 constant. 3. There is no change in the

kinetic or potential energy of the system. Fig. E2.2

56

Chapter 2 Energy and the First Law of Thermodynamics Analysis: An energy balance for the closed system takes the form

DKE 0 1 DPE 0 1 DU 5 Q 2 W where the kinetic and potential energy terms drop out by assumption 3. Then, writing DU in terms of specific internal energies, the energy balance becomes m(u2 2 u1) 5 Q 2 W where m is the system mass. Solving for Q Q 5 m(u2 2 u1) 1 W The value of the work for this process is determined in the solution to part (a) of Example 2.1: W 5 117.6 kJ. The change in internal energy is obtained using given data as m(u2 2 u1) 5 0.4 kg a255

kJ b 5 222 kJ kg

Substituting values ➋

Q 5 222 1 17.6 5 24.4 kJ

➊ The given relationship between pressure and volume allows the process to be represented by the path shown on the accompanying diagram. The area under the curve represents the work. Since they are not properties, the values of the work and heat transfer depend on the details of the process and cannot be determined from the end states only. ➋ The minus sign for the value of Q means that a net amount of energy has been transferred from the system to its surroundings by heat transfer.

✓ Skills Developed Ability to… ❑ define a closed system and

identify interactions on its boundary. ❑ apply the closed-system energy balance.

If the gas undergoes a process for which pV 5 constant and Du 5 0, determine the heat transfer, in kJ, keeping the initial pressure and given volumes fixed. Ans. 20.79 kJ.

In the next example, we follow up the discussion of Fig. 2.16 by considering two alternative systems. This example highlights the need to account correctly for the heat and work interactions occurring on the boundary as well as the energy change. cccc

EXAMPLE 2.3 c

Considering Alternative Systems Air is contained in a vertical piston–cylinder assembly fitted with an electrical resistor. The atmosphere exerts a pressure of 1 bar on the top of the piston, which has a mass of 45 kg and a face area of 0.09 m2. Electric current passes through the resistor, and the volume of the air slowly increases by 0.045 m3 while its pressure remains constant. The mass of the air is 0.27 kg, and its specific internal energy increases by 42 kJ/kg. The air and piston are at rest initially and finally. The piston–cylinder material is a ceramic composite and thus a good insulator. Friction between the piston and cylinder wall can be ignored, and the local acceleration of gravity is g 5 9.81 m/s2. Determine the heat transfer from the resistor to the air, in kJ, for a system consisting of (a) the air alone, (b) the air and the piston. SOLUTION Known: Data are provided for air contained in a vertical piston–cylinder fitted with an electrical resistor. Find: Considering each of two alternative systems, determine the heat transfer from the resistor to the air.

2.5 Energy Accounting: Energy Balance for Closed Systems

57

Schematic and Given Data: Piston

Piston

patm = 1 bar

System boundary for part (a)

m piston = 45 kg A piston = 0.09 m2 +

Air

+

Air

–

–

m air = 0.27 kg V2 – V1 = 0.045 m3 Δu air = 42 kJ/kg

(a)

System boundary for part (b)

(b)

Engineering Model: 1. Two closed systems are under consideration, as shown in the schematic. 2. The only significant heat transfer is from the resistor to the air, during which the air expands slowly and

its pressure remains constant. 3. There is no net change in kinetic energy; the change in potential energy of the air is negligible; and since

the piston material is a good insulator, the internal energy of the piston is not affected by the heat transfer. 4. Friction between the piston and cylinder wall is negligible. 5. The acceleration of gravity is constant; g 5 9.81 m/s2. Analysis: (a) Taking the air as the system, the energy balance, Eq. 2.35, reduces with assumption 3 to

(DKE 0 1 DPE 0 1 DU)air 5 Q 2 W Or, solving for Q Q 5 W 1 DUair For this system, work is done by the force of the pressure p acting on the bottom of the piston as the air expands. With Eq. 2.17 and the assumption of constant pressure W5

#

V2

p dV 5 p(V2 2 V1)

V1

To determine the pressure p, we use a force balance on the slowly moving, frictionless piston. The upward force exerted by the air on the bottom of the piston equals the weight of the piston plus the downward force of the atmosphere acting on the top of the piston. In symbols p Apiston 5 mpiston g 1 patmApiston Solving for p and inserting values p⫽

5

mpiston g Apiston

⫹ patm

(45 kg)(9.81 m/s2) 0.09 m2

`

1 bar 105 N/m2

` 1 1 bar 5 1.049 bar

Thus, the work is W 5 p(V2 2 V1) 5 (1.049 bar) (0.045 m2) `

105 N/m2 1 kJ ` 5 4.72 kJ ` ` 3 1 bar 10 N ? m

58

Chapter 2 Energy and the First Law of Thermodynamics With DUair 5 mair(Duair), the heat transfer is Q 5 W 1 mair(Duair) 5 4.72 kJ 1 11.07 kJ 5 15.8 kJ (b) Consider next a system consisting of the air and the piston. The energy change of the overall system is the

sum of the energy changes of the air and the piston. Thus, the energy balance, Eq. 2.35, reads (DKE 0 1 DPE 0 1 DU)air 1 (DKE 0 1 DPE 1 DU 0)piston 5 Q 2 W where the indicated terms drop out by assumption 3. Solving for Q Q 5 W 1 (DPE)piston 1 (DU)air For this system, work is done at the top of the piston as it pushes aside the surrounding atmosphere. Applying Eq. 2.17 W5

#

V2

p dV 5 patm (V2 2 V1)

V1

5 (1 bar) (0.045 m2) `

105 N/m2 1 kJ ` 5 4.5 kJ ` ` 3 1 bar 10 N ? m

The elevation change, Dz, required to evaluate the potential energy change of the piston can be found from the volume change of the air and the area of the piston face as Dz 5

V2 2 V1 0.045 m3 5 5 0.5 m Apiston 0.09 m2

Thus, the potential energy change of the piston is (DPE)piston 5 mpiston gDz 5 (45 kg) (9.81 m/s2) (0.5 m) 5 0.22 kJ Finally, Q 5 W 1 (DPE)piston 1 mair Duair 5 4.5 kJ 1 0.22 kJ 1 11.07 kJ 5 15.8 kJ

✓ Skills Developed Ability to… ❑ define alternative closed

systems and identify interactions on their boundaries. ❑ evaluate work using Eq. 2.17. ❑ apply the closed-system energy balance.

➊ which agrees with the result of part (a). ➊ Although the value of Q is the same for each system, observe that the values for W differ. Also, observe that the energy changes differ, depending on whether the air alone or the air and the piston is the system. What is the change in potential energy of the air, in kJ? Ans. ≈1.055 3 1023 kJ

2.5.3 Using the Energy Rate Balance: Steady-State Operation A system is at steady state if none of its properties change with time (Sec. 1.3). Many devices operate at steady state or nearly at steady state, meaning that property variations with time are small enough to ignore. The two examples to follow illustrate the application of the energy rate equation to closed systems at steady state.

2.5 Energy Accounting: Energy Balance for Closed Systems cccc

59

EXAMPLE 2.4 c

Evaluating Energy Transfer Rates of a Gearbox at Steady State During steady-state operation, a gearbox receives 60 kW through the input shaft and delivers power through the output shaft. For the gearbox as the system, the rate of energy transfer by convection is # Q 5 2hA(Tb 2 Tf) where h 5 0.171 kW/m2 ? K is the heat transfer coefficient, A 5 1.0 m2 is the outer surface area of the gearbox, Tb 5 300 K (27°C) is the temperature at the outer surface, and Tf 5 293 K (20°C) is the temperature of the surrounding air away from the immediate vicinity of the gearbox. For the gearbox, evaluate the heat transfer rate and the power delivered through the output shaft, each in kW. SOLUTION Known: A gearbox operates at steady state with a known power input. An expression for the heat transfer rate from the outer surface is also known. Find: Determine the heat transfer rate and the power delivered through the output shaft, each in kW. Schematic and Given Data: Tb = 300 K

Engineering Model: 1. The gearbox is a closed system at steady state.

W˙ 1 = –60 kW Tf = 293 K

1

h = 0.171 kW/m2 · K

2. For the gearbox, convection is the dominant heat

transfer mode.

W˙ 2

Input shaft

2 Gearbox

Outer surface A = 1.0 m2

Output shaft

Fig. E2.4 #

Analysis: Using the given expression for Q together with known data, the rate of energy transfer by heat is

➊

# Q 5 2hA(Tb 2 Tf)

kW b(1.0 m2)(300 2 293) K m2 ? K 5 21.2 kW # The minus sign for Q signals that energy is carried out of the gearbox by heat transfer. The energy rate balance, Eq. 2.37, reduces at steady state to 5 2a0.171

➋

0 # # # # dE 5 Q 2 W or W 5 Q dt

# # # The symbol W represents the net power from the system. The net power is the sum of W1 and the output power W2 # # # W 5 W1 1 W2 # With this expression for W, the energy rate balance becomes # # # W1 1 W2 5 Q # # # Solving for W2, inserting Q 5 21.2 kW, and W1 5 260 kW, where the minus sign is required because the input shaft brings energy into the system, we have # # # ➌ W2 5 Q 2 W1 5 (21.2 kW) 2 (260 kW) 5 +58.8 kW

60

Chapter 2 Energy and the First Law of Thermodynamics # ➍ The positive sign for W2 indicates that energy is transferred from the system through the output shaft, as expected. ➊ In accord with the sign convention for the heat transfer rate in the energy rate balance (Eq. 2.37), Eq. 2.34 # is written with a minus sign: Q is negative since Tb is greater than Tf. ➋ Properties of a system at steady state do not change with time. Energy E is a property, but heat transfer and work are not properties. ➌ For this system, energy transfer by work occurs at two different locations, and the signs associated with their values differ. ➍ At steady state, the rate of heat transfer from the gear box accounts for the difference between the input and output power. This can be summarized by the following energy rate “balance sheet” in terms of magnitudes: Input

Output

60 kW (input shaft)

58.8 kW (output shaft) 1.2 kW (heat transfer)

Total: 60 kW

60 kW

✓ Skills Developed

Ability to… ❑ define a closed system and

identify interactions on its boundary. ❑ evaluate the rate of energy transfer by convection. ❑ apply the energy rate balance for steady-state operation. ❑ develop an energy rate balance sheet.

For an emissivity of 0.8 and taking Ts 5 Tf , use Eq. 2.33 to determine the net rate at which energy is radiated from the outer surface of the gearbox, in kW. Ans. 0.03 kW.

cccc

EXAMPLE 2.5 c

Determining Surface Temperature of a Silicon Chip at Steady State A silicon chip measuring 5 mm on a side and 1 mm in thickness is embedded in a ceramic substrate. At steady state, the chip has an electrical power input of 0.225 W. The top surface of the chip is exposed to a coolant whose temperature is 208C. The heat transfer coefficient for convection between the chip and the coolant is 150 W/m2 ? K. If heat transfer by conduction between the chip and the substrate is negligible, determine the surface temperature of the chip, in 8C. SOLUTION Known: A silicon chip of known dimensions is exposed on its top surface to a coolant. The electrical power input and convective heat transfer coefficient are known. Find: Determine the surface temperature of the chip at steady state. Schematic and Given Data: Engineering Model:

Coolant h = 150 W/m2 · K

1. The chip is a closed system at steady state.

Tf = 20° C 5 mm +

2. There is no heat transfer between the chip and the substrate.

5 mm Tb

W˙ = –0.225 W

–

1 mm

Ceramic substrate

Fig. E2.5

2.5 Energy Accounting: Energy Balance for Closed Systems

61

Analysis: The surface temperature of the chip, Tb, can be determined using the energy rate balance, Eq. 2.37, which at steady state reduces as follows: 0 # # dE 5Q2W dt

➊

With assumption 2, the only heat transfer is by convection to the coolant. In this application, Newton’s law of cooling, Eq. 2.34, takes the form # ➋ Q 5 2h A(Tb 2 Tf) Collecting results

# 0 5 2hA(Tb 2 Tf) 2 W

Solving for Tb

# 2W Tb 5 1 Tf hA

# In this expression, W 5 20.225 W, A 5 25 3 1026 m2, h 5 150 W/m2 ? K, and Tf 5 293 K, giving 2(20.225 W) 1 293 K Tb 5 Ski lls Dev elo ped (150 W/m2 ? K)(25 3 1026 m2)

✓

5 353 K (80°C) ➊ Properties of a system at steady state do not change with time. Energy E is a property, but heat transfer and work are not properties. ➋ In accord with the sign convention for heat transfer in# the energy rate balance (Eq. 2.37), Eq. 2.34 is written with a minus sign: Q is negative since Tb is greater than Tf.

Ability to… ❑ define a closed system and

identify interactions on its boundary. ❑ evaluate the rate of energy transfer by convection. ❑ apply the energy rate balance for steady-state operation.

If the surface temperature of the chip must be no greater than 608C, what is the corresponding range of values required for the convective heat transfer coefficient, assuming all other quantities remain unchanged? Ans. h $ 225 W/m2 ? K.

2.5.4

Using the Energy Rate Balance: Transient Operation

Many devices undergo periods of transient operation where the state changes with time. This is observed during startup and shutdown periods. The next example illustrates the application of the energy rate balance to an electric motor during startup. The example also involves both electrical work and power transmitted by a shaft. cccc

EXAMPLE 2.6 c

Investigating Transient Operation of a Motor The rate of heat transfer between a certain electric motor and its surroundings varies with time as # Q 5 20.2[1 2 e(20.05t)] # where t is in seconds and Q is in kW. The shaft of the motor rotates at a constant speed of 𝜔 5 100 rad/s (about 955 revolutions per minute, or RPM) and applies a constant torque of 7 5 18 N# ? m to# an external load. The motor draws a constant electric power input equal to 2.0 kW. For the motor, plot Q and W, each in kW, and the change in energy DE, in kJ, as functions of time from t 5 0 to t 5 120 s. Discuss.

62

Chapter 2 Energy and the First Law of Thermodynamics SOLUTION Known: A motor operates with constant electric power input, shaft speed, and applied torque. The time-varying rate of heat transfer between the motor and its surroundings is given. # # Find: Plot Q, W, and DE versus time. Discuss. Schematic and Given Data: W˙ elec = –2.0 kW + –

Engineering Model: The system shown in the accompanying

= 18 N · m ω = 100 rad/s

sketch is a closed system.

W˙ shaft Motor

˙ = –0.2 [1 – e(–0.05t)] kW Q Fig. E2.6a Analysis: The time rate of change of system energy is

# # dE 5Q2W dt

# # W represents the net power from the system: the sum of the power associated with the rotating shaft, Wshaft, and the # power associated with the electricity flow, Welec: # # # W 5 Wshaft 1 Welec # # The rate Welec is known from the problem statement: Welec 5 22.0 kW, # where the negative sign is required because energy is carried into the system by electrical work. The term Wshaft can be evaluated with Eq. 2.20 as # Wshaft 5 7𝜔 5 (18 N ? m)(100 rad /s) 5 1800 W 5 11.8 kW

Because energy exits the system along the rotating shaft, this energy transfer rate is positive. In summary, # # # W 5 Welec 1 Wshaft 5 (22.0 kW) 1 (11.8 kW) 5 20.2 kW where the minus sign means that the # electrical power input is greater# than the power transferred out along the shaft. With the foregoing result for W and the given expression for Q, the energy rate balance becomes dE 5 20.2[1 2 e(20.05t)] 2 (20.2) 5 0.2e(20.05t) dt Integrating t

DE 5

# 0.2e

(20.05t)

dt

0

5

t 0.2 e(20.05t) d 5 4[1 2 e(20.05t)] (20.05) 0

➊ The accompanying # plots, Figs. E2.6b and c, are# developed using the given expression for Q and the expressions for W and DE obtained in the analysis. Because of our sign conventions for heat and work, the values # # of Q and W are negative. In the first few seconds, the net rate that energy is carried in by work greatly exceeds the rate that energy is carried out by heat transfer. Consequently, the energy stored in the motor increases# rapidly as the # motor “warms up.” As time elapses, the value of Q approaches W, and the rate of energy storage diminishes. After about

✓ Skills Developed Ability to… ❑ define a closed system and

identify interactions on its boundary. ❑ apply the energy rate balance for transient operation. deve lop and interpret graphical ❑ data.

2.6 Energy Analysis of Cycles ➋

63

5 –0.05

˙ , kW Q˙ , W

ΔE, kJ

4 3 2

Q˙

–0.15

˙ W

–0.20

1 0

–0.10

0

10 20 30 40 50 60 70 80 90 100

–0.25

0

10 20 30 40 50 60 70 80 90 100

Time, s

Time, s

Fig. E2.6b and c 100 s, this transient operating mode is nearly over, and there is little further change in the amount of energy stored, or in any other property. We may say that the motor is then at steady state. ➊ Figures E.2.6b and c can be developed using appropriate software or can be drawn by hand. # ➋ At steady state, the value of Q is constant at 20.2 kW. This constant value for the heat transfer rate can be thought of as the portion of the electrical power input that is not obtained as a mechanical power output because of effects within the motor such as electrical resistance and friction.

If the dominant mode of heat transfer from the outer surface of the motor is convection, determine at steady state the temperature Tb on the outer surface, in K, for h 5 0.17 kW/m2 ? K, A 5 0.3 m2, and Tf 5 293 K. Ans. 297 K.

2.6 Energy Analysis of Cycles In this section the energy concepts developed thus far are illustrated further by application to systems undergoing thermodynamic cycles. A thermodynamic cycle is a sequence of processes that begins and ends at the same state. At the conclusion of a cycle all properties have the same values they had at the beginning. Consequently, over the cycle the system experiences no net change of state. Cycles that are repeated periodically play prominent roles in many areas of application. For example, steam circulating through an electrical power plant executes a cycle. The study of systems undergoing cycles has played an important role in the development of the subject of engineering thermodynamics. Both the first and second laws of thermodynamics have roots in the study of cycles. Additionally, there are many important practical applications involving power generation, vehicle propulsion, and refrigeration for which an understanding of thermodynamic cycles is essential. In this section, cycles are considered from the perspective of the conservation of energy principle. Cycles are studied in greater detail in subsequent chapters, using both the conservation of energy principle and the second law of thermodynamics.

2.6.1 Cycle Energy Balance The energy balance for any system undergoing a thermodynamic cycle takes the form DEcycle 5 Qcycle 2 Wcycle

(2.39)

thermodynamic cycle

64

Chapter 2 Energy and the First Law of Thermodynamics

Hot body

Hot body Qout

Qin System

Wcycle = Qin – Qout

Qout Cold body

System Qin

Wcycle = Qout – Qin

Cold body

(a)

(b)

Fig. 2.17 Schematic diagrams of two important classes of cycles. (a) Power cycles. (b) Refrigeration and heat pump cycles.

where Qcycle and Wcycle represent net amounts of energy transfer by heat and work, respectively, for the cycle. Since the system is returned to its initial state after the cycle, there is no net change in its energy. Therefore, the left side of Eq. 2.39 equals zero, and the equation reduces to Wcycle 5 Qcycle

TAKE NOTE...

When analyzing cycles, we normally take energy transfers as positive in the directions of arrows on a sketch of the system and write the energy balance accordingly.

(2.40)

Equation 2.40 is an expression of the conservation of energy principle that must be satisfied by every thermodynamic cycle, regardless of the sequence of processes followed by the system undergoing the cycle or the nature of the substances making up the system. Figure 2.17 provides simplified schematics of two general classes of cycles considered in this book: power cycles and refrigeration and heat pump cycles. In each case pictured, a system undergoes a cycle while communicating thermally with two bodies, one hot and the other cold. These bodies are systems located in the surroundings of the system undergoing the cycle. During each cycle there is also a net amount of energy exchanged with the surroundings by work. Carefully observe that in using the symbols Qin and Qout on Fig. 2.17 we have departed from the previously stated sign convention for heat transfer. In this section it is advantageous to regard Qin and Qout as transfers of energy in the directions indicated by the arrows. The direction of the net work of the cycle, Wcycle, is also indicated by an arrow. Finally, note that the directions of the energy transfers shown in Fig. 2.17b are opposite to those of Fig. 2.17a.

2.6.2 Power Cycles power cycle

Systems undergoing cycles of the type shown in Fig. 2.17a deliver a net work transfer of energy to their surroundings during each cycle. Any such cycle is called a power cycle. From Eq. 2.40, the net work output equals the net heat transfer to the cycle, or Wcycle 5 Qin 2 Qout (power cycle)

(2.41)

where Qin represents the heat transfer of energy into the system from the hot body, and Qout represents heat transfer out of the system to the cold body. From Eq. 2.41 it is clear that Qin must be greater than Qout for a power cycle. The energy supplied by heat transfer to a system undergoing a power cycle is normally derived from the combustion of fuel or a moderated nuclear reaction; it can also be obtained from solar radiation. The energy Qout is generally discharged to the surrounding atmosphere or a nearby body of water.

2.6 Energy Analysis of Cycles The performance of a system undergoing a power cycle can be described in terms of the extent to which the energy added by heat, Qin, is converted to a net work output, Wcycle. The extent of the energy conversion from heat to work is expressed by the following ratio, commonly called the thermal efficiency 𝜂5

Wcycle Qin

(power cycle)

65

thermal efficiency

(2.42)

Introducing Eq. 2.41, an alternative form is obtained as 𝜂5

Qin 2 Qout Qout 51 2 (power cycle) Qin Qin

(2.43)

Since energy is conserved, it follows that the thermal efficiency can never be greater than unity (100%). However, experience with actual power cycles shows that the value of thermal efficiency is invariably less than unity. That is, not all the energy added to the system by heat transfer is converted to work; a portion is discharged to the cold body by heat transfer. Using the second law of thermodynamics, we will show in Chap. 5 that the conversion from heat to work cannot be fully accomplished by any power cycle. The thermal efficiency of every power cycle must be less than unity: 𝜂 , 1 (100%).

Power_Cycle A.9 – Tabs a & b

ENERGY & ENVIRONMENT Today fossil-fueled power plants can have thermal efficiencies of 40%, or more. This means that up to 60% of the energy added by heat transfer during the power plant cycle is discharged from the plant other than by work, principally by heat transfer. One way power plant cooling is achieved is to use water drawn from a nearby river or lake. The water is eventually returned to the river or lake but at a higher temperature, which is a practice having several possible environmental consequences. The return of large quantities of warm water to a river or lake can affect its ability to hold dissolved gases, including the oxygen required for aquatic life. If the return water temperature is greater than about 35°C (95°F), the dissolved oxygen may be too low to support some species of fish. If the return water temperature is too great, some species also can be stressed. As rivers and lakes become warmer, non-native species that thrive in the warmth can take over. Warmer water also fosters bacterial populations and algae growth. Regulatory agencies have acted to limit warm water discharges from power plants, which has made cooling towers (Sec. 12.9) adjacent to power plants a common sight.

2.6.3 Refrigeration and Heat Pump Cycles Next, consider the refrigeration and heat pump cycles shown in Fig. 2.17b. For cycles of this type, Qin is the energy transferred by heat into the system undergoing the cycle from the cold body, and Qout is the energy discharged by heat transfer from the system to the hot body. To accomplish these energy transfers requires a net work input, Wcycle. The quantities Qin, Qout, and Wcycle are related by the energy balance, which for refrigeration and heat pump cycles takes the form Wcycle 5 Qout 2 Qin

(refrigeration and heat pump cycles)

(2.44)

Since Wcycle is positive in this equation, it follows that Qout is greater than Qin.

refrigeration and heat pump cycles

A

66

Chapter 2 Energy and the First Law of Thermodynamics Although we have treated them as the same to this point, refrigeration and heat pump cycles actually have different objectives. The objective of a refrigeration cycle is to cool a refrigerated space or to maintain the temperature within a dwelling or other building below that of the surroundings. The objective of a heat pump is to maintain the temperature within a dwelling or other building above that of the surroundings or to provide heating for certain industrial processes that occur at elevated temperatures. Since refrigeration and heat pump cycles have different objectives, their performance parameters, called coefficients of performance, are defined differently. These coefficients of performance are considered next.

Refrigeration Cycles The performance of refrigeration cycles can be described as the ratio of the amount of energy received by the system undergoing the cycle from the cold body, Qin, to the net work into the system to accomplish this effect, Wcycle. Thus, the coefficient of performance, 𝛽, is coefficient of performance: refrigeration

𝛽5

Qin (refrigeration cycle) Wcycle

(2.45)

Introducing Eq. 2.44, an alternative expression for β is obtained as 𝛽5

Qin (refrigeration cycles) Qout 2 Qin

(2.46)

For a household refrigerator, Qout is discharged to the space in which the refrigerator is located. Wcycle is usually provided in the form of electricity to run the motor that drives the refrigerator. in a refrigerator the inside compartment acts as the cold body and the ambient air surrounding the refrigerator is the hot body. Energy Qin passes to the circulating refrigerant from the food and other contents of the inside compartment. For this heat transfer to occur, the refrigerant temperature is necessarily below that of the refrigerator contents. Energy Qout passes from the refrigerant to the surrounding air. For this heat transfer to occur, the temperature of the circulating refrigerant must necessarily be above that of the surrounding air. To achieve these effects, a work input is required. For a refrigerator, Wcycle is provided in the form of electricity. b b b b b

Heat Pump Cycles The performance of heat pumps can be described as the ratio of the amount of energy discharged from the system undergoing the cycle to the hot body, Qout, to the net work into the system to accomplish this effect, Wcycle. Thus, the coefficient of performance, 𝛾, is coefficient of performance: heat pump

𝛾5

Qout (heat pump cycle) Wcycle

(2.47)

Introducing Eq. 2.44, an alternative expression for this coefficient of performance is obtained as

A

Refrig_Cycle A.10 – Tabs a & b Heat_Pump_Cycle A.11 – Tabs a & b

𝛾5

Qout (heat pump cycle) Qout 2 Qin

(2.48)

From this equation it can be seen that the value of γ is never less than unity. For residential heat pumps, the energy quantity Qin is normally drawn from the surrounding atmosphere, the ground, or a nearby body of water. Wcycle is usually provided by electricity.

2.7 Energy Storage The coefficients of performance 𝛽 and 𝛾 are defined as ratios of the desired heat transfer effect to the cost in terms of work to accomplish that effect. Based on the definitions, it is desirable thermodynamically that these coefficients have values that are as large as possible. However, as discussed in Chap. 5, coefficients of performance must satisfy restrictions imposed by the second law of thermodynamics.

2.7 Energy Storage In this section we consider energy storage, which is deemed a critical national need today and likely will continue to be so in years ahead. The need is widespread, including use with conventional fossil- and nuclear-fueled power plants, power plants using renewable sources like solar and wind, and countless applications in transportation, industry, business, and the home.

2.7.1 Overview While aspects of the present discussion of energy storage are broadly relevant, we are mainly concerned here with storage and recapture of electricity. Electricity can be stored as internal energy, kinetic energy, and gravitational potential energy and converted back to electricity when needed. Owing to thermodynamic limitations associated with such conversions, the effects of friction and electrical resistance for instance, an overall input-to-output loss of electricity is always observed, however. Among technically feasible storage options, economics usually determines if, when, and how storage is implemented. For power companies, consumer demand for electricity is a key issue in storage decisions. Consumer demand varies over the day and typically is greatest in the 8 a.m. to 8 p.m. period, with demand spikes during that interval. Demand is least in nighttime hours, on weekends, and on major holidays. Accordingly, power companies must decide which option makes the greatest economic sense: marketing electricity as generated, storing it for later use, or a combination—and if stored, how to store it.

2.7.2 Storage Technologies The focus in this section is on five storage technologies: batteries, ultra-capacitors, superconducting magnets, flywheels, and hydrogen production. Thermal storage is considered in Sec. 3.8. Pumped-hydro and compressed-air storage are considered in Sec. 4.8.3. Batteries are a widely deployed means of electricity storage appearing in cell phones, laptop computers, automobiles, power-generating systems, and numerous other applications. Yet battery makers struggle to keep up with demands for lighterweight, greater-capacity, longer-lasting, and more quickly recharged units. For years batteries have been the subject of vigorous research and development programs. Through these efforts, batteries have been developed providing significant improvements over the lead-acid batteries used for decades. These include utility-scale sodiumsulfur batteries and the lithium-ion and nickel-metal hydride types seen in consumer products and hybrid vehicles. Novel nanotechnology-based batteries promise even better performance: greater capacity, longer service life, and a quicker recharge time, all of which are essential for use in hybrid vehicles. Ultra-capacitors are energy storage devices that work like large versions of common electrical capacitors. When an ultra-capacitor is charged electrically, energy is stored as a charge on the surface of a material. In contrast to batteries, ultra-capacitors require no chemical reactions and consequently enjoy a much longer service life. This

67

68

Chapter 2 Energy and the First Law of Thermodynamics storage type is also capable of very rapid charging and discharging. Applications today include starting railroad locomotives and Diesel trucks. Ultra-capacitors are also used in hybrid vehicles, where they work in tandem with batteries. In hybrids, ultra-capacitors are best suited for performing short-duration tasks, such as storing electricity from regenerative braking and delivering power for acceleration during start–stop driving, while batteries provide energy needed for sustained vehicle motion, all with less total mass and longer service life than with batteries alone. Superconducting magnetic systems store an electrical input in the magnetic field created by flow of electric current in a coil of cryogenically cooled, superconducting material. This storage type provides power nearly instantaneously, and with very low input-to-output loss of electricity. Superconducting magnetic systems are used by high-speed magnetic-levitation trains, by utilities for power-quality control, and by industry for special applications such as microchip fabrication. Flywheels provide another way to store an electrical input—as kinetic energy. When electricity is required, kinetic energy is drained from the spinning flywheel and provided to a generator. Flywheels typically exhibit low input-to-output loss of electricity. Flywheel storage is used, for instance, by Internet providers to protect equipment from power outages. Hydrogen has also been proposed as an energy storage medium for electricity. With this approach, electricity is used to dissociate water to hydrogen via the electrolysis reaction, H2O S H2 1 1⁄2 O2. Hydrogen produced this way can be stored to meet various needs, including generating electricity by fuel cells via the inverse reaction: H2 1 1⁄2 O2 S H2O. A shortcoming of this type of storage is its characteristically significant input-to-output loss of electricity. For discussion of hydrogen production for use in fuel cell vehicles, see Horizons in Sec. 5.3.3.

c CHAPTER SUMMARY AND STUDY GUIDE In this chapter, we have considered the concept of energy from an engineering perspective and have introduced energy balances for applying the conservation of energy principle to closed systems. A basic idea is that energy can be stored within systems in three macroscopic forms: internal energy, kinetic energy, and gravitational potential energy. Energy also can be transferred to and from systems. Energy can be transferred to and from closed systems by two means only: work and heat transfer. Work and heat transfer are identified at the system boundary and are not properties. In mechanics, work is energy transfer associated with macroscopic forces and displacements. The thermodynamic definition of work introduced in this chapter extends the notion of work from mechanics to include other types of work. Energy transfer by heat to or from a system is due to a temperature difference between the system and its surroundings, and occurs in the direction of decreasing temperature. Heat transfer modes include conduction, radiation, and convection. These sign conventions are used for work and heat transfer: # . o: work done by the system c W, W e , o: work done on the system # . o: heat transfer to the system c Q, Q e , o: heat transfer from the system Energy is an extensive property of a system. Only changes in the energy of a system have significance. Energy changes

are accounted for by the energy balance. The energy balance for a process of a closed system is Eq. 2.35 and an accompanying time rate form is Eq. 2.37. Equation 2.40 is a special form of the energy balance for a system undergoing a thermodynamic cycle. The following checklist provides a study guide for this chapter. When your study of the text and end-of-chapter exercises has been completed, you should be able to c write out the meanings of the terms listed in the margins

throughout the chapter and understand each of the related concepts. The subset of key concepts listed below is particularly important in subsequent chapters. c evaluate these energy quantities: –kinetic and potential energy changes using Eqs. 2.5 and 2.10, respectively. –work and power using Eqs. 2.12 and 2.13, respectively. –expansion or compression work using Eq. 2.17. c apply closed-system energy balances in each of several alternative forms, appropriately modeling the case at hand, correctly observing sign conventions for work and heat transfer, and carefully applying SI units. c conduct energy analyses for systems undergoing thermodynamic cycles using Eq. 2.40, and evaluating, as appropriate, the thermal efficiencies of power cycles and coefficients of performance of refrigeration and heat pump cycles.

Key Equations

69

c KEY ENGINEERING CONCEPTS sign convention for heat kinetic energy, p. 32 transfer, p. 47 gravitational potential energy, p. 33 heat transfer, p. 48 work, p. 35 adiabatic, p. 48 sign convention for work, p. 36 first law of thermodynamics, p. 51 power, p. 37 energy balance, p. 52 internal energy, p. 46

thermodynamic cycle, p. 63 power cycle, p. 64 refrigeration cycle, p. 66 heat pump cycle, p. 66

c KEY EQUATIONS DE 5 DU 1 DKE 1 DPE 1 DKE 5 KE2 2 KE1 5 m(V22 2 V21) 2 DPE 5 PE2 2 PE1 5 mg(z2 2 z1)

(2.27) p. 46

Change in total energy of a system.

(2.5) p. 32

Change in kinetic energy of a mass m.

(2.10) p. 33

Change in gravitational potential energy of a mass m at constant g.

E2 2 E1 5 Q 2 W # # dE 5Q 2W dt

(2.35a) p. 52

Energy balance for closed systems.

(2.37) p. 53

Energy rate balance for closed systems.

(2.12) p. 35

Work due to action of a force F.

(2.13) p. 37

Power due to action of a force F.

(2.17) p. 39

Expansion or compression work related to fluid pressure. See Fig. 2.4.

W5

#

s2

F ? ds

s1

# W5F?V W5

#

V2

p dV

V1

Thermodynamic Cycles Wcycle 5 Qin 2 Qout

𝜂5

Wcycle Qin

Wcycle 5 Qout 2 Qin

(2.41) p. 64

Energy balance for a power cycle. As in Fig. 2.17a, all quantities are regarded as positive.

(2.42) p. 65

Thermal efficiency of a power cycle.

(2.44) p. 65

Energy balance for a refrigeration or heat pump cycle. As in Fig. 2.17b, all quantities are regarded as positive.

𝛽5

Qin Wcycle

(2.45) p. 66

Coefficient of performance of a refrigeration cycle.

𝛾5

Qout Wcycle

(2.47) p. 66

Coefficient of performance of a heat pump cycle.

70

Chapter 2 Energy and the First Law of Thermodynamics

c EXERCISES: THINGS ENGINEERS THINK ABOUT 1. What forces act on the bicycle and rider considered in Sec. 2.2.2? Sketch a free-body diagram. 2. Why is it incorrect to say that a system contains heat? 3. What examples of heat transfer by conduction, radiation, and convection do you encounter when using a charcoal grill? 4. When a meteor impacts the earth and comes to rest, what happens to its kinetic and potential energies? 5. After running 8 km on a treadmill at her campus rec center, Ashley observes that the treadmill belt is warm to the touch. Why is the belt warm? 6. When cooking ingredients are mixed in a blender, what happens to the energy transferred to the ingredients? Is the energy transfer by work, by heat transfer, or by work and heat transfer? 7. When microwaves are beamed onto a tumor during cancer therapy to increase the tumor’s temperature, this interaction is considered work and not heat transfer.Why? 8. Experimental molecular motors are reported to exhibit movement upon the absorption of light, thereby achieving

a conversion of electromagnetic radiation into motion. Should the incident light be considered work or heat transfer? 9. Why are the symbols DU, DKE, and DPE used to denote the energy change during a process, but the work and heat transfer for the process are represented, respectively, simply as W and Q? 10. If the change in energy of a closed system is known for a process between two end states, can you determine if the energy change was due to work, to heat transfer, or to some combination of work and heat transfer? 11. Referring to Fig. 2.8, which process, A or B, has the greater heat transfer? 12. What form does the energy balance take for an isolated system? 13. How would you define an efficiency for the motor of Example 2.6? 14. How many tons of CO2 are produced annually by a conventional automobile?

c PROBLEMS: DEVELOPING ENGINEERING SKILLS Exploring Energy Concepts 2.1 A baseball has a mass of 0.20 kg. What is the kinetic energy relative to home plate of a 160 km/hr fastball, in kJ?

of 600 km/h at an altitude of 9,000 m. For g 5 9.8 m/s2, determine the change in kinetic energy and the change in gravitational potential energy of the airliner, each in kJ. 2.5 An object whose mass is 0.5 kg has a velocity of 30 m/s. Determine (a) the final velocity, in m/s, if the kinetic energy of the object decreases by 130 J. (b) the change in elevation, in m, associated with a 130 J change in potential energy. Let g 5 9.81 m/s2.

Fig. P2.1 2.2 An object whose mass is 50 kg experiences a decrease in kinetic energy of 0.80 kJ and an increase in potential energy of 4 kJ The initial velocity and elevation of the object, each relative to the surface of the earth, are 15 m/s and 12 m, espectively. If g 5 9.8 m/s2, determine (a) the final velocity, in m/s (b) the final elevation, in m

2.6 An object whose mass is 2 kg is accelerated from a velocity of 200 m/s to a final velocity of 500 m/s by the action of a resultant force F. Determine the work done by the resultant force, in kJ, if there are no other interactions between the object and its surroundings. 2.7 An object whose mass is 136 kg experiences changes in its kinetic and potential energies owing to the action of a resultant force R. The work done on the object by the resultant force is 148 kJ. There are no other interactions between the object and its surroundings. If the object’s elevation increases by 30.5 m and its final velocity is 61 m/s, what is its initial velocity, in m/s? Let g 5 9.81 m/s2.

2.3 A body is initially at rest then an external force is applied on it which accelerates it with a uniform acceleration of 1 m/s2. Mass of the body is 10 kg. Calculate the work done on the body in 10 s.

2.8 Using KE 5 I𝜔 2/2, how fast would a flywheel whose moment of inertia is 9.4 kg ? m2 have to spin, in RPM, to store an amount of kinetic energy equivalent to the potential energy of a 50 kg mass raised to an elevation of 10 m above the surface of the earth? Let g 5 9.81 m/s2.

2.4 A 30-seat turboprop airliner whose mass is 12,000 kg takes off from an airport and eventually achieves its cruising speed

2.9 A block of mass 10 kg moves along a surface inclined 308 relative to the horizontal. The center of gravity of the block

Problems: Developing Engineering Skills is elevated by 3.0 m and the kinetic energy of the block decreases by 50 J. The block is acted upon by a constant force R parallel to the incline, and by the force of gravity. Assume frictionless surfaces and let g 5 9.81 m/s2. Determine the magnitude and direction of the constant force R, in N. 2.10 Beginning from rest, an object of mass 100 kg slides down a 12-m-long ramp. The ramp is inclined at an angle of 308 from the horizontal. If air resistance and friction between the object and the ramp are negligible, determine the velocity of the object, in m/s, at the bottom of the ramp. Let g 5 9.81 m/s2.

Evaluating Work 2.11 A system with a mass of 5 kg, initially moving horizontally with a velocity of 40 m/s, experiences a constant horizontal deceleration of 2 m/s2 due to the action of a resultant force. As a result, the system comes to rest. Determine the length of time, in s, the force is applied and the amount of energy transfer by work, in kJ. 2.12 An object of mass 36 kg, initially at rest, experiences a constant horizontal acceleration of 3.7 m/s2 due to the action of a resultant force applied for 6.5 s. Determine the work of the resultant force, in N ? m, and in kJ. 2.13 The drag force, Fd, imposed by the surrounding air on a vehicle moving with velocity V is given by Fd 5 CdA12𝜌V2 where Cd is a constant called the drag coefficient, A is the projected frontal area of the vehicle, and 𝜌 is the air density. Determine the power, in kW, required to overcome aerodynamic drag for a truck moving at 110 km/h, if Cd 5 0.65, A 5 10 m2, and 𝜌 5 1.1 kg/m3. 2.14 A major force opposing the motion of a vehicle is the rolling resistance of the tires, Fr, given by Fr 5 f 0 where f is a constant called the rolling resistance coefficient and 0 is the vehicle weight. Determine the power, in kW, required to overcome rolling resistance for a truck weighing 375.5 kN that is moving at 90 km/h. Let f 5 0.0085. 2.15 The two major forces opposing the motion of a vehicle moving on a level road are the rolling resistance of the tires, Fr, and the aerodynamic drag force of the air flowing around the vehicle, Fd, given respectively by Fr 5 f mg, Fd 5 CdA12𝜌V2 where f and Cd are constants known as the rolling resistance coefficient and drag coefficient, respectively, m and A are the vehicle mass and projected frontal area, respectively, V is the vehicle velocity, and 𝜌 is the air density. For a passenger car with m 5 1,610 kg, A 5 2.2 m2, and Cd 5 0.34, and when f 5 0.02 and 𝜌 5 1.28 kg/m3 (a) determine the power required, in kW, to overcome rolling resistance and aerodynamic drag when V is 88.5 km/hr.

71

(b) plot versus vehicle velocity ranging from 0 to 120.7 km/hr (i) the power to overcome rolling resistance, (ii) the power to overcome aerodynamic drag, and (iii) the total power, all in kW. What implication for vehicle fuel economy can be deduced from the results of part (b)? 2.16 In a constant pressure process gas expands from initial volume (V1) of 0.01 m3 to the final volume (V2) of 0.03 m3. The pressure of the system is 2 bar. Determine the work done by the gas. 2.17 One-fourth kg of a gas contained within a piston– cylinder assembly undergoes a constant-pressure process at 5 bar beginning at 𝜐 1 5 0.20 m3/kg3. For the gas as the system, the work is 215 kJ. Determine the final volume of the gas, in m3. 2.18 For a process taking place in a closed system containing gas, the volume and pressure relationship is pV1.4 5 constant. The process starts with initial conditions, p1 5 1.5 bar, V1 5 0.03 m3 and ends with final volume, V2 5 0.05 m3. Determine the work done by the gas. 2.19 A gas is compressed from V1 5 0.3 m3, p1 5 1 bar to V2 5 0.1 m3, p2 5 3 bar. Pressure and volume are related linearly during the process. For the gas, find the work, in kJ. 2.20 Nitrogen (N2) gas within a piston–cylinder assembly undergoes a compression from p1 5 0.5 MPa, V1 5 3.25 m3 to a state where p2 5 3 MPa. The relationship between pressure and volume during the process is pV1.35 5 constant. For the N2, determine (a) the volume at state 2, in m3, and (b) the work, in kJ. 2.21 A closed system consisting of 0.09 kg of air undergoes a polytropic process from p1 5 138 kPa, 𝜐 1 5 0.72 m3/kg to a final state where p2 5 552 kPa, 𝜐 2 5 0.25 m3/kg. Determine the amount of energy transfer by work, in kJ, for the process. 2.22 Warm air is contained in a piston–cylinder assembly oriented horizontally as shown in Fig. P2.22. The air cools slowly from an initial volume of 0.003 m3 to a final volume of 0.002 m3. During the process, the spring exerts a force that varies linearly from an initial value of 900 N to a final value of zero. The atmospheric pressure is 100 kPa, and the area of the piston face is 0.018 m2. Friction between the piston and the cylinder wall can be neglected. For the air, determine the initial and final pressures, in kPa, and the work, in kJ. A = 0.018 m2

patm = 100 kPa Air Spring force varies linearly from 900 N when V1 = 0.003 m3 to zero when V2 = 0.002 m3

Fig. P2.22

72

Chapter 2 Energy and the First Law of Thermodynamics

2.23 Air contained within a piston–cylinder assembly is slowly heated. As shown in Fig. P2.23, during this process the pressure first varies linearly with volume and then remains constant. Determine the total work, in kJ.

2

75 p (kPa)

3 Wall

1

50

κ = 0.55×10–3 kW/m . K

25 0

surface to T2 on the outer surface. The outside ambient air temperature is T0 5 23.9°C and the convective heat transfer coefficient is 28.97 3 1023 kW/m2 ? K. Determine (a) the temperature T2, in °C, and (b) the rate of heat transfer through the wall, in kW/m2 per of surface area.

T1 = 21.1°C

0

0.020

0.035 V (m3)

T0 = –3.9°C h = 28.97×10–3 kW/m2K

0.060 T2

Fig. P2.23 152 mm

2.24 Air contained within a piston–cylinder assembly undergoes three processes in series: Process 1–2: Compression at constant pressure from p1 5 69 kPa, V1 5 0.11 m3 to state 2. Process 2–3: Constant-volume heating to state 3, where p3 5 345 kPa. Process 3–1: Expansion to the initial state, during which the pressure–volume relationship is pV 5 constant. Sketch the processes in series on p-V coordinates. Evaluate (a) the volume at state 2, in m3, and (b) the work for each process, in kJ. 2.25 The driveshaft of a building’s air-handling fan is turned at 300 RPM by a belt running on a 0.3-m-diameter pulley. The net force applied by the belt on the pulley is 2,000 N. Determine the torque applied by the belt on the pulley, in N ? m, and the power transmitted, in kW. 2.26 A 12-V automotive storage battery is charged with a constant current of 2 amp for 24 h. If electricity costs $0.08 per kW ? h, determine the cost of recharging the battery.

Evaluating Heat Transfer 2.27 A flat surface having an area of 2 m2 and a temperature of 350 K is cooled convectively by a gas at 300 K. Using data from Table 2.1, determine the largest and smallest heat transfer rates, in kW, that might be encountered for (a) free convection, (b) forced convection. 2.28 A 0.2-m-thick plane wall is constructed of concrete. At steady state, the energy transfer rate by conduction through a 1-m2 area of the wall is 0.15 kW. If the temperature distribution is linear through the wall, what is the temperature difference across the wall, in K? 2.29 As shown in Fig. P2.29, the 152 mm-thick exterior wall of a building has an average thermal conductivity of 0.55 3 1023 kW/m ? K. At steady state, the temperature of the wall decreases linearly from T1 5 21.1°C on the inner

Fig. P2.29

2.30 A composite plane wall consists of a 23 cm-thick layer of brick (𝜅b 5 2.4 3 1023 kW/m ? K) and a 10 cm-thick layer of insulation (𝜅i 5 0.09 3 1023 kW/m ? K). The outer surface temperatures of the brick and insulation are 700 K and 310 K, respectively, and there is perfect contact at the interface between the two layers. Determine at steady state the instantaneous rate of heat transfer, in kW/m2 of surface area, and the temperature, in K, at the interface between the brick and the insulation. 2.31 A composite plane wall consists of a 75-mm-thick layer of insulation (𝜅i 5 0.05 W/m ? K) and a 25-mm-thick layer of siding (𝜅 5 0.10 W/m ? K). The inner temperature of the insulation is 20°C. The outer temperature of the siding is 213°C. Determine at steady state (a) the temperature at the interface of the two layers, in °C, and (b) the rate of heat transfer through the wall, in W per m2 of surface area. 2.32 A body is placed in a room and the temperature of the walls of the room is 27°C. The surface area of the body is 0.4 m2 and its temperature is 100°C. The value of emissivity of the body is 0.75. Calculate the rate of heat exchanged between the body and the walls of the room. 2.33 Figure P2.33 shows a flat surface at temperature Ts, covered with a layer of insulation whose thermal conductivity is 𝜅. The top surface of the insulation is at temperature Ti and is exposed to air at temperature T0( ,Ts). The heat transfer coefficient for convection is h. Radiation can be ignored. (a) Show that at steady state, the temperature Ti is given by Ti 5 where B 5 hL/𝜅.

BT0 1 Ts B11

Problems: Developing Engineering Skills (b) For fixed Ts and T0, sketch the variation of Ti versus B. Discuss. (c) For Ts 5 300°C and T0 5 30°C, determine the minimum value of B such that Ti is no greater than 60°C. If h 5 4 W/m2 ? K and L 5 0.06 m, what restriction does this limit on B place on the allowed values for thermal conductivity? Repeat for h 5 20 W/m2 ? K.

specific internal energy is 1.9 kJ/kg. During the process, the mass receives 2 kJ of energy by heat transfer through its bottom surface and loses 1 kJ of energy by heat transfer through its top surface. The lateral surface experiences no heat transfer. For this process, evaluate (a) the change in kinetic energy of the mass in kJ, and (b) the work in kJ. 2.39 A mass of 5 kg undergoes a process during which there is heat transfer from the mass at a rate of 10 kJ per kg, an elevation decrease of 75 m, and an increase in velocity from 10 m/s to 20 m/s. The specific internal energy decreases by 10 kJ/kg and the acceleration of gravity is constant at 9.8 m/s2. Determine the work for the process, in kJ.

Air at T0, h Ti Insulation, κ

73

L

Surface Ts > T 0

Fig. P2.33

2.34 At steady state, a spherical interplanetary electronicsladen probe having a diameter of 0.6 m transfers energy by radiation from its outer surface at a rate of 160 W. If the probe does not receive radiation from the sun or deep space, what is the surface temperature, in K? 2.35 A body whose surface area is 0.25 m2, emissivity is 0.85, and temperature is 175°C is placed in a large, evacuated chamber whose walls are at 27°C. What is the rate at which radiation is emitted by the surface, in W? What is the net rate at which radiation is exchanged between the surface and the chamber walls, in W? 2.36 A chip with surface area of upper face of 25 mm2 is exposed to a convection environment that has a temperature equal to 25°C. An electrical input of 0.325 W is provided to the chip and under steady state heat is transferred from the upper face of the chip into the surrounding. The convective heat transfer coefficient between the chip and the surrounding is 125 W/m2 ? K. Determine the surface temperature of the chip.

2.40 As shown in Fig. P2.40, 5 kg of steam contained within a piston–cylinder assembly undergoes an expansion from state 1, where the specific internal energy is u1 5 2,709.9 kJ/kg, to state 2, where u2 5 2,659.6 kJ/kg. During the process, there is heat transfer to the steam with a magnitude of 80 kJ. Also, a paddle wheel transfers energy to the steam by work in the amount of 18.5 kJ. There is no significant change in the kinetic or potential energy of the steam. Determine the energy transfer by work from the steam to the piston during the process, in kJ.

5 kg of steam Wpiston = ?

Wpw = –18.5 kJ

Q = +80 kJ

u1 = 2,709.9 kJ/kg u2 = 2,659.6 kJ/kg

Fig. P2.40

Using the Energy Balance 2.37 Each line in the following table gives information about a process of a closed system. Each entry has the same energy units. Fill in the blank spaces in the table. Process a b c d e

Q

W

E1

220 150

150

120 260 290 1150

E2

DE

150 150 160 150

170 120 0

120

2.38 A vertical cylindrical mass of 5 kg undergoes a process during which the velocity decreases from 30 m/s to 15 m/s, while the elevation remains unchanged. The initial specific internal energy of the mass is 1.2 kJ/kg and the final

2.41 A gas contained within a piston–cylinder assembly undergoes two processes, A and B, between the same end states, 1 and 2, where p1 5 10 bar, V1 5 0.1 m3, U1 5 400 kJ and p2 5 1 bar, V2 5 1.0 m3, U2 5 200 kJ: Process A: Process from 1 to 2 during which the pressurevolume relation is pV 5 constant. Process B: Constant-volume process from state 1 to a pressure of 2 bar, followed by a linear pressure-volume process to state 2. Kinetic and potential energy effects can be ignored. For each of the processes A and B, (a) sketch the process on p-V coordinates, (b) evaluate the work, in kJ, and (c) evaluate the heat transfer, in kJ. 2.42 An electric generator coupled to a windmill produces an average electric power output of 15 kW. The power is used

74

Chapter 2 Energy and the First Law of Thermodynamics

to charge a storage battery. Heat transfer from the battery to the surroundings occurs at a constant rate of 1.8 kW. For 8 h of operation, determine the total amount of energy stored in the battery, in kJ. 2.43 An electric motor that is connected to a supply voltage of 100 V draws 10 amp current. The output shaft of the motor has a rotational speed of 800 RPM and develops a torque of 11 N ? m. For steady state operation of the motor, determine the rate of heat transfer. 2.44 In a rigid insulated container of volume 0.8 m3, 2.5 kg of air is filled. A paddle wheel is fitted in the container and it transfers energy to the contained air at a constant rate of 12 W for a period of 1 h. There is no change in the potential or kinetic energy of the system. Determine the energy transfer by the wheel to the air per kg of air. 2.45 An electric generator supplies electricity to a storage battery at a rate of 15 kW for a period of 12 hours. During this 12-h period there also is heat transfer from the battery to the surroundings at a rate of 1.5 kW. Then, during the next 12-h period the battery discharges electricity to an external load at a rate of 5 kW while heat transfer from the battery to the surroundings occurs at a rate of 0.5 kW. (a) For the first 12-h period, determine, in kW, the time rate of change of energy stored within the battery. (b) For the second 12-h period, determine, in kW, the time rate of change of energy stored in the battery. (c) For the full 24-h period, determine, in kJ, the overall change in energy stored in the battery. 2.46 A gas expands in a piston–cylinder assembly from p1 5 8 bar, V1 5 0.02 m3 to p2 5 2 bar in a process during which the relation between pressure and volume is pV1.2 5 constant. The mass of the gas is 0.25 kg. If the specific internal energy of the gas decreases by 55 kJ/kg during the process, determine the heat transfer, in kJ. Kinetic and potential energy effects are negligible. 2.47 Two kilograms of air is contained in a rigid well-insulated tank with a volume of 0.6 m3. The tank is fitted with a paddle wheel that transfers energy to the air at a constant rate of 10 W for 1 h. If no changes in kinetic or potential energy occur, determine (a) the specific volume at the final state, in m3/kg. (b) the energy transfer by work, in kJ. (c) the change in specific internal energy of the air, in kJ/kg. 2.48 Steam in a piston–cylinder assembly undergoes a polytropic process, with n 5 2, from an initial state where p1 5 3.45 MPa, v1 5 0.106 m3/kg, u1 5 3,171.1 kJ/kg, to a final state where u2 5 2,303.9 kJ/kg. During the process, there is a heat transfer from the steam of magnitude 361.76 kJ. The mass of steam is 0.54 kg. Neglecting changes in kinetic and potential energy, determine the work, in kJ, and the final specific volume, in m3/kg. 2.49 A gas undergoes a process from state 1, where p1 5 415 kPa, 𝜐1 5 0.37 m3/kg, to state 2, where p2 5 138 kPa, according to p𝜐1.3 5 constant. The relationship between pressure, specific volume, and specific internal energy is

u 5 (1.443)pv 2 221.557 where p is in kPa, v is in m3/kg, and u is in kJ/kg. The mass of gas is 4.5 kg. Neglecting kinetic and potential energy effects, determine the heat transfer, in kJ. 2.50 Air is contained in a vertical piston–cylinder assembly by a piston of mass 50 kg and having a face area of 0.01 m2. The mass of the air is 5 g, and initially the air occupies a volume of 5 liters. The atmosphere exerts a pressure of 100 kPa on the top of the piston. The volume of the air slowly decreases to 0.002 m3 as the specific internal energy of the air decreases by 260 kJ/kg. Neglecting friction between the piston and the cylinder wall, determine the heat transfer to the air, in kJ. 2.51 A gas is contained in a vertical piston–cylinder assembly by a piston weighing 4,450 N and having a face area of 75 cm2 The atmosphere exerts a pressure of 101.3 kPa on the top of the piston. An electrical resistor transfers energy to the gas in the amount of 5.3 kJ as the elevation of the piston increases by 0.6 m. The piston and cyclinder are poor thermal conductors and friction can be neglected. Determine the change in internal energy of the gas, in kJ, assuming it is the only significant internal energy change of any component present.

Analyzing Thermodynamic Cycles 2.52 The following table gives data, in kJ, for a system undergoing a thermodynamic cycle consisting of four processes in series. Determine (a) the missing table entries, each in kJ. (b) whether the cycle is a power cycle or a refrigeration cycle. Process

DU

DKE

DPE

1–2

76

25.3

26.3

2–3 3–4 4–1

67.5 102

0 0

DE

Q

W

0 95

0 3.2

97 0

2.53 A gas within a piston–cylinder assembly undergoes a thermodynamic cycle consisting of three processes: Process 1–2: Compression with pV 5 constant, from p1 5 1 bar, V1 5 1.6 m3 to V2 5 0.2 m3, U2 2 U1 5 0. Process 2–3: Constant pressure to V3 5 V1. Process 3–1: Constant volume, U1 2 U3 5 23,549 kJ. There are no significant changes in kinetic or potential energy. Determine the heat transfer and work for Process 2–3, in kJ. Is this a power cycle or a refrigeration cycle? 2.54 A gas within a piston–cylinder assembly undergoes a thermodynamic cycle consisting of three processes: Process 1–2: 26.4 kJ.

Constant volume, V 5 0.028 m3, U2 2 U1 5

Process 2–3: Expansion with pV 5 constant, U3 5 U2. Process 3–1: Constant pressure, p 5 1.4 bar, W31 5 210.5 kJ. There are no significant changes in kinetic or potential energy.

Design & Open Ended Problems: Exploring Engineering Practice (a) (b) (c) (d)

Sketch the cycle on a p–V diagram. Calculate the net work for the cycle, in kJ. Calculate the heat transfer for process 2–3, in kJ. Calculate the heat transfer for process 3–1, in kJ.

Is this a power cycle or a refrigeration cycle? 2.55 As shown in Fig. P2.55, a gas within a piston–cylinder assembly undergoes a thermodynamic cycle consisting of three processes in series: Compression with U2 5 U1. Process 2–3: Constant-volume cooling to p3 5 120 kPa, V3 5 0.03 m3. Process 3–1: Constant-pressure expansion with W31 5 9.5 kJ.

Process 1–2:

For the cycle, Wcycle 5 27.5 kJ. There are no changes in kinetic or potential energy. Determine (a) the volume at state 1, in m3, (b) the work and heat transfer for process 1–2, each in kJ. (c) Can this be a power cycle? A refrigeration cycle? Explain.

75

2.61 Shown in Fig. P2.61 is a cogeneration power plant operating in a thermodynamic cycle at steady state. The plant provides electricity to a community at a rate of 100 MW. The energy discharged from the. power plant by heat transfer is denoted on the figure by Qout. Of this, 75 MW is provided to the community for water heating and the remainder is discarded to the environment without use. The electricity is valued at $0.09 per kW ? h. If the cycle thermal efficiency is 45%, . determine the (a) rate energy is added by heat transfer, Qin, in MW, (b) rate energy is discarded to the environment, in MW, and (c) value of the electricity generated, in $ per year.

. Qin Power plant

η = 45%

+ –

. Welect = 100 MW

. Qout

2

75 MW for water heating

U2 = U1 To environment

Fig. P2.61 p 3 p3 = 120 kPa V3 = 0.03 m3

1

V

Fig. P2.55

2.56 For a power cycle operating as in Fig. 2.17a, the heat transfers are Qin 5 50 kJ and Qout 5 35 kJ. Determine the net work, in kJ, and the thermal efficiency. 2.57 The thermal efficiency of a power cycle operating as shown in Fig. 2.17a is 35%, and Qout 5 40 MJ. Determine the net work developed and the heat transfer Qin, each in MJ. 2.58 For a power cycle operating as in Fig. 2.17a, Qin 5 2,743 kJ and Qout 5 1,900 kJ. What is the net work developed, in kJ, and the thermal efficiency? 2.59 For a power cycle operating as in Fig. 2.17a, Wcycle 5 834 kJ and Qout 5 1,800 kJ. What is the thermal efficiency? 2.60 A power cycle has a thermal efficiency of 40% and generates electricity at a rate of 100 MW. The electricity is valued at $0.08 per . kW ? h. Based on the cost of fuel, the cost to supply Qin is $4.50 per GJ. For 8,000 hours of operation annually, determine, in $, (a) the value of electricity generated per year. (b) the annual fuel cost. (c) Does the difference between the results of parts (a) and (b) represent profit? Discuss.

2.62 For each of the following, what plays the roles of the hot body and the cold body of the appropriate Fig. 2.17 schematic? (a) Window air conditioner (b) Nuclear submarine power plant (c) Ground-source heat pump 2.63 In what ways do automobile engines operate analogously to the power cycle shown in Fig. 2.17a? How are they different? Discuss. 2.64 A refrigeration cycle operating as shown in Fig. 2.17b has heat transfer Qout 5 2,530 kJ and net work of Wcycle 5 844 kJ. Determine the coefficient of performance for the cycle. 2.65 A refrigeration cycle operates as shown in Fig. 2.17b with a coefficient of performance 𝛽 5 1.5. For the cycle, Qout 5 500 kJ. Determine Qin and Wcycle, each in kJ. 2.66 A household refrigerator operating steadily and with a coefficient of performance of 2.5 removes energy from a refrigerated space at a rate of 650 kJ/h. Evaluating electricity at $0.06 per kW ? h, determine the cost of electricity in a month when the refrigerator operates for 360 hours. 2.67 In the cooling season an air-conditioning system operates 9 hours per day for a period of 120 days. It provides cooling at a rate of 5000 kJ/h, and the coefficient of performance of the system is 2.5. If electricity costs $0.1 per kWh, determine the total cost of electricity in dollars for the cooling season. 2.68 A heat pump cycle whose coefficient of performance is 2.5 delivers energy by heat transfer to a dwelling at a rate of 20 kW.

76

Chapter 2 Energy and the First Law of Thermodynamics

(a) Determine the net power required to operate the heat pump, in kW. (b) Evaluating electricity at $0.08 per kW ? h, determine the cost of electricity in a month when the heat pump operates for 200 hours. 2.69 A heat pump cycle delivers energy by heat transfer to a dwelling at a rate of 63,300 kJ/h. The power input to the cycle is 5.82 kW.

(b) Evaluating electricity at $0.08 per kW ? h, determine the cost of electricity in a month when the heat pump operates for 200 hours. 2.70 An air-conditioning unit with a coefficient of performance of 2.93 provides 5,275 kJ/h of cooling while operating during the cooling season 8 hours per day for 125 days. If you pay 10 cents per kW ? h for electricity, determine the cost, in dollars, for the cooling season.

(a) Determine the coefficient of performance of the cycle.

c DESIGN & OPEN ENDED PROBLEMS: EXPLORING ENGINEERING PRACTICE 2.1D Critically evaluate and compare the various hybrid electric automobiles on the market today. In a memorandum, summarize your findings and conclusions. 2.2D Design a go-anywhere, use-anywhere wind screen for outdoor recreational and casual-living activities, including sunbathing, reading, cooking, and picnicking. The wind screen must be lightweight, portable, easy to deploy, and low cost. A key constraint is that the wind screen can be set up anywhere, including hard surfaces such as parking lots for tailgating, wood decks, brick and concrete patios, and at the beach. A cost analysis should accompany the design.

be able to heat large rooms in minutes without having a high outer-surface temperature, reducing humidity and oxygen levels, or producing carbon monoxide. A typical deployment is shown in Fig. P2.7D. The heater is an enclosure containing electrically powered quartz infrared lamps that shine on copper tubes. Air drawn into the enclosure by a fan flows over the tubes and then is directed back into the living space. Heaters requiring 500 watts of power cost about $400 while the 1,000-W model costs about $500. Critically evaluate the technical and economic merit of such heaters. Write a report including at least three references.

2.3D In living things, energy is stored in the molecule adenosine triphosphate, called ATP for short. ATP is said to act like a battery, storing energy when it is not needed and instantly releasing energy when it is required. Investigate how energy is stored and the role of ATP in biological processes. Write a report including at least three references. 2.4D For a 2-week period, keep a diary of your calorie intake for food and drink and the calories burned due to your full range of activities over the period, each in kcal. Interpret your results using concepts introduced in this chapter. In a memorandum, summarize results and interpretations. 2.5D Develop a list of the most common home-heating options in your locale. For a 225 m2 dwelling, what is the annual fuel cost or electricity cost of each option? Also, what is the installed cost of each option? For a 15-year life, which option is the most economical?

Fig. P2.7D

2.6D Homeowners concerned about interruption of electrical power from their local utility because of weather-related and utility system outages can acquire a standby generator that produces electricity using the home’s existing natural gas or 2.8D Fossil-fuel power plants produce most of the electricity generated annually in the United States. The cost of electricity liquid propane (LP) fuel source. For a single-family dwelling is determined by several factors, including the power plant of your choice, identify a standby generator that would provide thermal efficiency, the unit cost of the fuel, in $ per kW ? h, electricity during an outage to a set of essential devices and and the plant capital cost, in $ per kW of power generated. appliances you specify. Summarize your findings in a Prepare a memorandum comparing typical ranges of these memorandum including installed-cost and operating-cost data. three factors for coal-fired steam power plants and natural 2.7D An advertisement describes a portable heater claimed to gas-fired gas turbine power plants. Which type of plant is cut home-heating bills by up to 50%. The heater is said to most prevalent in the United States?

Design & Open Ended Problems: Exploring Engineering Practice 2.9D According to the New York City Transit Authority, the operation of subways raise tunnel and station temperatures as much as 263–266 K above ambient temperature. Principal contributors to the temperature rise include train motor operation, lighting, and energy from the passengers themselves. Passenger discomfort can increase significantly in hot-weather periods if air conditioning is not provided. Still, because on-board air-conditioning units discharge energy by heat transfer to their surroundings, such units contribute to the overall tunnel and station energy management problem. Investigate the application to subways of alternative cooling strategies that provide substantial cooling with a minimal power requirement, including but not limited to thermal storage and nighttime ventilation. Write a report with at least three references. 2.10D An inventor proposes borrowing water from municipal water mains and storing it temporarily in a tank on the premises of a dwelling equipped with a heat pump. As shown in Fig. P2.10D, the stored water serves as the cold body for the heat pump and the dwelling itself serves as the hot body. Since the energy of the stored water decreases as energy is removed from it by the heat pump, water is drawn from the mains periodically to restore the energy level and an equal amount of lower-energy water is returned to the mains. As the invention requires no net water from the mains, the inventor maintains that nothing should be paid for water usage. The inventor also maintains that this approach not only gives a coefficient of performance superior to those of air-source heat pumps but also avoids the installation costs associated with ground-source heat pumps. In all, significant

77

cost savings result, the inventor says. Critically evaluate the inventor’s claims. Write a report including at least three references.

Hot body: interior of dwelling

Qout

Heat pump

Water from mains Water to mains

Qin

Cold body: water storage tank

Fig. P2.10D

+ – Basement

3 Evaluating Properties ENGINEERING CONTEXT To apply the energy balance to a system of interest requires knowledge of the properties of the system and how the properties are related. The objectives of this chapter are to introduce property relations relevant to engineering thermodynamics and provide several examples illustrating the use of the closed system energy balance together with the property relations considered in this chapter.

LEARNING OUTCOMES When you complete your study of this chapter, you will be able to...

78

c

demonstrate understanding of key concepts . . . including phase and pure substance, state principle for simple compressible systems, p –𝜐–T surface, saturation temperature and saturation pressure, two-phase liquid-vapor mixture, quality, enthalpy, and specific heats.

c

apply the closed system energy balance with property data.

c

sketch T– 𝜐, p – 𝜐, and phase diagrams, and locate states on these diagrams.

c

retrieve property data from Tables A-1 through A-23.

c

apply the ideal gas model for thermodynamic analysis, including determining when use of the model is warranted.

3.2 p–ʋ–T Relation

79

3.1 Getting Started In this section, we introduce concepts that support our study of property relations, including phase, pure substance, and the state principle for simple systems.

3.1.1

Phase and Pure Substance

The term phase refers to a quantity of matter that is homogeneous throughout in both chemical composition and physical structure. Homogeneity in physical structure means that the matter is all solid, or all liquid, or all vapor (or equivalently all gas). A system can contain one or more phases. a system of liquid water and water vapor (steam) contains two phases. A system of liquid water and ice, including the case of slush, also contains two phases. Gases, oxygen and nitrogen for instance, can be mixed in any proportion to form a single gas phase. Certain liquids, such as alcohol and water, can be mixed to form a single liquid phase. But liquids such as oil and water, which are not miscible, form two liquid phases. b b b b b Two phases coexist during the changes in phase called vaporization, melting, and sublimation. A pure substance is one that is uniform and invariable in chemical composition. A pure substance can exist in more than one phase, but its chemical composition must be the same in each phase.

phase

Phase_PureSubstance A.12 – Tabs a & b

A

pure substance

if liquid water and water vapor form a system with two phases, the system can be regarded as a pure substance because each phase has the same composition. A uniform mixture of gases can be regarded as a pure substance provided it remains a gas and doesn’t react chemically. Air can be regarded as a pure substance as long as it is a mixture of gases; but if a liquid phase should form on cooling, the liquid would have a different composition than the gas phase, and the system would no longer be considered a pure substance. b b b b b Changes in composition due to chemical reaction are considered in Chap. 13.

3.1.2

Fixing the State

The intensive state of a closed system at equilibrium is its condition as described by the values of its intensive thermodynamic properties. From observation of many thermodynamic systems, we know that not all of these properties are independent of one another, and the state can be uniquely determined by giving the values of a subset of the independent intensive properties. Values for all other intensive thermodynamic properties are determined once this independent subset is specified. A general rule known as the state principle has been developed as a guide in determining the number of independent properties required to fix the state of a system. For the applications considered in this book, we are interested in what the state principle says about the intensive states of systems of commonly encountered pure substances, such as water or a uniform mixture of nonreacting gases. These systems are called simple compressible systems. Experience shows simple compressible systems occur in a wide range of engineering applications. For such systems, the state principle indicates that specification of the values for any two independent intensive thermodynamic properties will fix the values of all other intensive thermodynamic properties.

TAKE NOTE...

Temperature T, pressure p, specific volume 𝜐, specific internal energy u, and specific enthalpy h are all intensive properties. See Secs. 1.3.3, 1.5–1.7, and 3.6.1.

state principle

simple compressible systems

80

Chapter 3 Evaluating Properties

TAKE NOTE...

For a simple compressible system, specification of the values for any two independent intensive thermodynamic properties will fix the values of all other intensive thermodynamic properties.

in the case of a gas, temperature and another intensive property such as a specific volume might be selected as the two independent properties. The state principle then affirms that pressure, specific internal energy, and all other pertinent intensive properties are functions of T and 𝜐: p 5 p(T, 𝜐), u 5 u(T, 𝜐), and so on. The functional relations would be developed using experimental data and would depend explicitly on the particular chemical identity of the substances making up the system. The development of such functions is discussed in Chap. 11. b b b b b Intensive properties such as velocity and elevation that are assigned values relative to datums outside the system are excluded from present considerations. Also, as suggested by the name, changes in volume can have a significant influence on the energy of simple compressible systems. The only mode of energy transfer by work that can occur as a simple compressible system undergoes quasiequilibrium processes (Sec. 2.2.5) is associated with volume change, and is given by ep dV . For further discussion of simple systems and the state principle, see the box.

State Principle for Simple Systems Based on empirical evidence, there is one independent property for each way a system’s energy can be varied independently. We saw in Chap. 2 that the energy of a closed system can be altered independently by heat or by work. Accordingly, an independent property can be associated with heat transfer as one way of varying the energy, and another independent property can be counted for each relevant way the energy can be changed through work. On the basis of experimental evidence, therefore, the state principle asserts that the number of independent properties is one plus the number of relevant work interactions. When counting the number of relevant work interactions, only those that would be significant in quasiequilibrium processes of the system need to be considered. The term simple system is applied when there is only one way the system energy can be significantly altered by work as the system undergoes quasiequilibrium processes. Therefore, counting one independent property for heat transfer and another for the single work mode gives a total of two independent properties needed to fix the state of a simple system. This is the state principle for simple systems. Although no system is ever truly simple, many systems can be modeled as simple systems for the purpose of thermodynamic analysis. The most important of these models for the applications considered in this book is the simple compressible system. Other types of simple systems are simple elastic systems and simple magnetic systems.

Evaluating Properties: General Considerations The first part of the chapter is concerned generally with the thermodynamic properties of simple compressible systems consisting of pure substances. A pure substance is one of uniform and invariable chemical composition. In the second part of this chapter, we consider property evaluation for a special case: the ideal gas model. Property relations for systems in which composition changes by chemical reaction are considered in Chap. 13.

3.2 p–𝝊–T Relation We begin our study of the properties of pure, simple compressible substances and the relations among these properties with pressure, specific volume, and temperature. From experiment it is known that temperature and specific volume can be regarded

3.2 p–ʋ –T Relation

Liquid

Pressure

Critical point Liq uid Tri va ple po lin r e

Solid

So

Va p

lid

Sp

eci

fic

Tc

or

-va

por

re

atu

er mp

vol u

Te

me

Solid

(a)

Solid

Critical point Critical Liquid point L

V Vapor S Triple point V Temperature (b)

Pressure

Pressure

S L

Liquidvapor Triple line Solid-vapor

Vapor

T > Tc Tc T < Tc

Specific volume (c)

Fig. 3.1 p–𝝊–T surface and projections for a substance that expands on freezing. (a) Three-dimensional view. (b) Phase diagram. (c) p–ʋ diagram.

as independent and pressure determined as a function of these two: p 5 p(T, ʋ ). The graph of such a function is a surface, the p–𝝊–T surface.

p–𝝊–T surface

3.2.1 p–𝝊–T Surface Figure 3.1 is the p–ʋ –T surface of a substance such as water that expands on freezing. Figure 3.2 is for a substance that contracts on freezing, and most substances exhibit this characteristic. The coordinates of a point on the p–𝜐–T surfaces represent the values that pressure, specific volume, and temperature would assume when the substance is at equilibrium. There are regions on the p–𝜐 –T surfaces of Figs. 3.1 and 3.2 labeled solid, liquid, and vapor. In these single-phase regions, the state is fixed by any two of the properties: pressure, specific volume, and temperature, since all of these are independent when there is a single phase present. Located between the single-phase regions are two-phase regions where two phases exist in equilibrium: liquid–vapor, solid–liquid, and solid–vapor. Two phases can coexist during changes in phase such as vaporization, melting, and sublimation. Within the two-phase regions pressure and temperature are

two-phase regions

81

82

Chapter 3 Evaluating Properties

Liquid

Pressure

Solid-Liq uid

Solid

Critical point

Constantpressure line

Va p

or

So

lid

Sp

eci

fic

-va

po

re

r

tu era

Tc

p

vo lu

m Te

me

Solid Solid-liquid

(a)

S

Liquid Critical point

Solid

L V S V

Triple point

Vapor

Critical point

Pressure

Pressure

L

Liquidvapor Triple line Solid-vapor

Temperature

Specific volume

(b)

(c)

Vapor

T > Tc Tc T < Tc

Fig. 3.2 p–ʋ –T surface and projections for a substance that contracts on freezing. (a) Three-dimensional view. (b) Phase diagram. (c) p –ʋ diagram.

triple line saturation state vapor dome critical point

not independent; one cannot be changed without changing the other. In these regions the state cannot be fixed by temperature and pressure alone; however, the state can be fixed by specific volume and either pressure or temperature. Three phases can exist in equilibrium along the line labeled triple line. A state at which a phase change begins or ends is called a saturation state. The dome-shaped region composed of the two-phase liquid–vapor states is called the vapor dome. The lines bordering the vapor dome are called saturated liquid and saturated vapor lines. At the top of the dome, where the saturated liquid and saturated vapor lines meet, is the critical point. The critical temperature Tc of a pure substance is the maximum temperature at which liquid and vapor phases can coexist in equilibrium. The pressure at the critical point is called the critical pressure, pc. The specific volume at this state is the critical specific volume. Values of the critical point properties for a number of substances are given in Tables A-1 located in the Appendix. The three-dimensional p–ʋ –T surface is useful for bringing out the general relationships among the three phases of matter normally under consideration. However, it is often more convenient to work with two-dimensional projections of the surface. These projections are considered next.

3.2 p–𝝊–T Relation

3.2.2 Projections of the p–𝝊–T Surface The Phase Diagram If the p–𝜐–T surface is projected onto the pressure–temperature plane, a property diagram known as a phase diagram results. As illustrated by Figs. 3.1b and 3.2b, when the surface is projected in this way, the two-phase regions reduce to lines. A point on any of these lines represents all two-phase mixtures at that particular temperature and pressure. The term saturation temperature designates the temperature at which a phase change takes place at a given pressure, and this pressure is called the saturation pressure for the given temperature. It is apparent from the phase diagrams that for each saturation pressure there is a unique saturation temperature, and conversely. The triple line of the three-dimensional p–𝜐 –T surface projects onto a point on the phase diagram. This is called the triple point. Recall that the triple point of water is used as a reference in defining temperature scales (Sec. 1.7.3). By agreement, the temperature assigned to the triple point of water is 273.16 K. The measured pressure at the triple point of water is 0.6113 kPa. The line representing the two-phase solid–liquid region on the phase diagram slopes to the left for substances that expand on freezing and to the right for those that contract. Although a single solid phase region is shown on the phase diagrams of Figs. 3.1 and 3.2, solids can exist in different solid phases. For example, seven different crystalline forms have been identified for water as a solid (ice).

phase diagram

saturation temperature saturation pressure

triple point

p–𝝊 Diagram Projecting the p–𝜐–T surface onto the pressure–specific volume plane results in a p–𝝊 diagram, as shown by Figs. 3.1c and 3.2c. The figures are labeled with terms that have already been introduced. When solving problems, a sketch of the p–𝜐 diagram is frequently convenient. To facilitate the use of such a sketch, note the appearance of constant-temperature lines (isotherms). By inspection of Figs. 3.1c and 3.2c, it can be seen that for any specified temperature less than the critical temperature, pressure remains constant as the two-phase liquid–vapor region is traversed, but in the single-phase liquid and vapor regions the pressure decreases at fixed temperature as specific volume increases. For temperatures greater than or equal to the critical temperature, pressure decreases continuously at fixed temperature as specific volume increases. There is no passage across the two-phase liquid–vapor region. The critical isotherm passes through a point of inflection at the critical point and the slope is zero there.

p–𝝊 diagram

T–𝝊 Diagram Projecting the liquid, two-phase liquid–vapor, and vapor regions of the p–𝜐–T surface onto the temperature–specific volume plane results in a T–𝝊 diagram as in Fig. 3.3. Since consistent patterns are revealed in the p–𝜐–T behavior of all pure substances, Fig. 3.3 showing a T–𝜐 diagram for water can be regarded as representative. As for the p–𝜐 diagram, a sketch of the T–𝜐 diagram is often convenient for problem solving. To facilitate the use of such a sketch, note the appearance of constant-pressure lines (isobars). For pressures less than the critical pressure, such as the 10 MPa isobar on Fig. 3.3, the pressure remains constant with temperature as the two-phase region is traversed. In the single-phase liquid and vapor regions the temperature increases at fixed pressure as the specific volume increases. For pressures greater than or equal to the critical pressure, such as the one marked 30 MPa on Fig. 3.3, temperature increases continuously at fixed pressure as the specific volume increases. There is no passage across the two-phase liquid–vapor region.

T–𝝊 diagram

83

84

Chapter 3 Evaluating Properties pc = 22.09 MPa 30 MPa 10 MPa

Temperature

Tc Liquid

Critical point

Vapor 1.014 bar

Liquid-vapor

s

Fig. 3.3 Sketch of a temperature–specific volume diagram for water showing the liquid, twophase liquid–vapor, and vapor regions (not to scale).

100°C g

f 20°C

l Specific volume

The projections of the p–𝜐–T surface used in this book to illustrate processes are not generally drawn to scale. A similar comment applies to other property diagrams introduced later.

3.3 Studying Phase Change It is instructive to study the events that occur as a pure substance undergoes a phase change. To begin, consider a closed system consisting of a unit mass (1 kg) of liquid water at 20°C contained within a piston–cylinder assembly, as illustrated in Fig. 3.4a. This state is represented by point l on Fig. 3.3. Suppose the water is slowly heated while its pressure is kept constant and uniform throughout at 1.014 bar.

Liquid States

subcooled liquid compressed liquid

As the system is heated at constant pressure, the temperature increases considerably while the specific volume increases slightly. Eventually, the system is brought to the state represented by f on Fig. 3.3. This is the saturated liquid state corresponding to the specified pressure. For water at 1.014 bar the saturation temperature is 100°C. The liquid states along the line segment l–f of Fig. 3.3 are sometimes referred to as subcooled liquid states because the temperature at these states is less than the saturation temperature at the given pressure. These states are also referred to as compressed liquid states because the pressure at each state is higher than the saturation pressure corresponding to the temperature at the state. The names liquid, subcooled liquid, and compressed liquid are used interchangeably.

Water vapor Liquid water

Liquid water

(a)

(b)

Water vapor

(c)

Fig. 3.4 Illustration of constant-pressure change from liquid to vapor for water.

3.3 Studying Phase Change

85

Two-Phase, Liquid–Vapor Mixture When the system is at the saturated liquid state (state f of Fig. 3.3), additional heat transfer at fixed pressure results in the formation of vapor without any change in temperature but with a considerable increase in specific volume. As shown in Fig. 3.4b, the system would now consist of a two-phase liquid–vapor mixture. When a mixture of liquid and vapor exists in equilibrium, the liquid phase is a saturated liquid and the vapor phase is a saturated vapor. If the system is heated further until the last bit of liquid has vaporized, it is brought to point g on Fig. 3.3, the saturated vapor state. The intervening two-phase liquid–vapor mixture states can be distinguished from one another by the quality, an intensive property. For a two-phase liquid–vapor mixture, the ratio of the mass of vapor present to the total mass of the mixture is its quality, x. In symbols, x5

mvapor

two-phase liquid–vapor mixture quality

(3.1)

mliquid 1 mvapor

The value of the quality ranges from zero to unity: at saturated liquid states, x 5 0, and at saturated vapor states, x 5 1.0. Although defined as a ratio, the quality is frequently given as a percentage. Examples illustrating the use of quality are provided in Sec. 3.5. Similar parameters can be defined for two-phase solid–vapor and twophase solid–liquid mixtures.

Vapor States

Pressure

Let us return to Figs. 3.3 and 3.4. When the system is at the saturated vapor state (state g on Fig. 3.3), further heating at fixed pressure results in increases in both temperature and specific volume. The condition of the system would now be as shown in Fig. 3.4c. The state labeled s on Fig. 3.3 is representative of the states that would be attained by further heating while keeping the pressure constant. A state such as s is often referred to as a superheated vapor state because the system would be at a tempera- superheated vapor ture greater than the saturation temperature corresponding to the given pressure. Consider next the same thought experiment at the other constant pressures labeled on Fig. 3.3, 10 MPa, 22.09 MPa, and 30 MPa. The first of these pressures is less than the critical pressure of water, the second is the critical pressure, and the third is greater than the critical pressure. As before, let the system initially contain a liquid at 20°C. First, let us study the system if it were heated slowly at 10 MPa. At this pressure, vapor would form at a higher temperature than in the previous example, because the saturation pressure is higher (refer to Fig. 3.3). In addition, there would be somewhat less of an increase in specific volume from saturated liquid to saturated liquid vapor vapor, as evidenced by the narrowing of the vapor dome. Apart from this, Liquid Critical point the general behavior would be the same as before. a b c Consider next the behavior of the system if it were heated at the Melting critical pressure, or higher. As seen by following the critical isobar on Vaporization, Fig. 3.3, there would be no change in phase from liquid to vapor. At Condensation a´ b´ c´ all states there would be only one phase. As shown by line a-b-c of the phase diagram sketched in Fig. 3.5, vaporization and the inverse Solid Vapor process of condensation can occur only when the pressure is less than Triple point Sublimation the critical pressure. Thus, at states where pressure is greater than the critical pressure, the terms liquid and vapor tend to lose their signifia´´ b´´ c´´ cance. Still, for ease of reference to such states, we use the term liquid when the temperature is less than the critical temperature and vapor when the temperature is greater than the critical temperature. This convention is labeled on Fig. 3.5. Temperature While condensation of water vapor to liquid and further cooling to lower-temperature liquid are easily imagined and even a part of our Fig. 3.5 Phase diagram for water everyday experience, liquefying gases other than water vapor may not (not to scale).

86

Chapter 3 Evaluating Properties

Nitrogen, Unsung Workhorse Nitrogen is obtained using commercial air-separation technology that extracts oxygen and nitrogen from air. While applications for oxygen are widely recognized, uses for nitrogen tend to be less heralded but still touch on things people use every day. Liquid nitrogen is used to fast-freeze foods. Tunnel freezers employ a conveyer belt to pass food through a liquid-nitrogen spray, while batch freezers immerse food in a liquid-nitrogen bath. Each freezer type operates at temperatures less than about 2185°C. Liquid nitrogen is also used to preserve specimens employed in medical research and by dermatologists to remove lesions (see BIOCONNECTIONS in the next box). As a gas, nitrogen, with other gases, is inserted into food packaging to replace oxygen-bearing air, thereby prolonging shelf life—examples include gas-inflated bags of potato chips, salad greens, and shredded cheese. For improved tire performance, nitrogen is used to inflate the tires of airplanes and race cars. Nitrogen is among several alternative substances injected into underground rock formations to stimulate flow of trapped oil and natural gas to the surface—a procedure known as hydraulic fracturing. Chemical plants and refineries use nitrogen gas as a blanketing agent to prevent explosion. Laser-cutting machines also use nitrogen and other specialty gases.

A

Liq_to_Vapor A.13 – Tabs a & b Vapor_to_Liq A.14 – Tabs a & b

be so familiar. Still, there are important applications for liquefied gases. See the above box for applications of nitrogen in liquid and gas forms.

Melting and Sublimation Although the phase changes from liquid to vapor (vaporization) and vapor to liquid (condensation) are of principal interest in this book, it is also instructive to consider the phase changes from solid to liquid (melting) and from solid to vapor (sublimation). To study these transitions, consider a system consisting of a unit mass of ice at a temperature below the triple point temperature. Let us begin with the case where the pressure is greater than the triple point pressure and the system is at state a9 of Fig. 3.5. Suppose the system is slowly heated while maintaining the pressure constant and uniform throughout. The temperature increases with heating until point b9 on Fig. 3.5 is attained. At this state the ice is a saturated solid. Additional heat transfer at fixed pressure results in the formation of liquid without any change in temperature. As the system is heated further, the ice continues to melt until eventually the last bit melts, and the system contains only saturated liquid. During the melting process the temperature and pressure remain constant. For most substances, the specific volume increases during melting, but for water the specific volume of the liquid is less than the specific volume of the solid. Further heating at fixed pressure results in an increase in temperature as the system is brought to point c9 on Fig. 3.5. Next, consider the case where the pressure is less than the triple point pressure and the system is at state a0 of Fig. 3.5. In this case, if the system is heated at constant pressure it passes through the two-phase solid–vapor region into the vapor region along the line a0–b0–c0 shown on Fig. 3.5. That is, sublimation occurs.

BIOCONNECTIONS As discussed in the box devoted to nitrogen in this section, nitrogen has many applications, including medical applications. One medical application is the practice of cryosurgery by dermatologists. Cryosurgery entails the localized freezing of skin tissue for the removal of unwanted lesions, including precancerous lesions. For this type of surgery, liquid nitrogen is applied as a spray or with a probe. Cryosurgery is quickly performed and generally without anesthetic. Dermatologists store liquid nitrogen required for up to several months in containers called Dewar flasks that are similar to vacuum bottles.

3.3 Studying Phase Change

3.4 Retrieving Thermodynamic Properties Thermodynamic property data can be retrieved in various ways, including tables, graphs, equations, and computer software. The emphasis of Secs. 3.5 and 3.6 to follow is on the use of tables of thermodynamic properties, which are commonly available for pure, simple compressible substances of engineering interest. The use of these tables is an important skill. The ability to locate states on property diagrams is an important related skill. The software available with this text, Interactive Thermodynamics: IT, is introduced in Sec. 3.7. IT is used selectively in examples and end-of-chapter problems throughout the book. Skillful use of tables and property diagrams is prerequisite for the effective use of software to retrieve thermodynamic property data. Since tables for different substances are frequently set up in the same general format, the present discussion centers mainly on Tables A-2 through A-6 giving the properties of water; these are commonly referred to as the steam tables. Tables A-7 through A-9 for Refrigerant 22, Tables A-10 through A-12 for Refrigerant 134a, Tables A-13 through A-15 for ammonia, and Tables A-16 through A-18 for propane are used similarly, as are tables for other substances found in the engineering literature. Tables are provided in the Appendix in SI units. The substances for which tabulated data are provided in this book have been selected because of their importance in current practice. Still, they are merely representative of a wide range of industrially important substances. To meet changing requirements and address special needs, new substances are frequently introduced while others become obsolete.

ENERGY & ENVIRONMENT Development in the twentieth century of chlorine-containing refrigerants such as Refrigerant 12 helped pave the way for the refrigerators and air conditioners we enjoy today. Still, concern over the effect of chlorine on the earth’s protective ozone layer led to international agreements to phase out these refrigerants. Substitutes for them also have come under criticism as being environmentally harmful. Accordingly, a search is on for alternatives, and natural refrigerants are getting a close look. Natural refrigerants include ammonia, certain hydrocarbons—propane, for example— carbon dioxide, water, and air. Ammonia, once widely used as a refrigerant for domestic applications but dropped owing to its toxicity, is receiving renewed interest because it is both effective as a refrigerant and chlorine free. Refrigerators using propane are available on the global market despite lingering worries over flammability. Carbon dioxide is well suited for small, lightweight systems such as automotive and portable air-conditioning units. Although CO2 released to the environment contributes to global climate change, only a tiny amount is present in a typical unit, and ideally even this would be contained under proper maintenance and refrigeration unit disposal protocols.

3.5 Evaluating Pressure, Specific Volume, and Temperature 3.5.1 Vapor and Liquid Tables The properties of water vapor are listed in Tables A-4 and of liquid water in Tables A-5. These are often referred to as the superheated vapor tables and compressed liquid tables, respectively. The sketch of the phase diagram shown in Fig. 3.6 brings out the structure of these tables. Since pressure and temperature are independent properties in the single-phase liquid and vapor regions, they can be used to fix the state

steam tables

87

88

Chapter 3 Evaluating Properties Compressed liquid tables give v, u, h, s versus p, T Critical point

Pressure

Liquid

Solid

Vapor Superheated vapor tables give v, u, h, s versus p, T

Temperature

Fig. 3.6 Sketch of the phase diagram for water used to discuss the structure of the superheated vapor and compressed liquid tables (not to scale).

linear interpolation

in these regions. Accordingly, Tables A-4 and A-5 are set up to give values of several properties as functions of pressure and temperature. The first property listed is specific volume. The remaining properties are discussed in subsequent sections. For each pressure listed, the values given in the superheated vapor table (Tables A-4) begin with the saturated vapor state and then proceed to higher temperatures. The data in the compressed liquid table (Tables A-5) end with saturated liquid states. That is, for a given pressure the property values are given as the temperature increases to the saturation temperature. In these tables, the value shown in parentheses after the pressure in the table heading is the corresponding saturation temperature. in Tables A-4 and A-5, at a pressure of 10.0 MPa, the saturation temperature is listed as 311.06°C. b b b b b to gain more experience with Tables A-4 and A-5 verify the following: Table A-4 gives the specific volume of water vapor at 10.0 MPa and 600°C as 0.03837 m3/kg. At 10.0 MPa and 100°C, Table A-5 gives the specific volume of liquid water as 1.0385 3 10−3 m3/kg. b b b b b

The states encountered when solving problems often do not fall exactly on the grid of values provided by property tables. Interpolation between adjacent table entries then becomes necessary. Care always must be exercised when interpolating table values. The tables provided in the Appendix are extracted from more extensive tables that are set up so that linear interpolation, illustrated in the following example, can be used with acceptable accuracy. Linear interpolation is assumed to remain valid when using the abridged tables of the text for the solved examples and end-of-chapter problems. let us determine the specific volume of water vapor at a state where p 5 10 bar and T 5 215°C. Shown in Fig. 3.7 is a sampling of data from Table A-4. At a pressure of 10 bar, the specified temperature of 215°C falls between the table values of 200 and 240°C, which are shown in boldface. The corresponding specific volume values are also shown in boldface. To determine the specific volume 𝜐 corresponding to 215°C, we think of the slope of a straight line joining the adjacent table entries, as follows: slope 5

(0.2275 2 0.2060) m3/kg (𝜐 2 0.2060) m3/kg 5 (240 2 200) °C (215 2 200) °C

Solving for 𝜐, the result is 𝜐 5 0.2141 m3/kg. b

b b b b

3

v (m3/kg)

m (240°C, 0.2275 —— kg )

p = 10 bar T(°C) v (m3/kg) 200 0.2060 215 v=? 240 0.2275

(215°C, v)

3

m (200°C, 0.2060 —— kg )

Fig. 3.7 Illustration of linear interpolation.

200

215 T(°C)

240

3.5 Evaluating Pressure, Specific Volume, and Temperature

89

The following example features the use of sketches of p–𝜐 and T–𝜐 diagrams in conjunction with tabular data to fix the end states of a process. In accord with the state principle, two independent intensive properties must be known to fix the states of the system under consideration. cccc

EXAMPLE 3.1 c

Heating Ammonia at Constant Pressure A vertical piston–cylinder assembly containing 0.05 kg of ammonia, initially a saturated vapor, is placed on a hot plate. Due to the weight of the piston and the surrounding atmospheric pressure, the pressure of the ammonia is 1.5 bars. Heating occurs slowly, and the ammonia expands at constant pressure until the final temperature is 25°C. Show the initial and final states on T–𝜐 and p–𝜐 diagrams, and determine (a) the volume occupied by the ammonia at each end state, in m3. (b) the work for the process, in kJ. SOLUTION Known: Ammonia is heated at constant pressure in a vertical piston–cylinder assembly from the saturated vapor state to a known final temperature. Find:

Show the initial and final states on T–𝜐 and p–𝜐 diagrams, and determine the volume at each end state and the work for the process.

Schematic and Given Data: T

25°C

2 Ammonia

–25.2°C

1

+ v p

–

Hot plate

Engineering Model: 1. The ammonia is a closed system. 2. States 1 and 2 are equilibrium states. 3. The process occurs at constant pressure. 25°C 2

1.5 bars 1

Fig. E3.1

v

Analysis: The initial state is a saturated vapor condition at 1.5 bars. Since the process occurs at constant pres-

sure, the final state is in the superheated vapor region and is fixed by 1.5 bars and T2 5 25°C. The initial and final states are shown on the T–𝜐 and p–𝜐 diagrams above. (a) The volumes occupied by the ammonia at states 1 and 2 are obtained using the given mass and the respective specific volumes. From Table A-14 at P2 5 1.5 bars and corresponding to Sat. in the temperature column, we get 𝜐1 5 𝜐g 5 0.7787 m3/kg. Thus V1 5 m𝜐1 5 (0.05 kg)(0.7787 m3/kg) 5 0.0389 m3

90

Chapter 3 Evaluating Properties Interpolating in Table A-14 at p2 5 1.5 bar and T2 5 25°C, we get 𝜐2 5 0.9553 m3/kg. Thus V2 5 m𝜐2 5 (0.05 kg)(0.9553 m3/kg) 5 0.0478 m3 (b) In this case, the work can be evaluated using Eq. 2.17. Since the pressure is

constant W5

#

V2

Ability to… ❑ define a closed system and

p dV 5 p(V2 2 V1)

V1

identify interactions on its boundary. ❑ sketch T-υ and p-υ diagrams and locate states on them. ❑ evaluate work using Eq. 2.17. ❑ retrieve property data for ammonia at vapor states.

Inserting values W 5 (1.5 bars)(0.0478 2 0.0389)m3 ` ➊

✓ Skills Developed

105 N/m2 kJ ` ` ` 3 1 bar 10 N ~ m

5 1.335 kJ ➊ Note the use of conversion factors in this calculation. If heating continues at 1.5 bars from T2 5 25°C to T3 5 32°C, determine the work for Process 2-3, in kJ. Ans. 0.16 kJ

3.5.2 Saturation Tables Tables A-2, A-3, and A-6 provide property data for water at saturated liquid, saturated vapor, and saturated solid states. Tables A-2 and A-3 are the focus of the present discussion. Each of these tables gives saturated liquid and saturated vapor data. Property values at saturated liquid and saturated vapor states are denoted by the subscripts f and g, respectively. Table A-2 is called the temperature table, because temperatures are listed in the first column in convenient increments. The second column gives the corresponding saturation pressures. The next two columns give, respectively, the specific volume of saturated liquid, 𝜐f, and the specific volume of saturated vapor, 𝜐g. Table A-3 is called the pressure table, because pressures are listed in the first column in convenient increments. The corresponding saturation temperatures are given in the second column. The next two columns give 𝜐f and 𝜐g, respectively. The specific volume of a two-phase liquid–vapor mixture can be determined by using the saturation tables and the definition of quality given by Eq. 3.1 as follows. The total volume of the mixture is the sum of the volumes of the liquid and vapor phases V 5 Vliq 1 Vvap Dividing by the total mass of the mixture, m, an average specific volume for the mixture is obtained Vliq Vvap V 5 1 m m m Since the liquid phase is a saturated liquid and the vapor phase is a saturated vapor, Vliq 5 mliq𝜐f and Vvap 5 mvap𝜐g, so 𝜐5

𝜐5a

mliq m

b 𝜐f 1 a

mvap m

b 𝜐g

3.5 Evaluating Pressure, Specific Volume, and Temperature

91

Introducing the definition of quality, x 5 mvap/m, and noting that mliq/m 5 1 2 x, the above expression becomes 𝜐 5 (1 2 x)𝜐f 1 x𝜐g 5 𝜐f 1 x(𝜐g 2 𝜐f)

(3.2)

The increase in specific volume on vaporization (𝜐g 2 𝜐f) is also denoted by 𝜐fg. consider a system consisting of a two-phase liquid–vapor mixture of water at 100°C and a quality of 0.9. From Table A-2 at 100°C, 𝜐f 5 1.0435 3 10−3 m3/kg and 𝜐g 5 1.673 m3/kg. The specific volume of the mixture is 𝜐 5 𝜐f 1 x(𝜐g 2 𝜐f) 5 1.0435 3 1023 1 (0.9)(1.673 2 1.0435 3 1023) 5 1.506 m3/kg where the 𝜐f and 𝜐g values are obtained from Table A-2. b

b b b b

To facilitate locating states in the tables, it is often convenient to use values from the saturation tables together with a sketch of a T–𝜐 or p–𝜐 diagram. For example, if the specific volume 𝜐 and temperature T are known, refer to the appropriate temperature table, Table A-2, and determine the values of 𝜐f and 𝜐g. A T–𝜐 diagram illustrating these data is given in Fig. 3.8. If the given specific volume falls between 𝜐f and 𝜐g, the system consists of a two-phase liquid–vapor mixture, and the pressure is the saturation pressure corresponding to the given temperature. The quality can be found by solving Eq. 3.2. If the given specific volume is greater than 𝜐g, the state is in the superheated vapor region. Then, by interpolating in Table A-4 or, the pressure and other properties listed can be determined. If the given specific volume is less than 𝜐f, Table A-5 or would be used to determine the pressure and other properties. let us determine the pressure of water at each of three states defined by a temperature of 100°C and specific volumes, respectively, of 𝜐1 5 2.434 m3/kg, ʋ2 5 1.0 m3/kg, and ʋ3 5 1.0423 3 10−3 m3/kg. Using the known temperature, Table A-2 provides the values of ʋf and 𝜐g: 𝜐f 5 1.0435 3 10−3 m3/kg, 𝜐g 5 1.673 m3/kg. Since 𝜐1 is greater than ʋg, state 1 is in the vapor region. Table A-4 gives the pressure as 0.70 bar. Next, since 𝜐2 falls between 𝜐f and 𝜐g, the pressure is the saturation pressure corresponding to 100°C, which is 1.014 bar. Finally, since 𝜐3 is less than 𝜐f, state 3 is in the liquid region. Table A-5 gives the pressure as 25 bar. b b b b b The following example features the use of a sketch of the T–𝜐 diagram in conjunction with tabular data to fix the end states of processes. In accord with the state principle, two independent intensive properties must be known to fix the states of the system under consideration.

Temperature

Critical point

Liquid

Saturated liquid v < vf

Saturated vapor

vf < v < vg

f

Vapor v > vg

g

vf

vg Specific volume

Fig. 3.8 Sketch of a T–𝝊 diagram for water used to discuss locating states in the tables.

T 100°C

3 f 2

g 1

v

92 cccc

Chapter 3 Evaluating Properties

EXAMPLE 3.2 c

Heating Water at Constant Volume A closed, rigid container of volume 0.5 m3 is placed on a hot plate. Initially, the container holds a two-phase mixture of saturated liquid water and saturated water vapor at p1 5 1 bar with a quality of 0.5. After heating, the pressure in the container is p2 5 1.5 bar. Indicate the initial and final states on a T–𝜐 diagram, and determine (a) the temperature, in 8C, at states 1 and 2. (b) the mass of vapor present at states 1 and 2, in kg. (c) If heating continues, determine the pressure, in bar, when the container holds only saturated vapor. SOLUTION Known: A two-phase liquid–vapor mixture of water in a closed, rigid container is heated on a hot plate. The initial pressure and quality and the final pressure are known. Find: Indicate the initial and final states on a T–𝜐 diagram and determine at each state the temperature and the

mass of water vapor present. Also, if heating continues, determine the pressure when the container holds only saturated vapor. Schematic and Given Data:

T

Engineering Model: 1. The water in the con-

tainer is a closed system. p1 x1 p2 x3

= 1 bar = 0.5 = 1.5 bar = 1.0

2. States 1, 2, and 3 are 3

equilibrium states.

1.5 bar

3. The volume of the con-

V = 0.5 m3 2

tainer remains constant.

1 bar

1

+

–

Hot plate

v

Fig. E3.2

Analysis: Two independent properties are required to fix states 1 and 2. At the initial state, the pressure and quality are known. As these are independent, the state is fixed. State 1 is shown on the T–𝜐 diagram in the twophase region. The specific volume at state 1 is found using the given quality and Eq. 3.2. That is,

𝜐1 5 𝜐f1 1 x(𝜐g1 2 𝜐f1) From Table A-3 at p1 5 1 bar, 𝜐f1 5 1.0432 3 1023 m3/kg and 𝜐g1 5 1.694 m3/kg. Thus, 𝜐1 5 1.0432 3 10 23 1 0.5(1.694 2 1.0432 3 10 23) 5 0.8475 m3/kg At state 2, the pressure is known. The other property required to fix the state is the specific volume 𝜐2. Volume and mass are each constant, so 𝜐2 5 𝜐1 5 0.8475 m3/kg. For p2 5 1.5 bar, Table A-3 gives 𝜐f2 5 1.0582 3 1023 m3/kg and 𝜐g2 5 1.59 m3/kg. Since ➊

𝜐f , 𝜐2 , 𝜐g2

➋ state 2 must be in the two-phase region as well. State 2 is also shown on the T–𝜐 diagram above.

3.5 Evaluating Pressure, Specific Volume, and Temperature

93

(a) Since states 1 and 2 are in the two-phase liquid–vapor region, the temperatures correspond to the saturation temperatures for the given pressures. Table A-3 gives

T1 5 99.63°C and T2 5 111.4°C (b) To find the mass of water vapor present, we first use the volume and the specific volume to find the total

mass, m. That is m5

0.5 m3 V 5 5 0.59 kg 𝜐 0.8475 m3/kg

Then, with Eq. 3.1 and the given value of quality, the mass of vapor at state 1 is mg1 5 x1m 5 0.5(0.59 kg) 5 0.295 kg The mass of vapor at state 2 is found similarly using the quality x2. To determine x2, solve Eq. 3.2 for quality and insert specific volume data from Table A-3 at a pressure of 1.5 bar, along with the known value of 𝜐, as follows x2 5 5

𝜐 2 𝜐f 2 𝜐g2 2 𝜐f 2 0.8475 2 1.0528 3 10 23 5 0.731 1.159 2 1.0528 3 10 23

Then, with Eq. 3.1

✓ Skills Developed

mg2 5 0.731(0.59 kg) 5 0.431 kg (c) If heating continued, state 3 would be on the saturated vapor line, as shown on the T–𝜐 diagram of Fig. E3.2. Thus, the pressure would be the corresponding saturation pressure. Interpolating in Table A-3 at 𝜐g 5 0.8475 m3/kg, we get p3 5 2.11 bar.

➊ The procedure for fixing state 2 is the same as illustrated in the discussion of Fig. 3.8. ➋ Since the process occurs at constant specific volume, the states lie along a vertical line.

Ability to… ❑ define a closed system and

identify interactions on its boundary. ❑ sketch a T–υ diagram and locate states on it. retr ieve property data for ❑ water at liquid–vapor states, using quality.

If heating continues at constant specific volume from state 3 to a state where pressure is 3 bar, determine the temperature at that state, in 8C. Ans. 2828C

3.6 Evaluating Specific Internal Energy and Enthalpy 3.6.1

Introducing Enthalpy

In many thermodynamic analyses the sum of the internal energy U and the product of pressure p and volume V appears. Because the sum U 1 pV occurs so frequently in subsequent discussions, it is convenient to give the combination a name, enthalpy, and a distinct symbol, H. By definition H 5 U 1 pV

(3.3)

Since U, p, and V are all properties, this combination is also a property. Enthalpy can be expressed on a unit mass basis h 5 u 1 p𝜐 (3.4)

enthalpy

94

Chapter 3 Evaluating Properties and per mole h 5 u 1 p𝜐

(3.5)

Units for enthalpy are the same as those for internal energy.

3.6.2

Retrieving u and h Data

The property tables introduced in Sec. 3.5 giving pressure, specific volume, and temperature also provide values of specific internal energy u, enthalpy h, and entropy s. Use of these tables to evaluate u and h is described in the present section; the consideration of entropy is deferred until it is introduced in Chap. 6. Data for specific internal energy u and enthalpy h are retrieved from the property tables in the same way as for specific volume. For saturation states, the values of uf and ug, as well as hf and hg, are tabulated versus both saturation pressure and saturation temperature. The specific internal energy for a two-phase liquid–vapor mixture is calculated for a given quality in the same way the specific volume is calculated u 5 (1 2 x)uf 1 xug 5 uf 1 x(ug 2 uf)

(3.6)

The increase in specific internal energy on vaporization (ug 2 uf) is often denoted by ufg. Similarly, the specific enthalpy for a two-phase liquid–vapor mixture is given in terms of the quality by h 5 (1 2 x)hf 1 xhg 5 hf 1 x(hg 2 hf)

(3.7)

The increase in enthalpy during vaporization (hg 2 hf) is often tabulated for convenience under the heading hfg. to illustrate the use of Eqs. 3.6 and 3.7, we determine the specific enthalpy of Refrigerant 22 when its temperature is 128C and its specific internal energy is 144.58 kJ/kg. Referring to Table A-7, the given internal energy value falls between uf and ug at 128C, so the state is a two-phase liquid–vapor mixture. The quality of the mixture is found by using Eq. 3.6 and data from Table A-7 as follows: x5

u 2 uf 144.58 2 58.77 5 5 0.5 ug 2 uf 230.38 2 58.77

Then, with the values from Table A-7, Eq. 3.7 gives h 5 (1 2 x)hf 1 xhg 5 (1 2 0.5)(59.35) 1 0.5(253.99) 5 156.67 kJ/kg

b b b b b

In the superheated vapor tables, u and h are tabulated along with 𝜐 as functions of temperature and pressure. let us evaluate T, 𝜐, and h for water at 0.10 MPa and a specific internal energy of 2537.3 kJ/kg. Turning to Table A-3, note that the given value of u is greater than ug at 0.1 MPa (ug 5 2506.1 kJ/kg). This suggests that the state lies in the superheated vapor region. By inspection of Table A-4 we get T 5 1208C, 𝜐 5 1.793 m3/kg, and h 5 2716.6 kJ/kg. Alternatively, h and u are related by the definition of h h 5 u 1 p𝜐 kJ N m3 1 kJ ` 5 2537.3 1 a105 2 b a1.793 b ` 3 kg kg 10 N ? m m 5 2537.3 1 179.3 5 2716.6 kJ/kg

3.6 Evaluating Specific Internal Energy and Enthalpy Specific internal energy and enthalpy data for liquid states of water are presented in Tables A-5. The format of these tables is the same as that of the superheated vapor tables considered previously. Accordingly, property values for liquid states are retrieved in the same manner as those of vapor states. For water, Tables A-6 give the equilibrium properties of saturated solid and saturated vapor. The first column lists the temperature, and the second column gives the corresponding saturation pressure. These states are at pressures and temperatures below those at the triple point. The next two columns give the specific volume of saturated solid, 𝜐i, and saturated vapor, 𝜐g, respectively. The table also provides the specific internal energy, enthalpy, and entropy values for the saturated solid and the saturated vapor at each of the temperatures listed.

3.6.3 Reference States and Reference Values The values of u, h, and s given in the property tables are not obtained by direct measurement but are calculated from other data that can be more readily determined experimentally. The computational procedures require use of the second law of thermodynamics, so consideration of these procedures is deferred to Chap. 11 after the second law has been introduced. However, because u, h, and s are calculated, the matter of reference states and reference values becomes important and is considered briefly in the following paragraphs. When applying the energy balance, it is differences in internal, kinetic, and potential energy between two states that are important, and not the values of these energy quantities at each of the two states. consider the case of potential energy. The numerical value of potential energy determined relative to the surface of the earth is not the same as the value relative to the top of a tall building at the same location. However, the difference in potential energy between any two elevations is precisely the same regardless of the datum selected, because the datum cancels in the calculation. b b b b b Similarly, values can be assigned to specific internal energy and enthalpy relative to arbitrary reference values at arbitrary reference states. As for the case of potential energy considered above, the use of values of a particular property determined relative to an arbitrary reference is unambiguous as long as the calculations being performed involve only differences in that property, for then the reference value cancels. When chemical reactions take place among the substances under consideration, special attention must be given to the matter of reference states and values, however. A discussion of how property values are assigned when analyzing reactive systems is given in Chap. 13. The tabular values of u and h for water, ammonia, propane, and Refrigerants 22 and 134a provided in the Appendix are relative to the following reference states and values. For water, the reference state is saturated liquid at 0.018C (32.028F). At this state, the specific internal energy is set to zero. Values of the specific enthalpy are calculated from h 5 u 1 p𝜐, using the tabulated values for p, 𝜐, and u. For ammonia, propane, and the refrigerants, the reference state is saturated liquid at 2408C. At this reference state the specific enthalpy is set to zero. Values of specific internal energy are calculated from u 5 h 2 p𝜐 by using the tabulated values for p, 𝜐, and h. Notice in Table A-7 that this leads to a negative value for internal energy at the reference state, which emphasizes that it is not the numerical values assigned to u and h at a given state that are important but their differences between states. The values assigned to particular states change if the reference state or reference values change, but the differences remain the same.

reference states reference values

95

96

Chapter 3 Evaluating Properties

3.7 Evaluating Properties Using Computer Software The use of computer software for evaluating thermodynamic properties is becoming prevalent in engineering. Computer software falls into two general categories: those that provide data only at individual states and those that provide property data as part of a more general simulation package. The software available with this text, Interactive Thermodynamics: IT, is a tool that can be used not only for routine problem solving by providing data at individual state points, but also for simulation and analysis. Software other than IT also can be used for these purposes. See the box for discussion of software use in engineering thermodynamics. IT provides data for substances represented in the Appendix tables. Generally, data are retrieved by simple call statements that are placed in the workspace of the program. consider the two-phase, liquid–vapor mixture at state 1 of Example 3.2 for which p 5 1 bar, 𝜐 5 0.8475 m3/kg. The following illustrates how data for saturation temperature, quality, and specific internal energy are retrieved using IT. The

Using Software In Thermodynamics The computer software tool Interactive Thermodynamics: IT is available for use with this text. Used properly, IT provides an important adjunct to learning engineering thermodynamics and solving engineering problems. The program is built around an equation solver enhanced with thermodynamic property data and other valuable features. With IT you can obtain a single numerical solution or vary parameters to investigate their effects. You also can obtain graphical output, and the Windowsbased format allows you to use any Windows word-processing software or spreadsheet to generate reports. Additionally, functions in IT can be called from Excel through use of the Excel Add-in Manager, allowing you to use these thermodynamic functions while working within Excel. Other features of IT include: c

a guided series of help screens and a number of sample solved examples to help you learn how to use the program.

c

drag-and-drop templates for many of the standard problem types, including a list of assumptions that you can customize to the problem at hand.

c

predetermined scenarios for power plants and other important applications.

c

thermodynamic property data for water, refrigerants 22 and 134a, ammonia, air–water vapor mixtures, and a number of ideal gases.

c

the capability to input user-supplied data.

c

the capability to interface with user-supplied routines.

Many features of IT are found in the popular Engineering Equation Solver (EES). Readers already proficient with EES may prefer its use for solving problems in this text. The use of computer software for engineering analysis is a powerful approach. Still, there are some rules to observe: c

Software complements and extends careful analysis, but does not substitute for it.

c

Computer-generated values should be checked selectively against hand-calculated or otherwise independently determined values.

c

Computer-generated plots should be studied to see if the curves appear reasonable and exhibit expected trends.

3.7 Evaluating Properties Using Computer Software functions for T, 𝜐, and u are obtained by selecting Water/Steam from the Properties menu. Choosing SI units from the Units menu, with p in bar, T in 8C, and amount of substance in kg, the IT program is p 5 1//bar v 5 0.8475//m3/kg T 5 Tsat_P(“Water/Steam”,p) v 5 vsat_Px(“Water/Steam”,p,x) u 5 usat_Px(“Water/Steam”,p,x) Clicking the Solve button, the software returns values of T 5 99.638C, x 5 0.5, and u 5 1462 kJ/kg. These values can be verified using data from Table A-3. Note that text inserted between the symbol // and a line return is treated as a comment. b b b b b The previous example illustrates an important feature of IT. Although the quality, x, is implicit in the list of arguments in the expression for specific volume, there is no need to solve the expression algebraically for x. Rather, the program can solve for x as long as the number of equations equals the number of unknowns. IT also retrieves property values in the superheat region. consider the superheated ammonia vapor at state 2 in Example 3.1, for which p 5 1.5 bar and T 5 258C. Selecting Ammonia from the Properties menu and choosing SI units from the Units menu, data for specific volume, internal energy, and enthalpy are obtained from IT as follows: p 5 1.5//bar T 5 25//°C v 5 v_PT(“Ammonia”,p,T) u 5 u_PT(“Ammonia”,p,T) h 5 h_PT(“Ammonia”,p,T) Clicking the Solve button, the software returns values of 𝜐 5 0.9553 m3/kg, u 5 1380.93 kJ/kg, and h 5 1524.21 kJ/kg, respectively. These values agree closely with the respective values obtained by interpolation in Table A-14. b b b b b

3.8 Applying the Energy Balance Using Property Tables and Software The energy balance for closed systems is introduced in Sec. 2.5. Alternative expressions are given by Eqs. 2.35a and 2.35b, which are forms applicable to processes between end states denoted 1 and 2, and by Eq. 2.37, the time rate form. In applications where changes in kinetic energy and gravitational potential energy between the end states can be ignored, Eq. 2.35b reduces to U2 2 U1 5 Q 2 W

(a)

where Q and W account, respectively, for the transfer of energy by heat and work between the system and its surroundings during the process. The term U2 2 U1 accounts for change in internal energy between the end states. Taking water for simplicity, let’s consider how the internal energy term is evaluated in three representative cases of systems involving a single substance. Case 1: Consider a system consisting initially and finally of a single phase of water, vapor or liquid. Then Eq. (a) takes the form m(u2 2 u1) 5 Q 2 W

(b)

97

98

Chapter 3 Evaluating Properties where m is the system mass and u1 and u2 denote, respectively, the initial and final specific internal energies. When the initial and final temperatures T1, T2 and pressures p1, p2 are known, for instance, the internal energies u1 and u2 can be readily obtained from the steam tables or using computer software. Case 2: Consider a system consisting initially of water vapor and finally as a two-phase mixture of liquid water and water vapor. As in Case 1, we write U1 5 mu1 in Eq. (a), but now U2 5 (Uliq 1 Uvap) 5 mliquf 1 mvapug (c) where mliq and mvap account, respectively, for the masses of saturated liquid and saturated vapor present finally, and uf and ug are the corresponding specific internal energies determined by the final temperature T2 (or final pressure p2). If quality x2 is known, Eq. 3.6 can be invoked to evaluate the specific internal energy of the two-phase liquid–vapor mixture, u2. Then, U2 5 mu2, thereby preserving the form of the energy balance expressed by Eq. (b). Case 3: Consider a system consisting initially of two separate masses of water vapor that mix to form a total mass of water vapor. In this case U1 5 m¿u(T¿, p¿) 1 m–u(T–, p–) U2 5 (m¿ 1 m–)u(T2, p2) 5 mu(T2, p2)

(d) (e)

where m¿ and m– are masses of water vapor initially separate at T¿, p¿ and T–, p–, respectively, that mix to form a total mass, m 5 m¿ 1 m–, at a final state where temperature is T2 and pressure is p2. When temperatures and pressures at the respective states are known, for instance, the specific internal energies of Eqs. (d) and (e) can be readily obtained from the steam tables or using computer software. These cases show that when applying the energy balance, an important consideration is whether the system has one or two phases. A pertinent application is that of thermal energy storage, considered in the box.

Thermal Energy Storage

TAKE NOTE...

On property diagrams, solid olid lines are reserved for processes that pass through equilibrium states: quasiequilibrium processes (Sec. 2.2.5). A dashed line on a property diagram signals only that a process has occurred between initial and final equilibrium states, and does not define a path for the process.

Energy is often available at one time but more valuable or used more effectively at another. These considerations underlie various means for storing energy, including methods introduced in Sec. 2.7 and those discussed here. Solar energy is collected during daylight hours but often needed at other times of the day—to heat buildings overnight, for example. Accordingly, thermal energy storage systems have been developed to meet solar and other similar energy storage needs. The term thermal energy here should be understood as internal energy. Mediums used in thermal energy storage systems change temperature and/or change phase. Some storage systems simply store energy by heating water, mineral oil, or other substances held in a storage tank, usually pressurized, until the stored energy is needed. Solids such as concrete can also be the medium. Phase-change systems store energy by melting or freezing a substance, often water or a molten (eutectic) salt. The choice of storage medium is determined by the temperature requirements of the storage application at hand together with capital and operating costs related to the storage system. The availability of relatively inexpensive electricity generated in low-demand periods, usually overnight or during weekends, gives rise to storage strategies. In one approach, low-cost electricity is provided to a refrigeration system that chills water and/or produces ice during cooler nighttime hours when less refrigerator power is required. The chilled water and/or ice are stored in tanks until needed—for instance, to satisfy building-cooling needs during the warmest part of summer days when electricity is more costly.

3.8 Applying the Energy Balance Using Property Tables and Software

99

3.8.1 Using Property Tables In Examples 3.3 and 3.4, closed systems undergoing processes are analyzed using the energy balance. In each case, sketches of p–𝜐 and/or T–𝜐 diagrams are used in conjunction with appropriate tables to obtain the required property data. Using property diagrams and table data introduces an additional level of complexity compared to similar problems in Chap. 2.

cccc

EXAMPLE 3.3 c

Stirring Water at Constant Volume A well-insulated rigid tank having a volume of 0.25 m3 contains saturated water vapor at 100°C. The water is rapidly stirred until the pressure is 1.5 bars. Determine the temperature at the final state, in °C, and the work during the process, in kJ. SOLUTION Known: By rapid stirring, water vapor in a well-insulated rigid tank is brought from the saturated vapor state at 100°C to a pressure of 1.5 bars. Find:

Determine the temperature at the final state and the work.

Schematic and Given Data:

Engineering Model: 1. The water is a

closed system. Water

2. The initial and final

states are at equilibrium. There is no net change in kinetic or potential energy.

Boundary p

T 2 1.5 bars

2

➊

1.5 bars T2

1.014 bars 100°C

1.014 bars 1

3. There is no heat

T2

transfer with the surroundings. 4. The tank volume

remains constant.

1

100°C v

v

Analysis:

Fig. E3.3

To determine the final equilibrium state, the values of two independent intensive properties are required. One of these is pressure, p2 5 1.5 bars, and the other is the specific volume: 𝜐2 5 𝜐1. The initial and final specific volumes are equal because the total mass and total volume are unchanged in the process. The initial and final states are located on the accompanying T–υ and p–υ diagrams. From Table A-2, 𝜐1 5 𝜐g(100°C) 5 1.673 m3/kg, u1 5 ug(100°C) 5 2,506.5 kJ/kg. By using 𝜐2 5 𝜐1 and interpolating in Table A-4 at p2 5 1.5 bar, T2 5 273°C,

u2 5 2,767.8 kJ/kg

100

Chapter 3 Evaluating Properties Next, with assumptions 2 and 3 an energy balance for the system reduces to 0

0

0

DU 1 DKE 1 DPE 5 Q 2 W On rearrangement W 5 2(U2 2 U1) 5 2m(u2 2 u1) To evaluate W requires the system mass. This can be determined from the volume and specific volume m5

V 0.25 m3 b 5 0.149 kg 5a 𝜐1 1.673 m3/kg

Finally, by inserting values into the expression for W W 5 2(1.49 kg)(2,767.8 2 2,506.5) kJ/kg 5 238.9 kJ where the minus sign signifies that the energy transfer by work is to the system. ➊ Although the initial and final states are equilibrium states, the intervening states are not at equilibrium. To emphasize this, the process has been indicated on the T–𝜐 and p–𝜐 diagrams by a dashed line. Solid lines on property diagrams are reserved for processes that pass through equilibrium states only (quasiequilibrium processes). The analysis illustrates the importance of carefully sketched property diagrams as an adjunct to problem solving.

✓ Skills Developed Ability to… ❑ define a closed system and

identify interactions on its boundary. ❑ apply the energy balance with steam table data. ❑ sketch T–𝜐 and p–𝜐 diagrams and locate states on them.

If insulation were removed from the tank and the water cooled at constant volume from T2 5 229.48C to T3 5 148.898C while no stirring occurs, determine the heat transfer, in kJ. Ans. 220.57 kJ.

cccc

EXAMPLE 3.4 c

Analyzing Two Processes in Series Water contained in a piston–cylinder assembly undergoes two processes in series from an initial state where the pressure is 10 bar and the temperature is 4008C. Process 1–2: The water is cooled as it is compressed at a constant pressure of 10 bar to the saturated vapor state. Process 2–3: The water is cooled at constant volume to 1508C. (a) Sketch both processes on T–𝜐 and p–𝜐 diagrams. (b) For the overall process determine the work, in kJ/kg. (c) For the overall process determine the heat transfer, in kJ/kg. SOLUTION Known: Water contained in a piston–cylinder assembly undergoes two processes: It is cooled and compressed while keeping the pressure constant, and then cooled at constant volume. Find: Sketch both processes on T–𝜐 and p–𝜐 diagrams. Determine the net work and the net heat transfer for the

overall process per unit of mass contained within the piston–cylinder assembly.

3.8 Applying the Energy Balance Using Property Tables and Software

101

Schematic and Given Data: Engineering Model: 1. The water is a closed

Water

system. 10 bar

Boundary p

1

T

2. The piston is the only

400°C

work mode. 3. There are no changes

10 bar

2

400°C

179.9°C

1

in kinetic or potential energy.

2

179.9°C

4.758 bar 150°C

4.758 bar 3

150°C

3

v

v

Fig. E3.4

Analysis: (a) The accompanying T–𝜐 and p–𝜐 diagrams show the two processes. Since the temperature at state 1, T1 5 4008C, is greater than the saturation temperature corresponding to p1 5 10 bar: 179.98C, state 1 is located in the superheat region. (b) Since the piston is the only work mechanism

W5

#

1

3

p dV 5

#

2

3

p dV 1

1

# p dV

0

2

The second integral vanishes because the volume is constant in Process 2–3. Dividing by the mass and noting that the pressure is constant for Process 1–2 W 5 p(𝜐2 2 𝜐1) m The specific volume at state 1 is found from Table A-4 using p1 5 10 bar and T1 5 4008C: 𝜐1 5 0.3066 m3/kg. Also, u1 5 2957.3 kJ/kg. The specific volume at state 2 is the saturated vapor value at 10 bar: 𝜐2 5 0.1944 m3/kg, from Table A-3. Hence W 1 kJ m3 105 N/m2 ` ` ` 5 (10 bar)(0.1944 2 0.3066)a b ` 3 m kg 1 bar 10 N ? m 5 2112.2 kJ/kg The minus sign indicates that work is done on the water vapor by the piston. (c) An energy balance for the overall process reduces to

m(u3 2 u1) 5 Q 2 W By rearranging Q W 5 (u3 2 u1) 1 m m To evaluate the heat transfer requires u3, the specific internal energy at state 3. Since T3 is given and 𝜐3 5 𝜐2, two independent intensive properties are known that together fix state 3. To find u3, first solve for the quality x3 5

𝜐3 2 𝜐f3 0.1944 2 1.0905 3 10 23 5 5 0.494 𝜐g3 2 𝜐f3 0.3928 2 1.0905 3 10 23

102

Chapter 3 Evaluating Properties

where 𝜐f3 and 𝜐g3 are from Table A-2 at 1508C. Then u3 5 uf3 1 x3(ug3 2 uf3) 5 631.68 1 0.494(2559.5 2 631.68) 5 1584.0 kJ/kg where uf3 and ug3 are from Table A-2 at 1508C. Substituting values into the energy balance Q 5 1584.0 2 2957.3 1 (2112.2) 5 21485.5 kJ/kg m The minus sign shows that energy is transferred out by heat transfer.

✓ Skills Developed Ability to… ❑ define a closed system and

identify interactions on its boundary. ❑ evaluate work using Eq. 2.17. ❑ apply the energy balance with steam table data. ❑ sketch T–𝜐 and p–𝜐 diagrams and locate states on them.

If the two specified processes were followed by Process 3-4, during which the water expands at a constant temperature of 1508C to saturated vapor, determine the work, in kJ/kg, for the overall process from 1 to 4. Ans. W/m 5 217.8 kJ/kg.

3.8.2 Using Software Example 3.5 illustrates the use of Interactive Thermodynamics: IT for solving problems. In this case, the software evaluates the property data, calculates the results, and displays the results graphically. cccc

EXAMPLE 3.5 c

Plotting Thermodynamic Data Using Software For the system of Example 3.2, plot the heat transfer, in kJ, and the mass of saturated vapor present, in kg, each versus pressure at state 2 ranging from 1 to 2 bar. Discuss the results. SOLUTION Known: A two-phase liquid–vapor mixture of water in a closed, rigid container is heated on a hot plate. The initial pressure and quality are known. The pressure at the final state ranges from 1 to 2 bar. Find: Plot the heat transfer and the mass of saturated vapor present, each versus pressure at the final state. Discuss. Schematic and Given Data: See Fig. E3.2. Engineering Model: 1. There is no work. 2. Kinetic and potential energy effects are negligible. 3. See Example 3.2 for other assumptions. Analysis: The heat transfer is obtained from the energy balance. With assumptions 1 and 2, the energy balance reduces to 0 0 0 DU 1 DKE 1 DPE 5 Q 2 W or Q 5 m(u2 2 u1)

Selecting Water/Steam from the Properties menu and choosing SI Units from the Units menu, the IT program for obtaining the required data and making the plots is // Given data—State 1 p1 5 1//bar x1 5 0.5 V 5 0.5//m3

3.8 Applying the Energy Balance Using Property Tables and Software // Evaluate property data—State 1 v1 5 vsat_Px(“Water/Steam”, p1,x1) u1 5 usat_Px(“Water/Steam”, p1,x1) // Calculate the mass m 5 V/v1 // Fix state 2 v2 5 v1 p2 5 1.5//bar // Evaluate property data—State 2 v2 5 vsat_Px (“Water/Steam”, p2,x2) u2 5 usat_Px(“Water/Steam”, p2,x2)

103

// Calculate the mass of saturated vapor present mg2 5 x2 * m // Determine the pressure for which the quality is unity v3 5 v1 ➊ v3 5 vsat_Px( “Water/Steam”,p3,1) // Energy balance to determine the heat transfer m * (u2 – u1) 5 Q – W W50

600

0.6

500

0.5

400

0.4 mg, kg

Q, kJ

Click the Solve button to obtain a solution for p2 5 1.5 bar. The program returns values of 𝜐1 5 0.8475 m3/kg and m 5 0.59 kg. Also, at p2 5 1.5 bar, the program gives mg2 5 0.4311 kg. These values agree with the values determined in Example 3.2. Now that the computer program has been verified, use the Explore button to vary pressure from 1 to 2 bar in steps of 0.1 bar. Then, use the Graph button to construct the required plots. The results are:

300

0.3

200

0.2

100

0.1

0

1

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Pressure, bar

2

0

1

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Pressure, bar

We conclude from the first of these graphs that the heat transfer to the water varies directly with the pressure. The plot of mg shows that the mass of saturated vapor present also increases as the pressure increases. Both of these results are in accord with expectations for the process.

2

Fig. E3.5

✓ Skills Developed Ability to… ❑ apply the closed system

energy balance.

➊ Using the Browse button, the computer solution indicates that the pressure for which the quality becomes unity is 2.096 bar. Thus, for pressures ranging from 1 to 2 bar, all of the states are in the two-phase liquid–vapor region.

If heating continues at constant specific volume to a state where the pressure is 3 bar, modify the IT program to give the temperature at that state, in 8C. Ans. v4 5 v1 p4 5 3//bar v4 5 v_PT (“Water/Steam”, p4, T4) T4 5 282.48C

❑ use IT to retrieve property

data for water and plot calculated data.

104

Chapter 3 Evaluating Properties

BIOCONNECTIONS What do first responders, military flight crews, costumed characters at theme parks, and athletes have in common? They share a need to avoid heat stress while performing their duty, job, and past-time, respectively. To meet this need, wearable coolers have been developed such as cooling vests and cooling collars. Wearable coolers may feature ice pack inserts, channels through which a cool liquid is circulated, encapsulated phase-change materials, or a combination. A familiar example of a phase-change material (PCM) is ice, which on melting at 0°C absorbs energy of about 334 kJ/kg. When worn close to the body, PCM-laced apparel absorbs energy from persons working or exercising in hot environments, keeping them cool. When specifying a PCM for a wearable cooler, the material must change phase at the desired cooler operating temperature. Hydrocarbons known as paraffins are frequently used for such duty. Many coolers available today employ PCM beads with diameters as small as 0.5 microns, encapsulated in a durable polymer shell. Encapsulated phase-change materials also are found in other products.

3.9 Introducing Specific Heats cυ and cp specific heats

Several properties related to internal energy are important in thermodynamics. One of these is the property enthalpy introduced in Sec. 3.6.1. Two others, known as specific heats, are considered in this section. The specific heats, denoted c𝜐 and cp, are particularly useful for thermodynamic calculations involving the ideal gas model introduced in Sec. 3.12. The intensive properties c𝜐 and cp are defined for pure, simple compressible substances as partial derivatives of the functions u(T, υ) and h(T, p), respectively 𝜕u b 𝜕T 𝜐 𝜕h cp 5 a b 𝜕T p c𝜐 5 a

(3.8) (3.9)

where the subscripts υ and p denote, respectively, the variables held fixed during differentiation. Values for c𝜐 and cp can be obtained via statistical mechanics using spectroscopic measurements. They also can be determined macroscopically through exacting property measurements. Since u and h can be expressed either on a unit mass basis or per mole, values of the specific heats can be similarly expressed. SI units are kJ/kg ? K or kJ/kmol ? K. The property k, called the specific heat ratio, is simply the ratio k5

cp c𝜐

(3.10)

The properties cυ and cp are referred to as specific heats (or heat capacities) because under certain special conditions they relate the temperature change of a system to the amount of energy added by heat transfer. However, it is generally preferable to think of c𝜐 and cp in terms of their definitions, Eqs. 3.8 and 3.9, and not with reference to this limited interpretation involving heat transfer. In general, c𝜐 is a function of 𝜐 and T (or p and T ), and cp depends on both p and T (or υ and T). Figure 3.9 shows how cp for water vapor varies as a function of temperature and pressure. The vapor phases of other substances exhibit similar behavior. Note that the figure gives the variation of cp with temperature in the limit as pressure tends to zero. In this limit, cp increases with increasing temperature, which is a characteristic exhibited by other gases as well. We will refer again to such zero-pressure values for c𝜐 and cp in Sec. 3.13.2. Specific heat data are available for common gases, liquids, and solids. Data for gases are introduced in Sec. 3.13.2 as a part of the discussion of the ideal gas model.

3.9 Introducing Specific Heats c𝜐 and cp

105

9

60

8

70

7

80 or ted vap

90 100

Satura

cp , kJ/kg·K

6

5

Pa M 50 0 4

4

60 70 80 90 100 MPa

30 25 20

15 10

3

5 1

2

0 Zero pressure limit

1.5 100

2

200

300

400

500

600

700

800

T, 8C

Fig. 3.9 cp of water vapor as a function of temperature and pressure. Specific heat values for some common liquids and solids are introduced in Sec. 3.10.2 as a part of the discussion of the incompressible substance model.

3.10

Evaluating Properties of Liquids and Solids

Special methods often can be used to evaluate properties of liquids and solids. These methods provide simple, yet accurate, approximations that do not require exact compilations like the compressed liquid tables for water, Tables A-5. Two such special methods are discussed next: approximations using saturated liquid data and the incompressible substance model.

3.10.1 Approximations for Liquids Using Saturated Liquid Data

T

Approximate values for 𝜐, u, and h at liquid states can be obtained using saturated liquid data. To illustrate, refer to the compressed liquid tables, Tables A-5. These tables show that the specific volume and specific internal energy change very little with pressure at a fixed temperature. Because the values of 𝜐 and u vary only gradually as pressure changes at fixed temperature, the following approximations are reasonable for many engineering calculations: 𝜐(T, p) < 𝜐f (T) u(T, p) < uf(T)

(3.11) (3.12)

p = constant Saturated liquid p = constant

f T = constant v(T, p) < vf (T) u(T, p) < uf (T) v v vf

106

Chapter 3 Evaluating Properties That is, for liquids 𝜐 and u may be evaluated at the saturated liquid state corresponding to the temperature at the given state. An approximate value of h at liquid states can be obtained by using Eqs. 3.11 and 3.12 in the definition h 5 u 1 p𝜐; thus h(T, p) < uf(T) 1 p𝜐f(T) This can be expressed alternatively as h(T, p) < hf(T) 1 𝜐f(T)[p 2 psat(T)]

(3.13)

where psat denotes the saturation pressure at the given temperature. The derivation is left as an exercise. When the contribution of the underlined term of Eq. 3.13 is small, the specific enthalpy can be approximated by the saturated liquid value, as for 𝜐 and u. That is h(T, p) < hf(T)

(3.14)

Although the approximations given here have been presented with reference to liquid water, they also provide plausible approximations for other substances when the only liquid data available are for saturated liquid states. In this text, compressed liquid data are presented only for water (Tables A-5). Also note that Interactive Thermodynamics: IT does not provide compressed liquid data for any substance, but uses Eqs. 3.11, 3.12, and 3.14 to return liquid values for 𝜐, u, and h, respectively. When greater accuracy is required than provided by these approximations, other data sources should be consulted for more complete property compilations for the substance under consideration.

3.10.2 Incompressible Substance Model

incompressible substance model

TAKE NOTE...

For a substance modeled as incompressible, 𝜐 5 constant u 5 u (T )

As noted above, there are regions where the specific volume of liquid water varies little and the specific internal energy varies mainly with temperature. The same general behavior is exhibited by the liquid phases of other substances and by solids. The approximations of Eqs. 3.11–3.14 are based on these observations, as is the incompressible substance model under present consideration. To simplify evaluations involving liquids or solids, the specific volume (density) is often assumed to be constant and the specific internal energy assumed to vary only with temperature. A substance idealized in this way is called incompressible. Since the specific internal energy of a substance modeled as incompressible depends only on temperature, the specific heat c is also a function of temperature alone c𝜐(T) 5

du (incompressible) dT

(3.15)

This is expressed as an ordinary derivative because u depends only on T. Although the specific volume is constant and internal energy depends on temperature only, enthalpy varies with both pressure and temperature according to h(T, p) 5 u(T) 1 p𝜐 (incompressible)

(3.16)

For a substance modeled as incompressible, the specific heats cυ and cp are equal. This is seen by differentiating Eq. 3.16 with respect to temperature while holding pressure fixed to obtain a

𝜕h du b 5 𝜕T p dT

The left side of this expression is cp by definition (Eq. 3.9), so using Eq. 3.15 on the right side gives cp 5 c𝜐 (incompressible) (3.17)

3.10 Evaluating Properties of Liquids and Solids

107

Thus, for an incompressible substance it is unnecessary to distinguish between cp and c𝜐, and both can be represented by the same symbol, c. Specific heats of some common liquids and solids are given in Tables A-19. Over limited temperature intervals the variation of c with temperature can be small. In such instances, the specific heat c can be treated as constant without a serious loss of accuracy. Using Eqs. 3.15 and 3.16, the changes in specific internal energy and specific enthalpy between two states are given, respectively, by u 2 2 u1 5

#

T2

c(T) dT

(incompressible)

(3.18)

c(T) dT 1 𝜐(p2 2 p1) (incompressible)

(3.19)

T1

h2 2 h1 5 u2 2 u1 1 𝜐(p2 2 p1) 5

#

T2

T1

If the specific heat c is taken as constant, Eqs. 3.18 and 3.19 become, respectively, u2 2 u1 5 c(T2 2 T1) h2 2 h1 5 c(T2 2 T1) 1 𝜐(p2 2 p1)

(incompressible, constant c)

(3.20a) (3.20b)

In Eq. 3.20b, the underlined term is often small relative to the first term on the right side and then may be dropped. The next example illustrates use of the incompressible substance model in an application involving the constant-volume calorimeter considered in the box on p. 60. cccc

EXAMPLE 3.6 c

Measuring the Calorie Value of Cooking Oil One-tenth milliliter of cooking oil is placed in the chamber of a constant-volume calorimeter filled with sufficient oxygen for the oil to be completely burned. The chamber is immersed in a water bath. The mass of the water bath is 2.15 kg. For the purpose of this analysis, the metal parts of the apparatus are modeled as equivalent to an additional 0.5 kg of water. The calorimeter is well-insulated, and initially the temperature throughout is 25°C. The oil is ignited by a spark. When equilibrium is again attained, the temperature throughout is 25.3°C. Determine the change in internal energy of the chamber contents, in kcal per mL of cooking oil and in kcal per tablespoon of cooking oil. Known: Data are provided for a constant-volume calorimeter testing cooking oil for caloric value. Find: Determine the change in internal energy of the contents of the calorimeter chamber. Schematic and Given Data: Engineering Model:

Thermometer

1. The closed system is shown by the dashed line in Insulation

– +

the accompanying figure. 2. The total volume remains constant, including the

chamber, water bath, and the amount of water modeling the metal parts. 3. Water is modeled as incompressible with constant

O2

specific heat c.

Igniter

4. Heat transfer with the surroundings is negligible,

and there is no change in kinetic or potential energy.

Oil Water bath

Contents

Boundary

Fig. E3.6

108

Chapter 3 Evaluating Properties

Analysis: With the assumptions listed, the closed system energy balance reads

DU 1 DKE 0 1 DPE 0 5 Q 0 2 W 0 or (DU)contents 1 (DU)water 5 0 thus (a)

(DU)contents 5 2(DU)water

The change in internal energy of the contents is equal and opposite to the change in internal energy of the water. Since water is modeled as incompressible, Eq. 3.20a is used to evaluate the right side of Eq. (a), giving ➊➋

(DU)contents 5 2mwcw(T2 2 T1)

(b)

With mw 5 2.15 kg 1 0.5 kg 5 2.65 kg, (T2 2 T1) 5 0.3 K, and cw 5 4.18 kJ/kg ⴢ K from Table A-19, Eq. (b) gives (DU)contents 5 2(2.65 kg)(4.18 kJ/kg ? K)(0.3 K) 5 23.32 kJ Converting to kcal, and expressing the result on a per milliliter of oil basis using the oil volume, 0.1 mL, we get (DU)contents 23.32 kJ 1 kcal 5 ` ` Voil 0.1 mL 4.1868 kJ 5 27.9 kcal/mL The calorie value of the cooking oil is the magnitude—that is, 7.9 kcal/mL. Labels on cooking oil containers usually give calorie value for a serving size of 1 tablespoon (15 mL). Using the calculated value, we get 119 kcal per tablespoon. ➊ The change in internal energy for water can be found alternatively using Eq. 3.12 together with saturated liquid internal energy data from Table A-2. ➋ The change in internal energy of the chamber contents cannot be evaluated using a specific heat because specific heats are defined (Sec. 3.9) only for pure substances—that is, substances that are unchanging in composition.

✓ Skills Developed Ability to… ❑ define a closed system and

identify interactions within it and on its boundary. ❑ apply the energy balance using the incompressible substance model.

Using Eq. 3.12 together with saturated liquid internal energy data from Table A-2, find the change in internal energy of the water, in kJ, and compare with the value obtained assuming water is incompressible. Ans. 3.32 kJ

BIOCONNECTIONS Is your diet bad for the environment? It could be. The fruits, vegetables, and animal products found in grocery stores require a lot of fossil fuel just to get there. While study of the linkage of the human diet to the environment is in its infancy, some preliminary findings are interesting. One study of U.S. dietary patterns evaluated the amount of fossil fuel—and implicitly, the level of greenhouse gas production—required to support several different diets. Diets rich in meat and fish were found to require the most fossil fuel, owing to the significant energy resources required to produce these products and bring them to market. But for those who enjoy meat and fish, the news is not all bad. Only a fraction of the fossil fuel needed to get food to stores is used to grow the food; most is spent on processing and distribution. Accordingly, eating favorite foods originating close to home can be a good choice environmentally. Still, the connection between the food we eat, energy resource use, and accompanying environmental impact requires further study, including the vast amounts of agricultural land needed, huge water requirements, emissions related to fertilizer production and use, methane emitted from waste produced by billions of animals raised for food annually, and fuel for transporting food to market.

3.10 Evaluating Properties of Liquids and Solids

3.11

109

Generalized Compressibility Chart

The object of the present section is to gain a better understanding of the relationship among pressure, specific volume, and temperature of gases. This is important not only as a basis for analyses involving gases but also for the discussions of the second part of the chapter, where the ideal gas model is introduced. The current presentation is conducted in terms of the compressibility factor and begins with the introduction of the universal gas constant.

pv T

Measured data extrapolated to zero pressure

T1 T2

R T3

3.11.1 Universal Gas Constant, R

T4

Let a gas be confined in a cylinder by a piston and the entire assembly held at a constant temperature. The piston can be moved to various positions so that a series of equilibrium states at constant temperature can be visited. Suppose the pressure and specific volume are measured at each state and the value of the ratio p𝜐/T (𝜐 is volume per mole) determined. These ratios can then be plotted versus pressure at constant temperature. The results for several temperatures are sketched in Fig. 3.10. When the ratios are extrapolated to zero pressure, precisely the same limiting value is obtained for each curve. That is, p𝜐 5R pS0 T lim

Fig. 3.10 Sketch of p𝜐 T versus pressure for a gas at several specified values of temperature.

(3.21)

where R denotes the common limit for all temperatures. If this procedure were repeated for other gases, it would be found in every instance that the limit of the ratio p𝜐/T as p tends to zero at fixed temperature is the same, namely R. Since the same limiting value is exhibited by all gases, R is called the universal gas constant. Its value as determined experimentally is R 5 8.314 kJ/kmol ? K

p

universal gas constant

(3.22)

Having introduced the universal gas constant, we turn next to the compressibility factor.

3.11.2 Compressibility Factor, Z The dimensionless ratio p𝜐/RT is called the compressibility factor and is denoted by Z. That is, Z5

p𝜐 RT

(3.23)

As illustrated by subsequent calculations, when values for p, 𝜐, R, and T are used in consistent units, Z is unitless. With 𝜐 5 Mυ (Eq. 1.9), where M is the atomic or molecular weight, the compressibility factor can be expressed alternatively as Z5

p𝜐 RT

(3.24)

R5

R M

(3.25)

where

compressibility factor

110

Chapter 3 Evaluating Properties TABLE 3.1

Values of the Gas Constant R of Selected Elements and Compounds Substance

Chemical Formula

R (kJ/kg ? K)

— NH3 Ar CO2 CO He H2 CH4 N2 O2 H2O

0.2870 0.4882 0.2082 0.1889 0.2968 2.0769 4.1240 0.5183 0.2968 0.2598 0.4614

Air Ammonia Argon Carbon dioxide Carbon monoxide Helium Hydrogen Methane Nitrogen Oxygen Water

– Source: R values are calculated in terms of the universal gas constant R 5 8.314 kJ/kmol – and the molecular weight M provided in Table A-1 using R 5 R/M (Eq. 3.25).

1.5

K 5 1.986 Btu/Ibmol

? 8R

R is a constant for the particular gas whose molecular weight is M. Alternative units for R are kJ/kg ⴢ K, Btu/lb ⴢ 8R, and ft ⴢ lbf/lb ⴢ 8R. Table 3.1 provides a sampling of values for the gas constant R calculated from Eq. 3.25. Equation 3.21 can be expressed in terms of the compressibility factor as

35 K

100 K

?

50 K 60 K 200 K

1.0

lim Z 5 1

300 K

pS0

Z

(3.26)

That is, the compressibility factor Z tends to unity as pressure tends to zero at fixed temperature. This can be illustrated by reference to Fig. 3.11, which shows Z for hydrogen plotted versus pressure at a number of different temperatures. In general, at states of a gas where pressure is small relative to the critical pressure, Z is approximately 1.

0.5

3.11.3 Generalized Compressibility Data, Z Chart 0

100 p (atm)

200

Figure 3.11 gives the compressibility factor for hydrogen versus pressure at specified values of temperature. Similar charts have been Fig. 3.11 Variation of the compressibility prepared for other gases. When these charts are studied, they are factor of hydrogen with pressure at found to be qualitatively similar. Further study shows that when the constant temperature. coordinates are suitably modified, the curves for several different gases coincide closely when plotted together on the same coordinate axes, and so quantitative similarity also can be achieved. This is referred to as the principle of corresponding states. In one such approach, the compressibility factor Z is plotted versus a dimensionless reduced pressure pR and reduced temperature TR, defined as reduced pressure and temperature

generalized compressibility chart

pR 5 p/pc

(3.27)

TR 5 T/Tc

(3.28)

where pc and Tc denote the critical pressure and temperature, respectively. This results in a generalized compressibility chart of the form Z 5 f(pR, TR). Figure 3.12 shows experimental data for 10 different gases on a chart of this type. The solid lines denoting reduced isotherms represent the best curves fitted to the data. Observe that Tables A-1 provide the critical temperature and critical pressure for a sampling of substances.

3.11 Generalized Compressibility Chart

111

1.1 TR = 2.00

1.0 0.9

TR = 1.50

pv pv Z = ––– = ––– RT RT

0.8 TR = 1.30

0.7

TR = 1.20

0.6 0.5

Legend Methane Isopentane Ethylene n-Heptane Ethane Nitrogen Propane Carbon dioxide n-Butane Water Average curve based on data on hydrocarbons

TR = 1.10

0.4 TR = 1.00

0.3 0.2 0.1

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Reduced pressure pR

Fig. 3.12 Generalized compressibility chart for various gases.

A generalized chart more suitable for problem solving than Fig. 3.12 is given in the Appendix as Figs. A-1, A-2, and A-3. In Fig. A-1, pR ranges from 0 to 1.0; in Fig. A-2, pR ranges from 0 to 10.0; and in Fig. A-3, pR ranges from 10.0 to 40.0. At any one temperature, the deviation of observed values from those of the generalized chart increases with pressure. However, for the 30 gases used in developing the chart, the deviation is at most on the order of 5% and for most ranges is much less.1 Values of specific volume are included on the generalized chart through the variable υ9R, called the pseudoreduced specific volume, defined by 𝜐¿R 5

𝜐 RTcypc

pseudoreduced specific volume

(3.29)

The pseudoreduced specific volume gives a better correlation of the data than does the reduced specific volume 𝜐R 5 𝜐/𝜐c, where 𝜐c is the critical specific volume. Using the critical pressure and critical temperature of a substance of interest, the generalized chart can be entered with various pairs of the variables TR, pR, and 𝜐9R: (TR, pR), (pR, 𝜐9R), or (TR, 𝜐9R). The merit of the generalized chart for relating p, 𝜐, and T data for gases is simplicity coupled with accuracy. However, the generalized compressibility chart should not be used as a substitute for p–𝜐–T data for a given substance as provided by tables or computer software. The chart is mainly useful for obtaining reasonable estimates in the absence of more accurate data. The next example provides an illustration of the use of the generalized compressibility chart.

1

TAKE NOTE...

Study of Fig. A-2 shows that the value of Z tends to unity at fixed reduced temperature TR as reduced pressure pR tends to zero. That is, Z S 1 as pR S 0 at fixed TR. Figure A-2 also shows that Z tends to unity at fixed reduced pressure as reduced temperature becomes large.

To determine Z for hydrogen, helium, and neon above a TR of 5, the reduced temperature and pressure should be calculated using TR 5 T/(Tc 1 8) and pR 5 p/(pc 1 8), where temperatures are in K and pressures are in atm.

Ideal_Gas A.15 – Tab a

A

112

EXAMPLE 3.7 c

Using the Generalized Compressibility Chart A closed, rigid tank filled with water vapor, initially at 20 MPa, 5208C, is cooled until its temperature reaches 4008C. Using the compressibility chart, determine (a) the specific volume of the water vapor in m3/kg at the initial state. (b) the pressure in MPa at the final state.

Compare the results of parts (a) and (b) with the values obtained from the superheated vapor table, Table A-4. SOLUTION Known: Water vapor is cooled at constant volume from 20 MPa, 5208C to 4008C. Find: Use the compressibility chart and the superheated vapor table to determine the specific volume and final

pressure and compare the results. Schematic and Given Data: p1 = 20 MPa T1 = 520°C T2 = 400°C Closed, rigid tank

1.0 1

Z1

2

Water vapor TR = 1.05

Engineering Model: 1. The water vapor is

a closed system. 2. The initial and

TR = 1.3 TR = 1.2

v´R = 1.2 v´R = 1.1

pv Z = ––– RT

cccc

Chapter 3 Evaluating Properties

Cooling

final states are at equilibrium. 3. The volume is

constant.

pR2

0.5

Block of ice

0

0.5 pR

1.0

Fig. E3.7 Analysis: (a) From Table A-1, Tc 5 647.3 K and pc 5 22.09 MPa for water. Thus

793 20 5 1.23, pR1 5 5 0.91 647.3 22.09 With these values for the reduced temperature and reduced pressure, the value of Z obtained from Fig. A-1 is approximately 0.83. Since Z 5 p𝜐/RT, the specific volume at state 1 can be determined as follows: ➊

TR1 5

𝜐1 5 Z 1 ➋

RT1 R T1 5 0.83 p1 Mp1

N?m kmol ? K 793 K 5 0.83 ± ≤ 5 0.0152 m3/kg ° ¢ kg 6 N 18.02 20 3 10 2 kmol m 8314

The molecular weight of water is from Table A-1.

3.11 Generalized Compressibility Chart

113

Turning to Table A-4, the specific volume at the initial state is 0.01551 m3/kg. This is in good agreement with the compressibility chart value, as expected. (b) Since both mass and volume remain constant, the water vapor cools at constant specific volume, and thus

at constant 𝜐9R. Using the value for specific volume determined in part (a), the constant 𝜐9R value is

𝜐¿R 5

𝜐pc 5 RTc

a0.0152

m3 N b a22.09 3 106 2 b kg m

8314 N ? m a b(647.3 K) 18.02 kg ? K

5 1.12

At state 2 TR2 ⫽

673 ⫽ 1.04 647.3

Locating the point on the compressibility chart where 𝜐9R 5 1.12 and TR 5 1.04, the corresponding value for pR is about 0.69. Accordingly p2 5 pc(pR2) 5 (22.09 MPa)(0.69) 5 15.24 MPa Interpolating in the superheated vapor tables gives p2 5 15.16 MPa. As before, the compressibility chart value is in good agreement with the table value. ➊ Absolute temperature and absolute pressure must be used in evaluating the compressibility factor Z, the reduced temperature TR, and reduced pressure pR. ➋ Since Z is unitless, values for p, 𝜐, R, and T must be used in consistent units.

✓ Skills Developed Ability to… ❑ retrieve p–𝜐–T data from the

generalized compressibility chart. ❑ retrieve p–𝜐–T data from the steam tables.

Using the compressibility chart, determine the specific volume, in m3/kg, for water vapor at 14 MPa, 4408C. Compare with the steam table value. Ans. 0.0195 m3/kg

3.11.4 Equations of State Considering the curves of Figs. 3.11 and 3.12, it is reasonable to think that the variation with pressure and temperature of the compressibility factor for gases might be expressible as an equation, at least for certain intervals of p and T. Two expressions can be written that enjoy a theoretical basis. One gives the compressibility factor as an infinite series expansion in pressure: ^ (T)p 1 C ^(T)p2 1 D ^ (T)p3 1 . . . Z511B

(3.30)

^, C ^, D ^ , . . . depend on temperature only. The dots in Eq. 3.30 where the coefficients B represent higher-order terms. The other is a series form entirely analogous to Eq. 3.30 but expressed in terms of 1/ 𝜐 instead of p Z511

B(T) 𝜐

1

C(T) 𝜐

2

1

D(T) 𝜐3

1...

(3.31)

Equations 3.30 and 3.31 are known as virial equations of state, and the coefficients ^, C ^, D ^ , . . . and B, C, D, . . . are called virial coefficients. The word virial stems from B the Latin word for “force.” In the present usage it is force interactions among molecules that are intended.

virial equations of state

114

Chapter 3 Evaluating Properties The virial expansions can be derived by the methods of statistical mechanics, and physical significance can be attributed to the coefficients: B/ 𝜐 accounts for twomolecule interactions, C/ 𝜐2 accounts for three-molecule interactions, etc. In principle, the virial coefficients can be calculated by using expressions from statistical mechanics derived from consideration of the force fields around the molecules of a gas. The virial coefficients also can be determined from experimental p–𝜐–T data. The virial expansions are used in Sec. 11.1 as a point of departure for the further study of analytical representations of the p–𝜐–T relationship of gases known generically as equations of state. The virial expansions and the physical significance attributed to the terms making up the expansions can be used to clarify the nature of gas behavior in the limit as pressure tends to zero at fixed temperature. From Eq. 3.30 it is seen that if pressure ^ p, C ^p2, etc., accounting for various molecdecreases at fixed temperature, the terms B ular interactions tend to decrease, suggesting that the force interactions become weaker under these circumstances. In the limit as pressure approaches zero, these terms vanish, and the equation reduces to Z 5 1 in accordance with Eq. 3.26. Similarly, since specific volume increases when the pressure decreases at fixed temperature, the terms B/ 𝜐, C/ 𝜐2, etc., of Eq. 3.31 also vanish in the limit, giving Z 5 1 when the force interactions between molecules are no longer significant.

Evaluating Properties Using the Ideal Gas Model 3.12

Introducing the Ideal Gas Model

In this section the ideal gas model is introduced. The ideal gas model has many applications in engineering practice and is frequently used in subsequent sections of this text.

3.12.1 Ideal Gas Equation of State As observed in Sec. 3.11.3, study of the generalized compressibility chart Fig. A-2 shows that at states where the pressure p is small relative to the critical pressure pc (low pR) and/or the temperature T is large relative to the critical temperature Tc (high TR), the compressibility factor, Z 5 pυ/RT, is approximately 1. At such states, we can assume with reasonable accuracy that Z 5 1, or p𝜐 5 RT ideal gas equation of state

(3.32)

Known as the ideal gas equation of state, Eq. 3.32 underlies the second part of this chapter dealing with the ideal gas model. Alternative forms of the same basic relationship among pressure, specific volume, and temperature are obtained as follows. With 𝜐 5 V/m, Eq. 3.32 can be expressed as pV ⫽ mRT

(3.33)

In addition, using 𝜐 5 𝜐/M and R ⫽ R / M, which are Eqs. 1.9 and 3.25, respectively, where M is the molecular weight, Eq. 3.32 can be expressed as

A

Ideal_Gas A.15 – Tab b

p𝜐 5 RT

(3.34)

pV ⫽ nRT

(3.35)

or, with 𝜐 5 V/n, as

3.12 Introducing the Ideal Gas Model

115

3.12.2 Ideal Gas Model For any gas whose equation of state is given exactly by p𝜐 5 RT, the specific internal energy depends on temperature only. This conclusion is demonstrated formally in Sec. 11.4. It is also supported by experimental observations, beginning with the work of Joule, who showed in 1843 that the internal energy of air at low density (large specific volume) depends primarily on temperature. Further motivation from the microscopic viewpoint is provided shortly. The specific enthalpy of a gas described by p𝜐 5 RT also depends on temperature only, as can be shown by combining the definition of enthalpy, h 5 u 1 p𝜐, with u 5 u(T) and the ideal gas equation of state to obtain h 5 u(T) 1 RT. Taken together, these specifications constitute the ideal gas model, summarized as follows: p𝜐 5 RT u 5 u(T) h 5 h(T) 5 u(T) 1 RT

(3.32) (3.36) (3.37)

The specific internal energy and enthalpy of gases generally depend on two independent properties, not just temperature as presumed by the ideal gas model. Moreover, the ideal gas equation of state does not provide an acceptable approximation at all states. Accordingly, whether the ideal gas model is used depends on the error acceptable in a given calculation. Still, gases often do approach ideal gas behavior, and a particularly simplified description is obtained with the ideal gas model. To verify that a gas can be modeled as an ideal gas, the states of interest can be located on a compressibility chart to determine how well Z 5 1 is satisfied. As shown in subsequent discussions, other tabular or graphical property data can also be used to determine the suitability of the ideal gas model. The next example illustrates the use of the ideal gas equation of state and reinforces the use of property diagrams to locate principal states during processes.

cccc

ideal gas model

TAKE NOTE...

To expedite the solutions of many subsequent examples and end-of-chapter problems involving air, oxygen (O2), nitrogen (N2), carbon dioxide (CO2), carbon monoxide (CO), hydrogen (H2), and other common gases, we indicate in the problem statements that the ideal gas model should be used. If not indicated explicitly, the suitability of the ideal gas model should be checked using the Z chart or other data.

EXAMPLE 3.8 c

Air as an Ideal Gas Undergoing a Cycle One pound of air in a piston–cylinder assembly undergoes a thermodynamic cycle consisting of three processes. Process 1–2:

Constant specific volume

Process 2–3:

Constant-temperature expansion

Process 3–1:

Constant-pressure compression

At state 1, the temperature is 300 K, and the pressure is 1 atm. At state 2, the pressure is 2 bars. Employing the ideal gas equation of state, (a) sketch the cycle on p–𝜐 coordinates. (b) determine the temperature at state 2, in K. (c) determine the specific volume at state 3, in m3/kg. SOLUTION Known: Air executes a thermodynamic cycle consisting of three processes: Process 1–2, 𝜐 5 constant; Process 2–3,

T 5 constant; Process 3–1, p 5 constant. Values are given for T1, p1, and p2.

116

Chapter 3 Evaluating Properties

Find: Sketch the cycle on p–𝜐 coordinates and determine T2 and 𝜐3. Schematic and Given Data: p

Engineering Model: 1. The air is a closed system.

➊ 2. The air behaves as an ideal gas. 2

3. Volume change is the only work mode.

p2 = 2 bar

v=C

T=C

1 p1 = 1 bar

p=C

3 600 K

300 K v

Fig. E3.8 Analysis: (a) The cycle is shown on p–𝜐 coordinates in the accompanying figure. Note that since p 5 RT/𝜐 and temperature is constant, the variation of p with 𝜐 for the process from 2 to 3 is nonlinear. (b) Using p𝜐 5 RT, the temperature at state 2 is

T2 5 p2𝜐2 yR To obtain the specific volume 𝜐2 required by this relationship, note that 𝜐2 5 𝜐1, so ➋

𝜐2 5 RT1 yp1

Combining these two results gives T2 5

p2 p1

T1 5 a

2 bars b(300 K) 5 600 K 1 bar

(c) Since p𝜐 5 RT, the specific volume at state 3 is

𝜐3 5 RT3 yp3 Noting that T3 5 T2, p3 5 p1, and R ⫽ R/ M 𝜐3 5

RT2 Mp1

5 ±

kJ kmol ? J 600 k 1 bar 103 N ~ m ≤a ba 5 b a b kg 1 bar 10 N/m2 1 kJ 28.97 mol

8.314

5 1.72 m3/kg where the molecular weight of air is from Table A-1.

3.12 Introducing the Ideal Gas Model

117

➊ Table A-1 gives pc 5 37.3 bar, Tc 5 133 K for air. Therefore, pR2 5 0.054, TR2 5 4.52. Referring to Fig. A-1, the value of the compressibility factor at this state is Z ≈ 1. The same conclusion results when states 1 and 3 are checked. Accordingly, pυ 5 RT adequately describes the p–𝜐–T relation for the air at these states.

❑ evaluate p–υ–T data using the

➋ Carefully note that the equation of state p𝜐 5 RT requires the use of absolute temperature T and absolute pressure p.

❑ sketch processes on a p–υ

Is the cycle sketched in Fig. E3.8 a power cycle or a refrigeration cycle? Explain. Ans. A power cycle. As represented by enclosed area 1-2-3-1, the net work is positive.

3.12.3 Microscopic Interpretation A picture of the dependence of the internal energy of gases on temperature at low density (large specific volume) can be obtained with reference to the discussion of the virial equations: Eqs. 3.30 and 3.31. As p S 0 (𝜐 S `), the force interactions between molecules of a gas become weaker, and the virial expansions approach Z 5 1 in the limit. The study of gases from the microscopic point of view shows that the dependence of the internal energy of a gas on pressure, or specific volume, at a specified temperature arises primarily because of molecular interactions. Accordingly, as the density of a gas decreases (specific volume increases) at fixed temperature, there comes a point where the effects of intermolecular forces are minimal. The internal energy is then determined principally by the temperature. From the microscopic point of view, the ideal gas model adheres to several idealizations: The gas consists of molecules that are in random motion and obey the laws of mechanics; the total number of molecules is large, but the volume of the molecules is a negligibly small fraction of the volume occupied by the gas; and no appreciable forces act on the molecules except during collisions. Further discussion of the ideal gas using the microscopic approach is provided in Sec. 3.13.2.

3.13

Internal Energy, Enthalpy, and Specific Heats of Ideal Gases

3.13.1 Du, Dh, c𝝊, and cp Relations For a gas obeying the ideal gas model, specific internal energy depends only on temperature. Hence, the specific heat c𝜐, defined by Eq. 3.8, is also a function of temperature alone. That is, c𝜐(T) 5

du (ideal gas) dT

(3.38)

This is expressed as an ordinary derivative because u depends only on T. By separating variables in Eq. 3.38 du 5 c𝜐(T) dT

(3.39)

✓ Skills Developed Ability to…

ideal gas equation of state.

diagram

118

Chapter 3 Evaluating Properties On integration, the change in specific internal energy is u(T2) 2 u(T1) 5

#

T2

c𝜐(T) dT (ideal gas)

(3.40)

T1

Similarly, for a gas obeying the ideal gas model, the specific enthalpy depends only on temperature, so the specific heat cp, defined by Eq. 3.9, is also a function of temperature alone. That is cp(T) 5

dh (ideal gas) dT

(3.41)

Separating variables in Eq. 3.41 dh 5 cp(T) dT

(3.42)

On integration, the change in specific enthalpy is h(T2) 2 h(T1) 5

#

T2

cp(T) dT (ideal gas)

(3.43)

T1

An important relationship between the ideal gas specific heats can be developed by differentiating Eq. 3.37 with respect to temperature dh du ⫽ ⫹R dT dT and introducing Eqs. 3.38 and 3.41 to obtain cp(T) 5 c𝜐(T) 1 R (ideal gas)

(3.44)

On a molar basis, this is written as cp(T) 5 c𝜐(T) 1 R (ideal gas)

(3.45)

Although each of the two ideal gas specific heats is a function of temperature, Eqs. 3.44 and 3.45 show that the specific heats differ by just a constant: the gas constant. Knowledge of either specific heat for a particular gas allows the other to be calculated by using only the gas constant. The above equations also show that cp . c𝜐 and cp . c𝜐 , respectively. For an ideal gas, the specific heat ratio, k, is also a function of temperature only k5

cp(T) c𝜐(T)

(ideal gas)

(3.46)

Since cp . c𝜐, it follows that k . 1. Combining Eqs. 3.44 and 3.46 results in cp(T) 5

kR k21

(3.47a) (ideal gas)

c𝜐(T) 5

R k21

(3.47b)

Similar expressions can be written for the specific heats on a molar basis, with R being replaced by R.

3.13 Internal Energy, Enthalpy, and Specific Heats of Ideal Gases CO2 7

H2O

6

cp 5

O2

R

CO 4

H2

Air

3 Ar, Ne, He 0

1000

2000

3000

Temperature, K

–

Fig. 3.13 Variation of c– p /R with temperature for a number of gases modeled as ideal gases.

3.13.2 Using Specific Heat Functions The foregoing expressions require the ideal gas specific heats as functions of temperature. These functions are available for gases of practical interest in various forms, including graphs, tables, and equations. Figure 3.13 illustrates the variation of cp (molar basis) with temperature for a number of common gases. In the range of temperature shown, cp increases with temperature for all gases, except for the monatonic gases Ar, Ne, and He. For these, cp is constant at the value predicted by kinetic theory: cp ⫽ 52 R. Tabular specific heat data for selected gases are presented versus temperature in Tables A-20. Specific heats are also available in equation form. Several alternative forms of such equations are found in the engineering literature. An equation that is relatively easy to integrate is the polynomial form cp R

5 𝛼 1 𝛽T 1 𝛾T 2 1 𝛿T 3 1 𝜀T 4

(3.48)

Values of the constants α, β, δ, and ε are listed in Tables A-21 for several gases in the temperature range 300 to 1000 K. to illustrate the use of Eq. 3.48, let us evaluate the change in specific enthalpy, in kJ/kg, of air modeled as an ideal gas from a state where T1 5 400 K to a state where T2 5 900 K. Inserting the expression for cp(T) given by Eq. 3.48 into Eq. 3.43 and integrating with respect to temperature h 2 2 h1 5

5

R M

#

T2

(𝛼 1 𝛽T 1 𝛾T 2 1 𝛿T 3 1 𝜀T 4 )dT

T1

𝛾 𝛽 R 𝛿 𝜀 c 𝛼(T2 2 T1) 1 (T 22 2 T 21) 1 (T 32 2 T 31) 1 (T 42 2 T 41) 1 (T 52 2 T 51) d M 2 3 4 5

119

120

Chapter 3 Evaluating Properties where the molecular weight M has been introduced to obtain the result on a unit mass basis. With values for the constants from Table A-21 h2 2 h1 5

8.314 1.337 e3.653(900 2 400) 2 [(900)2 2 (400)2] 28.97 2(10)3 3.294 1.913 1 [(900)3 2 (400)3] 2 [(900)4 2 (400)4] 9 6 4(10) 3(10) 1

0.2763 [(900)5 2 (400)5]f 5 531.69 kJ/kg 12 5(10)

b b b b b

Specific heat functions cʋ(T) and cp(T) are also available in IT: Interactive Thermodynamics in the PROPERTIES menu. These functions can be integrated using the integral function of the program to calculate Du and Dh, respectively. let us repeat the immediately preceding example using IT. For air, the IT code is cp 5 cp_T (“Air”,T) delh 5 Integral(cp,T) Pushing SOLVE and sweeping T from 400 K to 900 K, the change in specific enthalpy is delh 5 531.7 kJ/kg, which agrees closely with the value obtained by integrating the specific heat function from Table A-21, as illustrated above. b b b b b The source of ideal gas specific heat data is experiment. Specific heats can be determined macroscopically from painstaking property measurements. In the limit as pressure tends to zero, the properties of a gas tend to merge into those of its ideal gas model, so macroscopically determined specific heats of a gas extrapolated to very low pressures may be called either zero-pressure specific heats or ideal gas specific heats. Although zero-pressure specific heats can be obtained by extrapolating macroscopically determined experimental data, this is rarely done nowadays because ideal gas specific heats can be readily calculated with expressions from statistical mechanics by using spectral data, which can be obtained experimentally with precision. The determination of ideal gas specific heats is one of the important areas where the microscopic approach contributes significantly to the application of engineering thermodynamics.

3.14

Applying the Energy Balance Using Ideal Gas Tables, Constant Specific Heats, and Software

Although changes in specific enthalpy and specific internal energy can be obtained by integrating specific heat expressions, as illustrated in Sec. 3.13.2, such evaluations are more easily conducted using ideal gas tables, the assumption of constant specific heats, and computer software, all introduced in the present section. These procedures are also illustrated in the present section via solved examples using the closed system energy balance.

3.14.1 Using Ideal Gas Tables For a number of common gases, evaluations of specific internal energy and enthalpy changes are facilitated by the use of the ideal gas tables, Tables A-22 and A-23, which give u and h (or u and h) versus temperature.

3.14 Applying the Energy Balance Using Ideal Gas Tables, Constant Specific Heats, and Software

121

To obtain enthalpy versus temperature, write Eq. 3.43 as h(T) 5

#

T

cp(T) dT 1 h(Tref)

Tref

where Tref is an arbitrary reference temperature and h(Tref) is an arbitrary value for enthalpy at the reference temperature. Tables A-22 and A-23 are based on the selection h 5 0 at Tref 5 0 K. Accordingly, a tabulation of enthalpy versus temperature is developed through the integral2 T

h(T) 5

# c (T) dT

(3.49)

p

0

Tabulations of internal energy versus temperature are obtained from the tabulated enthalpy values by using u 5 h 2 RT. For air as an ideal gas, h and u are given in Table A-22 with units of kJ/kg. Values of molar specific enthalpy h and internal energy u for several other common gases modeled as ideal gases are given in Tables A-23 with units of kJ/kmol or Btu/lbmol. Quantities other than specific internal energy and enthalpy appearing in these tables are introduced in Chap. 6 and should be ignored at present. Tables A-22 and A-23 are convenient for evaluations involving ideal gases, not only because the variation of the specific heats with temperature is accounted for automatically but also because the tables are easy to use. The next example illustrates the use of the ideal gas tables, together with the closed system energy balance. 2

The simple specific heat variation given by Eq. 3.48 is valid only for a limited temperature range, so tabular enthalpy values are calculated from Eq. 3.49 using other expressions that enable the integral to be evaluated accurately over wider ranges of temperature.

cccc

EXAMPLE 3.9 c

Using the Energy Balance and Ideal Gas Tables A piston–cylinder assembly contains 0.9 kg of air at a temperature of 300 K and a pressure of 1 bar. The air is compressed to a state where the temperature is 470 K and the pressure is 6 bars. During the compression, there is a heat transfer from the air to the surroundings equal to 20 kJ. Using the ideal gas model for air, determine the work during the process, in kJ. SOLUTION Known: 0.9 kg of air are compressed between two specified states while there is heat transfer from the air of a known amount. Find: Determine the work, in kJ. Schematic and Given Data: Engineering Model:

p 2

1. The air is a closed system.

p2 = 6 bars

2. The initial and final states

➊

T2 = 470 K p1 = 1 bar

1

T1 = 300 K v

Fig. E3.9

are equilibrium states. There is no change in kinetic or potential energy.

0.91 kg of air

➋

3. The air is modeled as an

ideal gas.

122

Chapter 3 Evaluating Properties

Analysis: An energy balance for the closed system is

DKE 0 1 DPE 0 1 DU 5 Q 2 W where the kinetic and potential energy terms vanish by assumption 2. Solving for W ➌

W 5 Q 2 DU 5 Q 2 m(u2 2 u1)

From the problem statement, Q 5 220 kJ. Also, from Table A-22, at T1 5 300 K, u1 5 214.07 kJ/kg, and at T2 5 470 K, u2 5 337.32 kJ/kg. Accordingly W 5 2 20 kJ 2 (0.9 kg)(337.32 2 214.07) 5 2130.9 kJ The minus sign indicates that work is done on the system in the process. ➊ Although the initial and final states are assumed to be equilibrium states, the intervening states are not necessarily equilibrium states, so the process has been indicated on the accompanying p–𝜐 diagram by a dashed line. This dashed line does not define a “path” for the process.

✓ Skills Developed Ability to… ❑ define a closed system and

➋ Table A-1 gives pc 5 37.7 bars, Tc 5 133 K for air. Therefore, at state 1, pR1 5 0.03, TR1 5 2.26, and at state 2, pR2 5 0.16, TR2 5 3.51. Referring to Fig. A-1, we conclude that at these states Z ⬇ 1, as assumed in the solution.

identify interactions on its boundary. ❑ apply the energy balance using the ideal gas model.

➌ In principle, the work could be evaluated through 兰 p dV, but because the variation of pressure at the piston face with volume is not known, the integration cannot be performed without more information. Replacing air by carbon dioxide, but keeping all other problem statement details the same, evaluate work, in kJ. Ans. 2131.99 kJ

3.14.2 Using Constant Specific Heats When the specific heats are taken as constants, Eqs. 3.40 and 3.43 reduce, respectively, to u(T2) 2 u(T1) 5 c𝜐(T2 2 T1) h(T2) 2 h(T1) 5 cp(T2 2 T1)

(3.50) (3.51)

Equations 3.50 and 3.51 are often used for thermodynamic analyses involving ideal gases because they enable simple closed-form equations to be developed for many processes. The constant values of c𝜐 and cp in Eqs. 3.50 and 3.51 are, strictly speaking, mean values calculated as follows:

# c𝜐 5

T2

#

c𝜐(T) dT

T1

T2 2 T1

, cp 5

T2

cp(T) dT

T1

T2 2 T1

However, when the variation of c𝜐 or cp over a given temperature interval is slight, little error is normally introduced by taking the specific heat required by Eq. 3.50 or 3.51 as the arithmetic average of the specific heat values at the two end temperatures. Alternatively, the specific heat at the average temperature over the

3.14 Applying the Energy Balance Using Ideal Gas Tables, Constant Specific Heats, and Software

123

interval can be used. These methods are particularly convenient when tabular specific heat data are available, as in Tables A-20, for then the constant specific heat values often can be determined by inspection. assuming the specific heat cʋ is a constant and using Eq. 3.50, the expression for work in the solution of Example 3.9 reads W 5 Q 2 mc𝜐(T2 2 T1) Evaluating cʋ at the average temperature, 383 K (110°C), Table A-20 gives cʋ 5 0.721 kJ/kg ? K. Inserting this value for cʋ together with other data from Example 3.9 W 5 220 kJ 2 (0.9 kg)(0.721 kJ/kg ? K)(470 2 300) K 5 2130.31 kJ which agrees closely with the answer obtained in Example 3.9 using Table A-22 data. b b b b b The following example illustrates the use of the closed-system energy balance, together with the ideal gas model and the assumption of constant specific heats.

cccc

EXAMPLE 3.10 c

Using the Energy Balance and Constant Specific Heats Two tanks are connected by a valve. One tank contains 2 kg of carbon monoxide gas at 77°C and 0.7 bar. The other tank holds 8 kg of the same gas at 27°C and 1.2 bar. The valve is opened and the gases are allowed to mix while receiving energy by heat transfer from the surroundings. The final equilibrium temperature is 42°C. Using the ideal gas model with constant cʋ, determine (a) the final equilibrium pressure, in bar, (b) the heat transfer for the process, in kJ. SOLUTION Known: Two tanks containing different amounts of carbon monoxide gas at initially different states are connected by a valve. The valve is opened and the gas allowed to mix while receiving energy by heat transfer. The final equilibrium temperature is known. Find: Determine the final pressure and the heat transfer for the process. Schematic and Given Data: Engineering Model: Carbon monoxide

Carbon monoxide 2 kg, 77°C, 0.7 bar Tank 1

Fig. E3.10

Valve

8 kg, 27°C, 1.2 bar

1. The total amount of carbon monoxide gas is a closed system. 2. The gas is modeled as an ideal gas with constant c𝜐.

3. The gas initially in each tank is in equilibrium. The final state

is an equilibrium state. Tank 2

➊

4. No energy is transferred to, or from, the gas by work. 5. There is no change in kinetic or potential energy.

Analysis: (a) The final equilibrium pressure pf can be determined from the ideal gas equation of state

pf 5

mRTf V

124

Chapter 3 Evaluating Properties

where m is the sum of the initial amounts of mass present in the two tanks, V is the total volume of the two tanks, and Tf is the final equilibrium temperature. Thus (m1 1 m2)RTf V1 1 V2 Denoting the initial temperature and pressure in tank 1 as T1 and p1, respectively, V1 5 m1RT1/p1. Similarly, if the initial temperature and pressure in tank 2 are T2 and p2, V2 5 m2RT2/p2. Thus, the final pressure is pf 5

pf 5

(m1 1 m2)Tf (m1 1 m2)RTf 5 m1RT1 m2RT2 m1T1 m2T2 a b1a b a b1a b p1 p2 p1 p2

Inserting values pf 5

(10 kg)(315 K) 5 1.05 bar (8 kg)(300 K) (2 kg)(350 K) 1 0.7 bar 1.2 bar

(b) The heat transfer can be found from an energy balance, which reduces with assumptions 4 and 5 to give

DU 5 Q 2 W 0 or Q 5 Uf 2 Ui Ui is the initial internal energy, given by Ui 5 m1u(T1) 1 m2u(T2) where T1 and T2 are the initial temperatures of the CO in tanks 1 and 2, respectively. The final internal energy is Uf Uf 5 (m1 1 m2)u(Tf) Introducing these expressions for internal energy, the energy balance becomes Q 5 m1[u(Tf) 2 u(T1)] 1 m2[u(Tf) 2 u(T2)] Since the specific heat c𝜐 is constant (assumption 2) Q 5 m1c𝜐(Tf 2 T1) 1 m2c𝜐(Tf 2 T2) Evaluating c𝜐 as the average of the values listed in Table A-20 at 300 K and 350 K, c𝜐 5 0.745 kJ/kg ⴢ K. Hence Q 5 (2 kg)a0.745

kJ kJ b(315 K 2 350 K) 1 (8 kg)a0.745 b(315 K 2 300 K)) kg ? K kg ? K

5 +37.25 kJ The plus sign indicates that the heat transfer is into the system. ➊ By referring to a generalized compressibility chart, it can be verified that the ideal gas equation of state is appropriate for CO in this range of temperature and pressure. Since the specific heat c𝜐 of CO varies little over the temperature interval from 300 to 350 K (Table A-20), it can be treated as constant with acceptable accuracy. Evaluate Q using specific internal energy values for CO from Table A-23. Compare with the result using constant c𝜐. Ans. 36.99 kJ

✓ Skills Developed Ability to… ❑ define a closed system and

identify interactions on its boundary. ❑ apply the energy balance using the ideal gas model when the specific heat c𝜐 is constant.

3.14 Applying the Energy Balance Using Ideal Gas Tables, Constant Specific Heats, and Software

3.14.3 Using Computer Software Interactive Thermodynamics: IT also provides values of the specific internal energy and enthalpy for a wide range of gases modeled as ideal gases. Let us consider the use of IT, first for air, and then for other gases.

AIR. For air, IT uses the same reference state and reference value as in Table A-22, and the values computed by IT agree closely with table data.

let us use IT to evaluate the change in specific enthalpy of air from a state where T1 5 400 K to a state where T2 5 900 K. Selecting Air from the Properties menu, the following code would be used by IT to determine Dh (delh), in kJ/kg h1 5 h_T(“Air”,T1) h2 5 h_T(“Air”,T2) T1 5 400//K T2 5 900//K delh 5 h2 2 h1 Choosing K for the temperature unit and kg for the amount under the Units menu, the results returned by IT are h1 5 400.8, h2 5 932.5, and Dh 5 531.7 kJ/kg, respectively. These values agree closely with those obtained from Table A-22: h1 5 400.98, h2 5 932.93, and Dh 5 531.95 kJ/kg. b b b b b

OTHER GASES. IT also provides data for each of the gases included in Table A-23. For these gases, the values of specific internal energy u and enthalpy h returned by IT are determined relative to a standard reference state that differs from that used in Table A-23. This equips IT for use in combustion applications; see Sec. 13.2.1 for further discussion. Consequently, the values of u and h returned by IT for the gases of Table A-23 differ from those obtained directly from the table. Still, the property differences between two states remain the same, for datums cancel when differences are calculated.

let us use IT to evaluate the change in specific enthalpy, in kJ/kmol, for carbon dioxide (CO2) as an ideal gas from a state where T1 5 300 K to a state where T2 5 500 K. Selecting CO2 from the Properties menu, the following code would be used by IT: h1 5 h_T(“CO2”,T1) h2 5 h_T(“CO2”,T2) T1 5 300//K T2 5 500//K delh 5 h2 2 h1 Choosing K for the temperature unit and moles for the amount under the Units menu, the results returned by IT are h1 5 23.935 3 105, h2 5 23.852 3 105 , and Dh 5 8238 kJ/mol, respectively. The large negative values for h1 and h2 are a consequence of the reference state and reference value used by IT for CO2.

125

126

Chapter 3 Evaluating Properties Although these values for specific enthalpy at states 1 and 2 differ from the corresponding values read from Table A-23: h1 ⫽ 9,431 and h2 ⫽ 17,678, which give Dh 5 8247 kJ/kmol, the difference in specific enthalpy determined with each set of data agree closely. b b b b b The next example illustrates the use of software for problem solving with the ideal gas model. The results obtained are compared with those determined assuming the specific heat c𝜐 is constant.

cccc

EXAMPLE 3.11 c

Using the Energy Balance and Software One kmol of carbon dioxide gas (CO2) in a piston–cylinder assembly undergoes a constant-pressure process at 1 bar from T1 5 300 K to T2. Plot the heat transfer to the gas, in kJ, versus T2 ranging from 300 to 1500 K. Assume the ideal gas model, and determine the specific internal energy change of the gas using (a) u data from IT. (b) a constant c𝜐 evaluated at T1 from IT. SOLUTION Known: One kmol of CO2 undergoes a constant-pressure process in a piston–cylinder assembly. The initial temperature, T1, and the pressure are known. Find: Plot the heat transfer versus the final temperature, T2. Use the ideal gas model and evaluate Du using

(a) u data from IT, (b) constant c𝜐 evaluated at T1 from IT. Schematic and Given Data:

Engineering Model: Carbon dioxide

T1 = 300 K n = 1 kmol p = 1 bar

1. The carbon dioxide is a closed system. 2. The piston is the only work mode, and the process occurs at constant

pressure. Fig. E3.11a

3. The carbon dioxide behaves as an ideal gas. 4. Kinetic and potential energy effects are negligible.

Analysis: The heat transfer is found using the closed system energy balance, which reduces to

U2 2 U1 5 Q 2 W Using Eq. 2.17 at constant pressure (assumption 2) W 5 p(V2 2 V1) 5 pn(𝜐2 2 𝜐1) Then, with DU 5 n(u2 2 u1), the energy balance becomes n(u2 2 u1) 5 Q 2 pn(𝜐2 2 𝜐1) Solving for Q

3.14 Applying the Energy Balance Using Ideal Gas Tables, Constant Specific Heats, and Software ➊

127

Q 5 n[(u2 2 u1) 1 p(𝜐2 2 𝜐1)]

With p𝜐 5 RT, this becomes Q 5 n[(u2 2 u1) 1 R(T2 2 T1)] The object is to plot Q versus T2 for each of the following cases: (a) values for u1 and u2 at T1 and T2, respectively, are provided by IT, (b) Eq. 3.50 is used on a molar basis, namely u2 2 u1 5 c𝜐(T2 2 T1) where the value of c𝜐 is evaluated at T1 using IT. The IT program follows, where Rbar denotes R, cvb denotes c𝜐, and ubar1 and ubar2 denote u1 and u2 , respectively. // Using the Units menu, select “mole” for the substance amount. //Given Data T1 5 300//K T2 5 1500//K n 5 1//kmol Rbar 5 8.314//kJ/kmol ? K // (a) Obtain molar specific internal energy data using IT. ubar1 5 u_T (“CO2”, T1) ubar2 5 u_T (“CO2”, T2) Qa 5 n*(ubar2 2 ubar1) 1 n*Rbar*(T2 2 T1) // (b) Use Eq. 3.50 with cv evaluated at T1. cvb 5 cv_T (“CO2”, T1) Qb 5 n*cvb*(T2 2 T1) 1 n*Rbar*(T2 2 T1) Use the Solve button to obtain the solution for the sample case of T2 5 1500 K. For part (a), the program returns Qa 5 6.16 3 104 kJ. The solution can be checked using CO2 data from Table A-23, as follows: Qa 5 n[(u2 2 u1) 1 R(T2 2 T1)] 5 (1 kmol)[(58,606 2 6939)kJ/kmol 1 (8.314 kJ/kmol ? K)(1500 2 300)K] 5 61,644 kJ Thus, the result obtained using CO2 data from Table A-23 is in close agreement with the computer solution for the sample case. For part (b), IT returns c𝜐 5 28.95 kJ/kmol ? K at T1, giving Qb 5 4.472 3 104 kJ when T2 5 1500 K. This value agrees with the result obtained using the specific heat c𝜐 at 300 K from Table A-20, as can be verified. 70,000 60,000

internal energy data cv at T1

Q, kJ

50,000 40,000 30,000 20,000 10,000 0 300

500

700

900 T2, K

1100

1300

1500

Fig. E3.11b

128

Chapter 3 Evaluating Properties

Now that the computer program has been verified, use the Explore button to vary T2 from 300 to 1500 K in steps of 10. Construct the following graph using the Graph button: As expected, the heat transfer is seen to increase as the final temperature increases. From the plots, we also see that using constant c𝜐 evaluated at T1 for calculating Du, and hence Q, can lead to considerable error when compared to using u data. The two solutions compare favorably up to about 500 K, but differ by approximately 27% when heating to a temperature of 1500 K. ➊ Alternatively, this expression for Q can be written as Q 5 n[(u2 1 p𝜐2) 2 (u1 1 p𝜐1)]

✓ Skills Developed Ability to… ❑ define a closed system and

identify interactions on its boundary. ❑ apply the energy balance using the ideal gas model. ❑ use IT to retrieve property data for CO2 as an ideal gas and plot calculated data.

Introducing h 5 u 1 p𝜐, the expression for Q becomes Q 5 n(h2 2 h1) Repeat part (b) using c𝜐 evaluated at Taverage 5 (T1 1 T2 )/2. Which approach gives better agreement with the results of part (a): evaluating c𝜐 at T1 or at Taverage? Ans. At Taverage.

3.15

Polytropic Process Relations

A polytropic process is a quasiequilibrium process (Sec. 2.2.5) described by pV n 5 constant

(3.52)

n

or, in terms of specific volumes, by p𝜐 5 constant. In these expressions, n is a constant. For a polytropic process between two states p1V 1n 5 p2V 2n or p2 p1

5a

V1 n b V2

(3.53)

The exponent n may take on any value from 2` to 1` depending on the particular process. When n 5 0, the process is an isobaric (constant-pressure) process, and when n 5 6` the process is an isometric (constant-volume) process. For a polytropic process 2

# p dV 5 1

p2 V 2 2 p 1 V 1 (n ? 1) 12n

(3.54)

for any exponent n except n 5 1. When n 5 1, 2

# p dV 5 p V ln V (n 5 1) V2

1

1

1

(3.55)

1

Example 2.1 provides the details of these integrations. Equations 3.52 through 3.55 apply to any gas (or liquid) undergoing a polytropic process. When the additional idealization of ideal gas behavior is appropriate, further relations can be derived. Thus, when the ideal gas equation of state is introduced into Eqs. 3.53, 3.54, and 3.55, the following expressions are obtained, respectively:

3.15 Polytropic Process Relations p2 (n21)/n T2 V1 n21 5a b 5a b p1 T1 V2 2

# p dV 5 1

mR(T2 2 T1) 12n

2

# p dV 5 mRT ln V

V2

1

(ideal gas)

(3.56)

(ideal gas, n ? 1)

(3.57)

(ideal gas, n 5 1)

(3.58)

129

1

For an ideal gas, the case n 5 1 corresponds to an isothermal (constant-temperature) process, as can readily be verified. Example 3.12 illustrates the use of the closed-system energy balance for a system consisting of an ideal gas undergoing a polytropic process.

cccc

EXAMPLE 3.12 c

Polytropic Process of Air as an Ideal Gas Air undergoes a polytropic compression in a piston–cylinder assembly from p1 5 1 bar, T1 5 22°C to p2 5 5 bars. Employing the ideal gas model with constant specific heat ratio k, determine the work and heat transfer per unit mass, in kJ/kg, if (a) n 5 1.3, (b) n 5 k. Evaluate k at T1. SOLUTION Known:

Air undergoes a polytropic compression process from a given initial state to a specified final

pressure. Find:

Determine the work and heat transfer, each in kJ/kg.

Schematic and Given Data:

p 5 bar

Engineering Model: 2

1. The air is a closed system. 2. The air behaves as an ideal gas with pvn

➊ 1 bar

= constant

Air p1 = 1 bar T1 = 22°C p2 = 5 bar

1

constant specific heat ratio k evaluated at the initial temperature. 3. The compression is polytropic. 4. There is no change in kinetic

orpotential energy. v

Fig. E3.12

Analysis: The work can be evaluated in this case from the expression 2

W5

# p dV 1

130

Chapter 3 Evaluating Properties

With Eq. 3.57 R(T2 2 T1) W 5 m 12n

(a)

The heat transfer can be evaluated from an energy balance. Thus Q W 5 1 (u2 2 u1) m m Inspection of Eq. 3.47b shows that when the specific heat ratio k is constant, c𝜐 is constant. Thus Q W 5 1 c𝜐(T2 2 T1) m m

(b)

(a) For n 5 1.3, the temperature at the final state, T2, can be evaluated from Eq. 3.56 as follows:

p2 (n21)/n 5 (1.321)/1.3 T2 5 T1a b 5 295 K a b 5 427.68 K (154.68°C) p1 1 Using Eq. (a), the work is then R(T2 2 T1) J 427.68 K 2 295 K W 5 a287.21 ba b 5 2127.02 kJ/kg 5 m 12n kg ? K 1 2 1.3 At 22°C, Table A-20 gives k 5 1.401 and c𝜐 5 0.716 kJ/kg ? K. Alternatively, c𝜐 can be found using Eq. 3.47b, as follows:

c𝜐 5 5

R k21

(c)

287.21 J/kg ? K 5 0.716 kJ/kg ? K (1.401 2 1)

Substituting values into Eq. (b), we get Q kJ kJ 1 a0.716 b (427.68 2 295) K 5 2127.02 m kg kg ? K 5 232.02

kJ kg

(b) For n 5 k, substituting Eqs. (a) and (c) into Eq. (b) gives R(T2 2 T1) R(T2 2 T1) Q 1 50 5 m 12k k21

✓ Skills Developed Ability to… ❑ evaluate work using Eq. 2.17. ❑ apply the energy balance

using the ideal gas model.

❑ apply the polytropic

process concept.

Chapter Summary and Study Guide

131

That is, no heat transfer occurs in the polytropic process of an ideal gas for which n 5 k. ➊ The states visited in a polytropic compression process are shown by the curve on the accompanying p–𝜐 diagram. The magnitude of the work per unit of mass is represented by the shaded area below the curve.

For n 5 k, evaluate the temperature at the final state, in K and °C. Ans. 466.67 K (193.67°C)

c CHAPTER SUMMARY AND STUDY GUIDE In this chapter, we have considered property relations for a broad range of substances in tabular, graphical, and equation form. Primary emphasis has been placed on the use of tabular data, but computer retrieval also has been considered. A key aspect of thermodynamic analysis is fixing states. This is guided by the state principle for pure, simple compressible systems, which indicates that the intensive state is fixed by the values of any two independent, intensive properties. Another important aspect of thermodynamic analysis is locating principal states of processes on appropriate diagrams: p–𝜐, T–𝜐, and p–T diagrams. The skills of fixing states and using property diagrams are particularly important when solving problems involving the energy balance. The ideal gas model is introduced in the second part of this chapter, using the compressibility factor as a point of departure. This arrangement emphasizes the limitations of the ideal gas model. When it is appropriate to use the ideal gas model, we stress that specific heats generally vary with temperature, and feature the use of the ideal gas tables in problem solving. The following checklist provides a study guide for this chapter. When your study of the text and end-of-chapter exercises has been completed, you should be able to

c write out the meanings of the terms listed in the margins

c

c c c c c c c

throughout the chapter and understand each of the related concepts. The subset of key concepts listed on the next page is particularly important in subsequent chapters. retrieve property data from Tables A-1 through A-23, using the state principle to fix states and linear interpolation when required. sketch T–𝜐, p–𝜐, and p–T diagrams, and locate principal states on such diagrams. apply the closed-system energy balance with property data. evaluate the properties of two-phase, liquid–vapor mixtures using Eqs. 3.1, 3.2, 3.6, and 3.7. estimate the properties of liquids using Eqs. 3.11–3.14. apply the incompressible substance model. use the generalized compressibility chart to relate p–𝜐–T data of gases. apply the ideal gas model for thermodynamic analysis, including determining when use of the ideal gas model is warranted, and appropriately using ideal gas table data or constant specific heat data to determine Du and Dh.

c KEY ENGINEERING CONCEPTS saturation pressure, p. 83 phase, p. 79 p–ʋ diagram, p. 83 pure substance, p. 79 state principle, p. 79 T–ʋ diagram, p. 83 simple compressible systems, p. 79 compressed liquid, p. 84 p–υ–T surface, p. 81 two-phase, liquid–vapor mixture, p. 85 quality, p. 85 phase diagram, p. 83 superheated vapor, p. 85 saturation temperature, p. 83

enthalpy, p. 93 specific heats, p. 104 incompressible substance model, p. 106 universal gas constant, p. 109 compressibility factor, p. 109 ideal gas model, p. 115

132

Chapter 3 Evaluating Properties

c KEY EQUATIONS x5

mvapor

(3.1) p. 85

Quality, x, of a two-phase, liquid–vapor mixture.

(3.2) p. 91 (3.6) p. 94 (3.7) p. 94

Specific volume, internal energy and enthalpy of a two-phase, liquid–vapor mixture.

(3.11) p. 105 (3.12) p. 105 (3.14) p. 106

Specific volume, internal energy, and enthalpy of liquids, approximated by saturated liquid values, respectively.

mliquid 1 mvapor

𝜐 5 (1 2 x)𝜐f 1 x𝜐g 5 𝜐f 1 x(𝜐g 2 𝜐f) u 5 (1 2 x)uf 1 xug 5 uf 1 x(ug 2 uf) h 5 (1 2 x)hf 1 xhg 5 hf 1 x(hg 2 hf) 𝜐(T, p) < 𝜐f(T) u(T, p) < uf(T) h(T, p) < hf(T)

Ideal Gas Model Relations

p𝜐 5 RT u 5 u(T) h 5 h(T) 5 u(T) 1 RT

u(T2) 2 u(T1) 5

#

T2

c𝜐(T) dT

(3.32) p. 114 (3.36) p. 115 (3.37) p. 115

Ideal gas model.

(3.40) p. 118

Change in specific internal energy.

(3.50) p. 122

For constant c𝜐.

(3.43) p. 118

Change in specific enthalpy.

(3.51) p. 122

For constant cp.

T1

u(T2) 2 u(T1) 5 c𝜐(T2 2 T1)

h(T2) 2 h(T1) 5

#

T2

cp(T) dT

T1

h(T2) 2 h(T1) 5 cp(T2 2 T1)

c EXERCISES: THINGS ENGINEERS THINK ABOUT 1. If water contracted on freezing, what implications might this have for aquatic life? 2. At what temperature does water boil at the top of Mount McKinley, Alaska? In Death Valley, California? 3. Why do frozen water pipes tend to burst? 4. Why do many foods have high-altitude cooking instructions? 5. Can a substance that contracts on freezing exist as a solid when at a temperature greater than its triple-point temperature? Repeat for a substance that expands on freezing. 6. The specific internal energy is arbitrarily set to zero in Table A-2 for saturated liquid water at 0.01°C. If the reference value for u at this reference state were specified differently, would there be any significant effect on thermodynamic analyses using u and h? 7. For liquid water at 20°C and 1.0 MPa, what percent difference would there be if its specific enthalpy were evaluated using Eq. 3.14 instead of Eq. 3.13?

8. How do I measure the specific heat cp of liquid water at atmospheric pressure and room temperature? 9. If a block of iron and a block of tin having equal volumes each received the same energy input by heat transfer, which block would experience the greater temperature increase? 10. How many minutes do I have to exercise to burn the calories in a helping of my favorite dessert? 11. What is my annual contribution to the emission of CO2 into the atmosphere? 12. How do I estimate the mass of air contained in a bicycle tire? 13. Specific internal energy and enthalpy data for water vapor are provided in two tables: Tables A-4 and A-23. When would Table A-23 be used?

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133

c PROBLEMS: DEVELOPING ENGINEERING SKILLS 3.1 For H2O, plot the following on a p–ʋ diagram drawn to scale on log–log coordinates: (a) the saturated liquid and saturated vapor lines from the triple point to the critical point, with pressure in MPa and specific volume in m3/kg. (b) lines of constant temperature at 100 and 300°C. 3.2 Determine the specific volume of a two-phase liquid vapor mixture of a substance. The pressure is 200 bar, and the mixture occupies a volume of 0.3 m3. The masses of saturated vapor and liquid are 3.5 kg and 4.5 kg, respectively. Also, determine the quality of the mixture.

3.11 As shown in Fig. P3.11, a closed, rigid cylinder contains different volumes of saturated liquid water and saturated water vapor at a temperature of 140°C. Determine the quality of the mixture, expressed as a percentage.

70 T = 140°C

50

3

3.3 Determine the mass, in kg, of 0.1 m of Refrigerant 134a at 4 bar, 100°C. 3.4 Determine the volume, in m3, occupied by 0.9 kg of H2O at a pressure of 6.9 MPa and (a) a temperature of 316°C. (b) a quality of 80%. (c) a temperature of 93°C. 3.5 A closed vessel with a volume of 0.054 m3 contains 2.27 kg of Refrigerant 134a. A pressure sensor in the tank wall reads 492.22 kPa (gage). If the atmospheric pressure is 99.3 kPa what is the temperature of the refrigerant, in °C? 3.6 Three kg of a two-phase, liquid–vapor mixture of carbon dioxide (CO2) exists at 260°C in a 0.07 m3 tank. Determine the quality of the mixture, if the values of specific volume for saturated liquid and saturated vapor CO2 at 260°C are 𝜐f 5 0.788 3 1023 m3/kg and 𝜐g 5 4.214 3 1022 m3/kg, respectively. 3.7 Determine the quality of a two-phase liquid–vapor mixture of (a) H2O at 20°C with a specific volume of 20 m3/kg. (b) Propane at 15 bar with a specific volume of 0.02997 m3/kg. (c) Refrigerant 134a at 60°C with a specific volume of 0.001 m3/kg. (d) Ammonia at 1 MPa with a specific volume of 0.1 m3/kg. 3.8 Determine the quality of a two-phase liquid–vapor mixture of (a) H2O at 101.325 kPa with a specific volume of 1.56 m3/kg. (b) Propane at 27°C with a specific volume of 0.0017 kg. (c) Refrigerant 134a at 345 kPa with a specific volume of 0.03 m3/kg. (d) Ammonia at 4°C with a specific volume of 125 m3/kg. 3.9 A two-phase liquid–vapor mixture of ammonia at 38°C has a specific volume of 0.06 m3/kg. Determine the quality of a two-phase liquid–vapor mixture at 217.8°C with the same specific volume. 3.10 A two-phase liquid–vapor mixture of a substance has a pressure of 160 bar and occupies a volume of 0.4 m3. The masses of saturated liquid and vapor present are 4.0 kg and 4.4 kg, respectively. Determine the specific volume of the mixture, in m3/kg.

60

Saturated vapor

40 30 20

Saturated liquid

10 0

Fig. P3.11

3.12 A closed system consists of a two-phase liquid–vapor mixture of H2O in equilibrium at 149°C. The quality of the mixture is 0.8 (80%) and the mass of saturated vapor present is 0.9 kg. Determine the mass of saturated liquid present, in kg, and the total volume of the system, in m3. 3.13 Water is contained in a closed, rigid, 0.2 m3 tank at an initial pressure of 5 bar and a quality of 50%. Heat transfer occurs until the tank contains only saturated vapor. Determine the final mass of vapor in the tank, in kg, and the final pressure, in bar. 3.14 A rigid tank contains 2.27 kg of a two-phase, liquid–vapor mixture of H2O, initially at 127°C with a quality of 0.6. Heat transfer to the contents of the tank occurs until the temperature is 160°C. Show the process on a p–𝜐 diagram. Determine the mass of vapor, in kg, initially present in the tank and the final pressure, in kPa. 3.15 Two thousand kg of water, initially a saturated liquid at 150°C, is heated in a closed, rigid tank to a final state where the pressure is 2.5 MPa. Determine the final temperature, in °C, the volume of the tank, in m3, and sketch the process on T–𝜐 and p–𝜐 diagrams. 3.16 Steam is contained in a closed rigid container with a volume of 1 m3. Initially, the pressure and temperature of the steam are 7 bar and 500°C, respectively. The temperature drops as a result of heat transfer to the surroundings. Determine the temperature at which condensation first occurs, in °C, and the fraction of the total mass that has condensed when the pressure reaches 0.5 bar. What is the volume, in m3, occupied by saturated liquid at the final state?

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Chapter 3 Evaluating Properties

3.17 Water vapor is heated in a closed, rigid tank from saturated vapor at 160°C to a final temperature of 400°C. Determine the initial and final pressures, in bar, and sketch the process on T–𝜐 and p–𝜐 diagrams. 3.18 Ammonia undergoes an isothermal process from an initial state at T1 5 27°C and 𝜐1 5 0.6 m2/kg to saturated vapor. Determine the initial and final pressures, in kPa, and sketch the process on T–𝜐 and p–𝜐 diagrams. 3.19 1.36 kg of saturated water vapor, contained in a closed rigid tank whose volume is 0.36 m3, is heated to a final temperature of 204°C. Sketch the process on a T–𝜐 diagram. Determine the pressures at the initial and final states, each in kPa. 3.20 One kg of water vapor is compressed at a constant pressure of 1.4 MPa from a volume of 0.2 m3 to a saturated vapor state. Determine the temperatures at the initial and final states, each in °C, and the work for the process, in kJ. 3.21 Water vapor initially at 10 bar and 400°C is contained within a piston–cylinder assembly. The water is cooled at constant volume until its temperature is 150°C. The water is then condensed isothermally to saturated liquid. For the water as the system, evaluate the work, in kJ/kg. 3.22 Two kg of Refrigerant 22 undergo a process for which the pressure–volume relation is p𝜐1.05 5 constant. The initial state of the refrigerant is fixed by p1 5 2 bar, T1 5 220°C, and the final pressure is p2 5 10 bar. Calculate the work for the process, in kJ. 3.23 Refrigerant 134a in a piston–cylinder assembly undergoes a process for which the pressure–volume relation is p𝜐1.058 5 constant. At the initial state, p1 5 200 kPa, T1 5 210°C. The final temperature is T2 5 50°C. Determine the final pressure, in kPa, and the work for the process, in kJ/kg of refrigerant. 3.24 A piston–cylinder assembly contains 0.02 kg of Refrigerant 134a. The refrigerant is compressed from an initial state where p1 5 68.9 kPa and T1 5 26.7°C to a final state where p2 5 1.1 MPa. During the process, the pressure and specific volume are related by pυ 5 constant. Determine the work, in kJ, for the refrigerant.

Using u–h Data 3.25 A quantity of water is at 15 MPa and 100°C. Evaluate the specific volume, in m3/kg, and the specific enthalpy, in kJ/kg, using (a) data from Table A-5. (b) saturated liquid data from Table A-2. 3.26 Evaluate the specific volume, in m3/kg, and the specific enthalpy, in kJ/kg, of water at 204°C and a pressure of 207 bars.

3.27 Evaluate the specific volume, in m3/kg, and the specific enthalpy, in kJ/kg, of Refrigerant 134a at 35°C and 10.3 bar. 3.28 Evaluate the specific volume, in m3/kg, and the specific enthalpy, in kJ/kg, of ammonia at 20°C and 1.0 MPa. 3.29 Evaluate the specific volume, in m3/kg, and the specific enthalpy, in kJ/kg, of propane at 800 kPa and 0°C. 3.30 Plot versus pressure the percent changes in specific volume, specific internal energy, and specific enthalpy for water at 20°C from the saturated liquid state to the state where the pressure is 300 bar. Based on the resulting plots, discuss the implications regarding approximating compressed liquid properties using saturated liquid properties at 20°C, as discussed in Sec. 3.10.1.

Applying the Energy Balance 3.31 A piston–cylinder assembly contains 1 kg of water, initially occupying a volume of 0.5 m3 at 1 bar. Energy transfer by heat to the water results in an expansion at constant temperature to a final volume of 1.694 m3. Kinetic and potential energy effects are negligible. For the water, (a) show the process on a T–𝜐 diagram, (b) evaluate the work, in kJ, and (c) evaluate the heat transfer, in kJ. 3.32 At 300°C and 100 kPa, steam has a volume of 0.8m3. In a constant-volume process, the pressure of steam changes to 50 kPa. Determine the heat transferred to the steam. 3.33 Refrigerant 134a is compressed with no heat transfer in a piston–cylinder assembly from 207 kPa, 26.7°C to 1.1 MPa. The mass of refrigerant is 0.02 kg. For the refrigerant as the system, W 5 20.59 kJ. Kinetic and potential energy effects are negligible. Determine the final temperature, in °C. 3.34 Water in a piston–cylinder assembly, initially at a temperature of 99.58°C and a quality of 60%, is heated at constant pressure to a temperature of 250°C. If the work during the process is +350 kJ, determine (a) the mass of water, in kg, and (b) the heat transfer, in kJ. Changes in kinetic and potential energy are negligible. 3.35 One kg of Refrigerant 22, initially at 0.9 MPa, 20°C, is contained within a rigid, closed tank. The tank is fitted with a paddle wheel that transfers energy to the refrigerant at a constant rate of 0.1 kW. Heat transfer from the refrigerant to its surroundings occurs at a rate Kt, in kW, where the constant K is in kW per minute and t is time, in minutes. After 20 minutes of stirring, the refrigerant is at a pressure of 1.2 MPa. No overall changes in kinetic or potential energy occur. For the Refrigerant 22, determine (a) the work, in kJ. (b) the heat transfer, in kJ. (c) Evaluate the constant K, in kW per minute.

Problems: Developing Engineering Skills 3.36 A closed, rigid tank contains 2 kg of water initially at 80°C and a quality of 0.6. Heat transfer occurs until the tank contains only saturated vapor at a higher pressure. Kinetic and potential energy effects are negligible. For the water as the system, determine the amount of energy transfer by heat, in kJ. 3.37 Initially steam is at 250°C and 300 kPa. It is compressed isothermally such that its new volume becomes 1/18th of the original volume. Determine the heat transfer to steam. 3.38 A rigid, insulated tank fitted with a paddle wheel is filled with water, initially a two-phase liquid–vapor mixture at 200 kPa, consisting of 0.04 kg of saturated liquid and 0.04 kg of saturated vapor. The tank contents are stirred by the paddle wheel until all of the water is saturated vapor at a pressure greater than 138 kPa. Kinetic and potential energy effects are negligible. For the water, determine the (a) (b) (c) (d)

volume occupied, in m3. initial temperature, in °C. final pressure, in kPa. work, in kJ.

3.39 At 300°C and 100 kPa air has a volume of 0.8 m3. In a constant-volume process the pressure of air changes to 50 kPa. Determine the heat transferred to the air and the final temperature of air. Take the value of specific heat capacity at constant volume for air equal to 0.718 kJ/kg ? K. 3.40 A rigid tank is filled with 2.27 kg of water, initially at T1 5 127°C and 𝜐1 5 0.44 m3/kg. The tank contents are heated until the temperature is T2 5 177°C. Kinetic and potential energy effects are negligible. For the water, determine (a) the initial and final pressure, each in kPa, and (b) the heat transfer, in kJ. 3.41 Saturated water vapor is contained in a closed rigid tank. The water is cooled to a temperature of 110°C. At this temperature the masses of saturated vapor and liquid are each 1000 kg. Determine the heat transfer for the process, in kJ. 3.42 A rigid, well-insulated tank contains a two-phase mixture consisting of 0.00014 m3 of saturated liquid water and 0.0324 m3 of saturated water vapor, initially at 101.3 kPa. A paddle wheel stirs the mixture until only saturated vapor at a higher pressure remains in the tank. Kinetic and potential energy effects are negligible. For the water, determine the amount of energy transfer by work, in kJ. 3.43 A two-phase liquid–vapor mixture of H2O, initially at 1.0 MPa with a quality of 90%, is contained in a rigid, wellinsulated tank. The mass of H2O is 2 kg. An electric resistance heater in the tank transfers energy to the water at a constant rate of 60 W for 1.95 h. Determine the final temperature of the water in the tank, in °C, and the final pressure, in bar. 3.44 Two kilograms of Refrigerant 134a, initially at 2 bar and occupying a volume of 0.12 m3, undergoes a process at

135

constant pressure until the volume has doubled. Kinetic and potential energy effects are negligible. Determine the work and heat transfer for the process, each in kJ. 3.45 0.9 kg of a two-phase liquid–vapor mixture of H2O, initially at 689 kPa, are confined to one side of a rigid, well-insulated container by a partition. The other side of the container has a volume of 0.189 m3 and is initially evacuated. The partition is removed and the water expands to fill the entire container. The pressure at the final equilibrium state is 275.8 kPa. Determine the quality of the mixture present initially and the overall volume of the container, in m3. 3.46 One kg of saturated solid water at the triple point is heated to saturated liquid while the pressure is maintained constant. Determine the work and the heat transfer for the process, each in kJ. Show that the heat transfer equals the change in enthalpy of the water in this case. 3.47 A closed system initially contains a three-phase mixture consisting of 0.45 kg of saturated solid water (ice), 0.45 kg of saturated liquid water, and 0.1 kg of saturated water vapor at the triple point temperature and pressure. Heat transfer occurs while the pressure is maintained constant until only saturated vapor remains. Determine the amounts of energy transfer by work and heat, each in kJ. 3.48 Water is stored in a closed tank of capacity 0.1m3. The quality of water is 60% and initial pressure is 7 bar. Heat is transferred to the water until the tank contains only saturated vapor. At the end of this process, determine the mass of vapor in the tank, and the final pressure inside the tank. 3.49 A piston–cylinder assembly contains 0.9 kg of water, initially at 149°C. The water undergoes two processes in series: constant-volume heating followed by a constantpressure process. At the end of the constant-volume process, the pressure is 690 kPa and the water is a two-phase, liquid–vapor mixture with a quality of 80%. At the end of the constant-pressure process, the temperature is 204°C. Neglect kinetic and potential energy effects. (a) Sketch T–υ and p–υ diagrams showing the key states and the processes. (b) Determine the work and heat transfer for each of the two processes, all in kJ. 3.50 A system consisting of 0.9 kg of water vapor, initially at 149°C and occupying a volume of 0.54 m3, is compressed isothermally to a volume of 0.24 m3. The system is then heated at constant volume to a final pressure of 827 kPa. During the isothermal compression there is energy transfer by work of magnitude 95.8 kJ into the system. Kinetic and potential energy effects are negligible. Determine the heat transfer, in kJ, for each process. 3.51 Ammonia in a piston–cylinder assembly undergoes two processes in series. At the initial state, p1 5 827 kPa and the quality is 100%. Process 1–2 occurs at constant volume until

136

Chapter 3 Evaluating Properties

the temperature is 38°C. The second process, from state 2 to state 3, occurs at constant temperature, with Q23 5 104 kJ, until the quality is again 100%. Kinetic and potential energy effects are negligible. For 1 kg of ammonia, determine (a) the heat transfer for Process 1–2 and (b) the work for Process 2–3, each in kJ. 3.52 1.4 kg of water is contained in a piston–cylinder assembly, initially occupying a volume V1 5 0.81 m3 at T1 5 149°C. The water undergoes two processes in series: Process 1–2: Constant-temperature compression to V2 5 0.3 m3, during which there is an energy transfer by heat from the water of 1345 kJ. Process 2–3:

Constant-volume heating to p3 5 827 kPa.

Sketch the two processes in series on a T–𝜐 diagram. Neglecting kinetic and potential energy effects, determine the work in Process 1–2 and the heat transfer in Process 2–3, each in Btu. 3.53 Initially air is at 2508C and 300 kPa. It is compressed isothermally such that its new volume becomes 1/8th of the original volume. Determine the final pressure and the work done on air. 3.54 A system consisting of 2 kg of ammonia undergoes a cycle composed of the following processes:

3.57 Ten kg of Refrigerant 22 contained in a piston–cylinder assembly undergoes a process for which the pressurespecific volume relationship is p𝜐 n 5 constant. The initial and final states of the refrigerant are fixed by p1 5 400 kPa, T1 5 25°C, and p2 5 2000 kPa, T2 5 70°C, respectively. Determine the work and heat transfer for the process, each in kJ. 3.58 Air undergoes a reversible adiabatic process, which begins at 3508C and 10 bar pressure and ends at a pressure of 1 bar. Determine the work done per kg of air. 3.59 A piston–cylinder assembly contains ammonia, initially at p1 5 0.1 MPa, T1 5 15°C. The ammonia is compressed to a final state where p2 5 1 MPa. During the process, the pressure and specific volume are related by pυ 5 constant and the work has a magnitude of 32 kJ. Sketch the process on a p–𝜐 diagram. Neglecting kinetic and potential energy effects, determine the heat transfer, in kJ. 3.60 As shown in Fig. P3.60, 0.5 kg of ammonia is contained in a piston–cylinder assembly, initially at T1 5 220°C and a quality of 25%. As the ammonia is slowly heated to a final state, where T2 5 20°C, p2 5 0.6 MPa, its pressure varies linearly with specific volume. There are no significant kinetic and potential energy effects. For the ammonia, (a) show the process on a sketch of the p–𝜐 diagram and (b) evaluate the work and heat transfer, each in kJ.

Process 1–2: Constant volume from p1 5 10 bar, x1 5 0.6 to saturated vapor. Process 2–3: Constant temperature to p3 5 p1, Q23 5 +228 kJ. Process 3–1:

Constant pressure.

Sketch the cycle on p–𝜐 and T–𝜐 diagrams. Neglecting kinetic and potential energy effects, determine the net work for the cycle and the heat transfer for each process, all in kJ. 3.55 A system consisting of 1 kg of H2O undergoes a power cycle composed of the following processes: Process 1–2: Constant-pressure heating at 10 bar from saturated vapor. Process 2–3: T3 5 160°C. Process 3–4:

Constant-volume cooling to

m = 0.5 kg Ammonia

Initially, T1 = –20°C, x = 25% Finally, T2 = 20°C, p2 = 0.6 MPa

p3 5 5 bar,

Isothermal compression with Q34 5 2815.8 kJ.

Process 4–1: Constant-volume heating. Sketch the cycle on T–𝜐 and p–𝜐 diagrams. Neglecting kinetic and potential energy effects, determine the thermal efficiency. 3.56 A system consisting of 0.45 kg of Refrigerant 22 undergoes a cycle composed of the following processes: Process 1–2: Constant pressure from p1 5 205 kPa, x1 5 0.95 to T2 5 4°C. Process 2–3: Constant temperature to saturated vapor with W23 5 212.47 kJ. Process 3–1: Adiabatic expansion. Sketch the cycle on p–𝜐 and T–𝜐 diagrams. Neglecting kinetic and potential energy effects, determine the net work for the cycle and the heat transfer for each process, all in kJ.

Q

Fig. P3.60

3.61 A well-insulated copper tank of mass 13 kg contains 4 kg of liquid water. Initially, the temperature of the copper is 27°C and the tem perature of the water is 50°C. An electrical resistor of neglible mass transfers 100 kJ of energy to the contents of the tank. The tank and its contents come to equilibrium. What is the final temperature, in °C? 3.62 A 5.4 3 1023 m3 aluminum bar, initially at 93°C, is placed in an open tank together with 0.108 m3 of liquid water, initially at 21°C. For the water and the bar as the

Problems: Developing Engineering Skills

137

system, determine the final equilibrium temperature, in °C, ignoring heat transfer between the tank and its surroundings.

the gas is heated to 80°C, determine the final pressure, expressed as a gage pressure, in kPa. The local atmospheric pressure is 1 atm.

3.63 A 22.7 kg iron casting with an initial temperature of 370°C is quenched in a tank filled with oil. The tank is 0.9 m in diameter and 1.5 m tall. The oil can be considered incompressible with a density of 960 kg/m3 and an initial temperature of 27°C. The mass of the tank is negligible. If the specific heat of iron is 420 J/kg ? K and that of oil is 1,884 J/kg ? K, what is the equilibrium temperature of the iron and oil, in °C? Neglect heat transfer between the tank and its surroundings.

3.74 A tank contains 4.5 kg of air at 21°C with a pressure of 207 kPa. Determine the volume of the air, in m3. Verify that ideal gas behavior can be assumed for air under these conditions.

Using Generalized Compressibility Data 3.64 Determine the compressibility factor for water vapor at 200 bar and 470°C, using (a) data from the compressibility chart. (b) data from the steam tables. 3.65 Determine the volume, in m3, occupied by 40 kg of nitrogen (N2) at 17 MPa, 180 K. 3.66 Oxygen (O2) occupies a volume of 0.135 m3 at 222 K. Determine the mass of oxygen, in kg. 3.67 Determine the pressure, in kPa, of ethane (C2H6) at 46°C and a specific volume of 0.02 m3/kg. 3.68 A tank contains 1 m3 of air at 287°C and a gage pressure of 1.78 MPa. Determine the mass of air, in kg. The local atmospheric pressure is 1 atm. 3.69 2.27 kmol of carbon dioxide (CO2), initially at 2.2 MPa, 367 K, is compressed at constant pressure in a piston–cylinder assembly. For the gas, W 5 22,110 kJ. Determine the final temperature, in K.

Working with the Ideal Gas Model 3.70 A tank contains 0.05 m3 of nitrogen (N2) at 221°C and 10 MPa. Determine the mass of nitrogen, in kg, using (a) the ideal gas model. (b) data from the compressibility chart. Comment on the applicability of the ideal gas model for nitrogen at this state.

3.75 Compare the densities, in kg/m3, of helium and air, each at 300 K, 100 kPa. Assume ideal gas behavior. 3.76 Assuming the ideal gas model, determine the volume, in m3, occupied by 0.45 kmol of argon (Ar) gas at 690 kPa and 305.6 K. 3.77 Show that the specific heat ratio of a monatomic ideal gas is equal to 5/3. 3.78 By integrating cp(T) obtained from Table A-21, determine the change in specific enthalpy, in kJ/kg, of methane (CH4) from T1 5 320 K, p1 5 2 bar to T2 5 800 K, p2 5 10 bar. Check your result using IT. 3.79 Assuming ideal gas behavior for air, plot to scale the isotherms 300, 500, 1,000, and 2,000 K on a p–𝜐 diagram. 3.80 Assuming ideal gas behavior for air, plot to scale the isotherms 278 K, 555.6 K, 1,111 K, and 2,222 K on a p–𝜐 diagram.

Using the Energy Balance with the Ideal Gas Model 3.81 Ammonia is contained in a rigid, well-insulated container. The initial pressure is 138 kPa, the mass is 0.05 kg, and the volume is 0.054 m3. The gas is stirred by a paddle wheel, resulting in an energy transfer to the ammonia of magnitude 21 kJ. Assuming the ideal gas model, determine the final temperature of the ammonia, in K. Neglect kinetic and potential energy effects. 3.82 One kilogram of nitrogen fills the cylinder of a pistoncylinder assembly, as shown in Fig. P3.82. There is no friction between the piston and the cylinder walls, and the surroundings are at 1 atm. The initial volume and pressure in the cylinder are 1 m3 and 1 atm, respectively. Heat transfer to the nitrogen occurs until the volume is doubled. Determine the heat transfer for the process, in kJ, assuming the specific heat ratio is constant, k 5 1.4.

3.71 Determine the percent error in using the ideal gas model to determine the specific volume of (a) (b) (c) (d) (e)

water vapor at 14 MPa, 370°C. water vapor at 6.9 kPa, 93°C. ammonia at 415 kPa, 71°C. air at 1 bar, 1,111 K. Refrigerant 22 at 2.1 MPa, 60°C.

Q

N2 m = 1 kg V1 = 1 m3 p1 = 1 atm

V2 = 2V1

patm = 1 atm

3.72 Determine the temperature, in K, of oxygen (O2) at 250 bar and a specific volume of 0.003 m3/kg using generalized compressibility data and compare with the value obtained using the ideal gas model.

Fig. P3.82

3.73 A closed, rigid tank is filled with a gas modeled as an ideal gas, initially at 30°C and a gage pressure of 350 kPa. If

3.83 A piston–cylinder assembly contains air at a pressure of 225 kPa and a volume of 0.03 m3. The air is heated at

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Chapter 3 Evaluating Properties

constant pressure until its volume is doubled. Assuming the ideal gas model with constant specific heat ratio, k 5 1.4, for the air, determine the work and heat transfer, each in kJ. 3.84 A piston–cylinder assembly contains nitrogen (N2), initially at 138 kPa, 27°C and occupying a volume of 0.054 m3. The nitrogen is compressed to a final state where the pressure is 1.1 MPa and the temperature is 149°C. During compression, heat transfer of magnitude 1.69 kJ occurs from the nitrogen to its surroundings. Assuming ideal gas behavior, determine for the nitrogen, (a) the mass, in kg, and (b) the work, in kJ. 3.85 As shown in Fig. P3.85, a piston–cylinder assembly whose piston is resting on a set of stops contains 0.5 kg of helium gas, initially at 100 kPa and 25°C. The mass of the piston and the effect of the atmospheric pressure acting on the piston are such that a pressure of 500 kPa is required to raise it. How much energy must be transferred by heat to the helium, in kJ, before the piston starts rising? Assume ideal gas behavior for the helium.

3.87 A closed, rigid tank fitted with a paddle wheel contains 0.2 kg of air, initially at 340 K, 0.2 MPa. The paddle wheel stirs the air for 25 minutes, with the power input varying # # with time according to W 5 215t, where W is in watts and t is time, in minutes. The final temperature of the air is 1090 K. Assuming ideal gas behavior and no change in kinetic or potential energy, determine for the air (a) the final pressure, in MPa, (b) the work, in kJ, and (c) the heat transfer, in kJ. 3.88 A rigid container has a removable partition that initially confines air on one side and creates a vacuum on the other side. At this stage, the air has following properties: p1 5 4 bar. T1 5 400 K. V1 5 0.25 m3. The partition is removed and new pressure (p2) is found to be one-fourth of p1 and the new volume (V2) is three times that of V1. Assuming air behaves as an ideal gas, determine the heat transfer in kJ. 3.89 Air is confined to one side of a rigid container divided by a partition, as shown in Fig. P3.89. The other side is initially evacuated. The air is initially at p1 5 4 bar, T1 5 600 K, and V1 5 0.25 m3. When the partition is removed, the air expands to fill the entire chamber. Measurements show that V2 5 2 V1 and p2 5 p1/4. Assuming the air behaves as an ideal gas, determine (a) the final temperature, in K, and (b) the heat transfer, kJ.

Piston

Helium, m = 0.5 kg p1 = 100 kPa T1 = 25°C

Removable partition

Q Initially:

Fig. P3.85 3.86 As shown in Fig. P3.86, a tank fitted with an electrical resistor of negligible mass holds 1 kg of nitrogen (N2), initially at 27°C, 0.2 MPa. Over a period of 5 minutes, electricity is provided to the resistor at a rate of 0.15 kW. During this same period, a heat transfer of magnitude 12.25 kJ occurs from the nitrogen to its surroundings. Assuming ideal gas behavior, determine the nitrogen’s final temperature, in °C, and final pressure, in MPa.

Nitrogen, N2 m = 1 kg T1 = 27°C p1 = 0.2 MPa

Fig. P3.86

+

–

Finally:

Air V1 = 0.25 m3 p1 = 4 bar T1 = 600 K

Vacuum

V2 = 2V1 p2 = 1– p1 4

Fig. P3.89 3.90 Two kilograms of air, initially at 5 bar, 350 K and 4 kg of carbon monoxide (CO) initially at 2 bar, 450 K are confined to opposite sides of a rigid, well-insulated container by a partition, as shown in Fig. P3.90. The partition is free to move and allows conduction

Problems: Developing Engineering Skills from one gas to the other without energy storage in the partition itself. The air and CO each behave as ideal gases with constant specific heat ratio, k 5 1.395. Determine at equilibrium (a) the temperature, in K, (b) the pressure, in bar, and (c) the volume occupied by each gas, in m3. Insulation

CO m = 4 kg p = 2 bar T = 450 K

139

patm

Piston CO2 at 225 kPa, 300 K. Initial volume = 0.04 m3

Movable partition

Air m = 2 kg p = 5 bar T = 350 K

Valve Leak

Fig. P3.90

3.91 Two uninsulated, rigid tanks contain air. Initially, tank A holds 0.45 kg of air at 800 K and tank B has 0.9 kg of air at 500 K. The initial pressure in each tank is 345 kPa. A valve in the line connecting the two tanks is opened and the contents are allowed to mix. Eventually, the contents of the tanks come to equilibrium at the temperature of the surroundings, 289 K. Assuming the ideal gas model, determine the amount of energy transfer by heat, in kJ, and the final pressure, in kPa. 3.92 Two kilograms of a gas with molecular weight 28 are contained in a closed, rigid tank fitted with an electric resistor. The resistor draws a constant current of 10 amps at a voltage of 12 V for 10 minutes. Measurements indicate that when equilibrium is reached, the temperature of the gas has increased by 40.3°C. Heat transfer to the surroundings is estimated to occur at a constant rate of 20 W. Assuming ideal gas behavior, determine an average value of the specific heat cp, in kJ/kg ? K, of the gas in this temperature interval based on the measured data. 3.93 As shown in Fig. P3.93, a rigid tank initially contains 4 kg of carbon dioxide (CO2) at 550 kPa. The tank is connected by a valve to a piston–cylinder assembly located vertically above, initially containing 0.04 m3 of CO2. Although the valve is closed, a slow leak allows CO2 to flow into the cylinder from the tank until the tank pressure falls to 225 kPa. The weight of the piston and the pressure of the atmosphere maintain a constant pressure of 225 kPa in the cylinder. Owing to heat transfer, the temperature of the CO2 throughout the tank and cylinder stays constant at 300 K. Assuming ideal gas behavior, determine for the CO2 the work and heat transfer, each in kJ. 3.94 A rigid tank, with a volume of 0.054 m3, contains air initially at 138 kPa, 278 K. If the air receives a heat transfer of magnitude 6.3 kJ, determine the final temperature, in K and the final pressure, in kPa. Assume ideal gas behavior, and use (a) a constant specific heat value from Table A-20. (b) a specific heat function from Table A-21. (c) data from Table A-22.

CO2 at 300 K Initially, 4 kg at 550 kPa. Finally, < 4 kg at 225 kPa.

Fig. P3.93 3.95 Air is compressed adiabatically from p1 5 1 bar, T1 5 300 K to p2 5 15 bar, 𝜐2 5 0.1227 m3/kg. The air is then cooled at constant volume to T3 5 300 K. Assuming ideal gas behavior, and ignoring kinetic and potential energy effects, calculate the work for the first process and the heat transfer for the second process, each in kJ per kg of air. Solve the problem each of two ways: (a) using data from Table A-22. (b) using a constant specific heat evaluated at 300 K. 3.96 Helium (He) gas initially at 2 bar, 200 K undergoes a polytropic process, with n 5 k, to a final pressure of 14 bar. Determine the work and heat transfer for the process, each in kJ per kg of helium. Assume ideal gas behavior. 3.97 A piston-cylinder assembly contains carbon monoxide modeled as an ideal gas with constant specific heat ratio, k 5 1.4. The carbon monoxide undergoes a polytropic expansion with n 5 k from an initial state, where T1 5 100°C and p1 5 300 kPa, to a final state, where the volume is twice the initial volume. Determine (a) the final temperature, in °C, and final pressure, in kPa, and (b) the work and heat transfer, each in kJ/kg. 3.98 One kg of air in a piston–cylinder assembly undergoes two processes in series from an initial state where p1 5 0.5 MPa, T1 5 227°C: Process 1–2: Constant–temperature expansion until the volume is twice the initial volume. Process 2–3: Constant-volume heating until the pressure is again 0.5 MPa. Sketch the two processes in series on a p–𝜐 diagram. Assuming ideal gas behavior, determine (a) the pressure at state 2, in MPa, (b) the temperature at state 3, in °C, and for each of the processes (c) the work and heat transfer, each in kJ.

140

Chapter 3 Evaluating Properties

3.99 A piston–cylinder assembly contains air modeled as an ideal gas with a constant specific heat ratio, k 5 1.4. The air undergoes a power cycle consisting of four processes in series:

3.100 One kg of air undergoes a power cycle consisting of the following processes:

Process 1–2: Constant-temperature expansion at 600 K from p1 5 0.5 MPa to p2 5 0.4 MPa.

Process 2–3: Adiabatic expansion to 𝜐2 5 1.4𝜐3

Process 2–3: Polytropic expansion with n 5 k to p3 5 0.3 MPa. Process 3–4:

Constant-pressure compression to V4 5 V1.

Process 4–1:

Constant-volume heating.

Sketch the cycle on a p–𝜐 diagram. Determine (a) the work and heat transfer for each process, in kJ/kg, and (d) the thermal efficiency.

Process 1–2: Constant volume from p1 5 138 kPa, T1 5 278 K to T2 5 455.6 K Process 3–1:

Constant-pressure compression

Sketch the cycle on a p–𝜐 diagram. Assuming ideal gas behavior, determine (a) the pressure at state 2, in kPa. (b) the temperature at state 3, in K. (c) the thermal efficiency of the cycle.

c DESIGN & OPEN ENDED PROBLEMS: EXPLORING ENGINEERING PRACTICE 3.1D Dermatologists remove skin blemishes from patients by applying sprays from canisters filled with liquid nitrogen (N2). Investigate how liquid nitrogen is produced and delivered to physicians, and how physicians manage liquid nitrogen in their practices. Also investigate the advantages and disadvantages of this approach for removing blemishes compared to alternative approaches used today. Write a report including at least three references.

3.6D Design a laboratory flask for containing up to 10 kmol of mercury vapor at pressures up to 3 MPa, and temperatures from 900 to 1000 K. Consider the health and safety of the technicians who would be working with such a mercury vapor-filled container.The p–𝜐–T relation for mercury vapor can be expressed as

3.2D Although used for refrigerators and freezers elsewhere on the globe, hydrocarbon refrigerants have not taken hold in the United States thus far owing to concerns about liability if there is an accident. Research hydrocarbon refrigerant safety. Write a report including at least three references.

3.7D One method of modeling gas behavior from the microscopic viewpoint is the kinetic theory of gases. Using kinetic theory, derive the ideal gas equation of state and explain the variation of the ideal gas specific heat c𝜐 with temperature. Is the use of kinetic theory limited to ideal gas behavior? Summarize your results as a memorandum.

3.3D Methane-laden gas generated by the decomposition of landfill trash has economic value. Research literature on the possible uses of landfill gas. Contact the manager of a large landfill in your locale concerning the capture and use, if any, of landfill gas generated by the facility. Write a report including at least three references. 3.4D Metallurgists use phase diagrams to study allotropic transformations, which are phase transitions within the solid region. What features of the phase behavior of solids are important in the fields of metallurgy and materials processing? Write a report including at least three references. 3.5D Use of the constant-volume calorimeter to measure the calorie value of foods and other substances, as illustrated in Example 3.6, is one of three general types of biological calorimetry. Two others are differential scanning calorimetry and isothermal titration calorimetry. For these two, investigate the objectives and instrumentation of each. Present your findings in a memorandum, including sketches of the respective instrumentation together with summaries of how the instrumentation is used in the laboratory.

p 5 RT/𝜐 2 T/𝜐2exp(10.3338 2 0.0312095/T 2 2.07950ln T) where T is in K, 𝜐 is in m3/kg, and p is in Pa.

3.8D A newspaper article reports that on a day when an airline cancelled eleven flights from Las Vegas because the local temperature was approaching the 47°C operating limit for its jets, another airline canceled seven flights from Denver because the local temperature there was above the 40°C operating level for its propeller planes. Prepare a 30-min. presentation suitable for a middle school science class explaining the technical considerations behind these cancellations. 3.9D Following a rash of vehicle rollover accidents linked to under-inflated tires, the U.S. Congress enacted legislation mandating that new vehicles weighing less than 4500 kg be equipped with a tire-pressure warning system, and directed the U.S. National Highway Traffic Safety Administration to issue a rule. The rule is expected to require car manufacturers to install a four-tire pressure-monitoring system that can warn drivers when a tire is dangerously under-inflated. Microelectromechanical transducers are expected to play a role in such systems. Investigate the technical options for tire pressure-monitoring systems, including the merits of batteryfree systems. Write a report including at least three references.

Design & Open Ended Problems: Exploring Engineering Practice 3.10D Carbon dioxide capture and storage is under consideration today for lessening the effects on global warming and climate change traceable to burning fossil fuels. One frequently mentioned approach, shown in Fig. 3.10D, is to remove CO2 from the exhaust gas of a power plant and inject it into an underground geological formation. Critically evaluate this approach for large-scale management of CO2

generated by human activity, including technical aspects and related costs. Consider the leading methods for CO2 capture from gas streams, issues related to injecting the CO2 at great depths, and the consequences of CO2 migration from storage. Formulate a position in favor of, or in opposition to, the deployment of such technology. Write a report including at least three references.

Electric power CO2 injection CO2 capture facility

Fossil fuel tation Power s

Exhaust gas

CO2 stored in salty aquifer CO2 stored in unminable coal seams CO2 stored in depleted oil and gas reservoirs

Fig. P3.10D

141

4 Control Volume Analysis Using Energy ENGINEERING CONTEXT The objective of this chapter is to develop and illustrate the use of the control volume forms of the conservation of mass and conservation of energy principles. Mass and energy balances for control volumes are introduced in Secs. 4.1 and 4.4, respectively. These balances are applied in Secs. 4.5–4.11 to control volumes at steady state and in Sec. 4.12 for time-dependent (transient) applications. Although devices such as turbines, pumps, and compressors through which mass flows can be analyzed in principle by studying a particular quantity of matter (a closed system) as it passes through the device, it is normally preferable to think of a region of space through which mass flows (a control volume). As in the case of a closed system, energy transfer across the boundary of a control volume can occur by means of work and heat. In addition, another type of energy transfer must be accounted for—the energy accompanying mass as it enters or exits.

LEARNING OUTCOMES When you complete your study of this chapter, you will be able to...

142

c

demonstrate understanding of key concepts related to control volume analysis, including distinguishing between steady-state and transient analysis, distinguishing between mass flow rate and volumetric flow rate, and the meanings of one-dimensional flow and flow work.

c

apply mass and energy balances to control volumes.

c

develop appropriate engineering models for control volumes, with particular attention to analyzing components commonly encountered in engineering practice such as nozzles, diffusers, turbines, compressors, heat exchangers, throttling devices, and integrated systems that incorporate two or more components.

c

use property data in control volume analysis appropriately.

4.1 Conservation of Mass for a Control Volume

Dashed line defines the control volume boundary

4.1 Conservation of Mass for a Control Volume In this section an expression of the conservation of mass principle for control volumes is developed and illustrated. As a part of the presentation, the onedimensional flow model is introduced.

143

Inlet i Exit e

Fig. 4.1 One-inlet, one-exit control volume.

4.1.1 Developing the Mass Rate Balance The mass rate balance for control volumes is introduced by reference to Fig. 4.1, which shows a control volume with mass flowing in at i and flowing out at e, respectively. When applied to such a control volume, the conservation of mass principle states

conservation of mass

time rate of change of time rate of flow time rate of flow of 5 mass contained within the of mass out acrosst mass in across 2 t t s s s control volume at time t exit e at time t inlet i at time t Denoting the mass contained within the control volume at time t by mcv (t), this statement of the conservation of mass principle can be expressed in symbols as dmcv # # 5 mi 2 me dt

(4.1)

# where# dmcvydt is the time rate of change of mass within the control volume, and mi and me are the instantaneous mass flow rates at the inlet and exit, respectively. As # # # # for the symbols W and Q, the dots in the quantities mi and me denote time rates of transfer. In SI, all terms in Eq. 4.1 are expressed in kg/s. For a discussion of the development of Eq. 4.1, see the box. In general, there may be several locations on the boundary through which mass enters or exits. This can be accounted for by summing, as follows: dmcv # # 5 a mi 2 a me dt i e

mass flow rates

(4.2)

Equation 4.2 is the mass rate balance for control volumes with several inlets and exits. It is a form of the conservation of mass principle commonly employed in engineering. Other forms of the mass rate balance are considered in discussions to follow.

Developing the Control Volume Mass Balance For each of the extensive properties mass, energy, and entropy (Chap. 6), the control volume form of the property balance can be obtained by transforming the corresponding closed system form. Let us consider this for mass, recalling that the mass of a closed system is constant. The figures in the margin at the top of the next page show a system consisting of a fixed quantity of matter m that occupies different regions at time t and a later time t 1 Dt. The mass under consideration is shown in color on the figures. At time t, the mass is the sum m 5 mcv(t) 1 mi, where mcv(t) is the mass contained within the control volume, and mi is the mass within the small region labeled i adjacent to the control volume. Let us study the fixed quantity of matter m as time elapses.

mass rate balance

144

Chapter 4 Control Volume Analysis Using Energy Dashed line defines the control volume boundary

mi

mcv(t)

Region i

Time t

In a time interval Dt all the mass in region i crosses the control volume boundary, while some of the mass, call it me, initially contained within the control volume exits to fill the region labeled e adjacent to the control volume. Although the mass in regions i and e as well as in the control volume differ from time t to t 1 Dt, the total amount of mass is constant. Accordingly mcv (t) 1 mi 5 mcv (t 1 D t) 1 me

(a)

mcv (t 1 Dt) 2 mcv (t) 5 mi 2 me

(b)

or on rearrangement

mcv(t + Δt)

me Region e

Equation (b) is an accounting balance for mass. It states that the change in mass of the control volume during time interval Dt equals the amount of mass that enters less the amount of mass that exits. Equation (b) can be expressed on a time rate basis. First, divide by Dt to obtain mcv (t 1 Dt) 2 mcv (t) me mi 5 2 Dt Dt Dt

Time t + Δt

(c)

Then, in the limit as Dt goes to zero, Eq. (c) becomes Eq. 4.1, the instantaneous control volume rate equation for mass # # dmcv 5 mi 2 me dt

(4.1)

where dmcv /dt denotes the time rate of change of mass within the control volume, # # and m i and m e are the inlet and exit mass flow rates, respectively, all at time t.

4.1.2 Evaluating the Mass Flow Rate Vn Δt

Volume of matter

V Δt

dA A

Fig. 4.2 Illustration used to develop an expression for mass flow rate in terms of local fluid properties.

# An expression for the mass flow rate m of the matter entering or exiting a control volume can be obtained in terms of local properties by considering a small quantity of matter flowing with velocity V across an incremental area dA in a time interval Dt, as shown in Fig. 4.2. Since the portion of the control volume boundary through which mass flows is not necessarily at rest, the velocity shown in the figure is understood to be the velocity relative to the area dA. The velocity can be resolved into components normal and tangent to the plane containing dA. In the following development Vn denotes the component of the relative velocity normal to dA in the direction of flow. The volume of the matter crossing dA during the time interval Dt shown in Fig. 4.2 is an oblique cylinder with a volume equal to the product of the area of its base dA and its altitude Vn Dt. Multiplying by the density 𝜌 gives the amount of mass that crosses dA in time Dt amount of mass £ crossing dA during § 5 𝜌(Vn Dt) dA the time interval Dt

Dividing both sides of this equation by Dt and taking the limit as Dt goes to zero, the instantaneous mass flow rate across incremental area dA is instantaneous rate £ of mass flow § 5 𝜌Vn dA across dA

4.1 Conservation of Mass for a Control Volume

145

When this is integrated over the area A through which mass passes, an expression for the mass flow rate is obtained # m5

# 𝜌V dA

(4.3)

n

A

Equation 4.3 can be applied at the inlets and exits to account for the rates of mass flow into and out of the control volume.

4.2 Forms of the Mass Rate Balance The mass rate balance, Eq. 4.2, is a form that is important for control volume analysis. In many cases, however, it is convenient to apply the mass balance in forms suited to particular objectives. Some alternative forms are considered in this section.

4.2.1 One-Dimensional Flow Form of the Mass Rate Balance When a flowing stream of matter entering or exiting a control volume adheres to the following idealizations, the flow is said to be one-dimensional:

one-dimensional flow

c The flow is normal to the boundary at locations where mass enters or exits the TAKE NOTE...

control volume. c All intensive properties, including velocity and density, are uniform with position

(bulk average values) over each inlet or exit area through which matter flows. Figure 4.3 illustrates the meaning of one-dimensional flow. The area through which mass flows is denoted by A. The symbol V denotes a single value that represents the velocity of the flowing air. Similarly T and 𝜐 are single values that represent the temperature and specific volume, respectively, of the flowing air. b b b b b When the flow is one-dimensional, Eq. 4.3 for the mass flow rate becomes # m 5 𝜌AV

(one-dimensional flow)

(4.4a)

# AV m5 (one-dimensional flow) 𝜐

(4.4b)

or in terms of specific volume

e

Area = A Air

V, T, v

Air compressor Air

i

–

+

Fig. 4.3 Figure illustrating the one-dimensional flow model.

In subsequent control volum mee analyses, we routinely assume that the idealizations of one-dimensional flow are appropriate. Accordingly the assumption of onedimensional flow is not listed explicitly in solved examples.

146

Chapter 4 Control Volume Analysis Using Energy

volumetric flow rate

When area is in m2, velocity is in m/s, and specific volume is in m3/kg, the mass flow rate found from Eq. 4.4b is in kg/s, as can be verified. The product AV in Eqs. 4.4 is the volumetric flow rate. The volumetric flow rate is expressed in units of m3/s. Substituting Eq. 4.4b into Eq. 4.2 results in an expression for the conservation of mass principle for control volumes limited to the case of one-dimensional flow at the inlets and exits AiVi A eVe dmcv 5 a 2a (one-dimensional flow) 𝜐 𝜐e dt i i e

(4.5)

Note that Eq. 4.5 involves summations over the inlets and exits of the control volume. Each individual term in these sums applies to a particular inlet or exit. The area, velocity, and specific volume appearing in each term refer only to the corresponding inlet or exit.

4.2.2 Steady-State Form of the Mass Rate Balance steady state

Many engineering systems can be idealized as being at steady state, meaning that all properties are unchanging in time. For a control volume at steady state, the identity of the matter within the control volume changes continuously, but the total amount present at any instant remains constant, so dmcv/dt 5 0 and Eq. 4.2 reduces to # a mi

# a me

5

i

(mass rate in)

(4.6)

e

(mass rate out)

That is, the total incoming and outgoing rates of mass flow are equal. Note that equality of total incoming and outgoing rates of mass flow does not necessarily imply that a control volume is at steady state. Although the total amount of mass within the control volume at any instant would be constant, other properties such as temperature and pressure might be varying with time. When a control volume is at steady state, every property is independent of time. Also note that the steadystate assumption and the one-dimensional flow assumption are independent idealizations. One does not imply the other.

4.2.3 Integral Form of the Mass Rate Balance We consider next the mass rate balance expressed in terms of local properties. The total mass contained within the control volume at an instant t can be related to the local density as follows: mcv(t) 5

# 𝜌 dV

(4.7)

V

where the integration is over the volume at time t. With Eqs. 4.3 and 4.7, the mass rate balance Eq. 4.2 can be written as d dt

mass flux

# 𝜌 dV 5 a a # 𝜌 V dAb 2 a a # 𝜌V dAb n

V

i

A

(4.8)

n

i

e

A

e

where the area integrals are over the areas through which mass enters and exits the control volume, respectively. The product 𝜌Vn appearing in this equation, known as the mass flux, gives the time rate of mass flow per unit of area. To evaluate the terms of the right side of Eq. 4.8 requires information about the variation of the mass flux

4.3 Applications of the Mass Rate Balance

147

over the flow areas. The form of the conservation of mass principle given by Eq. 4.8 is usually considered in detail in fluid mechanics.

4.3 Applications of the Mass Rate Balance 4.3.1 Steady-State Application For a control volume at steady state, the conditions of the mass within the control volume and at the boundary do not vary with time. The mass flow rates also are constant with time. Example 4.1 illustrates an application of the steady-state form of the mass rate balance to a control volume enclosing a mixing chamber called a feedwater heater. Feedwater heaters are components of the vapor power systems considered in Chap. 8.

cccc

EXAMPLE 4.1 c

Applying the Mass Rate Balance to a Feedwater Heater at Steady State A feedwater heater operating at steady state has two inlets and one exit. At inlet 1, water vapor enters at p1 5 7 bar, T1 5 2008C with a mass flow rate of 40 kg/s. At inlet 2, liquid water at p2 5 7 bar, T2 5 408C enters through an area A2 5 25 cm2. Saturated liquid at 7 bar exits at 3 with a volumetric flow rate of 0.06 m3/s. Determine the mass flow rates at inlet 2 and at the exit, in kg/s, and the velocity at inlet 2, in m/s. SOLUTION Known: A stream of water vapor mixes with a liquid water stream to produce a saturated liquid stream at the exit. The states at the inlets and exit are specified. Mass flow rate and volumetric flow rate data are given at one inlet and at the exit, respectively. Find: Determine the mass flow rates at inlet 2 and at the exit, and the velocity V2. Schematic and Given Data: Engineering Model: The control volume shown on the accompanying figure is at steady state. 2

1 T1 = 200°C p1 = 7 bar m1 = 40 kg/s

A2 = 25 cm2 T2 = 40°C p2 = 7 bar 3

Control volume boundary

Saturated liquid p3 = 7 bar (AV)3 = 0.06 m3/s

Fig. E4.1 #

Analysis: The principal relations to be employed are the mass rate balance (Eq. 4.2) and the expression m 5 AV/𝜐

(Eq. 4.4b). At steady state the mass rate balance becomes ➊

# Solving for m2,

dmcv0 # # # 5 m1 1 m2 2 m3 dt # # # m2 5 m3 2 m1

148

Chapter 4 Control Volume Analysis Using Energy

# The mass flow rate m1 is given. The mass flow rate at the exit can be evaluated from the given volumetric flow rate (AV)3 # m3 5 𝜐3 where 𝜐3 is the specific volume at the exit. In writing this expression, one-dimensional flow is assumed. From Table A-3, 𝜐3 5 1.108 3 1023 m3/kg. Hence, # m3 5

0.06 m3/s 5 54.15 kg/s (1.108 3 1023 m3/kg)

The mass flow rate at inlet 2 is then # # # m2 5 m3 2 m1 5 54.15 2 40 5 14.15 kg/s # For one-dimensional flow at 2, m2 5 A2V2 y𝜐 2, so # V2 5 m2 𝜐2 yA 2 State 2 is a compressed liquid. The specific volume at this state can be approximated by 𝜐2 < 𝜐f(T2) (Eq. 3.11). From Table A-2 at 408C, 𝜐2 5 1.0078 3 1023 m3/kg. So, (14.15 kg/s)(1.0078 3 1023 m3/kg) 104 cm2 V2 5 ` ` 5 5.7 m/s 25 cm2 1 m2

✓ Skills Developed Ability to… ❑ apply the steady-state

mass rate balance.

❑ apply the mass flow rate

➊ In accord with Eq. 4.6, the mass flow rate at the exit equals the sum of the mass flow rates at the inlets. It is left as an exercise to show that the volumetric flow rate at the exit does not equal the sum of the volumetric flow rates at the inlets.

expression, Eq. 4.4b.

❑ retrieve property data for

water.

Evaluate the volumetric flow rate, in m3/s, at each inlet. Ans. (AV)1 5 12 m3/s, (AV)2 5 0.01 m3/s

4.3.2 Time-Dependent (Transient) Application Many devices undergo periods of operation during which the state changes with time—for example, the startup and shutdown of motors. Other examples include containers being filled or emptied and applications to biological systems. The steadystate model is not appropriate when analyzing time-dependent (transient) cases. Example 4.2 illustrates a time-dependent, or transient, application of the mass rate balance. In this case, a barrel is filled with water. cccc

EXAMPLE 4.2 c

Filling a Barrel with Water Water flows into the top of an open barrel at a constant mass flow rate of 7 kg/s. Water exits through a pipe near # the base with a mass flow rate proportional to the height of liquid inside: me 5 1.4 L, where L is the instantaneous liquid height, in m. The area of the base is 0.2 m2, and the density of water is 1000 kg/m3. If the barrel is initially empty, plot the variation of liquid height with time and comment on the result. SOLUTION Known: Water enters and exits an initially empty barrel. The mass flow rate at the inlet is constant. At the exit, the mass flow rate is proportional to the height of the liquid in the barrel. Find: Plot the variation of liquid height with time and comment.

4.3 Applications of the Mass Rate Balance

149

Schematic and Given Data: Engineering Model: mi = 7 kg/s

1. The control volume is defined by the dashed line on

the accompanying diagram. 2. The water density is constant. Boundary of control volume

L (m)

A = 0.2 m2

me = 1.4 L kg/s

Fig. E4.2a Analysis: For the one-inlet, one-exit control volume, Eq. 4.2 reduces to

dmcv # # 5 mi 2 me dt The mass of water contained within the barrel at time t is given by mcv (t) 5 𝜌AL(t) where 𝜌 is density, A is the area of the base, and L(t) is the instantaneous liquid height. Substituting this into the mass rate balance together with the given mass flow rates d (𝜌AL) 5 7 2 1.4 L dt

Since density and area are constant, this equation can be written as dL 1.4 7 1a bL 5 dt 𝜌A 𝜌A which is a first-order, ordinary differential equation with constant coefficients. The solution is ➊

L 5 5 1 C exp a2

1.4t b 𝜌A

where C is a constant of integration. The solution can be verified by substitution into the differential equation. To evaluate C, use the initial condition: at t 5 0, L 5 0. Thus, C 5 25.0, and the solution can be written as L 5 5.0[1 2 exp (21.4ty𝜌A)] Substituting 𝜌 5 1000 kg/m and A 5 0.2 m2 results in 3

L 5 5[1 2 exp (20.007t)] This relation can be plotted by hand or using appropriate software. The result is

150

Chapter 4 Control Volume Analysis Using Energy 6

Height L, m

5 4 3 2 1 0

200

400 Time t, s

600

800

Fig. E4.2b

From the graph, we see that initially the liquid height increases rapidly and then levels out as steady-state operation is approached. After about 100 s, the height stays constant with time. At this point, the rate of water flow into the barrel equals the rate of flow out of the barrel. From the graph, the limiting value of L is 5 m, which also can be verified by taking the limit of the analytical solution as t S `. ➊ Alternatively, this differential equation can be solved using Interactive Thermodynamics: IT. The differential equation can be expressed as der(L, t) 1 (1.4 * L)/(rho * A) 5 7/(rho * A) rho 5 1000 // kg/m3

✓ Skills Developed Ability to… ❑ apply the time-dependent

mass rate balance.

A 5 0.2 // m2 where der(L,t) is dL/dt, rho is density 𝜌, and A is area. Using the Explore button, set the initial condition at L 5 0, and sweep t from 0 to 200 in steps of 0.5. Then, the plot can be constructed using the Graph button.

❑ solve an ordinary differen-

tial equation and plot the solution.

If the mass flow rate of the water flowing into the barrel were 12.25 kg/s while all other data remained the same, what would be the limiting value of the liquid height, L, in m? Ans. 0.9 m

BIOCONNECTIONS The human heart provides a good example of how biological systems can be modeled as control volumes. Figure 4.4 shows the cross section of a human heart. The flow is controlled by valves that intermittently allow blood to enter from veins and exit through arteries as the heart muscles pump. Work is done to increase the pressure of the blood leaving the heart to a level that will propel it through the cardiovascular system of the body. Observe that the boundary of the control volume enclosing the heart is not fixed but moves with time as the heart pulses.

Superior vena cava Aorta Artery

Left atrium Valve

Veins Right atrium

Valve

Valve

Left ventricle

Valve Right ventricle

Inferior vena cava

Boundary

Cardiac muscle

Fig. 4.4 Control volume enclosing the heart.

4.4 Conservation of Energy for a Control Volume

151

Understanding the medical condition known as arrhythmia requires consideration of the time-dependent behavior of the heart. An arrhythmia is a change in the regular beat of the heart. This can take several forms. The heart may beat irregularly, skip a beat, or beat very fast or slowly. An arrhythmia may be detectable by listening to the heart with a stethoscope, but an electrocardiogram offers a more precise approach. Although arrhythmia does occur in people without underlying heart disease, patients having serious symptoms may require treatment to keep their heartbeats regular. Many patients with arrhythmia may require no medical intervention at all.

4.4 Conservation of Energy for a Control Volume In this section, the rate form of the energy balance for control volumes is obtained. The energy rate balance plays an important role in subsequent sections of this book.

4.4.1 Developing the Energy Rate Balance for a Control Volume We begin by noting that the control volume form of the energy rate balance can be derived by an approach closely paralleling that considered in the box of Sec. 4.1.1, where the control volume mass rate balance is obtained by transforming the closed system form. The present development proceeds less formally by arguing that, like mass, energy is an extensive property, so it too can be transferred into or out of a control volume as a result of mass crossing the boundary. Since this is the principal difference between the closed system and control volume forms, the control volume energy rate balance can be obtained by modifying the closed-system energy rate balance to account for these energy transfers. Accordingly, the conservation of energy principle applied to a control volume states: time rate of change net rate at which net rate at which net rate of energy of the energy energy is being energy is being transfer into the contained within 5 transferred in 2 transferred out 1 control volume the control volume by heat transfer by work at accompanying at time t at time t time t mass flow

s

s

s

s

s

s

s

s

For the one-inlet one-exit control volume with one-dimensional flow shown in Fig. 4.5 the energy rate balance is # # dEcv V2i V2e # # 5 Q 2 W 1 mi aui 1 1 gzib 2 me aue 1 1 gzeb (4.9) dt 2 2 # where# Ecv denotes the energy of the control volume at time t. The terms Q and W account, respectively, for the net rate of energy transfer by heat and work across the boundary of the control volume at t. The underlined terms account for the rates of transfer of internal, kinetic, and potential energy of the entering and exiting streams. If there is no mass flow in or out, the respective mass flow rates vanish and the underlined terms of Eq. 4.9 drop out. The equation then reduces to the rate form of the energy balance for closed systems: Eq. 2.37. Next, we will place Eq. 4.9 in an alternative form that is more convenient for subsequent applications. This will be accomplished primarily

Q

Energy transfers can occur by heat and work W

Inlet i

Ve2 + gze ue + ___ 2

mi

me

Control volume Vi2 + gzi ui + ___ 2 zi

Exit e

ze

Dashed line defines the control volume boundary

Fig. 4.5 Figure used to develop Eq. 4.9.

152

A

Chapter 4 Control Volume Analysis Using Energy Energy_Rate_Bal_CV A.16 – Tab a

# by recasting the work term W, which represents the net rate of energy transfer by work across all portions of the boundary of the control volume.

4.4.2 Evaluating Work for a Control Volume Because work is always done on or by a control volume where matter flows across # the boundary, it is convenient to separate the work term W of Eq. 4.9 into two contributions: One contribution is the work associated with the fluid pressure as mass# is introduced at inlets and removed at exits. The other contribution, denoted by Wcv, includes all other work effects, such as those associated with rotating shafts, displacement of the boundary, and electrical effects. Consider the work at an exit e associated with the pressure of the flowing matter. Recall from Eq. 2.13 that the rate of energy transfer by work can be expressed as the product of a force and the velocity at the point of application of the force. Accordingly, the rate at which work is done at the exit by the normal force (normal to the exit area in the direction of flow) due to pressure is the product of the normal force, peAe, and the fluid velocity, Ve. That is time rate of energy transfer £ by work from the control § 5 ( pe A e)Ve volume at exit e

(4.10)

where pe is the pressure, Ae is the area, and Ve is the velocity at exit e, respectively. A similar expression can be written for the rate of energy transfer by work into the control volume at inlet i. # With these considerations, the work term W of the energy rate equation, Eq. 4.9, can be written as # # (4.11) W 5 Wcv 1 (pe A e)Ve 2 ( pi A i)Vi

flow work

where, in accordance with the sign convention for work, the term at the inlet has a negative sign because energy is transferred into the control volume there. A positive sign precedes the work term at the exit because energy is transferred out of the # control volume there. With AV 5 m𝜐 from Eq. 4.4b, the above expression for work can be written as # # # # W 5 Wcv 1 me( pe 𝜐e) 2 mi( pi 𝜐i) (4.12) # # where mi and me are the mass flow rates and 𝜐i and 𝜐e are the specific volumes # # evaluated at the inlet and exit, respectively. In Eq. 4.12, the terms mi( pi 𝜐i) and me(pe 𝜐e) account for the work associated with the pressure at the inlet and exit, respectively. # They are commonly referred to as flow work. The term Wcv accounts for all other energy transfers by work across the boundary of the control volume.

4.4.3 One-Dimensional Flow Form of the Control Volume Energy Rate Balance Substituting Eq. 4.12 in Eq. 4.9 and collecting all terms referring to the inlet and the exit into separate expressions, the following form of the control volume energy rate balance results: # # dEcv Vi2 Ve2 # # 5 Qcv 2 Wcv 1 mi aui 1 pi𝜐i 1 1 gzib 2 me aue 1 pe𝜐e 1 1 gzeb (4.13) dt 2 2 # The subscript “cv” has been added to Q to emphasize that this is the heat transfer rate over the boundary (control surface) of the control volume.

4.4 Conservation of Energy for a Control Volume

153

The last two terms of Eq. 4.13 can be rewritten using the specific enthalpy h introduced in Sec. 3.6.1. With h 5 u 1 p𝜐, the energy rate balance becomes # # dEcv V2i Ve2 # # 5 Qcv 2 Wcv 1 mi ahi 1 1 gzib 2 me ahe 1 1 gzeb dt 2 2

(4.14)

The appearance of the sum u 1 p𝜐 in the control volume energy equation is the principal reason for introducing enthalpy previously. It is brought in solely as a convenience: The algebraic form of the energy rate balance is simplified by the use of enthalpy and, as we have seen, enthalpy is normally tabulated along with other properties. In practice there may be several locations on the boundary through which mass enters or exits. This can be accounted for by introducing summations as in the mass balance. Accordingly, the energy rate balance is # # dEcv Vi2 Ve2 # # 5 Qcv 2 Wcv 1 a mi ahi 1 1 gzib 2 a me ahe 1 1 gzeb dt 2 2 i e

4.4.4 Integral Form of the Control Volume Energy Rate Balance As for the case of the mass rate balance, the energy rate balance can be expressed in terms of local properties to obtain forms that are more generally applicable. Thus, the term Ecv(t), representing the total energy associated with the control volume at time t, can be written as a volume integral

#

𝜌e dV 5

V

#

𝜌au 1

V

V2 1 gzb dV 2

(4.16)

Similarly, the terms accounting for the energy transfers accompanying mass flow and flow work at inlets and exits can be expressed as shown in the following form of the energy rate balance: d dt

#

V

# # 𝜌e dV 5 Qcv 2 Wcv 1 a c i

2a c e

# ah 1 A

#

ah 1

A

V2 1 gzb 𝜌Vn dA d 2 i

2

V 1 gzb 𝜌Vn dA d 2 e

A

energy rate balance

(4.15)

In writing Eq. 4.15, the one-dimensional flow model is assumed where mass enters and exits the control volume. Equation 4.15 is an accounting balance for the energy of the control volume. It states that the rate of energy increase or decrease within the control volume equals the difference between the rates of energy transfer in and out across the boundary. The mechanisms of energy transfer are heat and work, as for closed systems, and the energy that accompanies the mass entering and exiting.

Ecv(t) 5

Energy_Rate_Bal_CV A.16 – Tab b

(4.17)

Additional forms # of the energy rate balance can be obtained by expressing the heat transfer rate Qcv as the# integral of the heat flux over the boundary of the control volume, and the work Wcv in terms of normal and shear stresses at the moving portions of the boundary. In principle, the change in the energy of a control volume over a time period can be obtained by integration of the energy rate balance with respect to time. Such integrations require information about the time dependences of the work and heat transfer rates, the various mass flow rates, and the states at which mass enters and leaves the control volume. Examples of this type of analysis are presented in Sec. 4.12.

TAKE NOTE...

Equation 4.15 is the mosstt general form of the conservation of energy principle for control volumes used in this book. It serves as the starting point for applying the conservation of energy principle to control volumes in problem solving.

154

Chapter 4 Control Volume Analysis Using Energy

4.5 Analyzing Control Volumes at Steady State

A

System_Types A.1 – Tab e

In this section we consider steady-state forms of the mass and energy rate balances. These balances are applied to a variety of devices of engineering interest in Secs. 4.6–4.11. The steady-state forms considered here do not apply to the transient startup or shutdown periods of operation of such devices, but only to periods of steady operation. This situation is commonly encountered in engineering.

4.5.1 Steady-State Forms of the Mass and Energy Rate Balances For a control volume at steady state, the conditions of the mass within the control volume and at the boundary do not vary with time. The mass flow rates and the rates of energy transfer by heat and work are also constant with time. There can be no accumulation of mass within the control volume, so dmcv/dt 5 0 and the mass rate balance, Eq. 4.2, takes the form # a mi i

(mass rate in)

5

# me a e

(4.6)

(mass rate out)

Furthermore, at steady state dEcv/dt 5 0, so Eq. 4.15 can be written as # # Vi2 Ve2 # # 0 5 Qcv 2 Wcv 1 a mi ahi 1 1 gzib 2 a me ahe 1 1 gzeb 2 2 e i

(4.18)

Alternatively # # Vi2 V2e # # Qcv 1 a mi ahi 1 (4.19) 1 gzib 5 Wcv 1 a me ahe 1 1 gzeb 2 2 i e (energy rate in) (energy rate out) Equation 4.6 asserts that at steady state the total rate at which mass enters the control volume equals the total rate at which mass exits. Similarly, Eq. 4.19 asserts that the total rate at which energy is transferred into the control volume equals the total rate at which energy is transferred out. Many important applications involve one-inlet, one-exit control volumes at steady state. It is instructive to apply the mass and energy rate balances to this special case. # # The mass rate balance reduces simply to m1 5 m2. That is, the mass flow rate must be the same at the #exit, 2, as it is at the inlet, 1. The common mass flow rate is designated simply by m. Next, applying the energy rate balance and factoring the mass flow rate gives

A

Energy_Rate_Bal_CV A.16 – Tab c

# # (V 21 2 V 22) # 0 5 Qcv 2 Wcv 1 m c (h1 2 h2) 1 1 g(z1 2 z2) d 2

(4.20a)

Or, dividing by the mass flow rate # # (V21 2 V22) Qcv Wcv 0 5 # 2 # 1 (h1 2 h2) 1 1 g (z1 2 z2) 2 m m

(4.20b)

The enthalpy, kinetic energy, and potential energy terms all appear in Eqs. 4.20 as differences between their values at the inlet and exit. This illustrates that the datums

4.5 Analyzing Control Volumes at Steady State

155

used to assign values to enthalpy, velocity, and elevation cancel. In Eq. 4.20b, # specific # # # the ratios Qcv /m and Wcv /m are rates of energy transfer per unit mass flowing through the control volume. The foregoing steady-state forms of the energy rate balance relate only energy transfer quantities evaluated at the boundary of the control volume. No details concerning properties within the control volume are required by, or can be determined with, these equations. When applying the energy rate balance in any of its forms, it is necessary to use the same units for all terms in the equation. For instance, every term in Eq. 4.20b must have a unit such as kJ/kg. Appropriate unit conversions are emphasized in examples to follow.

4.5.2 Modeling Considerations for Control Volumes at Steady State In this section, we provide the basis for subsequent applications by considering the modeling of control volumes at steady state. In particular, several applications are given in Secs. 4.6–4.11 showing the use of the principles of conservation of mass and energy, together with relationships among properties for the analysis of control volumes at steady state. The examples are drawn from applications of general interest to engineers and are chosen to illustrate points common to all such analyses. Before studying them, it is recommended that you review the methodology for problem solving outlined in Sec. 1.9. As problems become more complicated, the use of a systematic problem-solving approach becomes increasingly important. When the mass and energy rate balances are applied to a control volume, simplifications are normally needed to make the analysis manageable. That is, the control volume of interest is modeled by making assumptions. The careful and conscious step of listing assumptions is necessary in every engineering analysis. Therefore, an important part of this section is devoted to considering various assumptions that are commonly made when applying the conservation principles to different types of devices. As you study the examples presented in Secs. 4.6–4.11, it is important to recognize the role played by careful assumption making in arriving at solutions. In each case considered, steady-state operation is assumed. The flow is regarded as one-dimensional at places where mass enters and exits the control volume. Also, at each of these locations equilibrium property relations are assumed to apply.

Smaller Can Be Better Engineers are developing miniature systems for use where weight, portability, and/or compactness are critically important. Some of these applications involve tiny micro systems with dimensions in the micrometer to millimeter range. Other somewhat larger meso-scale systems can measure up to a few centimeters. Microelectromechanical systems (MEMS) combining electrical and mechanical features are now widely used for sensing and control. Medical applications of MEMS include pressure sensors that monitor pressure within the balloon inserted into a blood vessel during angioplasty. Air bags are triggered in an automobile crash by tiny acceleration sensors. MEMS are also found in computer hard drives and printers.

Miniature versions of other technologies are being investigated. One study aims at developing an entire gas turbine power plant the size of a shirt button. Another involves micromotors with shafts the diameter of a human hair. Emergency workers wearing fire-, chemical-, or biological-protection suits might in the future be kept cool by tiny heat pumps imbedded in the suit material. As designers aim at smaller sizes, frictional effects and heat transfers pose special challenges. Fabrication of miniature systems is also demanding. Taking a design from the concept stage to high-volume production can be both expensive and risky, industry representatives say.

156

Chapter 4 Control Volume Analysis Using Energy p

p

pave

pave

t

t (a)

(b)

Fig. 4.6 Pressure variations about an average. (a) Fluctuation. (b) Periodic. # In several of the examples to follow, the heat transfer term Qcv is set to zero in the energy rate balance because it is small relative to other energy transfers across the boundary. This may be the result of one or more of the following factors: c The outer surface of the control volume is well insulated. c The outer surface area is too small for there to be effective heat transfer. c The temperature difference between the control volume and its surroundings is so

small that the heat transfer can be ignored. c The gas or liquid passes through the control volume so quickly that there is not

enough time for significant heat transfer to occur. # The work term Wcv drops out of the energy rate balance when there are no rotating shafts, displacements of the boundary, electrical effects, or other work mechanisms associated with the control volume being considered. The kinetic and potential energies of the matter entering and exiting the control volume are neglected when they are small relative to other energy transfers. In practice, the properties of control volumes considered to be at steady state do vary with time. The steady-state assumption would still apply, however, when properties fluctuate only slightly about their averages, as for pressure in Fig. 4.6a. Steady state also might be assumed in cases where periodic time variations are observed, as in Fig. 4.6b. For example, in reciprocating engines and compressors, the entering and exiting flows pulsate as valves open and close. Other parameters also might be time varying. However, the steady-state assumption can apply to control volumes enclosing these devices if the following are satisfied for each successive period of operation: (1) There is no net change in the total energy and the total mass within the control volume. (2) The time-averaged mass flow rates, heat transfer rates, work rates, and properties of the substances crossing the control surface all remain constant. Next, we present brief discussions and examples illustrating the analysis of several devices of interest in engineering, including nozzles and diffusers, turbines, compressors and pumps, heat exchangers, and throttling devices. The discussions highlight some common applications of each device and the modeling typically used in thermodynamic analysis.

4.6 Nozzles and Diffusers nozzle diffuser

A nozzle is a flow passage of varying cross-sectional area in which the velocity of a gas or liquid increases in the direction of flow. In a diffuser, the gas or liquid decelerates in the direction of flow. Figure 4.7 shows a nozzle in which the cross-sectional area decreases in the direction of flow and a diffuser in which the walls of the flow passage diverge. Observe that as velocity increases pressure decreases, and conversely.

4.6 Nozzles and Diffusers

1

V2 > V1 p2 < p1

V2 < V1 p2 > p1 2

157

2

1

Nozzle

Fig. 4.7 Illustration of a nozzle and a diffuser.

Diffuser

For many readers the most familiar application of a nozzle is its use with a garden hose. But nozzles and diffusers have several important engineering applications. In Fig. 4.8, a nozzle and diffuser are combined in a wind-tunnel test facility. Ducts with converging and diverging passages are commonly used in distributing cool and warm air in building air-conditioning systems. Nozzles and diffusers also are key components of turbojet engines (Sec. 9.11).

4.6.1 Nozzle and Diffuser Modeling Considerations For a control volume enclosing a nozzle or diffuser, the only work is# flow work at locations where mass enters and exits the control volume, so the term Wcv drops out of the energy rate balance. The change in potential energy from inlet to exit is negligible under most conditions. Thus, the underlined terms of Eq. 4.20a (repeated below) drop out, leaving the enthalpy, kinetic energy, and heat transfer terms, as shown by Eq. (a) # # (V12 2 V22) # 0 5 Qcv 2 Wcv 1 m c (h1 2 h2) 1 1 g(z1 2 z2) d 2 # (V 21 2 V 22) # 0 5 Qcv 1 m c (h1 2 h2) 1 d (a) 2 # # where m is the mass flow rate. The term Qcv representing heat transfer with the surroundings normally would be unavoidable (or stray) heat transfer, and this is often small enough relative to the enthalpy and kinetic energy terms that it also can be neglected, giving simply V12 2 V 22 0 5 (h1 2 h 2) 1 a b 2

(4.21)

4.6.2 Application to a Steam Nozzle The modeling introduced in Sec. 4.6.1 is illustrated in the next example involving a steam nozzle. Particularly note the use of unit conversion factors in this application. Flow-straightening screens Acceleration

Nozzle

Deceleration Test section

Diffuser

Fig. 4.8 Wind-tunnel test facility.

Nozzle A.17 – Tabs a, b, & c Diffuser A.18 – Tabs a, b, & c

A

158 cccc

Chapter 4 Control Volume Analysis Using Energy

EXAMPLE 4.3 c

Calculating Exit Area of a Steam Nozzle Steam enters a converging–diverging nozzle operating at steady state with p1 5 40 bar, T1 5 4008C, and a velocity of 10 m/s. The steam flows through the nozzle with negligible heat transfer and no significant change in potential energy. At the exit, p2 5 15 bar, and the velocity is 665 m/s. The mass flow rate is 2 kg/s. Determine the exit area of the nozzle, in m2. SOLUTION Known: Steam flows through a nozzle at steady state with known properties at the inlet and exit, a known mass flow rate, and negligible effects of heat transfer and potential energy. Find: Determine the exit area. Schematic and Given Data: T m = 2 kg/s

T1 = 400°C

1

Engineering Model: 1. The control volume

Insulation p = 40 bar

➊

2

p2 = 15 bar V2 = 665 m/s

p1 = 40 bar T1 = 400°C V1 = 10 m/s

Control volume boundary

2. Heat transfer # is negli-

gible and Wcv 5 0.

p = 15 bar 1

shown on the accompanying figure is at steady state.

3. The change in potential

2 v

energy from inlet to exit can be neglected.

Fig. E4.3 #

Analysis: The exit area can be determined from the mass flow rate m and Eq. 4.4b, which can be arranged to

read A2 5

# m𝜐2 V2

To evaluate A2 from this equation requires the specific volume 𝜐2 at the exit, and this requires that the exit state be fixed. The state at the exit is fixed by the values of two independent intensive properties. One is the pressure p2, which is known. The other is the specific enthalpy h2, determined from the steady-state energy rate balance Eq. 4.20a, as follows: #0 # 0 (V12 2 V22) # 0 5 Qcv 2 Wcv 1 m c (h1 2 h2) 1 1 g(z1 2 z2) d 2 # # The terms Qcv and Wcv are deleted by assumption 2. The change in specific potential energy drops out in accor# dance with assumption 3 and m cancels, leaving 0 5 (h1 2 h2) 1 a

V21 2 V22 b 2

Solving for h2 h2 5 h1 1 a

V12 2 V22 b 2

4.7 Turbines

159

From Table A-4, h1 5 3213.6 kJ/kg. The velocities V1 and V2 are given. Inserting values and converting the units of the kinetic energy terms to kJ/kg results in (10)2 2 (665)2 m2 1N 1 kJ ` da 2 b ` ` ` 3 2 2 s 1 kg ? m/s 10 N ? m 5 3213.6 2 221.1 5 2992.5 kJ/kg

➋

h2 5 3213.6 kJ/kg 1 c

Finally, referring to Table A-4 at p2 5 15 bar with h2 5 2992.5 kJ/kg, the specific volume at the exit is 𝜐2 5 0.1627 m3/kg. The exit area is then A2 5

✓ Skills Developed

(2 kg/s)(0.1627 m3/kg) 5 4.89 3 1024 m2 665 m/s

Ability to… ❑ apply the steady-state

➊ Although equilibrium property relations apply at the inlet and exit of the control volume, the intervening states of the steam are not necessarily equilibrium states. Accordingly, the expansion through the nozzle is represented on the T–𝜐 diagram as a dashed line. ➋ Care must be taken in converting the units for specific kinetic energy to kJ/kg.

energy rate balance to a control volume. ❑ apply the mass flow rate expression, Eq. 4.4b. ❑ develop an engineering model. ❑ retrieve property data for water.

Evaluate the nozzle inlet area, in m2. Ans. 1.47 3 1022 m2.

4.7 Turbines turbine

A turbine is a device in which power is developed as a result of a gas or liquid passing through a set of blades attached to a shaft free to rotate. A schematic of an axialflow steam or gas turbine is shown in Fig. 4.9. Such turbines are widely used for power generation in vapor power plants, gas turbine power plants, and aircraft engines (see Chaps. 8 and 9). In these applications, superheated steam or a gas enters the turbine and expands to a lower pressure as power is generated. A hydraulic turbine coupled to a generator installed in a dam is shown in Fig. 4.10. As water flows from higher to lower elevation through the turbine, the turbine provides shaft power to the generator. The generator converts shaft power to electricity. This type of generation is called hydropower. Today, hydropower is a leading renewable means for producing electricity, and it is one of the least expensive ways to do so. Electricity can also be produced from flowing water by using turbines to tap into currents in oceans and rivers. Turbines are also key components of wind-turbine power plants that, like hydropower plants, are renewable means for generating electricity.

Stationary blades

Rotating blades

Fig. 4.9 Schematic of an axial-flow steam or gas turbine.

160

Chapter 4 Control Volume Analysis Using Energy

Reservoir Dam

Intake

Generator

Hydraulic turbine

Fig. 4.10 Hydraulic turbine installed in a dam.

ENERGY & ENVIRONMENT Industrial-scale wind turbines can stand as tall as a 30-story building and produce electricity at a rate meeting the needs of hundreds of typical U.S. homes. The three-bladed rotors of these wind turbines have a diameter nearly the length of a football field and can operate in winds up to 90 km per hour. They feature microprocessor control of all functions, ensuring each blade is pitched at the correct angle for current wind conditions. Wind farms consisting of dozens of such turbines increasingly dot the landscape over the globe. In the United States, wind farms at favorable sites in several Great Plains states alone could supply much of the nation’s electricity needs—provided the electrical grid is upgraded and expanded (see Horizons on p. 372). Offshore wind farms along the U.S. coastline could also contribute significantly to meeting national needs. Experts say wind variability can be managed by producing maximum power when winds are strong and storing some, or all, of the power by various means, including pumped-hydro storage and compressed-air storage, for distribution when consumer demand is highest and electricity has its greatest economic value (see box in Sec. 4.8.3). Wind power can produce electricity today at costs competitive with all alternative means and within a few years is expected to be among the least costly ways to do it. Wind energy plants take less time to build than conventional power plants and are modular, allowing additional units to be added as warranted. While generating electricity, wind-turbine plants produce no global warming gases or other emissions. The industrial-scale wind turbines considered thus far are not the only ones available. Companies manufacture smaller, relatively inexpensive wind turbines that can generate electricity with wind speeds as low as 3 or 4 miles per hour. These low-wind turbines are suitable for small businesses, farms, groups of neighbors, or individual users.

4.7 Turbines

161

4.7.1 Steam and Gas Turbine Modeling Considerations With a proper selection of the control volume enclosing a steam or gas turbine, the net kinetic energy of the matter flowing across the boundary is usually small enough to be neglected. The net potential energy of the flowing matter also is typically negligible. Thus, the underlined terms of Eq. 4.20a (repeated below) drop out, leaving the power, enthalpy, and heat transfer terms, as shown by Eq. (a) # # (V12 2 V22) # 0 5 Qcv 2 Wcv 1 m c (h1 2 h2) 1 1 g(z1 2 z2) d 2 # # # 0 5 Qcv 2 Wcv 1 m(h1 2 h2)

(a)

# where m is the mass flow rate. The only heat transfer between the turbine and surroundings normally would be unavoidable (or stray) heat transfer, and this is often small enough relative to the power and enthalpy terms that it also can be neglected, giving simply # # (b) Wcv 5 m(h1 2 h2)

Turbine A.19 – Tabs a, b, & c

A

4.7.2 Application to a Steam Turbine In this section, modeling considerations for turbines are illustrated by application to a case involving the practically important steam turbine. Objectives in this example include assessing the significance of the heat transfer and kinetic energy terms of the energy balance and illustrating the appropriate use of unit conversion factors.

cccc

EXAMPLE 4.4 c

Calculating Heat Transfer from a Steam Turbine Steam enters a turbine operating at steady state with a mass flow rate of 4600 kg/h. The turbine develops a power output of 1000 kW. At the inlet, the pressure is 60 bar, the temperature is 4008C, and the velocity is 10 m/s. At the exit, the pressure is 0.1 bar, the quality is 0.9 (90%), and the velocity is 30 m/s. Calculate the rate of heat transfer between the turbine and surroundings, in kW. SOLUTION Known: A steam turbine operates at steady state. The mass flow rate, power output, and states of the steam at the inlet and exit are known. Find: Calculate the rate of heat transfer. Schematic and Given Data:

m· 1 = 4600 kg/h p1 = 60 bar T1 = 400°C 1 V1 = 10 m/s

T1 = 400°C

1. The control volume shown on

the accompanying figure is at steady state.

p = 60 bar

2. The change in potential en· Wcv = 1000 kW 2 p2 = 0.1 bar x2 = 0.9 (90%) V2 = 30 m/s

Fig. E4.4

Engineering Model:

1

T

ergy from inlet to exit can be neglected.

p = 0.1 bar 2 v

162

Chapter 4 Control Volume Analysis Using Energy

Analysis: To calculate the heat transfer rate, begin with the one-inlet, one-exit form of the energy rate balance for a control volume at steady state, Eq. 4.20a. That is

# # (V12 2 V22) # 0 5 Qcv 2 Wcv 1 m c (h1 2 h2) 1 1 g(z1 2 z2) d 2 # # where m is the mass flow rate. Solving for Qcv and dropping the potential energy change from inlet to exit # # V22 2 V12 # Qcv 5 Wcv 1 m c(h2 2 h1) 1 a bd 2

(a)

To compare the magnitudes of the enthalpy and kinetic energy terms, and stress the unit conversions needed, each of these terms is evaluated separately. First, the specific enthalpy difference h2 2 h1 is found. Using Table A-4, h1 5 3177.2 kJ/kg. State 2 is a twophase liquid–vapor mixture, so with data from Table A-3 and the given quality h2 5 hf 2 1 x2(hg2 2 hf 2) 5 191.83 1 (0.9)(2392.8) 5 2345.4 kJ/kg Hence h2 2 h1 5 2345.4 2 3177.2 5 2831.8 kJ/kg Consider next the specific kinetic energy difference. Using the given values for the velocities ➊

a

(30)2 2 (10)2 m2 V22 2 V12 1N 1 kJ ` b5 c da 2 b ` ` ` 3 2 2 2 s 1 kg ? m/s 10 N ? m 5 0.4 kJ/kg

# Calculating Qcv from Eq. (a)

➋

# kg kJ 1h 1 kW Qcv 5 (1000 kW) 1 a4600 b(2831.8 1 0.4)a b ` ` ` ` h kg 3600 s 1 kJ/s 5 262.3 kW

➊ The magnitude of the change in specific kinetic energy from inlet to exit is much smaller than the specific enthalpy change. Note the use of unit conver# sion factors here and in the calculation of Qcv to follow. # ➋ The negative value of Qcv means that there is heat transfer from the # turbine to its surroundings, as would be expected. The magnitude of Qcv is small relative to the power developed.

If the change in kinetic energy from inlet to exit were neglected, evaluate the heat transfer rate, in kW, keeping all other data unchanged. Comment. Ans. 262.9 kW

✓ Skills Developed Ability to… ❑ apply the steady-state

energy rate balance to a control volume. ❑ develop an engineering model. ❑ retrieve property data for water.

4.8 Compressors and Pumps

4.8 Compressors and Pumps Compressors and pumps are devices in which work is done on the substance

flowing through them in order to change the state of the substance, typically to increase the pressure and/or elevation. The term compressor is used when the substance is a gas (vapor) and the term pump is used when the substance is a liquid. Four compressor types are shown in Fig. 4.11. The reciprocating compressor of Fig. 4.11a features reciprocating motion while the others have rotating motion. The axial-flow compressor of Fig. 4.11b is a key component of turbojet engines (Sec. 9.11). Compressors also are essential components of refrigeration and heat pump systems (Chap. 10). In the study of Chap. 8, we find that pumps are important in vapor power systems. Pumps also are commonly used to fill water towers, remove water from flooded basements, and for numerous other domestic and industrial applications.

163

compressor, pump Inlet

Outlet (a) Reciprocating Inlet

Rotor Stator Outlet Outlet

4.8.1 Compressor and Pump Modeling Considerations

Inlet (b) Axial flow

For a control volume enclosing a compressor, the mass and energy rate balances reduce at steady state as for the case of turbines considered in Sec. 4.7.1. Thus, Eq. 4.20a reduces to read # # # 0 5 Qcv 2 Wcv 1 m(h1 2 h2) (a) Heat transfer with the surroundings is frequently a secondary effect that can be neglected, giving as for turbines # # Wcv 5 m(h1 2 h2) (b)

Outlet

Impeller

Inlet

Driveshaft (c) Centrifugal

For pumps, heat transfer is generally a secondary effect, but the kinetic and potential energy terms of Eq. 4.20a may be significant depending # on the application. Be sure to note that for compressors and pumps, the value of Wcv is negative because a power input is required.

Outlet

4.8.2 Applications to an Air Compressor and a Pump System In this section, modeling considerations for compressors and pumps are illustrated in Examples 4.5 and 4.6, respectively. Applications of compressors and pumps in energy storage systems are described in Sec. 4.8.3. In Example 4.5 the objectives include assessing the significance of the heat transfer and kinetic energy terms of the energy balance and illustrating the appropriate use of unit conversion factors.

Inlet (d) Roots type

Fig. 4.11 Compressor types.

cccc

EXAMPLE 4.5 c

Calculating Compressor Power Air enters a compressor operating at steady state at a pressure of 1 bar, a temperature of 290 K, and a velocity of 6 m/s through an inlet with an area of 0.1 m2. At the exit, the pressure is 7 bar, the temperature is 450 K, and the velocity is 2 m/s. Heat transfer from the compressor to its surroundings occurs at a rate of 180 kJ/min. Employing the ideal gas model, calculate the power input to the compressor, in kW.

164

Chapter 4 Control Volume Analysis Using Energy

SOLUTION Known: An air compressor operates at steady state with known inlet and exit states and a known heat transfer rate. Find: Calculate the power required by the compressor. Schematic and Given Data: · Wcv = ? p1 = 1 bar T1 = 290 K 1 V1 = 6 m/s A1 = 0.1m2

Air compressor

Engineering Model: 1. The control volume shown on the accompanying figure is at 2 p2 = 7 bar T2 = 450 K V2 = 2 m/s

· Qcv = –180 kJ/min

steady state. 2. The change in potential energy from inlet to exit can be neglected.

➊ 3. The ideal gas model applies for the air.

Fig. E4.5 Analysis: To calculate the power input to the compressor, begin with the one-inlet, one-exit form of the energy rate balance for a control volume at steady state, Eq. 4.20a. That is

# # (V12 2 V22) # 0 5 Qcv 2 Wcv 1 m c (h1 2 h2) 1 1 g(z1 2 z2) d 2 Solving # # V 21 2 V 22 # Wcv 5 Qcv 1 m c (h 1 2 h 2) 1 a bd 2 The change in potential energy from inlet to exit drops out by assumption 2. # The mass flow rate m can be evaluated with given data at the inlet and the ideal gas equation of state (0.1 m2)(6 m/s)(10 5 N/m2) A1V1 p1 A1 V 1 # 5 5 0.72 kg/s m5 5 𝜐1 (R /M)T1 8314 N ? m a b(290 K) 28.97 kg ? K A-22. At 290 K, h1 5 290.16 kJ/kg. At 450 K, h2 5 The specific enthalpies h1 and h2 can be found from Table # 451.8 kJ/kg. Substituting values into the expression for Wcv, and applying appropriate unit conversion factors, we get # kg kJ 1 min kJ Wcv 5 a2180 b` ` 1 0.72 c (290.16 2 451.8) s min 60 s kg

➋

(6)2 2 (2)2 m2 1N 1 kJ ` d ba 2 b ` ` ` 1a 2 s 1 kg ? m/s2 103 N ? m kg kJ kJ 5 23 1 0.72 (2161.64 1 0.02) s s kg kJ 1 kW 5 2119.4 ` ` 5 2119.4 kW s 1 kJ/s

➊ The applicability of the ideal gas model can be checked by reference to the generalized compressibility chart. # # ➋ In this example Qcv and Wcv have negative values, indicating that the direction of the heat transfer is from the compressor and work is done on the air passing through the compressor. The magnitude of the power input to the compressor is 119.4 kW. The change in kinetic energy does not contribute significantly. If the change in kinetic energy from inlet to exit were neglected, evaluate the compressor power, in kW, keeping all other data unchanged. Comment. Ans. 2119.4 kW.

✓ Skills Developed Ability to… ❑ apply the steady-state

energy rate balance to a control volume. ❑ apply the mass flow rate expression, Eq. 4.4b. ❑ develop an engineering model. ❑ retrieve property data of air modeled as an ideal gas.

4.8 Compressors and Pumps In Example 4.6, a pump is a component of an overall system that delivers a highvelocity stream of water at an elevation greater than at the inlet. Note the modeling considerations in this case, particularly the roles of kinetic and potential energy, and the use of appropriate unit conversion factors.

cccc

Compressor A.20 – Tabs a, b, & c

165

A

Pump A.21 – Tabs a, b, & c

EXAMPLE 4.6 c

Analyzing a Pump System A pump steadily draws water from a pond at a volumetric flow rate of 0.83 m3/min through a pipe having a 12-cm diameter inlet. The water is delivered through a hose terminated by a converging nozzle. The nozzle exit has a diameter of 3 cm and is located 10 m above the pipe inlet. Water enters at 208C, 1 atm and exits with no significant change in temperature or pressure. The magnitude of the rate of heat transfer from the pump to the surroundings is 5% of the power input. The acceleration of gravity is 9.81 m/s2. Determine (a) the velocity of the water at the inlet and exit, each in m/s, and (b) the power required by the pump, in kW. SOLUTION Known: A pump system operates at steady state with known inlet and exit conditions. The rate of heat transfer from the pump is specified as a percentage of the power input. Find: Determine the velocities of the water at the inlet and exit of the pump system and the power required. Schematic and Given Data: Engineering Model:

2 D2 = 3 cm

1. A control volume encloses the pump,

inlet pipe, and delivery hose. 2. The control volume is at steady state. 3. The magnitude of the heat transfer from 10 m

the control volume is 5% of the power input.

Pump + –

4. There is no significant change in tempera-

ture or pressure. 5. For liquid water, 𝜐 < 𝜐f (T) (Eq. 3.11) and

Eq. 3.13 is used to evaluate specific enthalpy.

1

6. g 5 9.81 m/s2.

T1 = 20°C p1 = 1 atm D1 = 12 cm (AV)1 = 0.83 m3/min

Fig. E4.6 Analysis:

#

#

(a) # A mass rate balance reduces at steady state to read m2 5 m1. The common mass flow rate at the inlet and exit,

m, can be evaluated using Eq. 4.4b together with 𝜐 < 𝜐f (20°C) 5 1.0018 3 1023 m3/kg from Table A-2. That is, 0.83 m3/min 1 min AV # 5a b` m5 ` 𝜐 1.0018 3 1023 m3/kg 60 s kg 5 13.8 s

166

Chapter 4 Control Volume Analysis Using Energy

Thus, the inlet and exit velocities are, respectively, # (13.8 kg/s)(1.0018 3 1023 m3/kg) m𝜐 ➊ 5 1.22 m/s V1 5 5 A1 𝜋(0.12 m)2/4 # (13.8 kg/s)(1.0018 3 1023 m3/kg) m𝜐 V2 5 5 19.56 m/s 5 A2 𝜋(0.03 m)2/4 (b) To calculate the power input, begin with the one-inlet, one-exit form of the energy rate balance for a control

volume at steady state, Eq. 4.20a. That is # # V12 2 V22 # 0 5 Qcv 2 Wcv 1 m c (h1 2 h2) 1 a b 1 g(z1 2 z2) d 2 # # # ➋ Introducing Qcv 5 (0.05)Wcv, and solving for Wcv, # # V 21 2 V 22 m Wcv 5 c(h1 2 h2) 1 a b 1 g(z1 2 z2)d 0.95 2

(a)

Using Eq. 3.13, the enthalpy term is expressed as h1 2 h2 5 [hf (T1) 1 𝜐f (T1)[p1 2 psat(T1)]] 2 [hf (T2) 1 𝜐f (T2)[p2 2 psat(T2)]]

(b)

Since there is no significant change in temperature, Eq. (b) reduces to h1 2 h2 5 𝜐f (T)(p1 2 p2) As there is also no significant change in pressure, the enthalpy term drops out of the present analysis. Next, evaluating the kinetic energy term 2 2 2 m 2 (19.56) ]a b [(1.22) s V12 2 V22 1N 1 kJ ` 5 20.191 kJ/kg 5 ` ` ` 3 2 2 2 1 kg ? m/s 10 N ? m

Finally, the potential energy term is g(z1 2 z2) 5 (9.81 m/s2)(0 2 10)m `

1N 1 kJ ` 5 20.098 kJ/kg ` ` 3 2 1 kg ? m/s 10 N ? m

Inserting values into Eq. (a) # 13.8 kg/s kJ 1 kW Wcv 5 a b [0 2 0.191 2 0.098] a b ` ` 0.95 kg 1 kJ/s 5 24.2 kW where the minus sign indicates that power is provided to the pump. ➊ Alternatively, V1 can be evaluated from the volumetric flow rate at 1. This is left as an exercise. # ➋ Since power is required to operate the pump, Wcv is negative in accord with our sign convention. The energy # transfer by heat is from the control volume # to the surroundings, and thus Q cv is # negative as well. Using the value of Wcv # found in part (b), Qcv 5 (0.05) Wcv 5 20.21 kW.

If the nozzle were removed and water exited directly from the hose, whose diameter is 5 cm, determine the velocity at the exit, in m/s, and the power required, in kW, keeping all other data unchanged. Ans. 7.04 m/s, 1.77 kW.

✓ Skills Developed Ability to… ❑ apply the steady-state

energy rate balance to a control volume. ❑ apply the mass flow rate expression, Eq. 4.4b. ❑ develop an engineering model. ❑ retrieve properties of liquid water.

4.8 Compressors and Pumps

167

4.8.3 Pumped-Hydro and Compressed-Air Energy Storage Owing to the dictates of supply and demand and other economic factors, the value of electricity varies with time. Both the cost to generate electricity and increasingly the price paid by consumers for electricity depend on whether the demand for it is on-peak or off-peak. The on-peak period is typically weekdays—for example from 8 a.m. to 8 p.m., while off-peak includes nighttime hours, weekends, and major holidays. Consumers can expect to pay more for on-peak electricity. Energy storage methods benefiting from variable electricity rates include thermal storage (see box on p. 98) and pumped-hydro and compressed-air storage introduced in the following box.

TAKE NOTE...

Cost refers to the amountt paid to produce a good or service. Price refers to what consumers pay to acquire that good or service.

Economic Aspects of Pumped-Hydro and Compressed-Air Energy Storage Despite the significant costs of owning and operating utility-scale energy storagy systems, various economic strategies, including taking advantage of differing on- and off-peak electricity rates, can make pumped-hydro and compressed-air storage good choices for power generators. In this discussion, we focus on the role of variable electricity rates. In pumped-hydro storage, water is pumped from a lower reservoir to an upper reservoir, thereby storing energy in the form of gravitational potential energy. (For simplicity, think of the hydropower plant of Fig. 4.10 operating in the reverse direction.) Off-peak electricity is used to drive the pumps that deliver water to the upper reservoir. Later, during an on-peak period, stored water is released from the upper reservoir to generate electricity as the water flows through turbines to the lower reservoir. For instance, in the summer water is released from the upper reservoir to generate power to meet a high daytime demand for air conditioning; while at night, when demand is low, water is pumped back to the upper reservoir for use the next day. Owing to friction and other nonidealities, an overall input-to-output loss of electricity occurs with pumped-hydro storage and this adds to operating costs. Still, differing daytime and nighttime electricity rates help make this technology viable. In compressed-air energy storage, compressors powered with off-peak electricity fill an underground salt cavern, hard-rock mine, or aquifer with pressurized air drawn from the atmosphere. See Fig. 4.12. When electricity demand peaks, high-pressure compressed air is released to the surface, heated by natural gas in combustors, and expanded through a turbine generator, generating electricity for distribution at on-peak rates.

Atmospheric air

Air and combustion products Fuel to combustor (not shown)

Off-peak + electricity – in

Compressor

Turbine Generator

Compressed air in

+ On-peak electricity – out

Compressed air out

Air in

Air out Cavern

Fig. 4.12 Compressed-air storage.

168

Chapter 4 Control Volume Analysis Using Energy

(a)

(b)

(c)

(d)

Fig. 4.13 Common heat exchanger types. (a) Direct contact heat exchanger. (b) Tube-within-a-tube counterflow heat exchanger. (c) Tube-within-a-tube parallel flow heat exchanger. (d ) Cross-flow heat exchanger.

4.9 Heat Exchangers heat exchanger

Heat exchangers have innumerable domestic and industrial applications, including use in home heating and cooling systems, automotive systems, electrical power generation, and chemical processing. Indeed, nearly every area of application listed in Table 1.1 (p. 3) involves heat exchangers. One common type of heat exchanger is a mixing chamber in which hot and cold streams are mixed directly as shown in Fig. 4.13a. The open feedwater heater, which is a component of the vapor power systems considered in Chap. 8, is an example of this type of device. Another common type of exchanger is one in which a gas or liquid is separated from another gas or liquid by a wall through which energy is conducted. These heat exchangers, known as recuperators, take many different forms. Counterflow and parallel tubewithin-a-tube configurations are shown in Figs. 4.13b and 4.13c, respectively. Other configurations include cross-flow, as in automobile radiators, and multiple-pass shell-and-tube condensers and evaporators. Figure 4.13d illustrates a cross-flow heat exchanger.

BIOCONNECTIONS Inflatable blankets such as shown in Fig. 4.14 are used to prevent subnormal body temperatures (hypothermia) during and after surgery. Typically, a heater and blower direct a stream of warm air into the blanket. Air exits the blanket through perforations in its surface. Such thermal blankets have been used safely and without incident in millions of surgical procedures. Still, there are obvious risks to patients if temperature controls fail and overheating occurs. Such risks can be anticipated and minimized with good engineering practices. Warming patients is not always the issue at hospitals; sometimes it is cooling, as in cases involving cardiac arrest, stroke, heart attack, and overheating of the body (hyperthermia). Cardiac arrest, for example, deprives the heart muscle of oxygen and blood, causing part of it to die. This often induces brain damage among survivors, including irreversible cognitive disability. Studies show when the core body temperature of cardiac

4.9 Heat Exchangers

169

patients is reduced to about 338C, damage is limited because vital organs function more slowly and require less oxygen. To achieve good outcomes, medical specialists say cooling should be done in about 20 minutes or less. A system approved for cooling cardiac arrest victims includes a disposable plastic body suit, pump, and chiller. The pump provides rapidly flowing cold water around the body, in direct contact with the skin of the patient wearing the suit, then recycles coolant to the chiller and back to the patient. These biomedical applications provide examples of how engineers well versed in thermodynamics principles can bring into the design process their knowledge of heat exchangers, temperature sensing and control, and safety and reliability requirements.

4.9.1 Heat Exchanger Modeling Considerations As shown by Fig. 4.13, heat exchangers can involve multiple inlets and exits. For a control volume enclosing a heat exchanger, the # only work is flow work at the places where matter enters and exits, so the term Wcv drops out of the energy rate balance. In addition, the kinetic and potential energies of the flowing streams usually can be ignored at the inlets and exits. Thus, the underlined terms of Eq. 4.18 (repeated below) drop out, leaving the enthalpy and heat transfer terms, as shown by Eq. (a). That is, # # Vi2 Ve2 # # 0 5 Qcv 2 Wcv 1 a mi ahi 1 1 gzib 2 a me ahe 1 1 gzeb 2 2 i e # # # 0 5 Qcv 1 a mi hi 2 a me he i

(a)

e

Although high rates of energy transfer within the heat exchanger occur, # heat transfer with the surroundings is often small enough to be neglected. Thus, the Qcv term of Eq. (a) would drop out, leaving just the enthalpy terms. The final form of the energy rate balance must be solved together with an appropriate expression of the mass rate balance, recognizing both the number and type of inlets and exits for the case at hand.

4.9.2 Applications to a Power Plant Condenser and Computer Cooling The next example illustrates how the mass and energy rate balances are applied to a condenser at steady state. Condensers are commonly found in power plants and refrigeration systems.

Heaterblower unit

Fig. 4.14 Inflatable thermal blanket.

Heat_Exchanger A.22 – Tabs a, b & c

A

170 cccc

Chapter 4 Control Volume Analysis Using Energy

EXAMPLE 4.7 c

Evaluating Performance of a Power Plant Condenser Steam enters the condenser of a vapor power plant at 0.1 bar with a quality of 0.95 and condensate exits at 0.1 bar and 458C. Cooling water enters the condenser in a separate stream as a liquid at 208C and exits as a liquid at 358C with no change in pressure. Heat transfer from the outside of the condenser and changes in the kinetic and potential energies of the flowing streams can be ignored. For steady-state operation, determine (a) the ratio of the mass flow rate of the cooling water to the mass flow rate of the condensing steam. (b) the rate of energy transfer from the condensing steam to the cooling water, in kJ per kg of steam passing

through the condenser. SOLUTION Known: Steam is condensed at steady state by interacting with a separate liquid water stream. Find: Determine the ratio of the mass flow rate of the cooling water to the mass flow rate of the steam and the

rate of energy transfer from the steam to the cooling water. Schematic and Given Data: Condensate 0.1 bar 2 45°C

Engineering Model:

Steam 0.1 bar x = 0.95

1

1. Each of the two control volumes shown

on the accompanying sketch is at steady state.

T Cooling water 20°C

3

4

Control volume for part (a)

Condensate

2

1

2. There is no significant heat transfer

Cooling water 35°C

0.1 bar 45.8°C

Steam

3. Changes in the kinetic and potential

1

energies of the flowing streams from inlet to exit can be ignored.

2 4 3

Energy transfer to cooling water

between the overall condenser and its # surroundings. Wcv 5 0.

v

4. At states 2, 3, and 4, h < hf(T) (see

Eq. 3.14).

Control volume for part (b)

Fig. E4.7 Analysis: The steam and the cooling water streams do not mix. Thus, the mass rate balances for each of the two streams reduce at steady state to give # # # # m1 5 m2 and m3 5 m4 # # (a) The ratio of the mass flow rate of the cooling water to the mass flow rate of the condensing steam, m3 /m1, can be found from the steady-state form of the energy rate balance, Eq. 4.18, applied to the overall condenser as follows:

# # V32 V12 # # 0 5 Qcv 2 Wcv 1 m 1 ah1 1 1 gz1b 1 m3 ah3 1 1 gz3b 2 2 V 22 V 24 # # 1 gz2b 2 m4 ah4 1 1 gz4b 2 m2 ah 2 1 2 2 The underlined terms drop out by assumptions 2 and 3. With these simplifications, together with the above mass flow rate relations, the energy rate balance becomes simply # # 0 5 m1(h1 2 h2) 1 m3(h3 2 h4) Solving, we get # m3 h1 2 h2 # 5 h4 2 h3 m1

4.9 Heat Exchangers

171

The specific enthalpy h1 can be determined using the given quality and data from Table A-3. From Table A-3 at 0.1 bar, hf 5 191.83 kJ/kg and hg 5 2584.7 kJ/kg, so h1 5 191.83 1 0.95(2584.7 2 191.83) 5 2465.1 kJ/kg ➊ Using assumption 4, the specific enthalpy at 2 is given by h2 < hf(T2) 5 188.45 kJ/kg. Similarly, h3 < hf(T3) and h4 < hf(T4), giving h4 2 h3 5 62.7 kJ/kg. Thus, # m3 2465.1 2 188.45 5 36.3 # 5 m1 62.7 (b) For a control volume enclosing the steam side of the condenser only, begin with the steady-state form of

energy rate balance, Eq. 4.20a: # # (V 21 2 V 22) # 0 5 Qcv 2 Wcv 1 m1 c(h1 2 h2) 1 1 g(z1 2 z2)d 2

➋

The underlined terms drop out by assumptions 2 and 3. The following expression for the rate of energy transfer between the condensing steam and the cooling water results: # # Qcv 5 m1(h2 2 h1) # Dividing by the mass flow rate of the steam, m1, and inserting values # Qcv # 5 h2 2 h1 5 188.45 2 2465.1 5 22276.7 kJ/kg m1 where the minus sign signifies that energy is transferred from the condensing steam to the cooling water. ➊ Alternatively, (h4 2 h3) can be evaluated using the incompressible liquid model via Eq. 3.20b. ➋ Depending on where the boundary of the control volume is located, two different formulations of the energy rate balance are obtained. In part (a), both streams are included in the control volume. Energy transfer between them occurs # internally and not across the boundary of the control volume, so the term Qcv drops out of the energy rate balance. With the control volume # of part (b), however, the term Qcv must be included.

✓ Skills Developed Ability to… ❑ apply the steady-state

mass and energy rate balances to a control volume. ❑ develop an engineering model. ❑ retrieve property data for water.

If the mass flow rate of the condensing steam is 125 kg/s, determine the mass flow rate of the cooling water, in kg/s. Ans. 4538 kg/s.

Excessive temperatures in electronic components are avoided by providing appropriate cooling. In the next example, we analyze the cooling of computer components, illustrating the use of the control volume form of energy rate balance together with property data for air.

cccc

EXAMPLE 4.8 c

Cooling Computer Components The electronic components of a computer are cooled by air flowing through a fan mounted at the inlet of the electronics enclosure. At steady state, air enters at 208C, 1 atm. For noise control, the velocity of the entering air cannot exceed 1.3 m/s. For temperature control, the temperature of the air at the exit cannot exceed 328C. The electronic components and fan receive, respectively, 80 W and 18 W of electric power. Determine the smallest fan inlet area, in cm2, for which the limits on the entering air velocity and exit air temperature are met.

172

Chapter 4 Control Volume Analysis Using Energy

SOLUTION Known: The electronic components of a computer are cooled by air flowing through a fan mounted at the inlet of the electronics enclosure. Conditions are specified for the air at the inlet and exit. The power required by the electronics and the fan are also specified. Find: Determine the smallest fan area for which the specified limits are met. Schematic and Given Data: +

Engineering Model:

–

1. The control volume shown on the accompa-

nying figure is at steady state.

T2 ≤ 32°C Air out 2

Electronic components

2. Heat transfer from the outer surface of the

electronics enclosure to the surroundings is # negligible. Thus, Qcv 5 0.

Fan

➊ 3. Changes in kinetic and potential energies can be ignored.

1

Air in

➋ 4. Air is modeled as an ideal gas with cp 5 1.005 kJ/kg ? K.

T1 = 20°C p1 = 1 atm V1 ≤ 1.3 m/s

Fig. E4.8 #

Analysis: The inlet area A1 can be determined from the mass flow rate m and Eq. 4.4b, which can be rearranged

to read

# m𝜐1 A1 5 V1

(a)

The mass flow rate can be evaluated, in turn, from the steady-state energy rate balance, Eq. 4.20a: # # V12 2 V 22 # 0 5 Qcv 2 Wcv 1 m c (h1 2 h2) 1 a b 1 g(z1 2 z2) d 2 The underlined terms drop out by assumptions 2 and 3, leaving # # 0 5 2Wcv 1 m(h1 2 h2) # # where Wcv accounts for the total electric power provided to the electronic components and the fan: Wcv 5 # (280 W) 1 (218 W) 5 298 W. Solving for m, and using assumption 4 with Eq. 3.51 to evaluate (h1 2 h2) # (2Wcv) # m5 cp(T2 2 T1) Introducing this into the expression for A1, Eq. (a), and using the ideal gas model to evaluate the specific volume 𝜐1 ?

(2Wcv) RT1 1 A1 5 c da b p1 V1 cp(T2 2 T1) From this expression we see that A1 increases when V1 and/or T2 decrease. Accordingly, since V1 # 1.3 m/s and T2 # 305 K (328C), the inlet area must satisfy

A1 $

1 ≥ 1.3 m/s

98 W a1.005

kJ b(305 2 293)K kg ? K

`

a

8314 N ? m b 293 K 28.97 kg ? K

1 kJ 1 J/s 104 cm2 ` ` `¥ ± ≤ ` ` 3 5 2 10 J 1 W 1 m2 1.01325 3 10 N/m

$ 52 cm2 For the specified conditions, the smallest fan area is 52 cm2.

4.10 Throttling Devices ➊ Cooling air typically enters and exits electronic enclosures at low velocities, and thus kinetic energy effects are insignificant. ➋ The applicability of the ideal gas model can be checked by reference to the generalized compressibility chart. Since the temperature of the air increases by no more than 128C, the specific heat cp is nearly constant (Table A-20).

✓ Skills Developed Ability to… ❑ apply the steady-state

energy rate balance to a control volume. ❑ apply the mass flow rate expression, Eq. 4.4b. deve lop an engineering model. ❑ ❑ retrieve property data of air modeled as an ideal gas.

If heat transfer occurs at a rate of 11 W from the outer surface of the computer case to the surroundings, determine the smallest fan inlet area for which the limits on entering air velocity and exit air temperature are met if the total power input remains at 98 W. Ans. 46 cm2.

4.10

Throttling Devices

A significant reduction in pressure can be achieved simply by introducing a restriction into a line through which a gas or liquid flows. This is commonly done by means of a partially opened valve or a porous plug. These throttling devices are illustrated in Fig. 4.15. An application of throttling occurs in vapor-compression refrigeration systems, where a valve is used to reduce the pressure of the refrigerant from the pressure at the exit of the condenser to the lower pressure existing in the evaporator. We consider this further in Chap. 10. Throttling also plays a role in the Joule–Thomson expansion considered in Chap. 11. Another application involves the throttling calorimeter, which is a device for determining the quality of a two-phase liquid–vapor mixture. The throttling calorimeter is considered in Example 4.9.

4.10.1 Throttling Device Modeling Considerations For a control volume enclosing a throttling device, the only work is flow work at # locations where mass enters and exits the control volume, so the term Wcv drops out of the energy rate balance. There is usually no significant heat transfer with the surroundings, and the change in potential energy from inlet to exit is negligible. Thus, the underlined terms of Eq. 4.20a (repeated below) drop out, leaving the enthalpy and kinetic energy terms, as shown by Eq. (a). That is, # # (V21 2 V22) # 0 5 Qcv 2 Wcv 1 m c (h1 2 h2) 1 1 g(z1 2 z2) d 2 V 21 2 V 22 0 5 (h 1 2 h 2) 1 2

1 1

Partially open valve

2

Fig. 4.15 Examples of throttling devices.

Porous plug

(a)

2

173

throttling calorimeter

174

Chapter 4 Control Volume Analysis Using Energy

A A.23 – Tabs a, b, & c Throttling_Dev

Although velocities may be relatively high in the vicinity of the restriction imposed by the throttling device on the flow through it, measurements made upstream and downstream of the reduced flow area show in most cases that the change in the specific kinetic energy of the flowing substance between these locations can be neglected. With this further simplification, Eq. (a) reduces to h2 5 h1

( p 2 , p1 )

(4.22)

When the flow through the valve or other restriction is idealized in this way, the process is called a throttling process.

throttling process

4.10.2 Using a Throttling Calorimeter to Determine Quality The next example illustrates use of a throttling calorimeter to determine steam quality. cccc

EXAMPLE 4.9 c

Measuring Steam Quality A supply line carries a two-phase liquid–vapor mixture of steam at 20 bar. A small fraction of the flow in the line is diverted through a throttling calorimeter and exhausted to the atmosphere at 1 bar. The temperature of the exhaust steam is measured as 120°C. Determine the quality of the steam in the supply line. SOLUTION Known: Steam is diverted from a supply line through a throttling calorimeter and exhausted to the atmosphere. Find: Determine the quality of the steam in the supply line. Schematic and Given Data: Steam line, 20 bar

p

Thermometer 1

1

p1 = 20 bar

Calorimeter p2 = 1 bar T2 = 120°C

2

p2 = 1 bar T2 = 120°C

v

Fig. E4.9

Engineering Model: 1. The control volume shown on the accompanying figure is at steady state. 2. The diverted steam undergoes a throttling process. Analysis: For a throttling process, the energy and mass balances reduce to give h2 5 h1, which agrees with Eq. 4.22.

Thus, with state 2 fixed, the specific enthalpy in the supply line is known, and state 1 is fixed by the known values of p1 and h1. ➊ As shown on the accompanying p–𝜐 diagram, state 1 is in the two-phase liquid–vapor region and state 2 is in the superheated vapor region. Thus, h2 5 h1 5 hf1 1 x1(hg1 2 hf1)

4.11 System Integration

175

Solving for x1,x1 5

h2 2 hf1 hg1 2 hf1

From Table A-3 at 20 bar, hf1 5 908.79 kJ/kg and hg1 5 2,799.5 kJ/kg. At 1 bar and 120°C, h2 5 2,766.6 kJ/kg from Table A-4. Inserting values into the above expression, the quality of the steam in the line is x1 5 0.956 (95.6%). ➊ For throttling calorimeters exhausting to the atmosphere, the quality of the steam in the line must be greater than about 94% to ensure that the steam leaving the calorimeter is superheated.

✓ Skills Developed Ability to… ❑ apply Eq. 4.22 for a throt-

tling process. ❑ retrieve property data for

water.

If the supply line carried saturated vapor at 20.5 bar, determine the temperature at the calorimeter exit, in 8C, for the same exit pressure, 1 bar. Ans. 162.28C.

4.11

˙ in Q

System Integration

Thus far, we have studied several types of components selected from those commonly seen in practice. These components are usually encountered in combination, rather than individually. Engineers often must creatively combine components to achieve some overall objective, subject to constraints such as minimum total cost. This important engineering activity is called system integration. In engineering practice and everyday life, integrated systems are regularly encountered. Many readers are already familiar with a particularly successful system integration: the simple power plant shown in Fig. 4.16. This system consists of four components in series: a turbinegenerator, condenser, pump, and boiler. We consider such power plants in detail in subsequent sections of the book.

Boiler

W˙ p

Pump

Turbine

Condenser

˙ out Q Fig. 4.16 Simple vapor power plant. system integration

BIOCONNECTIONS Living things also can be considered integrated systems. Figure 4.17 shows a control volume enclosing a tree receiving solar radiation. As indicated on the figure, a portion of the incident radiation is reflected to the surroundings. Of the net solar energy received by the tree, about 21% is returned to the surroundings by heat transfer, principally convection. Water management accounts for most of the remaining solar input. Trees sweat as do people; this is called evapotranspiration. As shown in Fig. 4.17, about 78% of the net solar energy received by the tree is used to pump liquid water from the surroundings, primarily the ground, convert it to a vapor, and discharge it to the surroundings through tiny pores (called stomata) in the leaves. Nearly all the water taken up is lost in this manner and only a small fraction is used within the tree. Applying an energy balance to the control volume enclosing the tree, just 1% of the net solar energy received by the tree is left for use in the production of biomass (wood and leaves). Evapotranspiration benefits trees but also contributes significantly to water loss from watersheds, illustrating that in nature as in engineering there are trade-offs.

W˙ t

176

Chapter 4 Control Volume Analysis Using Energy

Solar radiation

Reflected Vapor

Incident

Heat transfer (21%)

Evapotranspiration (78%)

Biomass production (1%)

Moisture

Fig. 4.17 Control volume enclosing a tree.

Example 4.10 provides another illustration of an integrated system. This case involves a waste heat recovery system.

cccc

EXAMPLE 4.10 c

Waste Heat Recovery System An industrial process discharges gaseous combustion products at 478°K, 1 bar with mass flow rate of 69.78 kg/s. As shown in Fig. E4.10, a proposed system for utilizing the combustion products combines a heat-recovery steam generator with a turbine. At steady state, combustion products exit the steam generator at 400°K, 1 bar and a separate stream of water enters at 0.275 MPa, 38.9°C with a mass flow rate of 2.079 kg/s. At the exit of the turbine, the pressure is 0.07 bar and the quality is 93%. Heat transfer from the outer surfaces of the steam generator and turbine can be ignored, as can the changes in kinetic and potential energies of the flowing streams. There is no significant pressure drop for the water flowing through the steam generator. The combustion products can be modeled as air as an ideal gas. (a) Determine the power developed by the turbine, in kJ/s. (b) Determine the turbine inlet temperature, in °C. (c) Evaluating the power developed at $0.08 per kW ? h, which is a typical rate for electricity, determine the

value of the power, in $/year, for 8000 hours of operation annually. SOLUTION Known: Steady-state operating data are provided for a system consisting of a heat-recovery steam generator and a turbine. Find: Determine the power developed by the turbine and the turbine inlet temperature. Evaluate the annual

value of the power developed.

4.11 System Integration

177

Schematic and Given Data: Engineering Model:

Combustion products in p1 = 1 bar T1 = 478°K m1 = 69.78 kg/s 1

1. The control volume shown on the

accompanying figure is at steady state. 2. Heat transfer is negligible, and Turbine 4

2 Combustion products out T2 = 400°K p2 = 1 bar

Steam generator

3. There is no pressure drop for water

flowing through the steam generator. 5

3 water in p3 = 0.275 MPa T3 = 38.9°C m3 = 2.08 kg/s

Power out

changes in kinetic and potential energy can be ignored.

water out p5 = 0.07 bar x5 = 93%

4. The combustion products are

modeled as air as an ideal gas.

Fig. E4.10 Analysis: (a) The power developed by the turbine is determined from a control volume enclosing both the steam generator and the turbine. Since the gas and water streams do not mix, mass rate balances for each of the streams reduce, respectively, to give # # # # m1 5 m 2, m3 5 m5

For this control volume, the appropriate form of the steady-state energy rate balance is Eq. 4.18, which reads # # V 23 V 21 # # 0 5 Qcv 2 Wcv 1 m1 ah1 1 1 gz1b 1 m3 ah3 1 1 gz3b 2 2 V 25 V 22 # # 2 m2 ah2 1 1 gz2b 2 m5 ah5 1 1 gz5b 2 2 The underlined terms drop out by assumption 2. With these simplifications, together with the above mass flow rate relations, the energy rate balance becomes # # # Wcv 5 m1(h1 2 h 2) 1 m3(h3 2 h5) # # where m1 5 69.78 kg/s, m3 5 2.08 kg/s. The specific enthalpies, h1 and h2, can be found from Table A-22: At 478 K, h1 5 480.35 kJ/kg, and at 400 K, h2 5 400.98 kJ/kg. At state 3, water is a liquid. Using Eq. 3.14 and saturated liquid data from Table A-2, h3 < hf (T 3) 5 162.9 kJ/kg. State 5 is a two-phase liquid–vapor mixture. With data from Table A-3 and the given quality h 5 5 hf 5 1 x5 (hg5 2 h f5) 5 161 1 0.93(2751.72 2 161) 5 2403 kJ/kg # Substituting values into the expression for Wcv # Wcv 5 (69.78 kg/s) (480.3 2 400.98) kJ/kg 1 (2.079kg/s)(162.9 2 2403) kJ/kg 5 876.8 kJ/s 5 876.8 kW

178

Chapter 4 Control Volume Analysis Using Energy

(b) To determine T4, it is necessary to fix the state at 4. This requires two independent property values. With assumption 3, one of these properties is pressure, p4 5 0.275 MPa. The other is the specific enthalpy h4, which can be found from an energy rate balance for volume # a control # # # enclosing just the steam generator. Mass rate balances for each of the two streams give m1 5 m2 and m3 5 m4. With assumption 2 and these mass flow rate relations, the steady-state form of the energy rate balance reduces to # # ➊ 0 5 m1(h1 2 h2) 1 m3(h3 2 h4)

Solving for h4

# m1 h4 5 h3 1 # (h1 2 h2) m3 kJ 69.78 kJ 5 162.9 1a b(480.3 2 400.98) kg 2.079 kg 5 2825 kJ/kg

Interpolating in Table A-4 at p4 5 0.275 MPa with h4, we get T 4 5 180°C. (c) Using the result of part (a), together with the given economic data and appropriate conversion factors, the

value of the power developed for 8000 hours of operation annually is Annual value h $ 5 (876.8 KW) a8000 b a0.08 b year kW ? h ➋

5 561,152

$ year

➊ Alternatively, to determine h4 a control volume enclosing just the turbine can be considered. ➋ The decision about implementing this solution to the problem of utilizing the hot combustion products discharged from an industrial process would necessarily rest on the outcome of a detailed economic evaluation, including the cost of purchasing and operating the steam generator, turbine, and auxiliary equipment.

✓ Skills Developed Ability to… ❑ apply the steady-state

mass and energy rate balances to a control volume. ❑ apply the mass flow rate expression, Eq. 4.4b. ❑ develop an engineering model. ❑ retrieve property data for water and for air modeled as an ideal gas. ❑ conduct an elementary economic evaluation.

Taking a control volume enclosing just the turbine, evaluate the turbine inlet temperature, in °C. Ans. 179°C.

4.12 transient

A

System_Types A.1 – Tab d

Transient Analysis

Many devices undergo periods of transient operation during which the state changes with time. Examples include the startup or shutdown of turbines, compressors, and motors. Additional examples include vessels being filled or emptied, as considered in Example 4.2 and in the discussion of Fig. 1.5. Because property values, work and heat transfer rates, and mass flow rates may vary with time during transient operation, the steady-state assumption is not appropriate when analyzing such cases. Special care must be exercised when applying the mass and energy rate balances, as discussed next.

4.12.1 The Mass Balance in Transient Analysis First, we place the control volume mass balance in a form that is suitable for transient analysis. We begin by integrating the mass rate balance, Eq. 4.2, from time 0 to a final

4.12 Transient Analysis time t. That is

#

0

t

a

dm cv b dt 5 dt

#

0

t

# a a mib dt 2 i

t

#

# a a m b dt e

e

0

This takes the form mcv(t) 2 mcv(0) 5 a a i

#

t

0

# mi dtb 2 a a e

t

#

# m dtb e

0

Introducing the following symbols for the underlined terms t

mi 5

#

# m dt i

0

t

me 5

#

# m dt e

0

amount of mass entering the control μ volume through inlet i, from time 0 to t amount of mass exiting the control μ volume through exit e, from time 0 to t

the mass balance becomes mcv(t) 2 mcv(0) 5 a mi 2 a me i

(4.23)

e

In words, Eq. 4.23 states that the change in the amount of mass contained in the control volume equals the difference between the total incoming and outgoing amounts of mass.

4.12.2 The Energy Balance in Transient Analysis Next, we integrate the energy rate balance, Eq. 4.15, ignoring the effects of kinetic and potential energy. The result is Ucv(t) 2 Ucv(0) 5 Qcv 2 Wcv 1 a a i

#

0

t

# mi hi dtb 2 a a e

t

#

# m h dtb e e

(4.24)

0

where Qcv accounts for the net amount of energy transferred by heat into the control volume and Wcv accounts for the net amount of energy transferred by work, except for flow work. The integrals shown underlined in Eq. 4.24 account for the energy carried in at the inlets and out at the exits. For the special case where the states at the inlets and exits are constant with time, the respective specific enthalpies, hi and he, are constant, and the underlined terms of Eq. 4.24 become t t # # mi hi dt 5 hi mi dt 5 hi mi

#

0 t

#

0

#

0 t

# # me he dt 5 he me dt 5 he me

#

0

Equation 4.24 then takes the following special form: Ucv(t) 2 Ucv(0) 5 Qcv 2 Wcv 1 a mihi 2 a me he i

(4.25)

e

where mi and me account, respectively, for the amount of mass entering the control volume through inlet i and exiting the control volume through exit e, each from time 0 to t. Whether in the general form, Eq. 4.24, or the special form, Eq. 4.25, these equations account for the change in the amount of energy contained within the control volume as the difference between the total incoming and outgoing amounts of energy. Another special case is when the intensive properties within the control volume are uniform with position at a particular time t. Accordingly, the specific volume and

179

180

Chapter 4 Control Volume Analysis Using Energy the specific internal energy are uniform throughout and can depend only on time— that is, 𝜐(t) and u(t), respectively. Then mcv(t) 5 Vcv(t)y𝜐(t) Ucv(t) 5 mcv(t)u(t)

(4.26) (4.27)

If the control volume comprises different phases at time t, the state of each phase is assumed uniform throughout. Equations 4.23 and 4.25–4.27 are applicable to a wide range of transient cases where inlet and exit states are constant with time and intensive properties within the control volume are uniform with position initially and finally. in cases involving the filling of containers having a single inlet and no exit, Eqs. 4.23, 4.25, and 4.27 combine to give mcv(t)u(t) 2 mcv(0)u(0) 5 Qcv 2 Wcv 1 hi(mcv(t) 2 mcv(0))

(4.28)

The details are left as an exercise. See Examples 4.12 and 4.13 for this type of transient application. b b b b b

4.12.3 Transient Analysis Applications The following examples provide illustrations of the transient analysis of control volumes using the conservation of mass and energy principles. In each case considered, to emphasize fundamentals we begin with general forms of the mass and energy balances and reduce them to forms suited for the case at hand, invoking the idealizations discussed in this section as warranted. The first example considers a vessel that is partially emptied as mass exits through a valve. cccc

EXAMPLE 4.11 c

Evaluating Heat Transfer for a Partially Emptying Tank A tank having a volume of 0.85 m3 initially contains water as a two-phase liquid–vapor mixture at 2608C and a quality of 0.7. Saturated water vapor at 2608C is slowly withdrawn through a pressure-regulating valve at the top of the tank as energy is transferred by heat to maintain the pressure constant in the tank. This continues until the tank is filled with saturated vapor at 2608C. Determine the amount of heat transfer, in kJ. Neglect all kinetic and potential energy effects. SOLUTION Known: A tank initially holding a two-phase liquid–vapor mixture is heated while saturated water vapor is slowly removed. This continues at constant pressure until the tank is filled only with saturated vapor. Find: Determine the amount of heat transfer. Schematic and Given Data: Pressure regulating valve e

T

1

2, e

260°C

v

e

Saturated watervapor removed, while the tank is heated

Initial: two-phase liquid-vapor mixture

Final: saturated vapor

Fig. E4.11

4.12 Transient Analysis

181

Engineering Model: 1. The control volume is defined by the dashed line on the accompanying diagram.

#

2. For the control volume, Wcv 5 0 and kinetic and potential energy effects can be neglected. 3. At the exit the state remains constant.

➊ 4. The initial and final states of the mass within the vessel are equilibrium states. Analysis: Since there is a single exit and no inlet, the mass rate balance Eq. 4.2 takes the form

dmcv # 5 2me dt With assumption 2, the energy rate balance Eq. 4.15 reduces to # dUcv # 5 Qcv 2 me he dt Combining the mass and energy rate balances results in # dm cv dUcv 5 Qcv 1 he dt dt By assumption 3, the specific enthalpy at the exit is constant. Accordingly, integration of the last equation gives DUcv 5 Qcv 1 he Dm cv Solving for the heat transfer Qcv, Qcv 5 DUcv 2 he Dmcv or ➋

Qcv 5 (m2u2 2 m1u1) 2 he(m2 2 m1)

where m1 and m2 denote, respectively, the initial and final amounts of mass within the tank. The terms u1 and m1 of the foregoing equation can be evaluated with property values from Table A-2 at 2608C and the given value for quality. Thus u1 5 uf 1 x1(ug 2 uf) 5 1128.4 1 (0.7)(2599.0 2 1128.4) 5 2157.8 kJ/kg Also, 𝜐1 5 𝜐f 1 x1(𝜐g 2 𝜐f) 5 1.2755 3 10 23 1 (0.7)(0.04221 2 1.2755 3 10 23) 5 29.93 3 10 23 m3/kg Using the specific volume 𝜐1, the mass initially contained in the tank is m1 5

V 0.85 m3 5 28.4 kg 5 𝜐1 (29.93 3 10 23 m3/kg)

The final state of the mass in the tank is saturated vapor at 2608C so Table A-2 gives u2 5 ug(260°C) 5 2599.0 kJ/kg,

𝜐2 5 𝜐g(260°C) 5 42.21 3 1023 m3/kg

The mass contained within the tank at the end of the process is m2 5

V 0.85 m3 5 20.14 kg 5 𝜐2 (42.21 3 1023 m3/kg)

Table A-2 also gives he 5 hg (2608C) 5 2796.6 kJ/kg.

182

Chapter 4 Control Volume Analysis Using Energy Substituting values into the expression for the heat transfer yields

✓ Skills Developed

Qcv 5 (20.14)(2599.0) 2 (28.4)(2157.8) 2 2796.6(20.14 2 28.4) 5 14,162 kJ

Ability to… ❑ apply the time-dependent

➊ In this case, idealizations are made about the state of the vapor exiting and the initial and final states of the mass contained within the tank. ➋ This expression for Qcv can be obtained by applying Eqs. 4.23, 4.25, and 4.27. The details are left as an exercise.

mass and energy rate balances to a control volume. ❑ develop an engineering model. ❑ retrieve property data for water.

If the initial quality were 90%, determine the heat transfer, in kJ, keeping all other data unchanged. Ans. 3707 kJ.

In the next two examples we consider cases where tanks are filled. In Example 4.12, an initially evacuated tank is filled with steam as power is developed. In Example 4.13, a compressor is used to store air in a tank.

cccc

EXAMPLE 4.12 c

Using Steam for Emergency Power Generation Steam at a pressure of 15 bar and a temperature of 3208C is contained in a large vessel. Connected to the vessel through a valve is a turbine followed by a small initially evacuated tank with a volume of 0.6 m3. When emergency power is required, the valve is opened and the tank fills with steam until the pressure is 15 bar. The temperature in the tank is then 4008C. The filling process takes place adiabatically and kinetic and potential energy effects are negligible. Determine the amount of work developed by the turbine, in kJ. SOLUTION Known: Steam contained in a large vessel at a known state flows from the vessel through a turbine into a small tank of known volume until a specified final condition is attained in the tank. Find: Determine the work developed by the turbine. Schematic and Given Data:

Valve

Control volume boundary

Engineering Model: 1. The control volume is defined by the dashed line on the

Steam at 15 bar, 320°C

Turbine

V= 0.6 m3

accompanying diagram.

#

2. For the control volume, Qcv 5 0 and kinetic and potential

energy effects are negligible. Initially evacuated tank

Fig. E4.12

➊ 3. The state of the steam within the large vessel remains constant. The final state of the steam in the smaller tank is an equilibrium state. 4. The amount of mass stored within the turbine and the

interconnecting piping at the end of the filling process is negligible.

4.12 Transient Analysis

183

Analysis: Since the control volume has a single inlet and no exits, the mass rate balance, Eq. 4.2, reduces to

dmcv # 5 mi dt The energy rate balance, Eq. 4.15, reduces with assumption 2 to # dUcv # 5 2Wcv 1 m i h i dt Combining the mass and energy rate balances gives # dmcv dUcv 5 2Wcv 1 h i dt dt Integrating DUcv 5 2Wcv 1 h i Dmcv In accordance with assumption 3, the specific enthalpy of the steam entering the control volume is constant at the value corresponding to the state in the large vessel. Solving for Wcv Wcv 5 h i Dmcv 2 DUcv DUcv and Dmcv denote, respectively, the changes in internal energy and mass of the control volume. With assumption 4, these terms can be identified with the small tank only. Since the tank is initially evacuated, the terms DUcv and Dmcv reduce to the internal energy and mass within the tank at the end of the process. That is DUcv 5 (m 2 u 2) 2 (m 1 u 1)0 ,

Dmcv 5 m 2 2 m01

where 1 and 2 denote the initial and final states within the tank, respectively. Collecting results yields ➋➌

Wcv 5 m2(hi 2 u2)

(a)

The mass within the tank at the end of the process can be evaluated from the known volume and the specific volume of steam at 15 bar and 4008C from Table A-4 m2 5

V 0.6 m3 5 2.96 kg 5 𝜐2 (0.203 m3/kg)

The specific internal energy of steam at 15 bar and 4008C from Table A-4 is 2951.3 kJ/kg. Also, at 15 bar and 3208C, h1 5 3081.9 kJ/kg. Substituting values into Eq. (a) Wcv 5 2.96 kg (3081.9 2 2951.3)kJ/kg 5 386.6 kJ ➊ In this case idealizations are made about the state of the steam entering the tank and the final state of the steam in the tank. These idealizations make the transient analysis manageable. ➋ A significant aspect of this example is the energy transfer into the control volume by flow work, incorporated in the p𝜐 term of the specific enthalpy at the inlet. ➌ This result can also be obtained by reducing Eq. 4.28. The details are left as an exercise.

If the turbine were removed, and the steam allowed to flow adiabatically into the small tank until the pressure in the tank is 15 bar, determine the final steam temperature in the tank, in 8C. Ans. 4778C.

✓ Skills Developed Ability to… ❑ apply the time-dependent

mass and energy rate balances to a control volume. ❑ develop an engineering model. ❑ retrieve property data for water.

184 cccc

Chapter 4 Control Volume Analysis Using Energy

EXAMPLE 4.13 c

Storing Compressed Air in a Tank An air compressor rapidly fills a 0.28 m3 tank, initially containing air at 21°C, 1 bar, with air drawn from the atmosphere at 21°C, 1 bar. During filling, the relationship between the pressure and specific volume of the air in the tank is pv1.4 5 constant. The ideal gas model applies for the air, and kinetic and potential energy effects are negligible. Plot the pressure, in atm, and the temperature, in °C, of the air within the tank, each versus the ratio m/m 1, where m 1 is the initial mass in the tank and m is the mass in the tank at time t . 0. Also, plot the compressor work input, in kJ, versus m/m1. Let m/m1 vary from 1 to 3. SOLUTION Known: An air compressor rapidly fills a tank having a known volume. The initial state of the air in the tank and the state of the entering air are known. Find: Plot the pressure and temperature of the air within the tank, and plot the air compressor work input, each

versus m/m 1 ranging from 1 to 3. Schematic and Given Data: Engineering Model:

Air

1. The control volume is defined by the dashed line on i

Ti = 21°C pi = 1 bar

the accompanying diagram.

#

2. Because the tank is filled rapidly, Qcv is ignored. 3. Kinetic and potential energy effects are negligible.

Tank

4. The state of the air entering the control volume

Air compressor V = 0.28 m3 T1 = 21°C p1 = 1 bar pv1.4 = constant

Fig. E4.13a

remains constant. 5. The air stored within the air compressor and inter–+

connecting pipes can be ignored.

➊ 6. The relationship between pressure and specific volume for the air in the tank is pv1.4 5 constant. 7. The ideal gas model applies for the air.

Analysis: The required plots are developed using Interactive Thermodynamics: IT. The IT program is based on the following analysis. The pressure p in the tank at time t . 0 is determined from

pv1.4 5 p1v1.4 1 where the corresponding specific volume v is obtained using the known tank volume V and the mass m in the tank at that time. That is, v 5 V/m. The specific volume of the air in the tank initially, v1, is calculated from the ideal gas equation of state and the known initial temperature, T1, and pressure, p1. That is 8314 N ? m a b (294°K) 28.97 kg ? °K RT1 1 bar v1 5 5 ` 5 ` 5 0.8437 m3/kg p1 (1 bar) 10 N/m2 Once the pressure p is known, the corresponding temperature T can be found from the ideal gas equation of state, T 5 pv/R. To determine the work, begin with the mass rate balance Eq. 4.2, which reduces for the single-inlet control volume to dmcv # 5 mi dt

4.12 Transient Analysis

185

Then, with assumptions 2 and 3, the energy rate balance Eq. 4.15 reduces to # dUcv # 5 2Wcv 1 m i h i dt Combining the mass and energy rate balances and integrating using assumption 4 gives DUcv 5 2Wcv 1 hi Dmcv Denoting the work input to the compressor by Win 5 2Wcv and using assumption 5, this becomes Win 5 mu 2 m1u1 2 (m 2 m1)hi

(a)

where m1 is the initial amount of air in the tank, determined from m1 5

V 0.28 m3 5 0.332 kg 5 v1 0.8437 m3/kg

As a sample calculation to validate the IT program below, consider the case m 5 0.664 kg, which corresponds to m/m1 5 2. The specific volume of the air in the tank at that time is v5

V 0.28 m3 5 0.422 m3/kg 5 m 0.664 kg

The corresponding pressure of the air is 0.8437 m3/kg 1.4 v1 1.4 b p 5 p1 a b 5 (1 bar) a v 0.422 m3/kg 5 2.64 atm and the corresponding temperature of the air is pv 5 ° T5 R

(2.64 bar)(0.422 m3/kg) a

8314 J b 28.97 kg ? °K

¢`

105 N/m2 ` 1 bar

5 388°K (114.9°C) Evaluating u1, u, and hi at the appropriate temperatures from Table A-22, u1 5 209.8 kJ/kg, u 5 27.75 kJ/kg, hi 5 294.2 kJ/kg. Using Eq. (a), the required work input is Win 5 mu 2 m1u1 2 (m 2 m1)hi kJ kJ kJ 5 (0.664 kg)a277.5 b 2 (0.332 kg) a209.8 b 2 (0.332 kg) a294.2 b kg kg kg 5 16.9 kJ IT Program: Choosing SI units from the Units menu, and selecting Air from the Properties menu, the IT program for solving the problem is //Given Data p1 5 1//bar T1 5 21//8C Ti 5 21//8C V 5 0.28//m3 n 5 1.4 // Determine the pressure and temperature for t . 0 v1 5 v_TP(“Air”, T1, p1) v 5 V/m p * v ^n = p1 * v1 ^n v 5 v_TP(“Air”, T, p) // Specify the mass and mass ratio r v1 5 V/m1 r 5 m/m1 r52

186

Chapter 4 Control Volume Analysis Using Energy

// Calculate the work using Eq. (a) Win 5 m * u – m1 * u1 – hi * (m – m1) u1 5 u_T(“Air”, T1) u 5 u_T(“Air”, T) hi 5 h_T(“Air”, Ti)

5

222

4

179.6

3

137.2

T, °C

p, bar

Using the Solve button, obtain a solution for the sample case r 5 m/m1 5 2 considered above to validate the program. Good agreement is obtained, as can be verified. Once the program is validated, use the Explore button to vary the ratio m/m1 from 1 to 3 in steps of 0.01. Then, use the Graph button to construct the required plots. The results are:

2

94.8

1 0

52.4 1

1.5

2 m/m1

2.5

10

3

1

1.4

1.8 2.2 m/m1

2.6

3

60 50 Win, kJ

40 30 20 10 0

1

1.5

2 m/m1

2.5

3

Fig. E4.13b We conclude from the first two plots that the pressure and temperature each increase as the tank fills. The work required to fill the tank increases as well. These results are as expected. ➊ This pressure-specific volume relationship is in accord with what might be measured. The relationship is also consistent with the uniform state idealization, embodied by Eq. 4.25.

✓ Skills Developed Ability to… ❑ apply the time-dependent

mass and energy rate balances to a control volume. ❑ develop an engineering model. ❑ retrieve property data for air modeled as an ideal gas. solv e iteratively and plot ❑ the results using IT.

As a sample calculation, for the case m 5 1 kg, evaluate p, in bar. Compare with the value read from the plot of Fig. E4.13b. Ans. 4.73 bar.

The final example of transient analysis is an application with a well-stirred tank. Such process equipment is commonly employed in the chemical and food processing industries.

4.12 Transient Analysis

EXAMPLE 4.14 c

Determining Temperature-Time Variation in a Well-Stirred Tank A tank containing 45 kg of liquid water initially at 458C has one inlet and one exit with equal mass flow rates. Liquid water enters at 458C and a mass flow rate of 270 kg/h. A cooling coil immersed in the water removes energy at a rate of 7.6 kW. The water is well mixed by a paddle wheel so that the water temperature is uniform throughout. The power input to the water from the paddle wheel is 0.6 kW. The pressures at the inlet and exit are equal and all kinetic and potential energy effects can be ignored. Plot the variation of water temperature with time. SOLUTION Known: Liquid water flows into and out of a well-stirred tank with equal mass flow rates as the water in the tank is cooled by a cooling coil. Find: Plot the variation of water temperature with time. Schematic and Given Data:

Mixing rotor

318

m1 = 270 kg/h T1 = 318 K (45°C)

Water temperature, K

cccc

187

Constant liquid level

Tank Boundary

296

Cooling coil

0

m2 = 270 kg/h

0.5 Time, h

1.0

Fig. E4.14

Engineering Model: 1. The control volume is defined by the dashed line on the accompanying diagram. 2. For the control volume, the only significant heat transfer is with the cooling coil. Kinetic and poten-

tial energy effects can be neglected. ➊ 3. The water temperature is uniform with position throughout and varies only with time: T 5 T(t). 4. The water in the tank is incompressible, and there is no change in pressure between inlet and exit. Analysis: The energy rate balance, Eq. 4.15, reduces with assumption 2 to

# # dUcv # 5 Qcv 2 Wcv 1 m(h1 2 h2) dt

# where m denotes the mass flow rate. The mass contained within the control volume remains constant with time, so the term on the left side of the energy rate balance can be expressed as d(mcvu) dUcv du 5 5 mcv dt dt dt Since the water is assumed incompressible, the specific internal energy depends on temperature only. Hence, the chain rule can be used to write du du dT dT 5 5c dt dT dt dt

188

Chapter 4 Control Volume Analysis Using Energy

where c is the specific heat. Collecting results dUcv dT 5 m cv c dt dt With Eq. 3.20b the enthalpy term of the energy rate balance can be expressed as 0

h 1 2 h2 5 c(T1 2 T 2) 1 𝜐(p1 2 p2 ) where the pressure term is dropped by assumption 4. Since the water is well mixed, the temperature at the exit equals the temperature of the overall quantity of liquid in the tank, so h 1 2 h 2 5 c(T 1 2 T ) where T represents the uniform water temperature at time t. With the foregoing considerations the energy rate balance becomes mcv c

# # dT # 5 Qcv 2 Wcv 1 m c(T1 2 T) dt

As can be verified by direct substitution, the solution of this first-order, ordinary differential equation is # # # Qcv 2 Wcv m b 1 T1 T 5 C1 exp a2 tb 1 a # mcv mc The constant C1 is evaluated using the initial condition: at t 5 0, T 5 T1. Finally # # # Qcv 2 Wcv m b c 1 2 exp a2 T 5 T1 1 a tb d # mcv mc Substituting given numerical values together with the specific heat c for liquid water from Table A-19 T 5 318 K 1

270 kg/h [27.6 2 (20.6)] kJ/s c 1 2 exp a2 tb d § £ 270 kg 45 kg kJ a b b a4.2 3600 s kg ? K

5 318 2 22[1 2 exp(26t)] where t is in hours. Using this expression, we construct the accompanying plot showing the variation of temperature with time. ➊ In this case idealizations are made about the state of the mass contained within the system and the states of the liquid entering and exiting. These idealizations make the transient analysis manageable.

✓ Skills Developed Ability to… ❑ apply the time-dependent

mass and energy rate balances to a control volume. ❑ develop an engineering model. ❑ apply the incompressible substance model for water. ❑ solve an ordinary differential equation and plot the solution.

What is the water temperature, in 8C, when steady state is achieved? Ans. 238C.

c CHAPTER SUMMARY AND STUDY GUIDE The conservation of mass and energy principles for control volumes are embodied in the mass and energy rate balances developed in this chapter. Although the primary emphasis is on cases in which one-dimensional flow is assumed, mass

and energy balances are also presented in integral forms that provide a link to subsequent fluid mechanics and heat transfer courses. Control volumes at steady state are featured, but discussions of transient cases are also provided.

Key Equations The use of mass and energy balances for control volumes at steady state is illustrated for nozzles and diffusers, turbines, compressors and pumps, heat exchangers, throttling devices, and integrated systems. An essential aspect of all such applications is the careful and explicit listing of appropriate assumptions. Such model-building skills are stressed throughout the chapter. The following checklist provides a study guide for this chapter. When your study of the text and end-of-chapter exercises has been completed you should be able to c write out the meanings of the terms listed in the margins

throughout the chapter and understand each of the related

189

concepts. The subset of key concepts listed below is particularly important in subsequent chapters. c list the typical modeling assumptions for nozzles and diffusers, turbines, compressors and pumps, heat exchangers, and throttling devices. c apply Eqs. 4.6, 4.18, and 4.20 to control volumes at steady state, using appropriate assumptions and property data for the case at hand. c apply mass and energy balances for the transient analysis of control volumes, using appropriate assumptions and property data for the case at hand.

c KEY ENGINEERING CONCEPTS flow work, p. 152 energy rate balance, p. 153 nozzle, p. 156 diffuser, p. 156 turbine, p. 159 compressor, p. 163

conservation of mass, p. 143 mass flow rates, p. 143 mass rate balance, p. 143 one-dimensional flow, p. 145 volumetric flow rate, p. 146 steady state, p. 146

pump, p. 163 heat exchanger, p. 168 throttling process, p. 174 system integration, p. 175 transient analysis, p. 178

c KEY EQUATIONS # AV m5 𝜐

(4.4b) p. 145

Mass flow rate, one-dimensional flow. (See Fig. 4.3.)

(4.2) p. 143

Mass rate balance.

(4.6) p. 146

Mass rate balance at steady state.

# # dEcv V 2i V 2e # # 5 Qcv 2 Wcv 1 a mi ahi 1 1 gzib 2 a me ahe 1 1 gzeb dt 2 2 i e

(4.15) p. 153

Energy rate balance.

# # V 2i V 2e # # 0 5 Qcv 2 Wcv 1 a mi ahi 1 1 gzib 2 a me ahe 1 1 gzeb 2 2 i e

(4.18) p. 154

Energy rate balance at steady state.

(4.20a) p. 154

Energy rate balance for one-inlet, one-exit control volumes at steady state.

dmcv # # 5 a mi 2 a me dt i e # # 5 a mi a me i

e

(mass rate in)

(mass rate out)

# # (V 21 2 V 22) # 0 5 Qcv 2 Wcv 1 m c (h1 2 h2) 1 1 g(z1 2 z2) d 2 # # (V12 2 V22) Qcv Wcv 0 5 # 2 # 1 (h1 2 h2) 1 1 g(z1 2 z2) 2 m m h2 5 h1

(p2 , p1)

(4.20b) p. 154

(4.22) p. 174

Throttling process. (See Fig. 4.15.)

190

Chapter 4 Control Volume Analysis Using Energy

c EXERCISES: THINGS ENGINEERS THINK ABOUT 1. Why does the relative velocity normal to the flow boundary, Vn, appear in Eqs. 4.3 and 4.8? 2. Why might a computer cooled by a constant-speed fan operate satisfactorily at sea level but overheat at high altitude? 3. By introducing enthalpy h to replace each of the (u 1 pv) terms of Eq. 4.13, we get Eq. 4.14. An even simpler algebraic form would result by replacing each of the (u 1 pv 1 V2/2 1 gz) terms by a single symbol, yet we have not done so. Why not? 4. When a slice of bread is placed in a toaster and the toaster is activated, is the toaster in steady-state operation, transient operation, both? 5. How is the work done by the heart measured? 6. Where do I encounter microelectromechanical systems in daily life? 7. Wind turbines and hydraulic turbines develop mechanical power from moving streams of air and water, respectively. In each case, what aspect of the stream is tapped for power? 8. For air flowing through a converging-diverging channel, sketch the variation of the air pressure as air accelerates in the converging section and decelerates in the diverging section.

9. Even though their outer surfaces would seem hot to the touch, large steam turbines in power plants might not be covered with much insulation. Why not? 10. One stream of a counterflow heat exchanger operating at steady state has saturated water vapor entering and saturated liquid exiting, each at 1 atm. The other stream has ambient air entering at 20°C and exiting at a higher temperature with no significant change in pressure. What is the variation of temperature with position of each stream as it passes through the heat exchanger? Sketch them. 11. Why does evapotranspiration in a tree require so much energy? 12. What are some examples of commonly encountered devices that undergo periods of transient operation? For each example, which type of system, closed system or control volume, would be most appropriate? 13. Why is it that when air at 1 atm is throttled to a pressure of 0.5 atm, its temperature at the valve exit is close to the temperature at the valve inlet, yet when air at 1 atm leaks into an insulated, rigid, initially evacuated tank until the tank pressure is 0.5 atm, the temperature of the air in the tank is greater than the air temperature outside the tank?

c PROBLEMS: DEVELOPING ENGINEERING SKILLS Applying Conservation of Mass Total volume = 800 m3

4.1 A 0.15 m3 tank, initially evacuated, develops a small hole, and air leaks in from the surroundings at a constant mass flow rate of 0.002 kg/s. Using the ideal gas model for air, determine the pressure, in kPa, in the tank after 30 s if the temperature is 20°C. 4.2 Liquid propane enters an initially empty cylindrical storage tank at a mass flow rate of 10 kg/s. The tank is 25-m long and has a 4-m diameter. The density of the liquid propane is 450 kg/m3. Determine the time, in h, to fill the tank. 4.3 A 0.5-m3 tank contains ammonia, initially at 240°C, 8 bar. A leak develops, and refrigerant flows out of the tank at a constant mass flow rate of 0.04 kg/s. The process occurs slowly enough that heat transfer from the surroundings maintains a constant temperature in the tank. Determine the time, in s, at which half of the mass has leaked out, and the pressure in the tank at that time, in bar. 4.4 Data are provided for the crude oil storage tank shown in Fig. P4.4. The tank initially contains 500 m3 of crude oil. Oil is pumped into the tank through a pipe at a rate of 1 m3/min and out of the tank at a velocity of 2 m/s through another pipe having a diameter of 0.1 m. The crude oil has a specific volume of 0.0016 m3/kg. Determine (a) the mass of oil in the tank, in kg, after 24 hours. (b) the volume of oil in the tank, in m3, at that time.

(AV)1 = 1m3/min

1

20 m

Initial volume of crude oil Vi = 500 m3 = 0.0016 m3/kg

V2 = 2 m/s D2 = 0.1 m 2

Fig. P4.4 4.5 Vegetable oil for cooking is dispensed from a cylindrical can fitted with a spray nozzle. According to the label, the can is able to deliver 560 sprays, each of duration 0.25 s and each having a mass of 0.25 g. Determine (a) the mass flow rate of each spray, in g/s. (b) the mass remaining in the can after 560 sprays, in g, if the initial mass in the can is 170 g.

Problems: Developing Engineering Skills 4.6 Figure P4.6 shows a mixing tank initially containing 1500 kg of liquid water. The tank is fitted with two inlet pipes, one delivering hot water at a mass flow rate of 0.5 kg/s and the other delivering cold water at a mass flow rate of 0.8 kg/s. Water exits through a single exit pipe at a mass flow rate of 1.4 kg/s. Determine the amount of water, in kg, in the tank after one hour. 1 hot water m· 1 = 0.5 kg/s

2

191

4.9 Air enters a one-inlet, one-exit control volume at 8 bar, 600 K, and 40 m/s through a flow area of 20 cm2. At the exit, the pressure is 2 bar, the temperature is 400 K, and the velocity is 350 m/s. The air behaves as an ideal gas. For steady-state operation, determine (a) the mass flow rate, in kg/s. (b) the exit flow area, in cm2. 4.10 A tank contains 500 kg of liquid water. The tank is fitted with two inlet pipes, and the water is entering through these pipes at the mass flow rates of 0.6 kg/s and 0.7 kg/s. It has one exit pipe through which water is leaving at a mass flow rate of 1 kg/s. Determine the amount of water that will be left in the tank after twenty minutes.

cold water m· 2 = 0.8 kg/s

4.11 Air enters a household electric furnace at 24°C, 1 bar, with a volumetric flow rate of 0.38 m3/s. The furnace delivers air at 50°C, 1 bar to a duct system with three branches consisting of two 150 mm diameter ducts and a 300 mm duct. If the velocity in each 150 mm duct is 3 m/s, determine for steady-state operation mi = 1,500 kg

(a) the mass flow rate of air entering the furnace, in kg/s. (b) the volumetric flow rate in each 150 mm duct, in m3/s. (c) the velocity in the 300 mm duct, in m/s.

3 m· 3 = 1.4 kg/s

Fig. P4.6 4.7 A cylindrical tank contains 1500 kg of liquid water. It has one inlet pipe through which water is entering at a mass flow rate of 1.2 kg/s. The tank is fitted with two outlet pipes and the water is flowing through these exit pipes at the mass flow rates of 0.5 kg/s and 0.8 kg/s. Determine the amount of water that will be left in the tank after thirty minutes. 4.8 A carburetor in an internal combustion engine mixes air with fuel to achieve a combustible mixture in which the airto-fuel ratio is 20 kg (air) per kg (fuel). For a fuel mass flow rate of 5 3 1023 kg/s, determine the mass flow rate of the mixture, in kg/s.

4.12 Refrigerant 134a enters the condenser of a refrigeration system operating at steady state at 9 bar, 50°C, through a 2.5-cm-diameter pipe. At the exit, the pressure is 9 bar, the temperature is 30°C, and the velocity is 2.5 m/s. The mass flow rate of the entering refrigerant is 6 kg/min. Determine (a) the velocity at the inlet, in m/s. (b) the diameter of the exit pipe, in cm. 4.13 Steam at 160 bar, 480°C, enters a turbine operating at steady state with a volumetric flow rate of 800 m3/min. Eighteen percent of the entering mass flow exits at 5 bar, 240°C, with a velocity of 25 m/s. The rest exits at another location with a pressure of 0.06 bar, a quality of 94%, and a velocity of 400 m/s. Determine the diameters of each exit duct, in m.

Air exiting through cracks at 20°C

Air infiltration through cracks at 0.03 m3/s, –9.5°C

Air infiltration through door openings at 0.02 m3/s, –9.5°C

Fig. P4.10

192

Chapter 4 Control Volume Analysis Using Energy Humid air m·4 = 1.764 kg/s

4.14 A substance flows through a 50 mm-diameter pipe with a velocity of 1 m/s at a particular location. Determine the mass flow rate, in kg/s, if the substance is (a) water at 345 kPa, 25°C. (b) nitrogen as an ideal gas at 345 kPa, 25°C. (c) Refrigerant 22 at 345 kPa, 25°C. 4.15 Air enters a compressor operating at steady state with a pressure of 1 bar, a temperature of 20°C, and a volumetric flow rate of 0.25 m3/s. The air velocity in the exit pipe is 210 m/s and the exit pressure is 1 MPa. If each unit mass of air passing from inlet to exit undergoes a process described by pv1.34 5 constant, determine the exit temperature, in °C, and the diameter of the exit pipe, in mm. 4.16 Ammonia enters a control volume operating at steady state at p1 5 14 bar, T1 5 28°C, with a mass flow rate of 0.5 kg/s. Saturated vapor at 4 bar leaves through one exit, with a volumetric flow rate of 1.036 m3/min, and saturated liquid at 4 bar leaves through a second exit. Determine

4

Cooling tower Fan

Warm water inlet m· 1 = 0.5 kg/s

Spray heads 1

Air-conditioning unit

T1 = 49°C Dry air T3 = 21°C 3 p = 1 bar 3 (AV)3 = 1.41 m3/s

Liquid

2

Return water T2 = 27°C m· = m· 2

5 Makeup water

1

Pump + –

Fig. P4.19

(a) the minimum diameter of the inlet pipe, in cm, so the ammonia velocity does not exceed 20 m/s. (b) the volumetric flow rate of the second exit stream, in m3/min. 4.17 Liquid water at 20°C enters a pump though an inlet pipe having a diameter of 160 mm. The pump operates at steady state and supplies water to two exit pipes having diameters of 80 mm and 120 mm, respectively. The velocity of the water exiting the 80 mm pipe is 0.5 m/s. At the exit of the 120 mm pipe the velocity is 0.1 m/s. The temperature of the water in each exit pipe is 22°C. Determine (a) the mass flow rate, in kg/s, in the inlet pipe and each of the exit pipes, and (b) the volumetric flow rate at the inlet, in m3/s. 4.18 At steady state, a stream of liquid water at 20°C, 1 bar is mixed with a stream of ethylene glycol (M 5 62.07) to form a refrigerant mixture that is 50% glycol by mass. The water molar flow rate is 4.2 kmol/min. The density of ethylene glycol is 1.115 times that of water. Determine (a) the molar flow rate, in kmol/min, and volumetric flow rate, in m3/min, of the entering ethylene glycol. (b) the diameters, in cm, of each of the supply pipes if the velocity in each is 2.5 m/s. 4.19 Figure P4.19 shows a cooling tower operating at steady state. Warm water from an air-conditioning unit enters at 49°C with a mass flow rate of 0.5 kg/s. Dry air enters the tower at 21°C, 1 bar with a volumetric flow rate of 1.41 m3/s. Because of evaporation within the tower, humid air exits at the top of the tower with a mass flow rate of 1.764 kg/s. Cooled liquid water is collected at the bottom of the tower for return to the air-conditioning unit together with makeup water. Determine the mass flow rate of the makeup water, in kg/s. 4.20 Figure P4.20 shows a cylindrical tank being drained through a duct whose cross-sectional area is 2 3 1024 m2 . The velocity of the water at the exit varies according to (2 gz)1/2 , where z is the water level, in m, and g is the acceleration of gravity, 9.81 m/s2. The tank initially contains 3000 kg of liquid water. Taking the density of the water as

z

mi = 3000 kg ρ = 103 kg/m3

e

Ve = (2gz)1/2 Ae = 2 × 10– 4 m2

Fig. P4.20

103 kg/m3, determine the time, in minutes, when the tank contains 600 kg of water.

Energy Analysis of Control Volumes at Steady State 4.21 Steam enters a horizontal pipe operating at steady state with a specific enthalpy of 3200 kJ/kg and a mass flow rate of 0.6 kg/s. At the exit, the specific enthalpy is 1900 kJ/kg. If there is no significant change in kinetic energy from inlet to exit, determine the rate of heat transfer between the pipe and its surroundings, in kW. 4.22 Carbon dioxide gas is heated as it flows steadily through a 2.5-cm-diameter pipe. At the inlet, the pressure is 2 bar, the temperature is 300 K, and the velocity is 100 m/s. At the exit, the pressure and velocity are 0.9413 bar and 400 m/s, respectively. The gas can be treated as an ideal gas with constant specific heat cp 5 0.94 kJ/kg ? K. Neglecting potential energy effects, determine the rate of heat transfer to the carbon dioxide, in kW.

Problems: Developing Engineering Skills 4.23 Steam with P1 5 3.5 MPa and T 1 5 350°C enters a converging-diverging nozzle operating at steady state. The velocity of steam at the inlet is 8 m/s. At the exit of the nozzle, velocity is 600 m/s and pressure is P2 5 1.6 MPa. There is negligible heat transfer during flow through the nozzle. Neglect the effect of change in potential energy. Determine the exit area of the nozzle if the mass flow rate is 2.5 kg/s. 4.24 Steam enters a well-insulated, horizontal nozzle operating at steady state with a velocity of 10 m/s. If the specific enthalpy decreases by 45 kJ/kg from inlet to exit, determine the velocity at the exit, in m/s. 4.25 Steam enters a nozzle operating at steady state at 30 bar, 320°C, with a velocity of 100 m/s. The exit pressure and temperature are 10 bar and 200°C, respectively. The mass flow rate is 2 kg/s. Neglecting heat transfer and potential energy, determine (a) the exit velocity, in m/s. (b) the inlet and exit flow areas, in cm2. 4.26 Steam enters a well-insulated nozzle at 2068 kPa, 316°C, with a velocity of 30 m/s and exits at 276 kPa with a velocity of 550 m/s. For steady-state operation, and neglecting potential energy effects, determine the exit temperature, in °C. 4.27 Air enters an uninsulated nozzle operating at steady state at 420°K with negligible velocity and exits the nozzle at 290°K with a velocity of 460 m/s. Assuming ideal gas behavior and neglecting potential energy effects, determine the heat transfer per unit mass of air flowing, in kJ/kg. 4.28 Air with a mass flow rate of 1 kg/s enters a horizontal nozzle operating at steady state at 450 K, 350 kPa and a velocity of 4 m/s. At the exit, the temperature is 320 K and the velocity is 450 m/s. Using the ideal gas model for air, determine (a) the area at the inlet, in m2, and (b) the heat transfer between the nozzle and its surroundings, in kJ/kg of air flowing. 4.29 Helium gas flows through a well-insulated nozzle at steady state. The temperature and velocity at the inlet are 333 K and 53 m/s, respectively. At the exit, the temperature is 256 K and the pressure is 345 kPa. The mass flow rate is 0.5 kg/s. Using the ideal gas model, and neglecting potential energy effects, determine the exit area, in m2. 4.30 As shown in Fig. P4.30, air enters the diffuser of a jet engine operating at steady state at 20 kPa, 240 K and a velocity of 270 m/s, all data corresponding to high-altitude Diffuser

p1 = 20 kPa T1 = 240 K V1 = 270 m/s T2 = 270 K

1

Fig. P4.30

Compressor

Combustors

Turbine

Air

Product

in

gases out

2

193

flight. The air flows adiabatically through the diffuser and achieves a temperature of 270 K at the diffuser exit. Using the ideal gas model for air, determine the velocity of the air at the diffuser exit, in m/s. 4.31 Air enters an insulated diffuser operating at steady state with a pressure of 1 bar, a temperature of 300 K, and a velocity of 250 m/s. At the exit, the pressure is 1.13 bar and the velocity is 140 m/s. Potential energy effects can be neglected. Using the ideal gas model, determine (a) the ratio of the exit flow area to the inlet flow area. (b) the exit temperature, in K. 4.32 Refrigerant 134a enters an insulated diffuser as a saturated vapor at 7 bar with a velocity of 370 m/s. At the exit, the pressure is 16 bar and the velocity is negligible. The diffuser operates at steady state and potential energy effects can be neglected. Determine the exit temperature, in °C. 4.33 Carbon dioxide gas enters a well-insulated diffuser at 138 kPa, 278 K, with a velocity of 244 m/s through a flow area of 9 cm2. At the exit, the flow area is 30 times the inlet area, and the velocity is 6 m/s. The potential energy change from inlet to exit is negligible. For steady-state operation, determine the exit temperature, in °K, the exit pressure, in kPa, and the mass flow rate, in kg/s. 4.34 Steam enters a well-insulated turbine operating at steady state at 5 MPa with a specific enthalpy of 3000 kJ/kg and a velocity of 9 m/s. The steam expands to the turbine exit where the pressure is 0.1 MPa, specific enthalpy is 2540 kJ/kg, and the velocity is 100 m/s. The mass flow rate is 12 kg/s. Neglecting potential energy effects, determine the power developed by the turbine, in kW. 4.35 Hot combustion gases, modeled as air behaving as an ideal gas, enter a turbine at 1100 kPa, 1600 K with a mass flow rate of 0.2 kg/s and exit at 300 kPa and 1000 K. If heat transfer from the turbine to its surroundings occurs at a rate of 16 kW, determine the power output of the turbine, in kW. 4.36 Air expands through a turbine from 10 bar, 900 K to 1 bar, 500 K. The inlet velocity is small compared to the exit velocity of 100 m/s. The turbine operates at steady state and develops a power output of 3200 kW. Heat transfer between the turbine and its surroundings and potential energy effects are negligible. Calculate the mass flow rate of air, in kg/s, and the exit area, in m2.

4.37 Air expands through a turbine operating at steady state on an instrumented test stand. At the inlet, p1 5 1103 kPa, T1 5 867 K, and at the exit, p2 5 102 kPa. The mass flow Nozzle rate of air entering the turbine is 4.8 kg/s, and the power developed is measured as 1900 kW. Neglecting heat transfer and kinetic and potential energy effects, determine the exit temperature, T2, in °K. 4.38 Steam enters a turbine operating at steady state at 445 K and 3447 kPa and leaves at 5.5 kPa with a quality of 93%. The turbine develops 11,185 kW, and heat transfer from the turbine to the surroundings occurs at a rate of 733 kW. Neglecting kinetic and potential energy changes from inlet to exit, determine the volumetric flow rate of the steam at the inlet, in m3/s.

194

Chapter 4 Control Volume Analysis Using Energy

4.39 A well-insulated turbine operating at steady state develops 23 MW of power for a steam flow rate of 40 kg/s. The steam enters at 360°C with a velocity of 35 m/s and exits as saturated vapor at 0.06 bar with a velocity of 120 m/s. Neglecting potential energy effects, determine the inlet pressure, in bar. 4.40 In a turbine operating at steady state, steam enters at a pressure of 4 MPa, specific enthalpy of the steam is 3018.5 kJ/kg and velocity is 8 m/s. The steam expands and exits with a velocity of 80 m/s and a specific enthalpy of 2458.6 kJ/kg. Pressure at the exit is 0.09 MPa. The mass flow rate of steam is 10 kg/s. Determine the power developed by the turbine. Neglect the effect of potential energy. 4.41 The intake to a hydraulic turbine installed in a flood control dam is located at an elevation of 10 m above the turbine exit. Water enters at 20°C with negligible velocity and exits from the turbine at 10 m/s. The water passes through the turbine with no significant changes in temperature or pressure between the inlet and exit, and heat transfer is negligible. The acceleration of gravity is constant at g 5 9.81 m/s2. If the power output at steady state is 500 kW, what is the mass flow rate of water, in kg/s? 4.42 In a steam power plant, steam flows steadily from the boiler to the turbine through a 0.25 m diameter pipe. Steam conditions at the boiler end are found to be P 5 4.5 Mpa, T 5 400°C, h 5 3204.7 kJ/kg and v 5 0.06475 m3/kg. At the turbine end, steam conditions are found to be P 5 4 MPa, T 5 393°C, h 5 3196.4 kJ/kg and v 5 0.073 m3/kg. There is a heat loss of 6 kJ/kg from the pipeline. Compute the steam flow rate. 4.43 Steam at 11032 kPa, 538°C, and a velocity of 0.6 m/s enters a turbine operating at steady state. As shown in Fig. P4.43, 22% of the entering mass flow is extracted at 1103 kPa, 232°C, with a velocity of 3 m/s. The rest of the steam exits as a two-phase liquid–vapor mixture at 7 kPa, with a quality of 85% and a velocity of 45 m/s. The turbine develops a power output of 2.6 3 105 kW. Neglecting potential energy effects and heat transfer between the turbine and its surroundings, determine (a) the mass flow rate of the steam entering the turbine, in kg/h. (b) the diameter of the extraction duct, in m. Wnet = 2.6 × 105 kW

1 p1 = 11032 kPa T1 = 538°C V1 = 0.6 m/s m· 2 = 0.22 m· 1 p2 = 1103 kPa T2 = 232°C V2 = 3 m/s

Turbine 2

3 p3 = 7 kPa x3 = 0.85 V1 = 45 m/s

Fig. P4.43 4.44 Air enters a compressor operating at steady state at 1 atm with a specific enthalpy of 280 kJ/kg and exits at a higher pressure with a specific enthalpy of 1025 kJ/kg. The mass flow rate is 0.2 kg/s. If the compressor power input is 80 kW,

determine the rate of heat transfer between the compressor and its surroundings, in kW. Neglect kinetic and potential energy effects. 4.45 Refrigerant 134a enters a compressor operating at steady state as saturated vapor at 0.14 MPa and exits at 1.4 MPa and 80°C at a mass flow rate of 0.106 kg/s. As the refrigerant passes through the compressor, heat transfer to the surroundings occurs at a rate of 20.40 kJ/s. Determine at steady state the power input to the compressor, in kW. 4.46 At steady state, a well-insulated compressor takes in air at 15°C, 98 kPa, with a volumetric flow rate of 0.57 m3/s, and compresses it to 260°C, 827 kPa. Kinetic and potential energy changes from inlet to exit can be neglected. Determine the compressor power, in kW, and the volumetric flow rate at the exit, in m3/s. 4.47 Air enters a compressor operating at steady state with a pressure of 100 kPa, a temperature of 27°C, and a volumetric flow rate of 0.57 m3/s. Air exits the compressor at 345 kPa. Heat transfer from the compressor to its surroundings occurs at a rate of 48 kJ/kg of air flowing. If the compressor power input is 78 kW, determine the exit temperature, in °C. 4.48 Refrigerant 134a enters an air-conditioner compressor at 3.2 bar, 10°C, and is compressed at steady state to 10 bar, 70°C. The volumetric flow rate of refrigerant entering is 3.0 m3/min. The power input to the compressor is 55.2 kJ per kg of refrigerant flowing. Neglecting kinetic and potential energy effects, determine the heat transfer rate, in kW. 4.49 Air is compressed at steady state from 1 bar, 300 K, to 6 bar with a mass flow rate of 4 kg/s. Each unit of mass passing from inlet to exit undergoes a process described by pv1.27 5 constant. Heat transfer occurs at a rate of 46.95 kJ per kg of air flowing to cooling water circulating in a water jacket enclosing the compressor. If kinetic and potential energy changes of the air from inlet to exit are negligible, calculate the compressor power, in kW. 4.50 Ammonia enters a refrigeration system compressor operating at steady state at 218°C, 138 kPa, and exits at 150°C, 1724 kPa. The magnitude of the power input to the compressor is 7.5 kW, and there is heat transfer from the compressor to the surroundings at a rate of 0.15 kW. Kinetic and potential energy effects are negligible. Determine the inlet volumetric flow rate, in m3/s, first using data from Table A-15, and then assuming ideal gas behavior for the ammonia. Discuss. 4.51 Air enters a water-jacketed air compressor operating at steady state with a volumetric flow rate of 37 m3/min at 136 kPa, 305 K and exits with a pressure of 680 kPa and a temperature of 400 K. The power input to the compressor is 155 kW. Energy transfer by heat from the compressed air to the cooling water circulating in the water jacket results in an increase in the temperature of the cooling water from inlet to exit with no change in pressure. Heat transfer from the outside of the jacket as well as all kinetic and potential energy effects can be neglected. (a) Determine the temperature increase of the cooling water, in K, if the cooling water mass flow rate is 82 kg/min. (b) Plot the temperature increase of the cooling water, in K, versus the cooling water mass flow rate ranging from 75 to 90 kg/min.

Problems: Developing Engineering Skills 4.52 Water is drawn steadily by a pump at a volumetric flow rate of 0.8 m3/min through a pipe with a 10 cm diameter inlet. The water is delivered to a tank placed at a height of 10 m above the pipe inlet. The water exits through a nozzle having a diameter of 2.5 cm. Water enters at 25°C and 1 atm pressure but there is no significant change in pressure and temperature at exit. Six percent of the power input to the pump is dissipated in the form of heat into the surroundings. Take the value of acceleration due to gravity (g) equal to 9.8 m/s2. Determine (a) the velocity of the water at the inlet and exit, and (b) the power required by the pump. 4.53 A pump steadily draws water through a pipe from a reservoir at a volumetric flow rate of 1.3 L/s. At the pipe inlet, the pressure is 101.4 kPa, the temperature is 18°C, and the velocity is 3 m/s. At the pump exit, the pressure is 240 kPa, the temperature is 18°C, and the velocity is 12 m/s. The pump exit is located 12 m above the pipe inlet. Ignoring heat transfer, determine the power required by the pump, in kJ/s and kW. The local acceleration of gravity is 9.8 m/s2. 4.54 A water pump operating at steady state has 76 mmdiameter inlet and exit pipes, each at the same elevation. The water can be modeled as incompressible and its temperature remains constant at 20°C. For a power input of 1.5 kW, plot the pressure rise from inlet to exit, in kPa, versus the volumetric flow rate ranging from 0.015 to 0.02 m3/s. 4.55 A pump steadily delivers water through a hose terminated by a nozzle. The exit of the nozzle has a diameter of 2.5 cm and is located 4 m above the pump inlet pipe, which has a diameter of 5.0 cm. The pressure is equal to 1 bar at both the inlet and the exit, and the temperature is constant at 20°C. The magnitude of the power input required by the pump is 8.6 kW, and the acceleration of gravity is g 5 9.81 m/s2. Determine the mass flow rate delivered by the pump, in kg/s. 4.56 Air flows steadily through an air compressor at a rate of 0.6 kg/s. Air enters the compressor at 8 m/s velocity, 100 kPa pressure and 0.96 m3/kg volume and leaves at 6 m/s velocity, 700 kPa pressure and 0.2 m3/kg volume. There is an increase in internal energy of 95 kJ/kg. Heat is absorbed by the cooling water in the compressor jackets at a rate of 60 kW. (a) Calculate the rate of shaft work input to the air in kW. (b) Determine the ratio of inlet pipe diameter to outlet pipe diameter.

195

heat exchanger and neglecting kinetic and potential energy effects, determine the mass flow rate of the air, in kg/min. 4.59 Refrigerant 134a enters a condenser operating at steady state at 4.1 bar and 60°C and is condensed to saturated liquid at 4.1 bar on the outside of tubes through which cooling water flows. In passing through the tubes, the cooling water increases in temperature by 28°C and experiences no significant pressure drop. The volumetric flow rate of cooling water is 132 L/m. Neglecting kinetic and potential energy effects and ignoring heat transfer from the outside of the condenser, determine (a) the mass flow rate of refrigerant, in kg/h. (b) the rate of energy transfer, in kJ/h, from the condensing refrigerant to the cooling water. 4.60 Steam at a pressure of 0.10 bar and a quality of 94.2% enters a shell-and-tube heat exchanger where it condenses on the outside of tubes through which cooling water flows, exiting as saturated liquid at 0.10 bar. The mass flow rate of the condensing steam is 4.2 3 105 kg/h. Cooling water enters the tubes at 20°C and exits at 40°C with negligible change in pressure. Neglecting stray heat transfer and ignoring kinetic and potential energy effects, determine the mass flow rate of the cooling water, in kg/h, for steady-state operation. 4.61 The cooling coil of an air-conditioning system is a heat exchanger in which air passes over tubes through which Refrigerant 22 flows. Air enters with a volumetric flow rate of 40 m3/min at 27°C, 1.1 bar, and exits at 15°C, 1 bar. Refrigerant enters the tubes at 7 bar with a quality of 16% and exits at 7 bar, 15°C. Ignoring heat transfer from the outside of the heat exchanger and neglecting kinetic and potential energy effects, determine at steady state (a) the mass flow rate of refrigerant, in kg/min. (b) the rate of energy transfer, in kJ/min, from the air to the refrigerant. 4.62 A 0.75 m3 capacity tank contains a two-phase liquid-vapor mixture at 240°C and a quality of 0.6. Heat is transferred to the mixture and saturated vapor from the tank are withdrawn at 240°C to maintain the constant pressure inside the tank by means of a pressure regulating value. This process continues until the tank has only saturated vapor left in it. Neglect the effect of kinetic and potential energies. Determine the amount of heat transfer in kJ.

4.57 Refrigerant 134a enters a heat exchanger in a refrigeration system operating at steady state as saturated vapor at 217°C and exits at 27°C with no change in pressure. A separate liquid stream of Refrigerant 134a passes in counterflow to the vapor stream, entering at 41°C, 11 bar, and exiting at a lower temperature while experiencing no pressure drop. The outside of the heat exchanger is well insulated, and the streams have equal mass flow rates. Neglecting kinetic and potential energy effects, determine the exit temperature of the liquid stream, in °C.

4.63 Refrigerant 134a enters a heat exchanger at 212°C and a quality of 42% and exits as saturated vapor at the same temperature with a volumetric flow rate of 0.85 m3/min. A separate stream of air enters at 22°C with a mass flow rate of 188 kg/min and exits at 17°C. Assuming the ideal gas model for air and ignoring kinetic and potential energy effects, determine (a) the mass flow rate of the Refrigerant 134a, in kg/min, and (b) the heat transfer between the heat exchanger and its surroundings, in kJ/min.

4.58 Ammonia enters a heat exchanger operating at steady state as a superheated vapor at 14 bar, 60°C, where it is cooled and condensed to saturated liquid at 14 bar. The mass flow rate of the ammonia is 450 kg/h. A separate stream of air enters the heat exchanger at 17°C, 1 bar and exits at 42°C, 1 bar. Ignoring heat transfer from the outside of the

4.64 A water heater operating under steady flow conditions receives water at the rate of 5 kg/s at 80°C temperature with specific enthalpy of 320.5 kJ/kg. Water is heated by mixing steam at temperature 100.5°C and specific enthalpy of 2650 kJ/kg. The mixture of water and steam leaves the heater in the form of liquid water at temperature 100°C with specific

196

Chapter 4 Control Volume Analysis Using Energy 1 Liquid, T1 = 30°C, p1 = 0.4 MPa m1 = 5 kg/min. Superheated vapor, T2 = 250°C, p2 = 0.4 MPa

Desuperheater 3

2

Fig. P4.66

enthalpy of 421 kJ/kg. Calculate the required steam flow rate to the heater per hour. 4.65 A feedwater heater operates at steady state with liquid water entering at inlet 1 at 7 bar, 42°C, and a mass flow rate of 70 kg/s. A separate stream of water enters at inlet 2 as a two-phase liquid–vapor mixture at 7 bar with a quality of 98%. Saturated liquid at 7 bar exits the feedwater heater at 3. Ignoring heat transfer with the surroundings and neglecting kinetic and potential energy effects, determine the mass flow rate, in kg/s, at inlet 2. 4.66 For the desuperheater shown in Fig. P4.66, liquid water at state 1 is injected into a stream of superheated vapor entering at state 2. As a result, saturated vapor exits at state 3. Data for steady state operation are shown on the figure. Ignoring stray heat transfer and kinetic and potential energy effects, determine the mass flow rate of the incoming superheated vapor, in kg/min. 4.67 Figure P4.67 provides steady-state data for the ducting ahead of the chiller coils in an air-conditioning system. Outside air at 32°C is mixed with return air at 24°C. Stray heat transfer is negligible, kinetic and potential energy effects can be ignored, and the pressure throughout is 1 bar. Outside air at T1 = 32°C V1 = 180 m/min. (AV)1 = 56 m3/min.

3

p = 1 bar

Return air at T2 = 24°C D2 = 1.2 m V2 = 120 m/min.

Fig. P4.67

Modeling the air as an ideal gas with cp 5 1 kJ/kg ? K, determine (a) the mixed-air temperature, in °C, and (b) the diameter of the mixed-air duct, in m. 4.68 The electronic components of Example 4.8 are cooled by air flowing through the electronics enclosure. The rate of energy transfer by forced convection from the electronic components to the air is hA(Ts 2 Ta), where hA 5 5 W/K, Ts denotes the average surface temperature of the components, and Ta denotes the average of the inlet and exit air temperatures. Referring to Example 4.8 as required, determine the largest value of Ts, in °C, for which the specified limits are met. 4.69 The electronic components of a computer consume 0.1 kW of electrical power. To prevent overheating, cooling air is supplied by a 25-W fan mounted at the inlet of the electronics enclosure. At steady state, air enters the fan at 20°C, 1 bar, and exits the electronics enclosure at 35°C. There is no significant energy transfer by heat from the outer surface of the enclosure to the surroundings, and the effects of kinetic and potential energy can be ignored. Determine the volumetric flow rate of the entering air, in m3/s. 4.70 Ammonia enters the expansion valve of a refrigeration system at a pressure of 1.4 MPa and a temperature of 32°C and exits at 0.08 MPa. If the refrigerant undergoes a throttling process, what is the quality of the refrigerant exiting the expansion valve? 4.71 Propane vapor enters a valve at 1.6 MPa, 70°C, and leaves at 0.5 MPa. If the propane undergoes a throttling process, what is the temperature of the propane leaving the valve, in °C?

1

2

Saturated vapor, p3 = 0.4 MPa

4.72 A large pipe carries steam as a two-phase liquid–vapor mixture at 1.0 MPa. A small quantity is withdrawn through a throttling calorimeter, where it undergoes a throttling process to an exit pressure of 0.1 MPa. For what range of exit temperatures, in °C, can the calorimeter be used to determine the quality of the steam in the pipe? What is the corresponding range of steam quality values?

Mixed air, V3 = 150 m/min. 4.73 Refrigerant 134a enters the expansion valve of an airT3 = ? conditioning unit at 965 kPa, 27°C, and exits at 345 kPa. D3 = ? If the refrigerant undergoes a throttling process, what are the

temperature, in °K, and the quality at the exit of the valve? 4.74 Figure P4.74 shows a turbine operating at a steady state that provides power to an air compressor and an electric generator. Air enters the turbine with a mass flow rate of 5.6 kg/s at 517°C and exits the turbine at 117°C, 2 bar. The turbine provides power at a rate of 800 kW to the compressor and at a rate of 1200 kW to the generator. Air can be modeled as an ideal gas, and kinetic and potential energy changes are negligible. Determine (a) the volumetric flow rate of the air at the turbine exit, in m3/s, and (b) the

Problems: Developing Engineering Skills Air 1 m1 = 5.6 kg/s T1 = 517°C

potential energy effects, and assuming ideal gas behavior for the air, determine (a) the mass flow rate of refrigerant, in kg/min, and (b) the compressor power, in kilowatt. Electric Generator

Turbine

Compressor

2 Air

+ –

T2 = 117°C p2 = 2 bar

Fig. P4.74 rate of heat transfer between the turbine and its surroundings, in kW. 4.75 As shown in Fig. P4.75, a steam turbine at steady state is operated at part load by throttling the steam to lower pressure before it enters the turbine. Ahead of the throttling valve, the pressure and temperature are 1.5 MPa and 320°C, respectively. After throttling, the pressure is 1 MPa. At the turbine exit, the steam is at 0.08 bar and the quality is 90%. Heat transfer to the surroundings and all kinetic and potential energy effects can be ignored. Determine (a) the temperature at the turbine inlet, °C, and (b) the power developed by the turbine, in kJ/kg, of steam flowing.

4.77 At steady state, water enters the waste heat recoverysteam generator shown in Fig. P4.77 at 290 kPa, 104°C, and exits at 276 kPa, 160°C. The steam is then fed into a turbine from which it exits at 7 kPa and a quality of 90%. Air from an oven exhaust enters the steam generator at 182°C, 1 bar, with a volumetric flow rate of 2 m3/s, and exits at 138°C, 1 bar. Ignore all stray heat transfer with the surroundings and all kinetic and potential energy effects. If the power developed is valued at 8 cents per kW ? h, do you recommend implementation of this waste-heat recovery system? Provide supporting calculations. Oven exhaust TA = 182°C (AV)A = 2 m3/s

(A) p2 = 276 kPa T2 = 160°C

1 p1 = 1.5 MPa T1 = 320°C

2

Turbine

(B)

1 p3 = 0.08 bar x3 = 90%

3

Steam generator

Power out

p2 = 1 MPa 3

Power out

Turbine

2 TB = 138°C

Valve

197

p3 = 7 kPa x 3 = 90%

p1 = 290 kPa T1 = 104°C

Water in

Fig. P4.75

Fig. P4.77 4.76 As shown in Fig. P4.76, Refrigerant 22 enters the compressor of an air-conditioning unit operating at steady state at 4°C, 5.5 bar and is compressed to 60°C, 14 bar. The refrigerant exiting the compressor enters a condenser where energy transfer to air as a separate stream occurs and the refrigerant exits as a liquid at 14 bar, 32°C. Air enters the condenser at 27°C, 1 bar with a volumetric flow rate of 20.25 m3/min and exits at 43°C. Neglecting stray heat transfer and kinetic and

4.78 A simple steam power plant operates at steady state with water circulating through the components with a mass flow rate of 60 kg/s. Figure P4.78 shows additional data at key · Q in

p1 = 3 MPa T1 = 400°C

Power out

Turbine

1 Condenser 5 T5 = 43°C 2

T2 = 60°C p2 = 14 bar

Compressor

4

Air at T4 = 27°C, p4 = 1 bar (AV)4 = 20.25 m3/min.

Steam generator

3 T3 = 32°C p3 = 14 bar

p2 = 50 kPa x2 = 95%

2

5

Condenser 4

6

p4 = 3 MPa T4 = 82°C

Cooling water out at T6 = T5 + 15°C

Pump 1

R22 at T1 = 4°C p1 = 5.5 bar

Fig. P4.76

Power in

Fig. P4.78

Cooling water in at T5

3 p3 = 50 kPa Saturated liquid

198

Chapter 4 Control Volume Analysis Using Energy Outside air at 32°C T3 = 40°C p3 = 1533.5 kPa

3

p2 = 1533.5 kPa h 2 = 300 kJ/kg

2

Condenser

Expansion valve

T 4 = 15°C

Air exiting at T > 32°C

Power input = 4.5 kW

Compressor

4

Supply air to residence at T < 24°C

Evaporator

p1 = 789.1 kPa T1 = 15°C

1

Return room air at 24°C

points in the cycle. Stray heat transfer and kinetic and potential effects are negligible. Determine (a) the thermal efficiency and (b) the mass flow rate of cooling water through the condenser, in kg/s. 4.79 A residential air-conditioning system operates at steady state, as shown in Fig. P4.79. Refrigerant 22 circulates through the components of the system. Property data at key locations are given on the figure. If the evaporator removes energy by heat transfer from the room air at a rate of 10 kW, determine (a) the rate of heat transfer between the compressor and the surroundings, in kW, and (b) the coefficient of performance.

Transient Analysis 4.80 A tiny hole develops in the wall of a rigid tank whose volume is 0.75 m3, and air from the surroundings at 1 bar, 25°C leaks in. Eventually, the pressure in the tank reaches 1 bar. The process occurs slowly enough that heat transfer between the tank and the surroundings keeps the temperature of the air inside the tank constant at 25°C. Determine the amount of heat transfer, in kJ, if initially the tank

Fig. P4.79

in K, and pressure, in kPa, in the container following injection of the air. 0.2 kg air at 2 bar, 300 K

Syringe

1 kg air at 1 bar, 300 K Stopcock Container

Fig. P4.82 4.83 As shown in Fig. P4.83, a 8 m3 tank contains H2O initially at 207 kPa and a quality of 80%. The tank is connected to a large steam line carrying steam at 1380 kPa, 232°C. Steam flows into the tank through a valve until the tank pressure reaches 690 kPa and the temperature is 204°C, at which time the valve is closed. Determine the amount of mass, in kg, that enters the tank and the heat transfer between the tank and its surroundings, in kJ.

(a) is evacuated. (b) contains air at 0.7 bar, 25°C. 4.81 A rigid tank of volume 0.75 m3 is initially evacuated. A hole develops in the wall, and air from the surroundings at 1 bar, 25°C flows in until the pressure in the tank reaches 1 bar. Heat transfer between the contents of the tank and the surroundings is negligible. Determine the final temperature in the tank, in °C. 4.82 Two-tenths of a pound of air at 2 bar and 300 K is trapped within a syringe by a plunger at one end and a stopcock at the other, as shown in Fig. P4.82. The stopcock is opened and the plunger moved to inject the trapped air into a container that initially holds 1 kg of air at 1.0 bar and 300 K. The plunger maintains the state of the injected air constant until all has passed the stopcock. Ignoring heat transfer with the surroundings, and kinetic and potential energy effects, determine the final equilibrium temperature,

Steam at 1380 kPa, 232°C

Valve

Tank V = 8 m3 Initially: 207 kPa, x = 80% Finally: 690 kPa, 204°C.

Fig. P4.83 4.84 A two-phase liquid–vapor mixture of Refrigerant 134a is contained in a 0.06 m3, cylindrical storage tank at 670 kPa. Initially, saturated liquid occupies 0.04 m3. The valve at the top of the tank develops a leak, allowing saturated vapor to escape slowly. Eventually, the volume of the liquid drops to

Problems: Developing Engineering Skills 0.02 m3. If the pressure in the tank remains constant, determine the mass of refrigerant that has escaped, in kg, and the heat transfer, in kJ. 3

4.85 A well-insulated rigid tank of volume 0.14 m contains oxygen (O2), initially at 275 kPa and 308C. The tank is connected to a large supply line carrying oxygen at 690 kPa 388C. A valve between the line and the tank is opened and gas flows into the tank until the pressure reaches 690 kPa, at which time the valve closes. The contents of the tank eventually cool back to 30°C. Determine (a) the temperature in the tank at the time when the valve closes, in °C. (b) the final pressure in the tank, in kPa.

when all the liquid has vaporized. Determine the amount of refrigerant admitted to the tank, in kg. 4.89 A well-insulated piston–cylinder assembly is connected by a valve to an air supply line at 8 bar, as shown in Fig. P4.89. Initially, the air inside the cylinder is at 1 bar, 300 K, and the piston is located 0.5 m above the bottom of the cylinder. The atmospheric pressure is 1 bar, and the diameter of the piston face is 0.3 m. The valve is opened and air is admitted slowly until the volume of air inside the cylinder has doubled. The weight of the piston and the friction between the piston and the cylinder wall can be ignored. Using the ideal gas model, plot the final temperature, in K, and the final mass, in kg, of the air inside the cylinder for supply temperatures ranging from 300 to 500 K.

4.86 A tank of volume 1 m3 initially contains steam at 6 MPa and 320°C. Steam is withdrawn slowly from the tank until the pressure drops to p. Heat transfer to the tank contents maintains the temperature constant at 320°C. Neglecting all kinetic and potential energy effects

patm = 1 bar

(a) determine the heat transfer, in kJ, if p 5 1.5 MPa. (b) plot the heat transfer, in kJ, versus p ranging from 0.5 to 6 MPa.

Diameter = 0.3 m L

4.87 A 0.5 m3 tank initially contains air at 300 kPa, 350 K. Air slowly escapes from the tank until the pressure drops to 100 kPa. The air that remains in the tank undergoes a process described by pv1.3 5 constant. For a control volume enclosing the tank, determine the heat transfer, in kJ. Assume ideal gas behavior with constant specific heats. 4.88 A well-insulated tank contains 25 kg of Refrigerant 134a, initially at 300 kPa with a quality of 0.8 (80%). The pressure is maintained by nitrogen gas acting against a flexible bladder, as shown in Fig. P4.88. The valve is opened between the tank and a supply line carrying Refrigerant 134a at 1.0 MPa, 120°C. The pressure regulator allows the pressure in the tank to remain at 300 kPa as the bladder expands. The valve between the line and the tank is closed at the instant

Pressure-regulating valve

N2

Nitrogen supply

Flexible bladder

Initially: L1 = 0.5 m T1 = 300 K p1 = 1 bar Valve Air supply line: 8 bar

Fig. P4.89 4.90 A well-insulated piston–cylinder assembly is connected by a valve to an air supply at 6.9 bar, 300 K, as shown in Fig. P4.90. The air inside the cylinder is initially at 1 bar, 300 K, and occupies a volume of 0.0027 m3. Initially, the piston face is located at x 5 0 and the spring exerts no force on the piston. The atmospheric pressure is 1 bar, and the area of the piston face is 0.0204 m2. The valve is opened, and air is admitted slowly until the volume of the air inside the cylinder is 0.0108 m3. During the process, the spring exerts a force on the piston that varies according to F 5 kx. The ideal gas model applies for the air, and there is no friction between the piston and the cylinder wall. For the air within the cylinder, plot the final pressure, in bar, and the final temperature, in 8C, versus k ranging from 9,500 to 10,950 N/m.

Refrigerant 134a 300 kPa Tank

Line: 1 MPa, 120°C

Valve Air supply 6.9 bar 300 K x x=0

Fig. P4.88

199

Fig. P4.90

patm = 1 bar

200

Chapter 4 Control Volume Analysis Using Energy V = 0.028 m3

Control valve

Air supply 207 kPa 93°C

Control valve pi = 101.4 kPa Ti = 38°C

Air discharge

Well-insulated chamber

4.91 A well-insulated chamber of volume 0.028 m3 is shown in Fig. P4.91. Initially, the chamber contains air at 101.4 kPa and 38°C. Connected to the chamber are supply and discharge pipes equipped with valves that control the flow rates into and out of the chamber. The supply air is at 207 kPa, 93°C. Both valves are opened simultaneously, # allowing air to flow with a mass flow rate m through each

Fig. P4.91

valve. The air within the chamber is well mixed, so the temperature and pressure at any time can be taken as uniform throughout. Neglecting kinetic and potential energy effects, and using the ideal gas model with constant specific heats for the air, plot the temperature, in °C, and the pressure, # in kPa, of the air in the chamber versus time for m 5 0.01, 0.02, and 0.04 kg/s.

c DESIGN & OPEN ENDED PROBLEMS: EXPLORING ENGINEERING PRACTICE 4.1D Using the Internet, identify at least five medical applications of MEMS technology. In each case, explain the scientific and technological basis for the application, discuss the state of current research, and determine how close the technology is in terms of commercialization. Write a report of your findings, including at least three references. 4.2D A group of cells called the sinus node is the natural pacemaker of the heart and controls the heartbeat. Sinus node dysfunction is one source of the medical condition known as heart arrhythmia: irregular heartbeat. Significant arrhythmias are treated in several ways, including the use of an artificial pacemaker, which is an electrical device that sends the signals needed to make the heart beat properly. Research how both natural and artificial pacemakers operate to achieve their goal of maintaining a regular heartbeat. Place your findings in a memorandum that includes annotated sketches of each type of pacemaker. 4.3D Identify sites in your state where wind turbines for utility-scale electrical generation are feasible but do not yet exist. Prepare a memorandum to an appropriate governing or corporate entity with your recommendations as to whether wind-turbine electrical generation should be developed at the most promising sites. Consider engineering, economic, and societal aspects. 4.4D Generating power by harnessing ocean tides and waves is being studied across the globe. Underwater turbines develop power from tidal currents. Wave-power devices develop power from the undulating motion of ocean waves. Although tides and waves long have been used to meet modest power generation needs, investigators today are aiming at large-scale power generation systems. Some see the oceans as providing a potentially unlimited renewable source of power. Critically evaluate the viability of tidal

and wave power, considering both technical and economic issues. Write a report, including at least three references. 4.5D Owing to their relatively compact size, simple construction, and modest power requirement, centrifugal-type blood pumps are considered promising for several medical applications. Still, centrifugal pumps have met with limited success for blood flow because they can cause damage to blood cells and are subject to mechanical failure. The goal of current development efforts is a device having sufficient long-term biocompatibility, performance, and reliability for widespread deployment. Investigate the status of centrifugal blood pump development, including identifying key technical challenges and prospects for overcoming them. Summarize your findings in a report, including at least three references. 4.6D Recent natural disasters, including major floods, hurricanes, and tsunamis, have revealed the vulnerability of municipal water distribution systems to water-borne contamination. For the water distribution system of a municipality of your choice, study the existing procedure for restoring the system to safe use after contamination. If no suitable decontamination procedure exists, make recommendations. Suggest easy-toimplement, cost-effective, environmentally responsible measures. Document your findings in a memorandum. 4.7D Prepare a memorandum providing guidelines for selecting fans for cooling electronic components. Consider the advantages and disadvantages of locating the fan at the inlet of the enclosure versus at the exit. Consider the relative merits of alternative fan types and of fixed versus variablespeed fans. Explain how characteristic curves assist in fan selection. 4.8D Forced-air warming systems involving inflatable thermal blankets are commonly used to prevent subnormal body

Design & Open Ended Problems: Exploring Engineering Practice temperature (hypothermia) during and following surgery. A heater and blower provide a stream of warmed air to the blanket. While the air temperature leaving the heater/blower is monitored by a temperature sensor, the temperature of the air providing warming to patients can vary widely, causing in some instances overheating and localized burning of patients. The object of this project is to develop costeffective modifications of existing thermal blankets that would control the air temperature and eliminate injurious “hot spots.” The modifications must conform to standards governing the safety of systems involving heating in medical applications. Summarize your conclusions in a memorandum. 4.9D Residential integrated systems capable of generating electricity and providing space heating and water heating will reduce reliance on electricity supplied from central power plants. For a 230 m2 dwelling in your locale, evaluate two alternative technologies for combined power and heating: a solar energy-based system and a natural gas fuel cell system. For each alternative, specify equipment and evaluate costs, including the initial system cost, installation cost, and operating cost. Compare total cost with that for conventional means for powering and heating the dwelling. Write a report summarizing your analysis and recommending either or both of the options if they are preferable to conventional means. 4.10D Figure P4.10D provides the schematic of a device for producing a combustible fuel gas from biomass. Owing to petroleum shortages, similar gasifiers were widely used in Europe during the 1940s war years, including use in tractors, trucks, fishing and ferry boats, and stationary engines. While several types of solid biomass can be employed in current gasifier designs, wood chips are commonly used. Wood chips are introduced at the top of the gasifier unit. Just below this

201

Gasifier Carburetor

Air

Wood chips Gas cooler Charcoal formation Engine Grate Ash

Filter

Fig. P4.10D level, the chips react with oxygen in the combustion air to produce charcoal. At the next depth, the charcoal reacts with hot combustion gases from the charcoal-formation stage to produce a fuel gas consisting mainly of hydrogen, carbon monoxide, and nitrogen from the combustion air. The fuel gas is then cooled, filtered, and ducted to the internal combustion engine served by the gasifier. Critically evaluate the suitability of this technology for use today in the event of a prolonged petroleum shortage in your locale. Document your conclusions in a memorandum.

5 The Second Law of Thermodynamics ENGINEERING CONTEXT The presentation to this point has considered thermodynamic analysis using the conservation of mass and conservation of energy principles together with property relations. In Chaps. 2 through 4 these fundamentals are applied to increasingly complex situations. The conservation principles do not always suffice, however, and the second law of thermodynamics is also often required for thermodynamic analysis. The objective of this chapter is to introduce the second law of thermodynamics. A number of deductions that may be called corollaries of the second law are also considered, including performance limits for thermodynamic cycles. The current presentation provides the basis for subsequent developments involving the second law in Chaps. 6 and 7.

LEARNING OUTCOMES When you complete your study of this chapter, you will be able to...

202

c

demonstrate understanding of key concepts related to the second law of thermodynamics, including alternative statements of the second law, the internally reversible process, and the Kelvin temperature scale.

c

list several important irreversibilities.

c

assess the performance of power cycles and refrigeration and heat pump cycles the corollaries of Secs. 5.6.2 and 5.7.2, together with Eqs. 5.9–5.11.

c

describe the Carnot cycle.

c

interpret the Clausius inequality as expressed by Eq. 5.13.

5.1 Introducing the Second Law

203

5.1 Introducing the Second Law The objectives of the present section are to 1. motivate the need for and the usefulness of the second law. 2. introduce statements of the second law that serve as the point of departure for its application.

5.1.1

Motivating the Second Law

It is a matter of everyday experience that there is a definite direction for spontaneous processes. This can be brought out by considering the three systems pictured in Fig. 5.1. c System a. An object at an elevated temperature Ti placed in contact with atmo-

spheric air at temperature T0 eventually cools to the temperature of its much larger surroundings, as illustrated in Fig. 5.1a. In conformity with the conservation of energy principle, the decrease in internal energy of the body appears as an increase in the internal energy of the surroundings. The inverse process would not take place spontaneously, even though energy could be conserved: The internal energy of the surroundings would not decrease spontaneously while the body warmed from T0 to its initial temperature. Atmospheric air at T0

Q

Time

Body at Ti > T0

Time

T0 < T < Ti

T0

(a)

Atmospheric air at p0 Valve

Air at pi > p0

Air at p0

Air p0 < p < pi

(b)

Mass zi

Mass 0 < z < zi

(c)

Mass

Fig. 5.1 Illustrations of spontaneous processes and the eventual attainment of equilibrium with the surroundings. (a) Spontaneous heat transfer. (b) Spontaneous expansion. (c) Falling mass.

204

Chapter 5 The Second Law of Thermodynamics c System b. Air held at a high pressure pi in a closed tank flows spontaneously to

the lower pressure surroundings at p0 when the interconnecting valve is opened, as illustrated in Fig. 5.1b. Eventually fluid motions cease and all of the air is at the same pressure as the surroundings. Drawing on experience, it should be clear that the inverse process would not take place spontaneously, even though energy could be conserved: Air would not flow spontaneously from the surroundings at p0 into the tank, returning the pressure to its initial value. c System c. A mass suspended by a cable at elevation zi falls when released, as illustrated in Fig. 5.1c. When it comes to rest, the potential energy of the mass in its initial condition appears as an increase in the internal energy of the mass and its surroundings, in accordance with the conservation of energy principle. Eventually, the mass also comes to the temperature of its much larger surroundings. The inverse process would not take place spontaneously, even though energy could be conserved: The mass would not return spontaneously to its initial elevation while its internal energy and/or that of its surroundings decreases. In each case considered, the initial condition of the system can be restored, but not in a spontaneous process. Some auxiliary devices would be required. By such auxiliary means the object could be reheated to its initial temperature, the air could be returned to the tank and restored to its initial pressure, and the mass could be lifted to its initial height. Also in each case, a fuel or electrical input normally would be required for the auxiliary devices to function, so a permanent change in the condition of the surroundings would result.

Further Conclusions The foregoing discussion indicates that not every process consistent with the principle of energy conservation can occur. Generally, an energy balance alone neither enables the preferred direction to be predicted nor permits the processes that can occur to be distinguished from those that cannot. In elementary cases, such as the ones considered in Fig. 5.1, experience can be drawn upon to deduce whether particular spontaneous processes occur and to deduce their directions. For more complex cases, where experience is lacking or uncertain, a guiding principle is necessary. This is provided by the second law. The foregoing discussion also indicates that when left alone systems tend to undergo spontaneous changes until a condition of equilibrium is achieved, both internally and with their surroundings. In some cases equilibrium is reached quickly, in others it is achieved slowly. For example, some chemical reactions reach equilibrium in fractions of seconds; an ice cube requires a few minutes to melt; and it may take years for an iron bar to rust away. Whether the process is rapid or slow, it must of course satisfy conservation of energy. However, that alone would be insufficient for determining the final equilibrium state. Another general principle is required. This is provided by the second law.

BIOCONNECTIONS Did you ever wonder why a banana placed in a closed bag or in the refrigerator quickly ripens? The answer is in the ethylene, C2H4, naturally produced by bananas, tomatoes, and other fruits and vegetables. Ethylene is a plant hormone that affects growth and development. When a banana is placed in a closed container, ethylene accumulates and stimulates the production of more ethylene. This positive feedback results in more and more ethylene, hastening ripening, aging, and eventually spoilage. In thermodynamic terms, if left alone, the banana tends to undergo spontaneous changes until equilibrium is achieved. Growers have learned to use this natural process to their advantage. Tomatoes picked while still green and shipped to distant markets may be red by the time they arrive; if not, they can be induced to ripen by means of an ethylene spray.

5.1 Introducing the Second Law

5.1.2

Opportunities for Developing Work

By exploiting the spontaneous processes shown in Fig. 5.1, it is possible, in principle, for work to be developed as equilibrium is attained. instead of permitting the body of Fig. 5.1a to cool spontaneously with no other result, energy could be delivered by heat transfer to a system undergoing a power cycle that would develop a net amount of work (Sec. 2.6). Once the object attained equilibrium with the surroundings, the process would cease. Although there is an opportunity for developing work in this case, the opportunity would be wasted if the body were permitted to cool without developing any work. In the case of Fig. 5.1b, instead of permitting the air to expand aimlessly into the lower-pressure surroundings, the stream could be passed through a turbine and work could be developed. Accordingly, in this case there is also a possibility for developing work that would not be exploited in an uncontrolled process. In the case of Fig. 5.1c, instead of permitting the mass to fall in an uncontrolled way, it could be lowered gradually while turning a wheel, lifting another mass, and so on. b b b b b These considerations can be summarized by noting that when an imbalance exists between two systems, there is an opportunity for developing work that would be irrevocably lost if the systems were allowed to come into equilibrium in an uncontrolled way. Recognizing this possibility for work, we can pose two questions: 1. What is the theoretical maximum value for the work that could be obtained? 2. What are the factors that would preclude the realization of the maximum value? That there should be a maximum value is fully in accord with experience, for if it were possible to develop unlimited work, few concerns would be voiced over our dwindling fossil fuel supplies. Also in accord with experience is the idea that even the best devices would be subject to factors such as friction that would preclude the attainment of the theoretical maximum work. The second law of thermodynamics provides the means for determining the theoretical maximum and evaluating quantitatively the factors that preclude attaining the maximum.

5.1.3

Aspects of the Second Law

We conclude our introduction to the second law by observing that the second law and deductions from it have many important uses, including means for: 1. predicting the direction of processes. 2. establishing conditions for equilibrium. 3. determining the best theoretical performance of cycles, engines, and other devices. 4. evaluating quantitatively the factors that preclude the attainment of the best theoretical performance level. Other uses of the second law include: 5. defining a temperature scale independent of the properties of any thermometric substance. 6. developing means for evaluating properties such as u and h in terms of properties that are more readily obtained experimentally. Scientists and engineers have found additional uses of the second law and deductions from it. It also has been used in philosophy, economics, and other disciplines far removed from engineering thermodynamics.

205

206

Chapter 5 The Second Law of Thermodynamics

TAKE NOTE...

No single statement of the he second law brings out each of its many aspects.

The six points listed can be thought of as aspects of the second law of thermodynamics and not as independent and unrelated ideas. Nonetheless, given the variety of these topic areas, it is easy to understand why there is no single statement of the second law that brings out each one clearly. There are several alternative, yet equivalent, formulations of the second law. In the next section, three statements of the second law are introduced as points of departure for our study of the second law and its consequences. Although the exact relationship of these particular formulations to each of the second law aspects listed above may not be immediately apparent, all aspects listed can be obtained by deduction from these formulations or their corollaries. It is important to add that in every instance where a consequence of the second law has been tested directly or indirectly by experiment, it has been unfailingly verified. Accordingly, the basis of the second law of thermodynamics, like every other physical law, is experimental evidence.

5.2 Statements of the Second Law Three alternative statements of the second law of thermodynamics are given in this section. They are the (1) Clausius, (2) Kelvin–Planck, and (3) entropy statements. The Clausius and Kelvin–Planck statements are traditional formulations of the second law. You have likely encountered them before in an introductory physics course. Although the Clausius statement is more in accord with experience and thus easier to accept, the Kelvin–Planck statement provides a more effective means for bringing out second law deductions related to thermodynamic cycles that are the focus of the current chapter. The Kelvin–Planck statement also underlies the entropy statement, which is the most effective form of the second law for an extremely wide range of engineering applications. The entropy statement is the focus of Chap. 6.

5.2.1 Clausius statement

Clausius Statement of the Second Law

The Clausius statement of the second law asserts that: It is impossible for any system to operate in such a way that the sole result would be an energy transfer by heat from a cooler to a hotter body. The Clausius statement does not rule out the possibility of transferring energy by heat from a cooler body to a hotter body, for this is exactly what refrigerators and heat pumps accomplish. However, as the words “sole result” in the statement suggest, when a heat transfer from a cooler body to a hotter body occurs, there must be other effects within the system accomplishing the heat transfer, its surroundings, or both. If the system operates in a thermodynamic cycle, its initial state is restored after each cycle, so the only place that must be examined for such other effects is its surroundings.

Q Hot

Metal bar Cold

Q

cooling of food is most commonly accomplished by refrigerators driven by electric motors requiring power from their surroundings to operate. The Clausius statement implies it is impossible to construct a refrigeration cycle that operates without a power input. b b b b b

5.2.2 thermal reservoir

Kelvin–Planck Statement of the Second Law

Before giving the Kelvin–Planck statement of the second law, the concept of a thermal reservoir is introduced. A thermal reservoir, or simply a reservoir, is a special kind of system that always remains at constant temperature even though energy is added or removed by heat transfer. A reservoir is an idealization of course, but such a system

5.2 Statements of the Second Law can be approximated in a number of ways—by the earth’s atmosphere, large bodies of water (lakes, oceans), a large block of copper, and a system consisting of two phases at a specified pressure (while the ratio of the masses of the two phases changes as the system is heated or cooled at constant pressure, the temperature remains constant as long as both phases coexist). Extensive properties of a thermal reservoir such as internal energy can change in interactions with other systems even though the reservoir temperature remains constant. Having introduced the thermal reservoir concept, we give the Kelvin–Planck statement of the second law: It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir. The Kelvin–Planck statement does not rule out the possibility of a system developing a net amount of work from a heat transfer drawn from a single reservoir. It only denies this possibility if the system undergoes a thermodynamic cycle. The Kelvin–Planck statement can be expressed analytically. To develop this, let us study a system undergoing a cycle while exchanging energy by heat transfer with a single reservoir, as shown by the adjacent figure. The first and second laws each impose constraints:

207

Kelvin–Planck statement Thermal reservoir

Qcycle

Wcycle

System undergoing a thermodynamic cycle

c A constraint is imposed by the first law on the net work and heat transfer between

the system and its surroundings. According to the cycle energy balance (see Eq. 2.40 in Sec. 2.6), Wcycle 5 Qcycle In words, the net work done by (or on) the system undergoing a cycle equals the net heat transfer to (or from) the system. Although the cycle energy balance allows the net work Wcycle to be positive or negative, the second law imposes a constraint, as considered next. c According to the Kelvin–Planck statement, a system undergoing a cycle while communicating thermally with a single reservoir cannot deliver a net amount of work to its surroundings: The net work of the cycle cannot be positive. However, the Kelvin–Planck statement does not rule out the possibility that there is a net work transfer of energy to the system during the cycle or that the net work is zero. Thus, the analytical form of the Kelvin–Planck statement is Wcycle # 0

(single reservoir)

(5.1)

where the words single reservoir are added to emphasize that the system communicates thermally only with a single reservoir as it executes the cycle. In Sec. 5.4, we associate the “less than” and “equal to” signs of Eq. 5.1 with the presence and absence of internal irreversibilities, respectively. The concept of irreversibilities is considered in Sec. 5.3. The equivalence of the Clausius and Kelvin–Planck statements can be demonstrated by showing that the violation of each statement implies the violation of the other. For details, see the box.

Demonstrating the Equivalence of the Clausius and Kelvin–Planck Statements The equivalence of the Clausius and Kelvin–Planck statements is demonstrated by showing that the violation of each statement implies the violation of the other. That a violation of the Clausius statement implies a violation of the Kelvin–Planck statement is readily shown using Fig. 5.2, which pictures a hot reservoir, a cold reservoir, and two systems.

analytical form of the Kelvin–Planck statement

208

Chapter 5 The Second Law of Thermodynamics System undergoing a thermodynamic cycle QC

Hot reservoir

QH

Wcycle = QH – QC

Fig. 5.2 Illustration used to demonstrate the equivalence of the Clausius and Kelvin– Planck statements of the second law.

QC

Cold reservoir

QC

Dotted line defines combined system

The system on the left transfers energy QC from the cold reservoir to the hot reservoir by heat transfer without other effects occurring and thus violates the Clausius statement. The system on the right operates in a cycle while receiving QH (greater than QC ) from the hot reservoir, rejecting QC to the cold reservoir, and delivering work Wcycle to the surroundings. The energy flows labeled on Fig. 5.2 are in the directions indicated by the arrows. Consider the combined system shown by a dotted line on Fig. 5.2, which consists of the cold reservoir and the two devices. The combined system can be regarded as executing a cycle because one part undergoes a cycle and the other two parts experience no net change in their conditions. Moreover, the combined system receives energy (QH 2 QC) by heat transfer from a single reservoir, the hot reservoir, and produces an equivalent amount of work. Accordingly, the combined system violates the Kelvin–Planck statement. Thus, a violation of the Clausius statement implies a violation of the Kelvin–Planck statement. The equivalence of the two second-law statements is demonstrated completely when it is also shown that a violation of the Kelvin–Planck statement implies a violation of the Clausius statement. This is left as an exercise (see end-of-chapter Problem 5.1).

5.2.3

Entropy Statement of the Second Law

Mass and energy are familiar examples of extensive properties of systems. Entropy is another important extensive property. We show how entropy is evaluated and applied for engineering analysis in Chap. 6. Here we introduce several important aspects. Just as mass and energy are accounted for by mass and energy balances, respectively, entropy is accounted for by an entropy balance. In words, the entropy balance states: change in the amount net amount of amount of entropy of entropy contained entropy transferred produced within the within the system 5 in across the system 1 system during the during some time boundary during the time interval interval time interval

(5.2)

Like mass and energy, entropy can be transferred across the system boundary. For closed systems, there is a single means of entropy transfer—namely, entropy transfer accompanying heat transfer. For control volumes entropy also is transferred in and out by streams of matter. These entropy transfers are considered further in Chap. 6.

5.3 Irreversible and Reversible Processes

Unlike mass and energy, which are conserved, entropy is produced (or generated) within systems whenever nonidealities (called irreversibilities) such as friction are present. The entropy statement of the second law states: It is impossible for any system to operate in a way that entropy is destroyed.

entropy statement of the second law

It follows that the entropy production term of Eq. 5.2 may be positive or zero but never negative. Thus, entropy production is an indicator of whether a process is possible or impossible.

5.2.4 Second Law Summary In the remainder of this chapter, we apply the Kelvin–Planck statement of the second law to draw conclusions about systems undergoing thermodynamic cycles. The chapter concludes with a discussion of the Clausius inequality (Sec. 5.11), which provides the basis for developing the entropy concept in Chap. 6. This is a traditional approach to the second law in engineering thermodynamics. However, the order can be reversed—namely, the entropy statement can be adopted as the starting point for study of the second law aspects of systems.

5.3 Irreversible and Reversible Processes One of the important uses of the second law of thermodynamics in engineering is to determine the best theoretical performance of systems. By comparing actual performance with the best theoretical performance, insights often can be gained into the potential for improvement. As might be surmised, the best performance is evaluated in terms of idealized processes. In this section such idealized processes are introduced and distinguished from actual processes that invariably involve irreversibilities.

5.3.1 Irreversible Processes A process is called irreversible if the system and all parts of its surroundings cannot be exactly restored to their respective initial states after the process has occurred. A process is reversible if both the system and surroundings can be returned to their initial states. Irreversible processes are the subject of the present discussion. Reversible processes are considered again in Sec. 5.3.3. A system that has undergone an irreversible process is not necessarily precluded from being restored to its initial state. However, were the system restored to its initial state, it would not be possible also to return the surroundings to the state they were in initially. As demonstrated in Sec. 5.3.2, the second law can be used to determine whether both the system and surroundings can be returned to their initial states after a process has occurred: the second law can be used to determine whether a given process is reversible or irreversible. It might be apparent from the discussion of the Clausius statement of the second law that any process involving spontaneous heat transfer from a hotter body to a cooler body is irreversible. Otherwise, it would be possible to return this energy from the cooler body to the hotter body with no other effects within the two bodies or their surroundings. However, this possibility is denied by the Clausius statement. Processes involving other kinds of spontaneous events, such as an unrestrained expansion of a gas or liquid, are also irreversible. Friction, electrical resistance, hysteresis, and inelastic deformation are examples of additional effects whose presence during a process renders it irreversible.

irreversible process reversible process

209

210

Chapter 5 The Second Law of Thermodynamics In summary, irreversible processes normally include one or more of the following

irreversibilities

irreversibilities:

1. Heat transfer through a finite temperature difference 2. Unrestrained expansion of a gas or liquid to a lower pressure 3. Spontaneous chemical reaction 4. Spontaneous mixing of matter at different compositions or states 5. Friction—sliding friction as well as friction in the flow of fluids 6. Electric current flow through a resistance 7. Magnetization or polarization with hysteresis 8. Inelastic deformation

internal and external irreversibilities

Hot, TH

Q Cold, TC

Area

Although the foregoing list is not exhaustive, it does suggest that all actual processes are irreversible. That is, every process involves effects such as those listed, whether it is a naturally occurring process or one involving a device of our construction, from the simplest mechanism to the largest industrial plant. The term irreversibility is used to identify any of these effects. The above list comprises a few of the irreversibilities that are commonly encountered. As a system undergoes a process, irreversibilities may be found within the system and its surroundings, although they may be located predominately in one place or the other. For many analyses it is convenient to divide the irreversibilities present into two classes. Internal irreversibilities are those that occur within the system. External irreversibilities are those that occur within the surroundings, often the immediate surroundings. As this distinction depends solely on the location of the boundary, there is some arbitrariness in the classification, for by extending the boundary to take in a portion of the surroundings, all irreversibilities become “internal.” Nonetheless, as shown by subsequent developments, this distinction between irreversibilities is often useful. Engineers should be able to recognize irreversibilities, evaluate their influence, and develop practical means for reducing them. However, certain systems, such as brakes, rely on the effect of friction or other irreversibilities in their operation. The need to achieve profitable rates of production, high heat transfer rates, rapid accelerations, and so on invariably dictates the presence of significant irreversibilities. Furthermore, irreversibilities are tolerated to some degree in every type of system because the changes in design and operation required to reduce them would be too costly. Accordingly, although improved thermodynamic performance can accompany the reduction of irreversibilities, steps taken in this direction are constrained by a number of practical factors often related to costs.

consider two bodies at different temperatures that are able to communicate thermally. With a finite temperature difference between them, a spontaneous heat transfer would take place and, as discussed previously, this would be a source of irreversibility. It might be expected that the importance of this irreversibility diminishes as the temperature difference between the bodies diminishes, and while this is the case, there are practical consequences: From the study of heat transfer (Sec. 2.4), we know that the transfer of a finite amount of energy by heat transfer between bodies whose temperatures differ only slightly requires a considerable amount of time, a large (costly) heat transfer surface area, or both. In the limit as the temperature difference between the bodies vanishes, the amount of time and/or surface area required approach infinity. Such options are clearly impractical; still, they must be imagined when thinking of heat transfer approaching reversibility. b b b b b

5.3 Irreversible and Reversible Processes

5.3.2

211

Demonstrating Irreversibility

Whenever an irreversibility is present during a process, that process must necessarily be irreversible. However, the irreversibility of a process can be demonstrated rigorously using the Kelvin–Planck statement of the second law and the following procedure: (1) Assume there is a way to return the system and surroundings to their respective initial states. (2) Show that as a consequence of this assumption, it is possible to devise a cycle that violates the Kelvin–Planck statement—namely, a cycle that produces work while interacting thermally with only a single reservoir. Since the existence of such a cycle is denied by the Kelvin–Planck statement, the assumption must be in error and it follows that the process is irreversible. This procedure can be used to demonstrate that processes involving friction, heat transfer through a finite temperature difference, the unrestrained expansion of a gas or liquid to a lower pressure, and other effects from the list given previously are irreversible. A case involving friction is discussed in the box. While use of the Kelvin–Planck statement to demonstrate irreversibility is part of a traditional presentation of thermodynamics, such demonstrations can be unwieldy. It is normally easier to use the entropy production concept (Sec. 6.7).

Block

zf (a) Initial state of the cycle.

Demonstrating Irreversibility: Friction Let us use the Kelvin–Planck statement to demonstrate the irreversibility of a process involving friction. Consider a system consisting of a block of mass m and an inclined plane. To begin, the block is at rest at the top of the incline. The block then slides down the plane, eventually coming to rest at a lower elevation. There is no significant work or heat transfer between the block–plane system and its surroundings during the process. Applying the closed system energy balance to the system, we get o

0

(Uf 2 Ui) 1 mg(zf 2 zi) 1 (KEf 2 KEi) 5 Q 2 W

zi

(b) Process 1.

0

or

Uf 2 Ui 5 mg(zi 2 zf)

(a)

where U denotes the internal energy of the block–plane system and z is the elevation of the block. Thus, friction between the block and plane during the process acts to convert the potential energy decrease of the block to internal energy of the overall system. Since no work or heat interactions occur between the block–plane system and its surroundings, the condition of the surroundings remains unchanged during the process. This allows attention to be centered on the system only in demonstrating that the process is irreversible, as follows: When the block is at rest after sliding down the plane, its elevation is zf and the internal energy of the block–plane system is Uf. To demonstrate that the process is irreversible using the Kelvin–Planck statement, let us take this condition of the system, shown in Fig. 5.3a, as the initial state of a cycle consisting of three processes. We imagine that a pulley–cable arrangement and a thermal reservoir are available to assist in the demonstration. Process 1: Assume the inverse process occurs with no change in the surroundings: As shown in Fig. 5.3b, the block returns spontaneously to the top of the plane while the internal energy of the system decreases to its initial value, Ui. (This is the process we want to demonstrate is impossible.) Process 2: As shown in Fig. 5.3c, we use the pulley–cable arrangement provided to lower the block from zi to zf, while allowing the block–plane system to do work by lifting another mass located in the surroundings. The work done equals the decrease in potential energy of the block. This is the only work for the cycle. Thus, Wcycle 5 mg(zi 2 zf).

Work

(c) Process 2. Reservoir Heat transfer from reservoir

(d) Process 3.

Fig. 5.3 Figure used to demonstrate the irreversibility of a process involving friction.

212

Chapter 5 The Second Law of Thermodynamics

Process 3: The internal energy of the system is increased from Ui to Uf by bringing it into communication with the reservoir, as shown in Fig. 5.3d. The heat transfer equals (Uf 2 Ui). This is the only heat transfer for the cycle. Thus, Qcycle 5 (Uf 2 Ui), which with Eq. (a) becomes Qcycle 5 mg(zi 2 zf). At the conclusion of this process the block is again at elevation zf and the internal energy of the block–plane system is restored to Uf. The net result of this cycle is to draw energy from a single reservoir by heat transfer, Qcycle, and produce an equivalent amount of work, Wcycle. There are no other effects. However, such a cycle is denied by the Kelvin–Planck statement. Since both the heating of the system by the reservoir (Process 3) and the lowering of the mass by the pulley–cable while work is done (Process 2) are possible, we conclude it is Process 1 that is impossible. Since Process 1 is the inverse of the original process where the block slides down the plane, it follows that the original process is irreversible.

5.3.3

Reversible Processes

A process of a system is reversible if the system and all parts of its surroundings can be exactly restored to their respective initial states after the process has taken place. It should be evident from the discussion of irreversible processes that reversible processes are purely hypothetical. Clearly, no process can be reversible that involves spontaneous heat transfer through a finite temperature difference, an unrestrained expansion of a gas or liquid, friction, or any of the other irreversibilities listed previously. In a strict sense of the word, a reversible process is one that is perfectly executed. All actual processes are irreversible. Reversible processes do not occur. Even so, certain processes that do occur are approximately reversible. The passage of a gas through a properly designed nozzle or diffuser is an example (Sec. 6.12). Many other devices also can be made to approach reversible operation by taking measures to reduce the significance of irreversibilities, such as lubricating surfaces to reduce friction. A reversible process is the limiting case as irreversibilities, both internal and external, are reduced further and further. Although reversible processes cannot actually occur, they can be imagined. In Sec. 5.3.1, we considered how heat transfer would approach reversibility as the temperature difference approaches zero. Let us consider two additional examples: c A particularly elementary example is a pendulum oscillating in an evacuated space.

Gas

The pendulum motion approaches reversibility as friction at the pivot point is reduced. In the limit as friction is eliminated, the states of both the pendulum and its surroundings would be completely restored at the end of each period of motion. By definition, such a process is reversible. c A system consisting of a gas adiabatically compressed and expanded in a frictionless piston–cylinder assembly provides another example. With a very small increase in the external pressure, the piston would compress the gas slightly. At each intermediate volume during the compression, the intensive properties T, p, 𝜐, etc. would be uniform throughout: The gas would pass through a series of equilibrium states. With a small decrease in the external pressure, the piston would slowly move out as the gas expands. At each intermediate volume of the expansion, the intensive properties of the gas would be at the same uniform values they had at the corresponding step during the compression. When the gas volume returned to its initial value, all properties would be restored to their initial values as well. The work done on the gas during the compression would equal the work done by the gas during the expansion. If the work between the system and its surroundings were delivered to, and received from, a frictionless pulley–mass assembly, or the equivalent, there also would be no net change in the surroundings. This process would be reversible.

5.3 Irreversible and Reversible Processes

213

Second Law Takes Big Bite from Hydrogen Hydrogen is not naturally occurring and thus must be produced. Hydrogen can be produced today from water by electrolysis and from natural gas by chemical processing called reforming. Hydrogen produced by these means and its subsequent utilization is burdened by the second law. In electrolysis, an electrical input is employed to dissociate water to hydrogen according to H2O → H2 1 1/2O2. When the hydrogen is subsequently used by a fuel cell to generate electricity, the cell reaction is H2 1 1/2O2 → H2O. Although the cell reaction is the inverse of that occurring in electrolysis, the overall loop from electrical input–to hydrogen–to fuel cellgenerated electricity is not reversible. Irreversibilities in the

5.3.4

electrolyzer and the fuel cell conspire to ensure that the fuel cell-generated electricity is much less than the initial electrical input. This is wasteful because the electricity provided for electrolysis could instead be fully directed to most applications envisioned for hydrogen, including transportation. Further, when fossil fuel is burned in a power plant to generate electricity for electrolysis, the greenhouse gases produced can be associated with fuel cells by virtue of the hydrogen they consume. Although technical details differ, similar findings apply to the reforming of natural gas to hydrogen. While hydrogen and fuel cells are expected to play a role in our energy future, second law barriers and other technical and economic issues stand in the way.

Internally Reversible Processes

A reversible process is one for which no irreversibilities are present within the system or its surroundings. An internally reversible process is one for which there are no irreversibilities within the system. Irreversibilities may be located within the surroundings, however.

internally reversible process

think of water condensing from saturated vapor to saturated liquid at 100°C while flowing through a copper tube whose outer surface is exposed to the ambient at 20°C. The water undergoes an internally reversible process, but there is heat transfer from the water to the ambient through the tube. For a control volume enclosing the water within the tube, such heat transfer is an external irreversibility. b b b b b At every intermediate state of an internally reversible process of a closed system, all intensive properties are uniform throughout each phase present. That is, the temperature, pressure, specific volume, and other intensive properties do not vary with position. If there were a spatial variation in temperature, say, there would be a tendency for a spontaneous energy transfer by conduction to occur within the system in the direction of decreasing temperature. For reversibility, however, no spontaneous processes can be present. From these considerations it can be concluded that the internally reversible process consists of a series of equilibrium states: It is a quasiequilibrium process. The use of the internally reversible process concept in thermodynamics is comparable to idealizations made in mechanics: point masses, frictionless pulleys, rigid beams, and so on. In much the same way as idealizations are used in mechanics to simplify an analysis and arrive at a manageable model, simple thermodynamic models of complex situations can be obtained through the use of internally reversible processes. Calculations based on internally reversible processes often can be adjusted with efficiencies or correction factors to obtain reasonable estimates of actual performance under various operating conditions. Internally reversible processes are also useful for investigating the best thermodynamic performance of systems. Finally, using the internally reversible process concept, we refine the definition of the thermal reservoir introduced in Sec. 5.2.2 as follows: In subsequent discussions we assume no internal irreversibilities are present within a thermal reservoir. That is, every process of a thermal reservoir is internally reversible.

TAKE NOTE...

The terms internally reversible ble process and quasiequilibrium process can be used interchangeably. However, to avoid having two terms that refer to the same thing, in subsequent sections we will refer to any such process as an internally reversible process.

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Chapter 5 The Second Law of Thermodynamics

5.4 Interpreting the Kelvin–Planck Statement In this section, we recast Eq. 5.1, the analytical form of the Kelvin–Planck statement, into a more explicit expression, Eq. 5.3. This expression is applied in subsequent sections to obtain a number of significant deductions. In these applications, the following idealizations are assumed: The thermal reservoir and the portion of the surroundings with which work interactions occur are free of irreversibilities. This allows the “less than” sign to be associated with irreversibilities within the system of interest and the “equal to” sign to apply when no internal irreversibilites are present. Accordingly, the analytical form of the Kelvin–Planck statement now takes the form analytical form: Kelvin–Planck statement

Wcycle # 0 e

, 0: Internal irreversibilities present. (single reservoir) 5 0: No internal irreversibilities.

(5.3)

For details, see the Kelvin–Planck box below.

Associating Signs with the Kelvin–Planck Statement Thermal reservoir Heat transfer

System Boundary Mass

Fig. 5.4 System undergoing a cycle while exchanging energy by heat transfer with a single thermal reservoir.

Consider a system that undergoes a cycle while exchanging energy by heat transfer with a single reservoir, as shown in Fig. 5.4. Work is delivered to, or received from, the pulley–mass assembly located in the surroundings. A flywheel, spring, or some other device also can perform the same function. The pulley–mass assembly, flywheel, or other device to which work is delivered, or from which it is received, is idealized as free of irreversibilities. The thermal reservoir is also assumed free of irreversibilities. To demonstrate the correspondence of the “equal to” sign of Eq. 5.3 with the absence of irreversibilities, consider a cycle operating as shown in Fig. 5.4 for which the equality applies. At the conclusion of one cycle,

c The system would necessarily be returned to its initial state. c Since Wcycle 5 0, there would be no net change in the elevation of the mass used to store energy in the surroundings.

c Since Wcycle 5 Qcycle, it follows that Qcycle 5 0, so there also would be no net change in the condition of the reservoir. Thus, the system and all elements of its surroundings would be exactly restored to their respective initial conditions. By definition, such a cycle is reversible. Accordingly, there can be no irreversibilities present within the system or its surroundings. It is left as an exercise to show the converse: If the cycle occurs reversibly, the equality applies (see end-of-chapter Problem 5.4). Since a cycle is reversible or irreversible and we have linked the equality with reversible cycles, we conclude the inequality corresponds to the presence of internal irreversibilities. Moreover, the inequality can be interpreted as follows: Net work done on the system per cycle is converted by action of internal irreversibilities to internal energy that is discharged by heat transfer to the thermal reservoir in an amount equal to net work.

Concluding Comment The Kelvin–Planck statement considers systems undergoing thermodynamic cycles while exchanging energy by heat transfer with one thermal reservoir. These restrictions must be strictly observed—see the thermal glider box.

Does the Thermal Glider Challenge the Kelvin–Planck Statement? A 2008 Woods Hole Oceanographic Institute news release, “Researchers Give New Hybrid Vehicle Its First Test-Drive in the Ocean,” announced the successful testing of an underwater thermal glider that “harvests . . . energy from the ocean (thermally) to propel

5.5 Applying the Second Law to Thermodynamic Cycles

itself.” Does this submersible vehicle challenge the Kelvin–Planck statement of the second law? Study of the thermal glider shows it is capable of sustaining forward motion underwater for weeks while interacting thermally only with the ocean and undergoing a mechanical cycle. Still, the glider does not mount a challenge to the Kelvin–Planck statement because it does not exchange energy by heat transfer with a single thermal reservoir and does not execute a thermodynamic cycle. The glider propels itself by interacting thermally with warmer surface waters and colder, deep-ocean layers to change its buoyancy to dive, rise toward the surface, and dive again, as shown on the accompanying figure. Accordingly, the glider does not interact thermally with a single reservoir as required by the Kelvin–Planck statement. The glider also does not satisfy all energy needs by interacting with the ocean: Batteries are required to power on-board electronics. Although these power needs are relatively minor, the batteries lose charge with use, and so the glider does not execute a thermodynamic cycle as required by the Kelvin–Planck statement.

Warmer surface water Thermal glider diving

Thermal glider rising

Colder deep-ocean layer

5.5 Applying the Second Law to Thermodynamic Cycles While the Kelvin–Planck statement of the second law (Eq. 5.3) provides the foundation for the rest of this chapter, application of the second law to thermodynamic cycles is by no means limited to the case of heat transfer with a single reservoir or even with any reservoirs. Systems undergoing cycles while interacting thermally with two thermal reservoirs are considered from a second-law viewpoint in Secs. 5.6 and 5.7, providing results having important applications. Moreover, the one- and tworeservoir discussions pave the way for Sec. 5.11, where the general case is considered—namely, what the second law says about any thermodynamic cycle without regard to the nature of the body or bodies with which energy is exchanged by heat transfer. In the sections to follow, applications of the second law to power cycles and refrigeration and heat pump cycles are considered. For this content, familiarity with rudimentary thermodynamic cycle principles is required. We recommend you review Sec. 2.6, where cycles are considered from an energy perspective and the thermal efficiency of power cycles and coefficients of performance for refrigeration and heat pump systems are introduced. In particular, Eqs. 2.40–2.48 and the accompanying discussions should be reviewed.

215

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Chapter 5 The Second Law of Thermodynamics

5.6 Second Law Aspects of Power Cycles Interacting with Two Reservoirs 5.6.1

Limit on Thermal Efficiency

A significant limitation on the performance of systems undergoing power cycles can be brought out using the Kelvin–Planck statement of the second law. Consider Fig. 5.5, which shows a system that executes a cycle while communicating thermally with two thermal reservoirs, a hot reservoir and a cold reservoir, and developing net work Wcycle. The thermal efficiency of the cycle is 𝜂5 TAKE NOTE...

The energy transfers labeled on Fig. 5.5 are positive in the directions indicated by the arrows.

Wcycle QH

512

QC QH

(5.4)

where QH is the amount of energy received by the system from the hot reservoir by heat transfer and QC is the amount of energy discharged from the system to the cold reservoir by heat transfer. If the value of QC were zero, the system of Fig. 5.5 would withdraw energy QH from the hot reservoir and produce an equal amount of work, while undergoing a cycle. The thermal efficiency of such a cycle would be unity (100%). However, this method of operation violates the Kelvin–Planck statement and thus is not allowed. It follows that for any system executing a power cycle while operating between two reservoirs, only a portion of the heat transfer QH can be obtained as work, and the remainder, QC, must be discharged by heat transfer to the cold reservoir. That is, the thermal efficiency must be less than 100%. In arriving at this conclusion it was not necessary to c identify the nature of the substance contained within the system, c specify the exact series of processes making up the cycle, c indicate whether the processes are actual processes or somehow idealized.

The conclusion that the thermal efficiency must be less than 100% applies to all power cycles whatever their details of operation. This may be regarded as a corollary of the second law. Other corollaries follow.

5.6.2

Carnot corollaries Hot reservoir

Since no power cycle can have a thermal efficiency of 100%, it is of interest to investigate the maximum theoretical efficiency. The maximum theoretical efficiency for systems undergoing power cycles while communicating thermally with two thermal reservoirs at different temperatures is evaluated in Sec. 5.9 with reference to the following two corollaries of the second law, called the Carnot corollaries. 1. The thermal efficiency of an irreversible power cycle is always less than the thermal efficiency of a reversible power cycle when each operates between the same two thermal reservoirs.

QH

Wcycle = QH – QC

Boundary

Cold reservoir

Corollaries of the Second Law for Power Cycles

QC

Fig. 5.5 System undergoing a power cycle while exchanging energy by heat transfer with two reservoirs.

2. All reversible power cycles operating between the same two thermal reservoirs have the same thermal efficiency. A cycle is considered reversible when there are no irreversibilities within the system as it undergoes the cycle and heat transfers between the system and reservoirs occur reversibly. The idea underlying the first Carnot corollary is in agreement with expectations stemming from the discussion of the second law thus far. Namely, the presence of irreversibilities during the execution of a cycle

5.6 Second Law Aspects of Power Cycles Interacting with Two Reservoirs is expected to exact a penalty: If two systems operating between the same reservoirs each receive the same amount of energy QH and one executes a reversible cycle while the other executes an irreversible cycle, it is in accord with intuition that the net work developed by the irreversible cycle will be less, and thus the irreversible cycle has the smaller thermal efficiency. The second Carnot corollary refers only to reversible cycles. All processes of a reversible cycle are perfectly executed. Accordingly, if two reversible cycles operating between the same reservoirs each receive the same amount of energy QH but one could produce more work than the other, it could only be as a result of more advantageous selections for the substance making up the system (it is conceivable that, say, air might be better than water vapor) or the series of processes making up the cycle (nonflow processes might be preferable to flow processes). This corollary denies both possibilities and indicates that the cycles must have the same efficiency whatever the choices for the working substance or the series of processes. The two Carnot corollaries can be demonstrated using the Kelvin–Planck statement of the second law. For details, see the box.

Demonstrating the Carnot Corollaries The first Carnot corollary can be demonstrated using the arrangement of Fig. 5.6. A reversible power cycle R and an irreversible power cycle I operate between the same two reservoirs and each receives the same amount of energy Q H from the hot reservoir. The reversible cycle produces work WR while the irreversible cycle produces work WI. In accord with the conservation of energy principle, each cycle discharges energy to the cold reservoir equal to the difference between QH and the work produced. Let R now operate in the opposite direction as a refrigeration (or heat pump) cycle. Since R is reversible, the magnitudes of the energy transfers WR, QH, and QC remain the same, but the energy transfers are oppositely directed, as shown by the dashed lines on Fig. 5.6. Moreover, with R operating in the opposite direction, the hot reservoir would experience no net change in its condition since it would receive QH from R while passing QH to I. The demonstration of the first Carnot corollary is completed by considering the combined system shown by the dotted line on Fig. 5.6, which consists of the two cycles and the hot reservoir. Since its parts execute cycles or experience no net change, the combined system operates in a cycle. Moreover, the combined system exchanges energy by heat transfer with a single reservoir: the cold reservoir. Accordingly, the combined system must satisfy Eq. 5.3 expressed as Wcycle , 0

(single reservoir)

where the inequality is used because the combined system is irreversible in its operation since irreversible cycle I is one of its parts. Evaluating Wcycle for the combined system in terms of the work amounts WI and WR, the above inequality becomes WI 2 WR , 0

Dotted line defines combined system Hot reservoir QH

WR

R

QH

I

QC = QH – WR Q′C = QH – WI Cold reservoir

WI

Fig. 5.6 Sketch for demonstrating that a reversible cycle R is more efficient than an irreversible cycle I when they operate between the same two reservoirs.

217

218

Chapter 5 The Second Law of Thermodynamics which shows that WI must be less than WR. Since each cycle receives the same energy input, QH, it follows that 𝜂 I , 𝜂 R and this completes the demonstration. The second Carnot corollary can be demonstrated in a parallel way by considering any two reversible cycles R1 and R2 operating between the same two reservoirs. Then, letting R1 play the role of R and R2 the role of I in the previous development, a combined system consisting of the two cycles and the hot reservoir may be formed that must obey Eq. 5.3. However, in applying Eq. 5.3 to this combined system, the equality is used because the system is reversible in operation. Thus, it can be concluded that WR1 5 WR2, and therefore, 𝜂 R1 5 𝜂 R2. The details are left as an exercise (see end-of-chapter Problem 5.5).

5.7 Second Law Aspects of Refrigeration and Heat Pump Cycles Interacting with Two Reservoirs 5.7.1

Limits on Coefficients of Performance

The second law of thermodynamics places limits on the performance of refrigeration and heat pump cycles as it does for power cycles. Consider Fig. 5.7, which shows a system undergoing a cycle while communicating thermally with two thermal reservoirs, a hot and a cold reservoir. The energy transfers labeled on the figure are in the directions indicated by the arrows. In accord with the conservation of energy principle, the cycle discharges energy QH by heat transfer to the hot reservoir equal to the sum of the energy QC received by heat transfer from the cold reservoir and the net work input. This cycle might be a refrigeration cycle or a heat pump cycle, depending on whether its function is to remove energy QC from the cold reservoir or deliver energy QH to the hot reservoir. For a refrigeration cycle the coefficient of performance is QC QC 𝛽5 5 (5.5) Wcycle QH 2 QC The coefficient of performance for a heat pump cycle is QH QH 𝛾5 5 Wcycle QH 2 QC

(5.6)

As the net work input to the cycle Wcycle tends to zero, the coefficients of performance given by Eqs. 5.5 and 5.6 approach a value of infinity. If Wcycle were identically zero, the system of Fig. 5.7 would withdraw energy QC from the cold reservoir and deliver that energy to the hot reservoir, while undergoing a cycle. However, this method of operation violates the Clausius statement of the second law and thus is not allowed. It follows that the coefficients of performance 𝛽 and 𝛾 must invariably be finite in value. This may be regarded as another corollary of the second law. Further corollaries follow. Hot reservoir

QH = QC + Wcycle

Wcycle = QH – QC

Boundary

Fig. 5.7 System undergoing a refrigeration or heat pump cycle while exchanging energy by heat transfer with two reservoirs.

Cold reservoir

QC

5.7 Second Law Aspects of Refrigeration and Heat Pump Cycles Interacting with Two Reservoirs

5.7.2

219

Corollaries of the Second Law for Refrigeration and Heat Pump Cycles

The maximum theoretical coefficients of performance for systems undergoing refrigeration and heat pump cycles while communicating thermally with two reservoirs at different temperatures are evaluated in Sec. 5.9 with reference to the following corollaries of the second law: 1. The coefficient of performance of an irreversible refrigeration cycle is always less than the coefficient of performance of a reversible refrigeration cycle when each operates between the same two thermal reservoirs. 2. All reversible refrigeration cycles operating between the same two thermal reservoirs have the same coefficient of performance. By replacing the term refrigeration with heat pump, we obtain counterpart corollaries for heat pump cycles. The first of these corollaries agrees with expectations stemming from the discussion of the second law thus far. To explore this, consider Fig. 5.8, QH = QC + WR Q′H = QC + WI which shows a reversible refrigeration cycle R and an irreversible refrigHot reservoir eration cycle I operating between the same two reservoirs. Each cycle removes the same energy QC from the cold reservoir. The net work input required to operate R is WR, while the net work input for I is WI. Each WR R I WI cycle discharges energy by heat transfer to the hot reservoir equal to the sum of QC and the net work input. The directions of the energy transfers are shown by arrows on Fig. 5.8. The presence of irreversibilities during Cold reservoir QC QC the operation of a refrigeration cycle is expected to exact a penalty: If two refrigerators working between the same reservoirs each receive an identical energy transfer from the cold reservoir, QC, and one executes a Fig. 5.8 Sketch for demonstrating that a reversible cycle while the other executes an irreversible cycle, we expect reversible refrigeration cycle R has a the irreversible cycle to require a greater net work input and thus have greater coefficient of performance than the smaller coefficient of performance. By a simple extension it follows an irreversible cycle I when they operate that all reversible refrigeration cycles operating between the same two between the same two reservoirs. reservoirs have the same coefficient of performance. Similar arguments apply to the counterpart heat pump cycle statements. These corollaries can be demonstrated formally using the Kelvin–Planck statement of the second law and a procedure similar to that employed for the Carnot corollaries. The details are left as an exercise (see end-of-chapter Problem 5.6).

ENERGY & ENVIRONMENT Warm blankets of pollution-laden air surround major cities. Sunlight-absorbing rooftops and expanses of pavement, together with little greenery, conspire with other features of city living to raise urban temperatures several degrees above adjacent suburban areas. Figure 5.9 shows the variation of surface temperature in the vicinity of a city as measured by infrared measurements made from low-level flights over the area. Health-care professionals worry about the impact of these “heat islands,” especially on the elderly. Paradoxically, the hot exhaust from the air conditioners city dwellers use to keep cool also make sweltering neighborhoods even hotter. Irreversibilities within air conditioners contribute to the warming effect. Air conditioners may account for as much as 20% of the urban temperature rise. Vehicles and commercial activity also are contributors. Urban planners are combating heat islands in many ways, including the use of highly reflective colored roofing products and the installation of roof-top gardens. The shrubs and trees of roof-top gardens absorb solar energy, leading to summer roof temperatures significantly below those of nearby buildings without roof-top gardens, reducing the need for air conditioning.

Chapter 5 The Second Law of Thermodynamics

Surface Temperature

220

Suburban

City center

Suburban

Fig. 5.9 Surface temperature variation in an urban area.

5.8 The Kelvin and International Temperature Scales The results of Secs. 5.6 and 5.7 establish theoretical upper limits on the performance of power, refrigeration, and heat pump cycles communicating thermally with two reservoirs. Expressions for the maximum theoretical thermal efficiency of power cycles and the maximum theoretical coefficients of performance of refrigeration and heat pump cycles are developed in Sec. 5.9 using the Kelvin temperature scale considered next.

5.8.1

The Kelvin Scale

From the second Carnot corollary we know that all reversible power cycles operating between the same two thermal reservoirs have the same thermal efficiency, regardless of the nature of the substance making up the system executing the cycle or the series of processes. Since the thermal efficiency is independent of these factors, its value can be related only to the nature of the reservoirs themselves. Noting that it is the difference in temperature between the two reservoirs that provides the impetus for heat transfer between them, and thereby for the production of work during the cycle, we reason that the thermal efficiency depends only on the temperatures of the two reservoirs. From Eq. 5.4 it also follows that for such reversible power cycles the ratio of the heat transfers QC /QH depends only on the temperatures of the two reservoirs. That is a

QC b rev 5 𝜓(𝜃C, 𝜃H) QH cycle

(a)

where 𝜃H and 𝜃C denote the temperatures of the reservoirs and the function 𝜓 is for the present unspecified. Note that the words “rev cycle” are added to this expression to emphasize that it applies only to systems undergoing reversible cycles while operating between two thermal reservoirs. Equation (a) provides a basis for defining a thermodynamic temperature scale: a scale independent of the properties of any substance. There are alternative

5.8 The Kelvin and International Temperature Scales choices for the function 𝜓 that lead to this end. The Kelvin scale is obtained by making a particularly simple choice, namely, 𝜓 5 TC/TH, where T is the symbol used by international agreement to denote temperatures on the Kelvin scale. With this, we get a

QC TC b rev 5 QH cycle TH

More on the Kelvin Scale Equation 5.7 gives only a ratio of temperatures. To complete the definition of the Kelvin scale, it is necessary to proceed as in Sec. 1.7.3 by assigning the value 273.16 K to the temperature at the triple point of water. Then, if a reversible cycle is operated between a reservoir at 273.16 K and another reservoir at temperature T, the two temperatures are related according to Q b rev Qtp cycle

Kelvin scale

(5.7)

Thus, two temperatures on the Kelvin scale are in the same ratio as the values of the heat transfers absorbed and rejected, respectively, by a system undergoing a reversible cycle while communicating thermally with reservoirs at these temperatures. If a reversible power cycle were operated in the opposite direction as a refrigeration or heat pump cycle, the magnitudes of the energy transfers QC and QH would remain the same, but the energy transfers would be oppositely directed. Accordingly, Eq. 5.7 applies to each type of cycle considered thus far, provided the system undergoing the cycle operates between two thermal reservoirs and the cycle is reversible.

T 5 273.16 a

221

(5.8)

where Qtp and Q are the heat transfers between the cycle and reservoirs at 273.16 K and temperature T, respectively. In the present case, the heat transfer Q plays the role of the thermometric property. However, since the performance of a reversible cycle is independent of the makeup of the system executing the cycle, the definition of temperature given by Eq. 5.8 depends in no way on the properties of any substance or class of substances. In Sec. 1.7 we noted that the Kelvin scale has a zero of 0 K, and lower temperatures than this are not defined. Let us take up these points by considering a reversible power cycle operating between reservoirs at 273.16 K and a lower temperature T. Referring to Eq. 5.8, we know that the energy rejected from the cycle by heat transfer Q would not be negative, so T must be nonnegative. Equation 5.8 also shows that the smaller the value of Q, the lower the value of T, and conversely. Accordingly, as Q approaches zero the temperature T approaches zero. It can be concluded that a temperature of zero on the Kelvin scale is the lowest conceivable temperature. This temperature is called the absolute zero, and the Kelvin scale is called an absolute temperature scale. When numerical values of the thermodynamic temperature are to be determined, it is not possible to use reversible cycles, for these exist only in our imaginations. However, temperatures evaluated using the constant-volume gas thermometer discussed in Sec. 5.8.2 to follow are identical to those of the Kelvin scale in the range of temperatures where the gas thermometer can be used. Other empirical approaches can be employed for temperatures above and below the range accessible to gas thermometry. The Kelvin scale provides a continuous definition of temperature valid over all ranges and provides an essential connection between the several empirical measures of temperature.

TAKE NOTE...

Some readers may prefer to proceed directly to Sec. 5.9, where Eq. 5.7 is applied.

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Chapter 5 The Second Law of Thermodynamics

5.8.2

The Gas Thermometer

The constant-volume gas thermometer shown in Fig. 5.10 is so exceptional in terms of precision and accuracy that it has been adopted internationally as the standard instrument for calibrating other thermometers. The thermometric substance is the gas Capillary (normally hydrogen or helium), and the thermometric property is the pressure Mercury exerted by the gas. As shown in the figure, the gas is contained in a bulb, reservoir L and the pressure exerted by it is measured by an open-tube mercury manometer. As temperature increases, the gas expands, forcing mercury up in the open tube. The gas is kept at constant volume by raising or lowering the Gas bulb reservoir. The gas thermometer is used as a standard worldwide by bureaus of standards and research laboratories. However, because gas thermometers require elaborate apparatus and are large, slowly responding devices that Manometer demand painstaking experimental procedures, smaller, more rapidly responding thermometers are used for most temperature measurements and they are calibrated (directly or indirectly) against gas thermometers. For further Fig. 5.10 Constant-volume gas discussion of gas thermometry, see the box. thermometer.

Measuring Temperature with the Gas Thermometer—The Gas Scale It is instructive to consider how numerical values are associated with levels of temperature by the gas thermometer shown in Fig. 5.10. Let p stand for the pressure in the bulb of a constant-volume gas thermometer in thermal equilibrium with a bath. A value can be assigned to the bath temperature by a linear relation T 5 𝛼p

(a)

where 𝛼 is an arbitrary constant. The value of 𝛼 is determined by inserting the thermometer into another bath maintained at the triple point of water and measuring the pressure, call it ptp, of the confined gas at the triple point temperature, 273.16 K. Substituting values into Eq. (a) and solving for 𝛼 𝛼5

273.16 ptp

Inserting this in Eq. (a), the temperature of the original bath, at which the pressure of the confined gas is p, is then T 5 273.16 a

p b ptp

(b)

However, since the values of both pressures, p and ptp, depend in part on the amount of gas in the bulb, the value assigned by Eq. (b) to the bath temperature varies with the amount of gas in the thermometer. This difficulty is overcome in precision thermometry by repeating the measurements (in the original bath and the reference bath) several times with less gas in the bulb in each successive attempt. For each trial the ratio p/ptp is calculated from Eq. (b) and plotted versus the corresponding reference pressure ptp of the gas at the triple point temperature. When several such points have been plotted, the resulting curve is extrapolated to the ordinate where ptp 5 0. This is illustrated in Fig. 5.11 for constant-volume thermometers with a number of different gases. Inspection of Fig. 5.11 shows that at each nonzero value of the reference pressure, the p/ptp values differ with the gas employed in the thermometer. However, as pressure decreases, the p/ptp values from thermometers with different gases approach one another, and in the limit as pressure tends to zero, the same value

5.9 Maximum Performance Measures for Cycles Operating between Two Reservoirs for p/ptp is obtained for each gas. Based on these general results, the gas temperature scale is defined by the relationship T 5 273.16 lim

p ptp

Measured data for a fixed level of temperature, extrapolated to zero pressure

(c)

where “lim” means that both p and ptp tend to zero. It should be evident that the determination of temperatures by this means requires extraordinarily careful and elaborate experimental procedures. Although the temperature scale of Eq. (c) is independent of the properties of any one gas, it still depends on the properties of gases in general. Accordingly, the measurement of low temperatures requires a gas that does not condense at these temperatures, and this imposes a limit on the range of temperatures that can be measured by a gas thermometer. The lowest temperature that can be measured with such an instrument is about 1 K, obtained with helium. At high temperatures gases dissociate, and therefore these temperatures also cannot be determined by a gas thermometer. Other empirical means, utilizing the properties of other substances, must be employed to measure temperature in ranges where the gas thermometer is inadequate. For further discussion see Sec. 5.8.3.

5.8.3 International Temperature Scale To provide a standard for temperature measurement taking into account both theoretical and practical considerations, the International Temperature Scale (ITS) was adopted in 1927. This scale has been refined and extended in several revisions, most recently in 1990. The International Temperature Scale of 1990 (ITS-90) is defined in such a way that the temperature measured on it conforms with the thermodynamic temperature, the unit of which is the kelvin, to within the limits of accuracy of measurement obtainable in 1990. The ITS-90 is based on the assigned values of temperature of a number of reproducible fixed points (Table 5.1). Interpolation between the fixed-point temperatures is accomplished by formulas that give the relation between readings of standard instruments and values of the ITS. In the range from 0.65 to 5.0 K, ITS-90 is defined by equations giving the temperature as functions of the vapor pressures of particular helium isotopes. The range from 3.0 to 24.5561 K is based on measurements using a helium constant-volume gas thermometer. In the range from 13.8033 to 1234.93 K, ITS-90 is defined by means of certain platinum resistance thermometers. Above 1234.93 K the temperature is defined using Planck’s equation for blackbody radiation and measurements of the intensity of visible-spectrum radiation.

5.9 Maximum Performance Measures for Cycles Operating between Two Reservoirs The discussion continues in this section with the development of expressions for the maximum thermal efficiency of power cycles and the maximum coefficients of performance of refrigeration and heat pump cycles in terms of reservoir temperatures evaluated on the Kelvin scale. These expressions can be used as standards of comparison for actual power, refrigeration, and heat pump cycles.

223

O2

N2 p p–– tp

p T = 273.16 lim p–– tp

ptp

Fig. 5.11 Readings of constant-volume gas thermometers, when several gases are used.

He H2

224

Chapter 5 The Second Law of Thermodynamics TABLE 5.1

Defining Fixed Points of the International Temperature Scale of 1990 T (K)

Substancea

3 to 5 13.8033 < 17 < 20.3 24.5561 54.3584 83.8058 234.3156 273.16 302.9146 429.7485 505.078 692.677 933.473 1234.93 1337.33 1357.77

He e-H2 e-H2 e-H2 Ne O2 Ar Hg H2O Ga In Sn Zn Al Ag Au Cu

Stateb

Vapor pressure point Triple point Vapor pressure point Vapor pressure point Triple point Triple point Triple point Triple point Triple point Melting point Freezing point Freezing point Freezing point Freezing point Freezing point Freezing point Freezing point

a He denotes 3He or 4He; e-H2 is hydrogen at the equilibrium concentration of the ortho- and para-molecular forms. b

Triple point: temperature at which the solid, liquid, and vapor phases are in equilibrium. Melting point, freezing point: temperature, at a pressure of 101.325 kPa, at which the solid and liquid phases are in equilibrium. Source: H. Preston-Thomas, “The International Temperature Scale of 1990 (ITS-90),” Metrologia 27, 3–10 (1990). See also www.ITS-90.com.

5.9.1

Power Cycles

The use of Eq. 5.7 in Eq. 5.4 results in an expression for the thermal efficiency of a system undergoing a reversible power cycle while operating between thermal reservoirs at temperatures TH and TC. That is 𝜂max 5 1 2

TC TH

(5.9)

which is known as the Carnot efficiency. As temperatures on the Rankine scale differ from Kelvin temperatures only by the factor 1.8, the T’s in Eq. 5.9 may be on either scale of temperature. Recalling the two Carnot corollaries, it should be evident that the efficiency given by Eq. 5.9 is the thermal efficiency of all reversible power cycles operating 1.0 between two reservoirs at temperatures TH and TC, and the maximum efficiency any power cycle can have while operating between b the two reservoirs. By inspection, the value of the Carnot efficiency η → 1 (100%) increases as TH increases and/or TC decreases. 0.5 Equation 5.9 is presented graphically in Fig. 5.12. The temperature TC used in constructing the figure is 298 K in recognition that actual power cycles ultimately discharge energy by heat transfer at about the a 0 temperature of the local atmosphere or cooling water drawn from a 298 3000 1000 2000 nearby river or lake. Note that the possibility of increasing the thermal Temperature, TH (K) efficiency by reducing TC below that of the environment is not practical, Fig. 5.12 Carnot efficiency versus TH, for for maintaining TC lower than the ambient temperature would require TC 5 298 K. a refrigerator that would have to be supplied work to operate.

TC ηmax = 1 – ––– TH

Carnot efficiency

5.9 Maximum Performance Measures for Cycles Operating between Two Reservoirs Figure 5.12 shows that the thermal efficiency increases with TH. Referring to segment a–b of the curve, where TH and 𝜂 are relatively low, we see that 𝜂 increases rapidly as TH increases, showing that in this range even a small increase in TH can have a large effect on efficiency. Though these conclusions, drawn as they are from Eq. 5.9, apply strictly only to systems undergoing reversible cycles, they are qualitatively correct for actual power cycles. The thermal efficiencies of actual cycles are observed to increase as the average temperature at which energy is added by heat transfer increases and/or the average temperature at which energy is discharged by heat transfer decreases. However, maximizing the thermal efficiency of a power cycle may not be the only objective. In practice, other considerations such as cost may be overriding. Conventional power-producing cycles have thermal efficiencies ranging up to about 40%. This value may seem low, but the comparison should be made with an appropriate limiting value and not 100%.

Power_Cycle A.9 – Tab c

225

A

consider a system executing a power cycle for which the average temperature of heat addition is 745 K and the average temperature at which heat is discharged is 298 K. For a reversible cycle receiving and discharging energy by heat transfer at these temperatures, the thermal efficiency given by Eq. 5.9 is 60%. When compared to this value, an actual thermal efficiency of 40% does not appear to be so low. The cycle would be operating at two-thirds of the theoretical maximum. b b b b b In the next example, we evaluate an inventor’s claim about the performance of a power cycle, illustrating the use of the Carnot corollaries (Sec. 5.6.2) and the Carnot efficiency, Eq. 5.9.

cccc

EXAMPLE 5.1 c

Evaluating a Power Cycle Performance Claim An inventor claims to have developed a power cycle capable of delivering a net work output of 410 kJ for an energy input by heat transfer of 1000 kJ. The system undergoing the cycle receives the heat transfer from hot gases at a temperature of 500 K and discharges energy by heat transfer to the atmosphere at 300 K. Evaluate this claim. SOLUTION Known: A system operates in a cycle and produces a net amount of work while receiving and discharging energy by heat transfer at fixed temperatures. Find: Evaluate the claim that the cycle can develop 410 kJ of work for an energy input by heat of 1000 kJ. Schematic and Given Data: Qin = 1000 kJ

Engineering Model: 1. The system shown on the accompanying figure executes a power cycle.

500 K

2. The hot gases and the atmosphere play the roles of hot and cold reservoirs,

Power cycle

Qout

W = 410 kJ

respectively.

300 K

Fig. E5.1

Analysis: Inserting the values supplied by the inventor into Eq. 5.4, the cycle thermal efficiency is

𝜂5

410 kJ 5 0.41 (41%) 1000 kJ

226

Chapter 5 The Second Law of Thermodynamics

The maximum thermal efficiency any power cycle can have while operating between reservoirs at TH 5 500 K and TC 5 300 K is given by Eq. 5.9. ➊

𝜂max 5 1 2

TC 300 K 512 5 0.40 (40%) TH 500 K

The Carnot corollaries provide a basis for evaluating the claim: Since the thermal efficiency of the actual cycle exceeds the maximum theoretical value, the claim cannot be valid. ➊ The temperatures TC and TH used in evaluating 𝜂max must be in K or 8R.

✓ Skills Developed Ability to… ❑ apply the Carnot corollaries,

using Eqs. 5.4 and 5.9 appropriately.

If the cycle receives heat transfer from a hot gas at 600 K while all other data remain unchanged, evaluate the inventor’s claim. Ans. Claim is in accord with the second law.

5.9.2

Refrigeration and Heat Pump Cycles

Equation 5.7 is also applicable to reversible refrigeration and heat pump cycles operating between two thermal reservoirs, but for these QC represents the heat added to the cycle from the cold reservoir at temperature TC on the Kelvin scale and QH is the heat discharged to the hot reservoir at temperature TH. Introducing Eq. 5.7 in Eq. 5.5 results in the following expression for the coefficient of performance of any system undergoing a reversible refrigeration cycle while operating between the two reservoirs 𝛽max 5

TC TH 2 TC

(5.10)

Similarly, substituting Eq. 5.7 into Eq. 5.6 gives the following expression for the coefficient of performance of any system undergoing a reversible heat pump cycle while operating between the two reservoirs: 𝛾max 5

A cccc

Refrig_Cycle A.10 – Tab c Heat_Pump_Cycle A.11 – Tab c

TH TH 2 TC

(5.11)

Note that the temperatures used to evaluate 𝛽 max and 𝛾max must be absolute temperatures on the Kelvin or Rankine scale. From the discussion of Sec. 5.7.2, it follows that Eqs. 5.10 and 5.11 are the maximum coefficients of performance that any refrigeration and heat pump cycles can have while operating between reservoirs at temperatures TH and TC. As for the case of the Carnot efficiency, these expressions can be used as standards of comparison for actual refrigerators and heat pumps. In the next example, we evaluate the coefficient of performance of a refrigerator and compare it with the maximum theoretical value, illustrating the use of the second law corollaries of Sec. 5.7.2 together with Eq. 5.10.

EXAMPLE 5.2 c

Evaluating Refrigerator Performance By steadily circulating a refrigerant at low temperature through passages in the walls of the freezer compartment, a refrigerator maintains the freezer compartment at 258C when the air surrounding the refrigerator is at 228C. The rate of heat transfer from the freezer compartment to the refrigerant is 8000 kJ/h and the power input

5.9 Maximum Performance Measures for Cycles Operating between Two Reservoirs

227

required to operate the refrigerator is 3200 kJ/h. Determine the coefficient of performance of the refrigerator and compare with the coefficient of performance of a reversible refrigeration cycle operating between reservoirs at the same two temperatures. SOLUTION Known: A refrigerator maintains a freezer compartment at a specified temperature. The rate of heat transfer from the refrigerated space, the power input to operate the refrigerator, and the ambient temperature are known. Find: Determine the coefficient of performance and compare with that of a reversible refrigerator operating

between reservoirs at the same two temperatures. Schematic and Given Data: Engineering Model:

Surroundings at 22°C (295 K) · QH

1. The system shown on the accompanying figure is at steady state. 2. The freezer compartment and the surrounding air play the roles of

· W cycle = 3200 kJ/h

cold and hot reservoirs, respectively.

System boundary · Q C = 8000 kJ/h Freezer compartment at –5°C (268 K)

Fig. E5.2

Analysis: Inserting the given operating data into Eq. 5.5 expressed on a time-rate basis, the coefficient of performance of the refrigerator is # QC 8000 kJ/h 𝛽5 # 5 5 2.5 3200 kJ/h Wcycle

Substituting values into Eq. 5.10 gives the coefficient of performance of a reversible refrigeration cycle operating between reservoirs at TC 5 268 K and TH 5 295 K ➊

𝛽max 5

TC 268 K 5 5 9.9 TH 2 TC 295 K 2 268 K

➋ In accord with the corollaries of Sec. 5.7.2, the coefficient of performance of the refrigerator is less than for a reversible refrigeration cycle operating between reservoirs at the same two temperatures. That is, irreversibilities are present within the system. ➊ The temperatures TC and TH used in evaluating 𝛽 max must be in K. ➋ The difference between the actual and maximum coefficients of performance suggests that there may be some potential for improving the thermodynamic performance. This objective should be approached judiciously, however, for improved performance may require increases in size, complexity, and cost.

An inventor claims the power required to operate the refrigerator can be reduced to 800 kJ/h while all other data remain the same. Evaluate this claim using the second law. Ans. 𝛽 5 10. Claim invalid.

✓ Skills Developed Ability to… ❑ apply the second law corol-

laries of Sec. 5.7.2, using Eqs. 5.5 and 5.10 appropriately.

228

Chapter 5 The Second Law of Thermodynamics In Example 5.3, we determine the minimum theoretical work input and cost for one day of operation of an electric heat pump, illustrating the use of the second law corollaries of Sec. 5.7.2 together with Eq. 5.11.

cccc

EXAMPLE 5.3 c

Evaluating Heat Pump Performance A dwelling requires 633 MJ per day to maintain its temperature at 21°C when the outside temperature is 0°C. (a) If an electric heat pump is used to supply this energy, determine the minimum theoretical work input for one day of operation, in kJ/day. (b) Evaluating electricity at 8 cents per kW ? h, determine the minimum theoretical cost to operate the heat pump, in $/day. SOLUTION Known: A heat pump maintains a dwelling at a specified temperature. The energy supplied to the dwelling, the ambient temperature, and the unit cost of electricity are known. Find: Determine the minimum theoretical work required by the heat pump and the corresponding electricity cost. Schematic and Given Data: Engineering Model: 1. The system shown on the accompanying figure executes a Heat pump W cycle

QH

Dwelling at 70°F (530°R)

heat pump cycle. 2. The dwelling and the outside air play the roles of hot and

cold reservoirs, respectively. 3. The value of electricity is 8 cents per kW ? h.

QC Surroundings at 32°F (492°R)

Fig. E5.3 Analysis: (a) Using Eq. 5.6, the work for any heat pump cycle can be expressed as Wcycle 5 QH/𝛾. The coefficient of performance 𝛾 of an actual heat pump is less than, or equal to, the coefficient of performance 𝛾 max of a reversible heat pump cycle when each operates between the same two thermal reservoirs: 𝛾 # 𝛾 max. Accordingly, for a given value of QH, and using Eq. 5.11 to evaluate 𝛾 max, we get

Wcycle $

QH 𝛾max

$ a1 2 Inserting values ➊

Wcycle $ a1 2

TC b QH TH

kJ kJ 273 K b a6.33 3 105 b 5 4.5 3 104 294 K day day

The minimum theoretical work input is 4.5 3 104 kJ/day. (b) Using the result of part (a) together with the given cost data and an appropriate conversion factor

➋

minimum $ $ kJ 1 kW ? h ` `b a0.08 b51 £ theoretical § 5 a4.5 3 104 day 3600 kJ kW ? h day cost per day

5.10 Carnot Cycle ➊ Note that the temperatures TC and TH must be in °R or K. ➋ Because of irreversibilities, an actual heat pump must be supplied more work than the minimum to provide the same heating effect. The actual daily cost could be substantially greater than the minimum theoretical cost.

229

✓ Skills Developed Ability to… ❑ apply the second law corollar-

ies of Sec. 5.7.2, using Eqs. 5.6 and 5.11 appropriately. ❑ conduct an elementary economic evaluation.

If the cost of electricity is 10 cents per kW ? h, evaluate the minimum theoretical cost to operate the heat pump, in $/day, keeping all other data the same. Ans. $1.26/day.

5.10

Carnot Cycle

The Carnot cycles introduced in this section provide specific examples of reversible cycles operating between two thermal reservoirs. Other examples are provided in Chap. 9: the Ericsson and Stirling cycles. In a Carnot cycle, the system executing the cycle undergoes a series of four internally reversible processes: two adiabatic processes alternated with two isothermal processes.

5.10.1

Carnot cycle

Carnot Power Cycle

Figure 5.13 shows the p–𝜐 diagram of a Carnot power cycle in which the system is a gas in a piston–cylinder assembly. Figure 5.14 provides details of how the cycle is executed. The piston and cylinder walls are nonconducting. The heat transfers are in the directions of the arrows. Also note that there are two reservoirs at temperatures TH and TC, respectively, and an insulating stand. Initially, the piston–cylinder assembly is on the insulating stand and the system is at state 1, where the temperature is TC. The four processes of the cycle are Process 1–2: The gas is compressed adiabatically to state 2, where the temperature is TH. Process 2–3: The assembly is placed in contact with the reservoir at TH. The gas expands isothermally while receiving energy QH from the hot reservoir by heat transfer. Process 3–4: The assembly is again placed on the insulating stand and p the gas is allowed to continue to expand adiabatically until the temperature drops to TC. Process 4–1: The assembly is placed in contact with the reservoir at TC. The gas is compressed isothermally to its initial state while it discharges energy QC to the cold reservoir by heat transfer. For the heat transfer during Process 2–3 to be reversible, the difference between the gas temperature and the temperature of the hot reservoir must be vanishingly small. Since the reservoir temperature remains constant, this implies that the temperature of the gas also remains constant during Process 2–3. The same can be concluded for the gas temperature during Process 4–1. For each of the four internally reversible processes of the Carnot cycle, the work can be represented as an area on Fig. 5.13. The area under the adiabatic process line 1–2 represents the work done per unit of mass to compress the gas in this process. The areas under process lines 2–3 and 3–4 represent the work done per unit of mass by the gas as it expands in these processes. The area under process line 4–1 is the work done per unit of mass

2

3

TC 1

TH

4 v

Fig. 5.13 p–𝝊 diagram for a Carnot gas power cycle.

230

Chapter 5 The Second Law of Thermodynamics

Isothermal expansion

Adiabatic compression

Adiabatic expansion

Isothermal compression

Gas QH

Insulating stand

Hot reservoir, TH

Process 1–2

Process 2–3

QC

Insulating stand

Cold reservoir, TC

Boundary Process 3–4

Process 4–1

Fig. 5.14 Carnot power cycle executed by a gas in a piston–cylinder assembly.

to compress the gas in this process. The enclosed area on the p–𝜐 diagram, shown shaded, is the net work developed by the cycle per unit of mass. The thermal efficiency of this cycle is given by Eq. 5.9. The Carnot cycle is not limited to processes of a closed system taking place in a piston–cylinder assembly. Figure 5.15 shows the schematic and accompanying p–𝜐 diagram of a Carnot cycle executed by water steadily circulating through a series of four interconnected components that has features in common with the simple vapor power plant shown in Fig. 4.16. As the water flows through the boiler, a change of phase from liquid to vapor at constant temperature TH occurs as a result of heat transfer from the hot reservoir. Since temperature remains constant, pressure also remains constant during the phase change. The steam exiting the boiler expands adiabatically through the turbine and work is developed. In this process the temperature decreases to the temperature of the cold reservoir, TC, and there is an accompanying decrease in pressure. As the steam passes through the condenser, a heat transfer to the cold reservoir occurs and some of the vapor condenses at constant temperature TC. Since temperature remains constant, pressure also remains constant as the water passes through the condenser. The fourth component is a pump (or compressor) that receives a two-phase liquid–vapor mixture from the condenser and returns it adiabatically

Hot reservoir, TH QH 4

1

Boiler

p Pump

Work

3

Turbine

Condenser

Work

TH TC

1

4

TH

2

QC Cold reservoir, TC

Fig. 5.15 Carnot vapor power cycle.

3

2

TC v

5.11 Clausius Inequality to the state at the boiler entrance. During this process, which requires a work input to increase the pressure, the temperature increases from TC to TH. The thermal efficiency of this cycle also is given by Eq. 5.9.

5.10.2

p

231

TH 4

Carnot Refrigeration and Heat Pump Cycles

If a Carnot power cycle is operated in the opposite direction, the magnitudes of all energy transfers remain the same but the energy transfers are oppositely directed. Such a cycle may be regarded as a reversible refrigeration or heat pump cycle, for which the coefficients of performance are given by Eqs. 5.10 and 5.11, respectively. A Carnot refrigeration or heat pump cycle executed by a gas in a piston–cylinder assembly is shown in Fig. 5.16. The cycle consists of the following four processes in series:

3

TC 1

2

v

Process 1–2: The gas expands isothermally at TC while receiving energy Fig. 5.16 p–υ diagram for a Carnot QC from the cold reservoir by heat transfer. gas refrigeration or heat pump Process 2–3: The gas is compressed adiabatically until its temperature cycle. is TH. Process 3–4: The gas is compressed isothermally at TH while it discharges energy QH to the hot reservoir by heat transfer. Process 4–1: The gas expands adiabatically until its temperature decreases to TC. A refrigeration or heat pump effect can be accomplished in a cycle only if a net work input is supplied to the system executing the cycle. In the case of the cycle shown in Fig. 5.16, the shaded area represents the net work input per unit of mass.

5.10.3

Carnot Cycle Summary

In addition to the configurations discussed previously, Carnot cycles also can be devised that are composed of processes in which a capacitor is charged and discharged, a paramagnetic substance is magnetized and demagnetized, and so on. However, regardless of the type of device or the working substance used, 1. the Carnot cycle always has the same four internally reversible processes: two adiabatic processes alternated with two isothermal processes. 2. the thermal efficiency of the Carnot power cycle is always given by Eq. 5.9 in terms of the temperatures evaluated on the Kelvin or Rankine scale. 3. the coefficients of performance of the Carnot refrigeration and heat pump cycles are always given by Eqs. 5.10 and 5.11, respectively, in terms of temperatures evaluated on the Kelvin or Rankine scale.

5.11

Clausius Inequality

Corollaries of the second law developed thus far in this chapter are for systems undergoing cycles while communicating thermally with one or two thermal energy reservoirs. In the present section a corollary of the second law known as the Clausius inequality is introduced that is applicable to any cycle without regard for the body, or bodies, from which the cycle receives energy by heat transfer or to which the cycle rejects energy by heat transfer. The Clausius inequality provides the basis for further development in Chap. 6 of the entropy, entropy production, and entropy balance concepts introduced in Sec. 5.2.3.

232

Chapter 5 The Second Law of Thermodynamics The Clausius inequality states that for any thermodynamic cycle 𝛿Q b #0 C T b a

Clausius inequality

(5.12)

where 𝛿Q represents the heat transfer at a part of the system boundary during a portion of the cycle, and T is the absolute temperature at that part of the boundary. The subscript “b” serves as a reminder that the integrand is evaluated at the boundary of the system executing the cycle. The symbol r indicates that the integral is to be performed over all parts of the boundary and over the entire cycle. The equality and inequality have the same interpretation as in the Kelvin–Planck statement: the equality applies when there are no internal irreversibilities as the system executes the cycle, and the inequality applies when internal irreversibilities are present. The Clausius inequality can be demonstrated using the Kelvin–Planck statement of the second law. See the box for details. The Clausius inequality can be expressed equivalently as 𝛿Q b 5 2𝜎cycle C T b a

(5.13)

where 𝜎cycle can be interpreted as representing the “strength” of the inequality. The value of 𝜎cycle is positive when internal irreversibilities are present, zero when no internal irreversibilities are present, and can never be negative. In summary, the nature of a cycle executed by a system is indicated by the value for 𝜎cycle as follows: 𝜎cycle 5 0 𝜎cycle . 0 𝜎cycle , 0

no irreversibilities present within the system irreversibilities present within the system impossible

(5.14)

applying Eq. 5.13 to the cycle of Example 5.1, we get 𝛿Q Qin Qout b 5 2 5 2𝜎cycle T T TC b H C 1000 kJ 590 kJ 5 2 5 0.033 kJ/K 500 K 300 K a

giving 𝜎cycle 5 20.033 kJ/K, where the negative value indicates the proposed cycle is impossible. This is in keeping with the conclusion of Example # 5.1. Applying Eq. 5.13 on a time-rate basis to the cycle of Example 5.2, we get 𝜎cycle 5 8.12 kJ/h ? K. The positive value indicates irreversibilities are present within the system undergoing the cycle, which is in keeping with the conclusions of Example 5.2. b b b b b In Sec. 6.7, Eq. 5.13 is used to develop the closed system entropy balance. From that development, the term 𝜎cycle of Eq. 5.13 can be interpreted as the entropy produced (generated) by internal irreversibilities during the cycle.

Developing the Clausius Inequality The Clausius inequality can be demonstrated using the arrangement of Fig. 5.17. A system receives energy 𝛿Q at a location on its boundary where the absolute temperature is T while the system develops work 𝛿W. In keeping with our sign convention for heat transfer, the phrase receives energy 𝛿Q includes the possibility

5.11 Clausius Inequality Reservoir at Tres δQ´

Intermediary cycle δW´

Combined system boundary δQ T System

δW

Fig. 5.17 Illustration used to System boundary

develop the Clausius inequality.

of heat transfer from the system. The energy 𝛿Q is received from a thermal reservoir at Tres. To ensure that no irreversibility is introduced as a result of heat transfer between the reservoir and the system, let it be accomplished through an intermediary system that undergoes a cycle without irreversibilities of any kind. The cycle receives energy 𝛿Q9 from the reservoir and supplies 𝛿Q to the system while producing work 𝛿W9. From the definition of the Kelvin scale (Eq. 5.7), we have the following relationship between the heat transfers and temperatures:

𝛿Q¿ 𝛿Q 5a b Tres T b

(a)

As temperature T may vary, a multiplicity of such reversible cycles may be required. Consider next the combined system shown by the dotted line on Fig. 5.17. An energy balance for the combined system is dEC 5 𝛿Q¿ 2 𝛿WC where 𝛿WC is the total work of the combined system, the sum of 𝛿W and 𝛿W9, and dEC denotes the change in energy of the combined system. Solving the energy balance for 𝛿WC and using Eq. (a) to eliminate 𝛿Q9 from the resulting expression yields 𝛿WC 5 Tres a

𝛿Q b 2 dEC T b

Now, let the system undergo a single cycle while the intermediary system undergoes one or more cycles. The total work of the combined system is WC 5

C

Tres a

𝛿Q 𝛿Q 0 b 2 dE C 5 Tres a b T b C C T b

(b)

Since the reservoir temperature is constant, Tres can be brought outside the integral. The term involving the energy of the combined system vanishes because the energy change for any cycle is zero. The combined system operates in a cycle because its parts execute cycles. Since the combined system undergoes a cycle and exchanges energy by heat transfer with a single reservoir, Eq. 5.3 expressing the Kelvin–Planck statement of the second law must be satisfied. Using this, Eq. (b) reduces to give Eq. 5.12, where the equality applies when there are no irreversibilities within the system as it executes the cycle and the inequality applies when internal irreversibilities are present. This interpretation actually refers to the combination of system plus intermediary cycle. However, the intermediary cycle is free of irreversibilities, so the only possible site of irreversibilities is the system alone.

233

234

Chapter 5 The Second Law of Thermodynamics

c CHAPTER SUMMARY AND STUDY GUIDE In this chapter, we motivate the need for and usefulness of the second law of thermodynamics, and provide the basis for subsequent applications involving the second law in Chaps. 6 and 7. Three statements of the second law, the Clausius, Kelvin–Planck, and entropy statements, are introduced together with several corollaries that establish the best theoretical performance for systems undergoing cycles while interacting with thermal reservoirs. The irreversibility concept is introduced and the related notions of irreversible, reversible, and internally reversible processes are discussed. The Kelvin temperature scale is defined and used to obtain expressions for maximum performance measures of power, refrigeration, and heat pump cycles operating between two thermal reservoirs. The Carnot cycle is introduced to provide a specific example of a reversible cycle operating between two thermal reservoirs. Finally, the Clausius inequality providing a bridge from Chap. 5 to Chap. 6 is presented and discussed.

The following checklist provides a study guide for this chapter. When your study of the text and end-of-chapter exercises has been completed, you should be able to c write out the meanings of the terms listed in the margins

c c c

c c

throughout the chapter and understand each of the related concepts. The subset of key concepts listed below is particularly important in subsequent chapters. give the Kelvin–Planck statement of the second law, correctly interpreting the “less than” and “equal to” signs in Eq. 5.3. list several important irreversibilities. apply the corollaries of Secs. 5.6.2 and 5.7.2 together with Eqs. 5.9, 5.10, and 5.11 to assess the performance of power cycles and refrigeration and heat pump cycles. describe the Carnot cycle. interpret the Clausius inequality.

c KEY ENGINEERING CONCEPTS Clausius statement, p. 206 thermal reservoir, p. 206 Kelvin–Planck statement, p. 207 analytical form of the Kelvin–Planck statement, p. 207 entropy statement of the second law, p. 209

irreversible process, p. 209 reversible process, p. 209 irreversibilities, p. 210 internal and external irreversibilities, p. 210 internally reversible process, p. 213 Carnot corollaries, p. 216

Kelvin scale, p. 221 Carnot efficiency, p. 224 Carnot cycle, p. 229 Clausius inequality, p. 232

c KEY EQUATIONS Wcycle # 0 e

, 0: 5 0:

Analytical form of the Internal irreversibilities present. (single reservoir) (5.3) p. 214 Kelvin–Planck statement. No internal irreversibilities.

𝜂max 5 1 2

TC TH

(5.9) p. 224

Maximum thermal efficiency: power cycle operating between two reservoirs.

𝛽max

TC 5 TH 2 TC

Maximum coefficient of performance: refrigeration (5.10) p. 226 cycle operating between two reservoirs.

𝛾max

TH 5 TH 2 TC

Maximum coefficient of performance: heat pump (5.11) p. 226 cycle operating between two reservoirs.

𝛿Q b 5 2𝜎cycle C T b a

(5.13) p. 232 Clausius inequality.

5.11 Clausius Inequality

235

c EXERCISES: THINGS ENGINEERS THINK ABOUT 1. Extending the discussion of Sec. 5.1.2, how might work be developed when (a) Ti is less than T0 in Fig. 5.1a, (b) pi is less than p0 in Fig. 5.1b?

9. When a power plant discharges cooling water to a river at a temperature higher than that of the river, what are the possible effects on the aquatic life of the river?

2. A system consists of an ice cube in a cup of tap water. The ice cube melts and eventually equilibrium is attained. How might work be developed as the ice and water come to equilibrium?

10. Referring to Eqs. 5.10 and 5.11, how might the coefficients of performance of refrigeration cycles and heat pumps be increased?

3. Describe a process that would satisfy the conservation of energy principle, but does not actually occur in nature.

11. Is it possible for the coefficient of performance of a refrigeration cycle to be less than 1? To be greater than 1? Answer the same questions for a heat pump cycle.

4. Are health risks associated with consuming tomatoes induced to ripen by an ethylene spray? 5. In years ahead, are we more likely to be driving plug-in hybrid electrical vehicles or fuel cell–powered vehicles operating on hydrogen? 6. Referring to Fig. 2.3, what are the internal irreversibilities associated with system A and system B, respectively? 7. For the gearbox of Example 2.4, what are the principal internal and external irreversibilities?

12. A hot combustion gas enters a turbine operating at steady state and expands adiabatically to a lower pressure. Would you expect the power output to be greater in an internally reversible expansion or an actual expansion? 13. Refrigerant 22 enters a compressor operating at steady state and is compressed adiabatically to a higher pressure. Would you expect the power input to the compressor to be greater in an internally reversible compression or an actual compression?

8. If a window air conditioner were placed on a table in a room and operated, would the room temperature increase, decrease, or remain the same? Explain.

c PROBLEMS: DEVELOPING ENGINEERING SKILLS Exploring the Second Law 5.1 Complete the demonstration of the equivalence of the Clausius and Kelvin–Planck statements of the second law given in Sec. 5.2 by showing that a violation of the Kelvin– Planck statement implies a violation of the Clausius statement. 5.2 Classify the following processes of a closed system as possible, impossible, or indeterminate.

(a) (b) (c) (d) (e) (f) (g)

Entropy Change

Entropy Transfer

.0 ,0 0 .0 0 .0 ,0

0

Entropy Production

.0 .0 .0 ,0 ,0 ,0

5.3 Answer the following true or false. (a) A process that violates the second law of thermodynamics violates the first law of thermodynamics. (b) When a net amount of work is done on a closed system undergoing an internally reversible process, a net heat transfer of energy from the system also occurs. (c) A closed system can experience an increase in entropy only when a net amount of entropy is transferred into the system.

(d) The change in entropy of a closed system is the same for every process between two specified end states. 5.4 Complete the discussion of the Kelvin–Planck statement of the second law in the box of Sec. 5.4 by showing that if a system undergoes a thermodynamic cycle reversibly while communicating thermally with a single reservoir, the equality in Eq. 5.3 applies. 5.5 Provide the details left to the reader in the demonstration of the second Carnot corollary given in the box of Sec. 5.6.2. 5.6 Using the Kelvin–Planck statement of the second law of thermodynamics, demonstrate the following corollaries: (a) The coefficient of performance of an irreversible refrigeration cycle is always less than the coefficient of performance of a reversible refrigeration cycle when both exchange energy by heat transfer with the same two reservoirs. (b) All reversible refrigeration cycles operating between the same two reservoirs have the same coefficient of performance. (c) The coefficient of performance of an irreversible heat pump cycle is always less than the coefficient of performance of a reversible heat pump cycle when both exchange energy by heat transfer with the same two reservoirs. (d) All reversible heat pump cycles operating between the same two reservoirs have the same coefficient of performance.

236

Chapter 5 The Second Law of Thermodynamics

5.7 The relation between resistance R and temperature T for a thermistor closely follows

R 5 R0 exp c 𝛽 a

1 1 2 bd T T0

where R0 is the resistance, in ohms (V), measured at temperature T0 (K) and 𝛽 is a material constant with units of K. For a particular thermistor R0 5 2.5 V at T0 5 320 K. From a calibration test, it is found that R 5 0.40 V at T 5 425 K. Determine the value of 𝛽 for the thermistor and make a plot of resistance versus temperature. 5.8 Over a limited temperature range, the relation between electrical resistance R and temperature T for a resistance temperature detector is

R 5 R0[1 1 𝛼(T 2 T0)] where R0 is the resistance, in ohms (V), measured at reference temperature T0 (in 8C) and 𝛼 is a material constant with units of (8C)21. The following data are obtained for a particular resistance thermometer: T (8C)

R (V)

0 95

52.44 53.88

Test 1 Test 2

What temperature would correspond to a resistance of 53.62 V on this thermometer?

Power Cycle Applications 5.9 A power cycle operates between hot and cold reservoirs at 5758C and 1008C, respectively. Determine the maximum theoretical thermal efficiency of the power cycle. 5.10 A carnot engine absorbs 250 J of heat from reservoir at 1008C and rejects heat to a reservoir at 208C. Find (a) the heat rejected, (b) work done by the engine, and (c) thermal efficiency. 5.11 A power cycle operating# at steady state receives energy by heat transfer at a rate QH at TH 5 1000 K and rejects # energy by heat transfer to a cold reservoir at a rate QC at TC 5 300 K. For each of the following cases, determine whether the cycle operates reversibly, operates irreversibly, or is impossible. # # (a) Q# H 5 500 kW, Q# C 5 100 kW # (b) QH 5 500 kW, Wcycle 5 250 kW, QC 5 200 kW # # (c) Wcycle 5 350 kW, QC 5 150 kW # # (d) QH 5 500 kW, QC 5 200 kW

Hot reservoir at TH

Boundary

R

Cold reservoir at TC

Wcycle

QC

Fig. P5.12 5.13 A reversible power cycle receives 100 kJ by heat transfer from a hot reservoir at 3278C and rejects 40 kJ by heat transfer to a cold reservoir at TC. Determine (a) the thermal efficiency and (b) the temperature TC of the cold reservoir, in 8C. 5.14 Determine the maximum theoretical thermal efficiency for any power cycle operating between hot and cold reservoirs at 8008C and 1208C, respectively. 5.15 A reversible power cycle operating as in Fig. 5.5 receives energy QH by heat transfer from a hot reservoir at TH and rejects energy QC by heat transfer to a cold reservoir at 88C. If Wcycle 5 2.5 QC, determine (a) the thermal efficiency and (b) TH, in 8C. 5.16 A reversible power cycle has the same thermal efficiency for hot and cold reservoirs at temperature T and 600 K, respectively, as for hot and cold reservoirs at 4000 and 2000 K, respectively. Determine T, in K. 5.17 As shown in Fig. P5.17, two reversible cycles arranged in series each produce the same net work, Wcycle. The first cycle receives energy QH by heat transfer from a hot reservoir at 555 K and rejects energy Q by heat transfer to a reservoir at an intermediate temperature, T. The second cycle receives energy Q by heat transfer from the reservoir at temperature T and rejects energy QC by heat transfer to a reservoir at 222 K. All energy transfers are positive in the directions of the arrows. Determine Hot reservoir at TH = 555 K QH

R1

Reservoir at T

5.12 As shown in Fig. P5.12, a reversible power cycle receives energy QH by heat transfer from a hot reservoir at TH and rejects energy QC by heat transfer to a cold reservoir at TC. (a) If TH 5 1200 K and TC 5 300 K, what is the thermal efficiency? (b) If TH 5 500°C, TC 5 20°C, and Wcycle 5 1000 kJ, what are QH and QC, each in kJ? (c) If 𝜂 5 60% and TC 5 408C, what is TH, in °C? (d) If 𝜂 5 40% and TH 5 7278C, what is TC, in °C?

QH

Q Q

R2

QC Cold reservoir at TC = 222 K

Fig. P5.17

Wcycle

Wcycle

5.11 Clausius Inequality (a) the intermediate temperature T, in K, and the thermal efficiency for each of the two power cycles. (b) the thermal efficiency of a single reversible power cycle operating between hot and cold reservoirs at 555 K and 222 K, respectively. Also, determine the net work developed by the single cycle, expressed in terms of the net work developed by each of the two cycles, Wcycle. 5.18 A reversible power cycle operating between hot and cold reservoirs at 900 K and 200 K, respectively, receives 120 kJ by heat transfer from the hot reservoir for each cycle of operation. Determine the net work developed in 12 cycles of operation, in kJ. 5.19 A cyclic heat engine operates between a source temperature of 7008C and a sink temperature of 258C. What is the least rate of heat rejection per kW net output of the engine? 5.20 Data for two reversible refrigeration is given below: Cycle 1: TH 5 258C, TC 5 2 108C Cycle 2: TH 5 258C, TC 5 2 258C where TH and Tc are the temperature of hot and cold reservoirs, respectively. Determine the ratio of net work input values of the two cycles if same amount of heat energy is removed from cold reservoirs by each refrigerator. 5.21 During January, at a location in Alaska winds at 230°C can be observed. Several meters below ground the temperature remains at 138C, however. An inventor claims to have devised a power cycle exploiting this situation that has a thermal efficiency of 14%. Evaluate this claim. 5.22 An inventor claims to have developed a power cycle operating between hot and cold reservoirs at 2000 K and 500 K, respectively, that develops net work equal to a multiple of the amount of energy, QC rejected to the cold reservoir—that is Wcycle 5 NQC, where all quantities are positive. What is the maximum theoretical value of the number N for any such cycle? 5.23 A power cycle operates between a reservoir at temperature T and a lower-temperature reservoir at 280 K. At steady state, the cycle develops 40 kW of power while rejecting 1000 kJ/min of energy by heat transfer to the cold reservoir. Determine the minimum theoretical value for T, in K. 5.24 At steady state, a new power cycle is claimed by its inventor to develop power at a rate of 65 kW for a heat addition rate of 4.5 3 105 kJ/h, while operating between hot and cold reservoirs at 800 and 400 K, respectively. Evaluate this claim. 5.25 At steady state, a power cycle develops a power output of 20 kW while receiving energy by heat transfer at the rate of 20 kJ per cycle of operation from a source at temperature T. The cycle rejects energy by heat transfer to cooling water at a lower temperature of 500 K. If there are 150 cycles per minute, what is the minimum theoretical value for T, in K?

237

5.26 At steady state, a power cycle having a thermal efficiency of 38% generates 100 MW of electricity while discharging energy by heat transfer to cooling water at an average temperature of 218C. The average temperature of the steam passing through the boiler is 4808C. Determine (a) the rate at which energy is discharged to the cooling water, in kJ/h. (b) the minimum theoretical rate at which energy could be discharged to the cooling water, in kJ/h. Compare with the actual rate and discuss. 5.27 Ocean temperature energy conversion (OTEC) power plants generate power by utilizing the naturally occurring decrease with depth of the temperature of ocean water. Near Florida, the ocean surface temperature is 278C, while at a depth of 700 m the temperature is 78C. (a) Determine the maximum thermal efficiency for any power cycle operating between these temperatures. (b) The thermal efficiency of existing OTEC plants is approximately 2%. Compare this with the result of part (a) and comment. 5.28 A power cycle operating at steady state receives energy by heat transfer from the combustion of fuel at an average temperature of 1000 K. Owing to environmental considerations, the cycle discharges energy by heat transfer to the atmosphere at 300 K at a rate no greater than 60 MW. Based on the cost of fuel, the cost to supply the heat transfer is $4.50 per GJ. The power developed by the cycle is valued at $0.08 per kW? h. For 8000 hours of operation annually, determine for any such cycle, in $ per year, (a) the maximum value of the power generated and (b) the minimum fuel cost. 5.29 At steady state, a power cycle receives energy by heat transfer at an average temperature of 4638C and discharges energy by heat transfer to a river. Upstream of the power plant the river has a volumetric flow rate of 67.87 m3/s and a temperature of 208C. From environmental considerations, the temperature of the river downstream of the plant can be no more than 228C. Determine the maximum theoretical power that can be developed, in MW, subject to this constraint. 5.30 The preliminary design of a space station calls for a power cycle that at steady state receives energy by heat transfer at TH 5 600 K from a nuclear source and rejects energy to space by thermal radiation according to Eq. 2.32. For the radiative surface, the temperature is TC, the emissivity is 0.6, and the surface receives no radiation from any source. The thermal efficiency of the power cycle is one-half that of a reversible power cycle operating between reservoirs at TH and TC. # (a) For TC 5 400 K determine Wcycle /A, the net power developed per unit of radiator surface area, in kW/m2, and the thermal # efficiency. (b) Plot Wcycle /A and the thermal efficiency versus TC, and # determine the maximum value of Wcycle /A. (c) # Determine the range of temperatures TC, in K, for which Wcycle /A is within 2% of the maximum value obtained in part (b).

238

Chapter 5 The Second Law of Thermodynamics

5.31 Two reversible power cycles are arranged in series. The first cycle receives energy by heat transfer from a hot reservoir at temperature TH and rejects energy by heat transfer to a reservoir at an intermediate temperature T , TH. The second cycle receives energy by heat transfer from the reservoir at temperature T and rejects energy by heat transfer to a cold reservoir at temperature TC , T. (a) Obtain an expression for the thermal efficiency of a single reversible power cycle operating between hot and cold reservoirs at TH and TC, respectively, in terms of the thermal efficiencies of the two cycles. (b) Obtain an expression for the intermediate temperature T in terms of TH and TC for the special case where the thermal efficiencies of the two cycles are equal.

Refrigeration and Heat Pump Cycle Applications 5.32 A refrigeration cycle operating between two reservoirs receives energy QC from a cold reservoir at TC 5 280 K and rejects energy QH to a hot reservoir at TH 5 320 K. For each of the following cases determine whether the cycle operates reversibly, operates irreversibly, or is impossible: (a) QC 5 1500 kJ, Wcycle 5 150 kJ. (b) QC 5 1400 kJ, QH 5 1600 kJ. (c) QH 5 1600 kJ, Wcycle 5 400 kJ. (d) 𝛽 5 5. 5.33 The freezer compartment of a refrigerator is maintained at 268C. The temperature of the surrounding air is 258C. The refrigerant absorbs heat from the freezer compartment at the rate of 10,000 kJ/h. The power input required to operate the refrigerator is 3500 kJ/h. Determine the coefficient of performance of the refrigerator, and determine the coefficient of performance of a reversible refrigeration cycle operating between the same given temperatures. 5.34 At steady state, a# reversible heat pump cycle discharges energy at the rate QH to a hot reservoir at temperature TH, # while receiving energy at the rate QC from a cold reservoir at temperature TC. (a) If TH 5 278C and TC 5 178C, determine the coefficient performance. # # (b) If QH 5 12.5 kW, QC 5 9.5 kW, and TC 5 78C, determine TH, in 8C. (c) If the coefficient of performance is 20 and TH 5 378C, determine TC, in 8C. 5.35 A refrigeration cycle rejects QH 5 540 kJ per cycle to a hot reservoir at TH 5 500 K, while receiving QC 5 400 kJ per cycle from a cold reservoir at temperature TC. For 20 cycles of operation, determine (a) the net work input, in kJ, and (b) the minimum theoretical temperature TC, in K. 5.36 A reversible heat pump # cycle operating at steady state receives energy at the rate QC 5 7.5 kW from a cold reservoir at# temperature TC 5 08C and discharges energy at the rate QH 5 9.5 kW to a hot reservoir at temperature TH. Determine the value of TH and the co efficient of performance of heat pump. 5.37 A reversible power cycle and a reversible heat pump cycle operate between hot and cold reservoirs at temperature

TH 5 600 K and TC, respectively. If the thermal efficiency of the power cycle is 65%, determine (a) TC, in K, and (b) the coefficient of performance of the heat pump. 5.38 An inventor has developed a refrigerator capable of maintaining its freezer compartment at 25°C while operating in a kitchen at 258C, and claims the device has a coefficient of performance of (a) 7, (b) 8.93, (c) 10. Evaluate the claim in each of the three cases. 5.39 An inventor claims to have developed a refrigerator that at steady state requires a net power input of 0.54 kW to remove 12,800 kJ/h of energy by heat transfer from the freezer compartment at 220°C and discharge energy by heat transfer to a kitchen at 278C. Evaluate this claim. 5.40 According to an inventor of a refrigerator, the refrigerator can remove heat from the freezer compartment at the rate of 13,000 kJ/h by net input power consumption of 0.65 kW. Heat is discharged into the room at 238C. The temperature of freezer compartment is 2158C. Evaluate this claim. 5.41 By removing energy by heat transfer from its freezer compartment at a rate of 1.5 kW, a refrigerator maintains the freezer at 2228C on a day when the temperature of the surroundings is 288C. Determine the minimum theoretical power, in kW, required by the refrigerator at steady state. 5.42 Determine the minimum theoretical power, in kJ/s, required at steady state by a refrigeration system to maintain a cryogenic sample at 2126°C in a laboratory at 218C, if energy leaks by heat transfer to the sample from its surroundings at a rate of 0.09 kJ/s. 5.43 For each kW of power input to an ice maker at steady state, determine the maximum rate that ice can be produced, in kg/h, from liquid water at 08C. Assume that 333 kJ/kg of energy must be removed by heat transfer to freeze water at 08C, and that the surroundings are at 208C. 5.44 At steady state, a reversible refrigeration cycle operates between hot and cold reservoirs at 300 K and 270 K, respectively. Determine the minimum theoretical net power input required, in kW per kW of heat transfer from the cold reservoir. 5.45 At steady state, a refrigeration cycle operating between hot and cold reservoirs at 320 K and 272 K, respectively, removes energy by heat transfer from the cold reservoir at a rate of 650 kW. (a) If the cycle’s coefficient of performance is 5, determine the power input required, in kW. (b) Determine the minimum theoretical power required, in kW, for any such cycle. 5.46 As shown in Fig P5.45, an air conditioner operating at steady state maintains a dwelling at 208C on a day when the outside temperature is 358C. If the rate of heat transfer into the dwelling through the walls and roof is 28,000 kJ/h, might a net power input to the air-conditioner compressor of 1.5 kW be sufficient? If yes, determine the coefficient of performance. If no, determine the minimum theoretical power input, in kW .

5.11 Clausius Inequality

Inside, 20°C

28,000 kJ/h

239

Outside, 35°C

Refrigerant loop Evaporator · Qin

Condenser · Qout

· Compressor Wc

Fig. P5.45 5.47 To maintain a dwelling at a temperature of 208C, 600 MJ energy is required per day when the temperature outside the dwelling is 48C. A heat pump is used to supply this energy. Determine the minimum theoretical cost to operate the heat pump, in $/day, if the cost of electricity is $0.10 per kW ? h.

Refrigerator β = 4.5

Surroundings, 20°C Coils, 28°C · QH

5.48 The coefficient of performance of a refrigeration cycle is 70% of the value for a reversible refrigeration cycle. The temperatures of the hot and cold reservoirs are 358C and 248C, respectively. Determine the power input per kW of cooling required in both the cycles. 5.49 By removing energy by heat transfer from a room, a window air conditioner maintains the room at 188C on a day when the outside temperature is 428C. (a) Determine, in kW per kW of cooling, the minimum theoretical power required by the air conditioner. (b) To achieve required rates of heat transfer with practicalsized units, air conditioners typically receive energy by heat transfer at a temperature below that of the room being cooled and discharge energy by heat transfer at a temperature above that of the surroundings. Consider the effect of this by determining the minimum theoretical power, in kW per kW of cooling, required when TC 5 218C and TH 5 488C, and compare with the value found in part (a). 5.50 The refrigerator shown in Fig. P5.50 operates at steady state with a coefficient of performance of 4.5 and a power input of 0.8 kW. Energy is rejected from the refrigerator to the surroundings at 208C by heat transfer from metal coils whose average surface temperature is 28 8 C. Determine (a) the rate energy that is rejected, in kW. (b) the lowest theoretical temperature inside the refrigerator, in K. (c) the maximum theoretical power, in kW, that could be developed by a power cycle operating between the coils and the surroundings. Would you recommend making use of this opportunity for developing power?

+ –

0.8 kW

Fig. P5.50

5.51 At steady state, a heat pump provides 31,650 kJ/h to maintain a dwelling at 208C on a day when the outside temperature is 1.78C. The power input to the heat pump is 3.73 kW. If electricity costs 8 cents per kW ? h, compare the actual operating cost with the minimum theoretical operating cost for each day of operation. 5.52 By supplying energy at an average rate of 21,100 kJ/h, a heat pump maintains the temperature of a dwelling at 218C. If electricity costs 8 cents per kW ? h, determine the minimum theoretical operating cost for each day of operation if the heat pump receives energy by heat transfer from (a) the outdoor air at 258C. (b) well water at 88C. 5.53 A heat pump with a coefficient of performance of 3.8 provides energy at an average rate of 75,000 kJ/h to maintain a building at 218C on a day when the outside temperature is 08C. If electricity costs 8 cents per kW ? h (a) determine the actual operating cost and the minimum theoretical operating cost, each in $/day. (b) compare the results of part (a) with the cost of electricalresistance heating.

240

Chapter 5 The Second Law of Thermodynamics

5.54 A heat pump maintains a dwelling at temperature T when the outside temperature averages 58C. The heat transfer rate through the walls and roof is 2000 kJ/h per degree of temperature difference between the inside and outside. If electricity costs 8 cents per kW ? h (a) determine the minimum theoretical operating cost for each day of operation when T 5 208C. (b) plot the minimum theoretical operating cost for each day of operation as a function of T ranging from 18 to 238C.

and 2108C. The refrigerator is driven by the heat engine. The heat transfer to the heat engine is 1500 kJ. A net work output of 300 kJ is obtained from the combined enginerefrigerator plant. Determine the net heat transfer to the reservoir at 358C and the heat transfer to the refrigerant from cold reservoir.

5.55 A heat pump maintains a dwelling at temperature T when the outside temperature is 27 8 C. The heat transfer rate through the walls and roof is 0.44 kJ/s per degree temperature difference between the inside and outside.

5.60 A reversible power cycle receives QH from a hot reservoir at temperature TH and rejects energy by heat transfer to the surroundings at temperature T0. The work developed by the power cycle is used to drive a reversible refrigeration cycle that removes QC from a cold reservoir at temperature TC and discharges energy by heat transfer to the same surroundings at T0. Develop an expression for the ratio QC/QH in terms of the temperature ratios TH/T0 and TC/T0.

(a) If electricity costs 8 cents per kW ? h, plot the minimum theoretical operating cost for each day of operation for T ranging from 20 to 248C (b) If T 5 218C, plot the minimum theoretical operating cost for each day of operation for a cost of electricity ranging from 4 to 12 cents per kW ? h.

5.61 A reversible power cycle receives energy QH from a reservoir at temperature TH and rejects QC to a reservoir at temperature TC. The work developed by the power cycle is used to drive a reversible heat pump that removes energy Q9C from a reservoir at temperature T9C and rejects energy Q9H to a reservoir at temperature T9H.

5.56 Two reversible refrigeration cycles operate in series. The first cycle receives energy by heat transfer from a cold reservoir at 310 K and rejects energy by heat transfer to a reservoir at an intermediate temperature T greater than 310 K. The second cycle receives energy by heat transfer from the reservoir at temperature T and rejects energy by heat transfer to a higher-temperature reservoir at 850 K. If the refrigeration cycles have the same coefficient of performance, determine (a) T, in K, and (b) the value of each coefficient of performance.

(a) Develop an expression for the ratio Q9H/QH in terms of the temperatures of the four reservoirs. (b) What must be the relationship of the temperatures T H, T C, T9 C, and T9 H for Q9 H/Q H to exceed a value of unity?

5.57 Two reversible heat pump cycles operate in series. The first cycle receives energy by heat transfer from a cold reservoir at 260 K and rejects energy by heat transfer to a reservoir at an intermediate temperature T greater than 260 K. The second cycle receives energy by heat transfer from the reservoir at temperature T and rejects energy by heat transfer to a higher-temperature reservoir at 1200 K. If the heat pump cycles have the same coefficient of performance, determine (a) T, in K, and (b) the value of each coefficient of performance. 5.58 Two reversible refrigeration cycles are arranged in series. The first cycle receives energy by heat transfer from a cold reservoir at temperature TC and rejects energy by heat transfer to a reservoir at an intermediate temperature T greater than TC. The second cycle receives energy by heat transfer from the reservoir at temperature T and rejects energy by heat transfer to a higher-temperature reservoir at TH. Obtain an expression for the coefficient of performance of a single reversible refrigeration cycle operating directly between cold and hot reservoirs at TC and TH, respectively, in terms of the coefficients of performance of the two cycles. 5.59 A reversible heat engine operates between two reservoirs at temperatures of 6508C and 358C. A reversible refrigerator operates between reservoirs at temperatures of 358 C

Carnot Cycle Applications 5.62 Two kg of water execute a Carnot power cycle. During the isothermal expansion, the water is heated until it is a saturated vapor from an initial state where the pressure is 40 bar and the quality is 15%. The vapor then expands adiabatically to a pressure of 1.5 bar while doing 491.5 kJ/kg of work. (a) Sketch the cycle on p–υ coordinates. (b) Evaluate the heat and work for each process, in kJ. (c) Evaluate the thermal efficiency. 5.63 One-half kg of water executes a Carnot power cycle. During the isothermal expansion, the water is heated at 3158C from a saturated liquid to a saturated vapor. The vapor then expands adiabatically to a temperature of 328C and a quality of 64.3%. (a) Sketch the cycle on p–υ coordinates. (b) Evaluate the heat and work for each process, in kJ. (c) Evaluate the thermal efficiency. 5.64 One kg of air as an ideal gas executes a Carnot power cycle having a thermal efficiency of 50%. The heat transfer to the air during the isothermal expansion is 50 kJ. At the end of the isothermal expansion, the pressure is 574 kPa and the volume is 0.3 m3. Determine (a) the maximum and minimum temperatures for the cycle, in K. (b) the pressure and volume at the beginning of the isothermal expansion in bar and m3, respectively.

5.11 Clausius Inequality (c) the work and heat transfer for each of the four processes, in kJ. (d) Sketch the cycle on p–υ coordinates.

Clausius Inequality Applications

241

(b) Process 4–1: constant-pressure at 8 MPa from saturated liquid to saturated vapor. Process 2–3: constant-pressure at 8 kPa from x2 5 67.5% to x3 5 34.2%. (c) Process 4–1: constant-pressure at 0.15 MPa from saturated liquid to saturated vapor. Process 2–3: constantpressure at 20 kPa from x2 5 90% to x3 5 30%

5.65 A system executes a power cycle while receiving 1690 kJ by heat transfer at a temperature of 1390 K and discharging 211 kJ by heat transfer at 278 K. A heat transfer from the system also occurs at a temperature of 833 K. There are no other heat transfers. If no internal irreversibilites are present, determine the thermal efficiency.

˙ in Q 4

5.66 As shown in Fig. P5.65, a system executes a power cycle while receiving 750 kJ by heat transfer at a temperature of 1500 K and discharging 100 kJ by heat transfer at a temperature of 500 K. Another heat transfer from the system occurs at a temperature of 1000 K. Using Eq. 5.13, determine the thermal efficiency if 𝜎cycle is (a) 0.1 kJ/K, (b) 0.2 kJ/K, (c) 0.35 kJ/K.

W˙ p

1

Boiler

Pump

Turbine

Condenser

3

W˙ t

2

˙ out Q Q1 = 750 kJ

Fig. P5.67

T1 = 1500 K Q3

Wcycle

T3 = 1000 K

T2 = 500 K Q2 = 100 kJ

Fig. P5.65

5.67 A simple vapor power plant operating at steady state is shown in Fig. P5.66. In each of the following three cases, use Eq. 5.13 to determine whether the cycle is possible, internally reversible, or impossible. Processes 1–2 and 3–4 are adiabatic. Kinetic and potential energy changes can be ignored. (a) Process 4–1: constant-pressure at 1 MPa from saturated liquid to saturated vapor. Process 2–3: constant-pressure at 20 kPa from x3 5 88% to x4 5 18%.

5.68 Figure P5.68 shows a system consisting of a power cycle driving a# heat pump. At steady state, the power cycle receives Qs by heat transfer at Ts from the high-temperature # source and delivers Q to a dwelling at Td# . The heat pump 1 # receives Q0 from the outdoors at T0, and Q2 to the dwelling. Using Eq. 5.13, obtain an expression for the maximum # # theoretical value of the performance parameter (Q1 1 Q2)/ # Qs in terms of the temperature ratios Ts/Td and T0/Td. · Q1

Dwelling at Td

· Qs

Air Ts

Heat pump

Power cycle Driveshaft

Fuel

· Q2

· Q0 Outdoors at T0

Fig. P5.68

c DESIGN & OPEN ENDED PROBLEMS: EXPLORING ENGINEERING PRACTICE 5.1D Experiments by Australian researchers suggest that the second law can be violated at the micron scale over time intervals of up to 2 seconds. If validated, some say these findings could place a limit on the engineering of nanomachines because such devices may not behave simply like miniaturized versions of their larger counterparts. Investigate the implications, if any, for nanotechnology of these experimental findings. Write a report including at least three references. 5.2D The U.S. Food and Drug Administration (FDA) has long permitted the application of citric acid, ascorbic acid, and

other substances to keep fresh meat looking red longer. In 2002, the FDA began allowing meat to be treated with carbon monoxide. Carbon monoxide reacts with myoglobin in the meat to produce a substance that resists the natural browning of meat, thereby giving meat a longer shelf life. Investigate the use of carbon monoxide for this purpose. Identify the nature of myoglobin and explain its role in the reactions that cause meat to brown or, when treated with carbon monoxide, allows the meat to appear red longer. Consider the hazards, if any, that may accompany this practice for consumers and for meat industry workers. Report your findings in a memorandum.

242

Chapter 5 The Second Law of Thermodynamics

5.3D In an Internet paper titled, “How Lightning Strikes are Produced and Why the Second Law of Thermodynamics Is Invalid,” the author claims that electricity can be produced economically by extracting “constant-temperature ambient heat from the atmosphere or the oceans and converting it fully into work.” Critically evaluate this claim, and report your findings in a memorandum. 5.4D Figure P5.4D shows one of those bobbing toy birds that seemingly takes an endless series of sips from a cup filled with water. Prepare a 30-min presentation suitable for a middle school science class explaining the operating principles of this device and whether or not its behavior is at odds with the second law.

5.5D Figure P5.5D shows that the typical thermal efficiency of U.S. power plants increased rapidly in the early and mid-1900s, but has increased only gradually since then. Investigate the most important factors contributing to this plateauing of thermal efficiency and the most promising near-term and long-term technologies that might lead to appreciable thermal efficiency gains. Report your findings in a memorandum. 40

Thermal efficiency, %

5.8D Insulin and several other pharmaceuticals required daily by those suffering from diabetes and other medical conditions have relatively low thermal stability. Those living and traveling in hot climates are especially at risk by heatinduced loss of potency of their pharmaceuticals. Design a wearable, lightweight, and reliable cooler for transporting temperature-sensitive pharmaceuticals. The cooler also must be solely powered by human motion. While the long-term goal is a moderately priced consumer product, the final project report need only provide the costing of a single prototype. 5.9D The International Temperature Scale was first adopted by the International Committee on Weights and Measures in 1927 to provide a global standard for temperature measurement. This scale has been refined and extended in several revisions, most recently in 1990 (International Temperature Scale of 1990, ITS-90). What are some reasons for revising the scale? What are some principal changes that have been made since 1927? Summarize your findings in a memorandum.

Fig. P5.4D

25

0

5.7D The heat transfer rate through the walls and roof of a building is 3570 kJ/h per degree temperature difference between the inside and outside. For outdoor temperatures ranging from 20 to 2208C, make a comparison of the daily cost of maintaining the building at 208C by means of an electric heat pump, direct electric resistance heating, and a conventional gas-fired furnace. Report your findings in a memorandum.

1925

2005 Year

Fig. P5.5D 5.6D Investigate adverse health conditions that might be exacerbated for persons living in urban heat islands. Write a report including at least three references.

5.10D A technical article considers hurricanes as an example of a natural Carnot engine: K. A. Emmanuel, “Toward a General Theory of Hurricanes,” American Scientist, 76, 371– 379, 1988. Also see Physics Today, 59, No. 8, 74–75, 2006, for a related discussion by this author. U.S. Patent (No. 4,885,913) is said to have been inspired by such an analysis. Does the concept have scientific merit? Engineering merit? Summarize your conclusions in a memorandum.

6 Using Entropy ENGINEERING CONTEXT Up to this point, our study of the second law has been concerned primarily with what it says about systems undergoing thermodynamic cycles. In this chapter means are introduced for analyzing systems from the second law perspective as they undergo processes that are not necessarily cycles. The property entropy and the entropy production concept introduced in Chap. 5 play prominent roles in these considerations. The objective of this chapter is to develop an understanding of entropy concepts, including the use of entropy balances for closed systems and control volumes in forms effective for the analysis of engineering systems. The Clausius inequality developed in Sec. 5.11, expressed as Eq. 5.13, provides the basis.

LEARNING OUTCOMES When you complete your study of this chapter, you will be able to… c

demonstrate understanding of key concepts related to entropy and the second law . . . including entropy transfer, entropy production, and the increase in entropy principle.

c

evaluate entropy, evaluate entropy change between two states, and analyze isentropic processes, using appropriate property data.

c

represent heat transfer in an internally reversible process as an area on a temperature-entropy diagram.

c

apply entropy balances to closed systems and control volumes.

c

evaluate isentropic efficiencies for turbines, nozzles, compressors, and pumps.

243

244

Chapter 6 Using Entropy

6.1 Entropy—A System Property The word energy is so much a part of the language that you were undoubtedly familiar with the term before encountering it in early science courses. This familiarity probably facilitated the study of energy in these courses and in the current course in engineering thermodynamics. In the present chapter you will see that the analysis of systems from a second law perspective is effectively accomplished in terms of the property entropy. Energy and entropy are both abstract concepts. However, unlike energy, the word entropy is seldom heard in everyday conversation, and you may never have dealt with it quantitatively before. Energy and entropy play important roles in the remaining chapters of this book.

6.1.1

Defining Entropy Change

2 C B A 1

Fig. 6.1 Two internally reversible cycles.

A quantity is a property if, and only if, its change in value between two states is independent of the process (Sec. 1.3.3). This aspect of the property concept is used in the present section together with the Clausius inequality to introduce entropy change as follows: Two cycles executed by a closed system are represented in Fig. 6.1. One cycle consists of an internally reversible process A from state 1 to state 2, followed by internally reversible process C from state 2 to state 1. The other cycle consists of an internally reversible process B from state 1 to state 2, followed by the same process C from state 2 to state 1 as in the first cycle. For the first cycle, Eq. 5.13 (the Clausius inequality) takes the form a

#

2

1

𝛿Q a b 5 2𝜎cycle C T b (Eq. 5.13)

𝛿Q b 1a T A

#

1

2

𝛿Q b 5 2𝜎 0cycle T C

(6.1a)

For the second cycle, Eq. 5.13 takes the form a

#

1

2

𝛿Q b 1a T B

#

1

2

𝛿Q b 5 2𝜎 0cycle T C

(6.1b)

In writing Eqs. 6.1, the term 𝜎cycle has been set to zero since the cycles are composed of internally reversible processes. When Eq. 6.1b is subtracted from Eq. 6.1a, we get a

#

1

2

𝛿Q b 5a T A

#

1

2

𝛿Q b T B

This shows that the integral of 𝛿Q/T is the same for both processes. Since A and B are arbitrary, it follows that the integral of 𝛿Q/T has the same value for any internally reversible process between the two states. In other words, the value of the integral depends on the end states only. It can be concluded, therefore, that the integral represents the change in some property of the system. Selecting the symbol S to denote this property, which is called entropy, the change in entropy is given by entropy change

S 2 2 S1 5 a

#

1

2

𝛿Q b int T rev

(6.2a)

where the subscript “int rev” is added as a reminder that the integration is carried out for any internally reversible process linking the two states. On a differential basis, the defining equation for entropy change takes the form dS 5 a Entropy is an extensive property.

𝛿Q b int T rev

(6.2b)

6.2 Retrieving Entropy Data The SI unit for entropy is J/K. However, in this book it is convenient to work in terms of kJ/K. Units in SI for specific entropy are kJ/kg ? K for s and kJ/kmol ? K for s. It should be clear that entropy is defined and evaluated in terms of a particular expression (Eq. 6.2a) for which no accompanying physical picture is given. We encountered this previously with the property enthalpy. Enthalpy is introduced without physical motivation in Sec. 3.6.1. Then, in Chap. 4, we learned how enthalpy is used for thermodynamic analysis of control volumes. As for the case of enthalpy, to gain an appreciation for entropy you need to understand how it is used and what it is used for. This is the aim of the rest of this chapter.

6.1.2

Evaluating Entropy

Since entropy is a property, the change in entropy of a system in going from one state to another is the same for all processes, both internally reversible and irreversible, between these two states. Thus, Eq. 6.2a allows the determination of the change in entropy, and once it has been evaluated, this is the magnitude of the entropy change for all processes of the system between the two states. The defining equation for entropy change, Eq. 6.2a, serves as the basis for evaluating entropy relative to a reference value at a reference state. Both the reference value and the reference state can be selected arbitrarily. The value of entropy at any state y relative to the value at the reference state x is obtained in principle from S y 5 Sx 1 a

#

x

y

𝛿Q b int T rev

(6.3)

where Sx is the reference value for entropy at the specified reference state. The use of entropy values determined relative to an arbitrary reference state is satisfactory as long as they are used in calculations involving entropy differences, for then the reference value cancels. This approach suffices for applications where composition remains constant. When chemical reactions occur, it is necessary to work in terms of absolute values of entropy determined using the third law of thermodynamics (Chap. 13).

6.1.3

Entropy and Probability

The presentation of engineering thermodynamics provided in this book takes a macroscopic view as it deals mainly with the gross, or overall, behavior of matter. The macroscopic concepts of engineering thermodynamics introduced thus far, including energy and entropy, rest on operational definitions whose validity is shown directly or indirectly through experimentation. Still, insights concerning energy and entropy can result from considering the microstructure of matter. This brings in the use of probability and the notion of disorder. Further discussion of entropy, probability, and disorder is provided in Sec. 6.8.2.

6.2 Retrieving Entropy Data In Chap. 3, we introduced means for retrieving property data, including tables, graphs, equations, and the software available with this text. The emphasis there is on evaluating the properties p, 𝜐, T, u, and h required for application of the conservation of mass and energy principles. For application of the second law, entropy values are usually required. In this section, means for retrieving entropy data for water and several refrigerants are considered.

units for entropy

245

246

Chapter 6 Using Entropy Tables of thermodynamic data are introduced in Secs. 3.5 and 3.6 (Tables A-2 through A-18). Specific entropy is tabulated in the same way as considered there for the properties 𝜐, u, and h, and entropy values are retrieved similarly. The specific entropy values given in Tables A-2 through A-18 are relative to the following reference states and values. For water, the entropy of saturated liquid at 0.018C is set to zero. For the refrigerants, the entropy of the saturated liquid at 2408C is assigned a value of zero.

6.2.1

Vapor Data

In the superheat regions of the tables for water and the refrigerants, specific entropy is tabulated along with 𝜐, u, and h versus temperature and pressure. consider two states of water. At state 1 the pressure is 3 MPa and the temperature is 5008C. At state 2, the pressure is 0.3 MPa and the specific entropy is the same as at state 1, s2 5 s1. The object is to determine the temperature at state 2. Using T1 and p1, we find the specific entropy at state 1 from Table A-4 as s1 5 7.2338 kJ/kg ? K. State 2 is fixed by the pressure, p2 5 0.3 MPa, and the specific entropy, s2 5 7.2338 kJ/kg ? K. Returing to Table A-4 at 0.3 MPa and interpolating with s2 between 160 and 2008C results in T2 5 1838C. b b b b b

6.2.2

Saturation Data

For saturation states, the values of sf and sg are tabulated as a function of either saturation pressure or saturation temperature. The specific entropy of a two-phase liquid–vapor mixture is calculated using the quality s 5 (1 2 x)sf 1 xsg 5 sf 1 x(sg 2 sf)

(6.4)

These relations are identical in form to those for 𝜐, u, and h (Secs. 3.5 and 3.6). let us determine the specific entropy of Refrigerant 134a at a state where the temperature is 08C and the specific internal energy is 138.43 kJ/kg. Referring to Table A-10, we see that the given value for u falls between uf and ug at 08C, so the system is a two-phase liquid–vapor mixture. The quality of the mixture can be determined from the known specific internal energy u 2 uf 138.43 2 49.79 x5 5 5 0.5 ug 2 uf 227.06 2 49.79 Then with values from Table A-10, Eq. 6.4 gives s 5 (1 2 x)sf 1 xsg 5 (0.5)(0.1970) 1 (0.5)(0.9190) 5 0.5580 kJ/kg ? K

6.2.3

b b b b b

Liquid Data

Compressed liquid data are presented for water in Table A-5. In this table, s, 𝜐, u, and h are tabulated versus temperature and pressure as in the superheat tables, and the tables are used similarly. In the absence of compressed liquid data, the value of the specific entropy can be estimated in the same way as estimates for 𝜐 and u are obtained for liquid states (Sec. 3.10.1), by using the saturated liquid value at the given temperature s(T, p) < sf (T)

(6.5)

6.2 Retrieving Entropy Data

247

suppose the value of specific entropy is required for water at 25 bar, 2008C. The specific entropy is obtained directly from Table A-5 as s 5 2.3294 kJ/ kg ? K. Using the saturated liquid value for specific entropy at 2008C from Table A-2, the specific entropy is approximated with Eq. 6.5 as s 5 2.3309 kJ/kg ? K, which agrees closely with the previous value. b b b b b

6.2.4

Computer Retrieval

The software available with this text, Interactive Thermodynamics: IT, provides data for the substances considered in this section. Entropy data are retrieved by simple call statements placed in the workspace of the program. consider a two-phase liquid–vapor mixture of H2O at p 5 1 bar, 𝜐 5 0.8475 m3/kg. The following illustrates how specific entropy and quality x are obtained using IT: p v v s

5 5 5 5

1 // bar 0.8475 // m3/kg vsat_Px(“Water/Steam”,p,x) ssat_Px(“Water/Steam”,p,x)

The software returns values of x 5 0.5 and s 5 4.331 kJ/kg ? K, which can be checked using data from Table A-3. Note that quality x is implicit in the expression for specific volume, and it is not necessary to solve explicitly for x. As another example, consider superheated ammonia vapor at p 5 1.5 bar, T 5 88C. Specific entropy is obtained from IT as follows: p 5 1.5 // bar T 5 8 // 8C s 5 s_PT (“Ammonia”, p,T) The software returns s 5 5.981 kJ/kg ? K, which agrees closely with the value obtained by interpolation in Table A-15. b b b b b

6.2.5

TAKE NOTE...

Note that IT does not provide compressed liquid data for any substance. IT returns liquid entropy data using the approximation of Eq. 6.5. Similarly, Eqs. 3.11, 3.12, and 3.14 are used to return liquid values for 𝜐, u, and h, respectively.

Using Graphical Entropy Data

The use of property diagrams as an adjunct to problem solving is emphasized throughout this book. When applying the second law, it is frequently helpful to locate states and plot processes on diagrams having entropy as a coordinate. Two commonly used figures having entropy as one of the coordinates are the temperature–entropy diagram and the enthalpy–entropy diagram.

Temperature–Entropy Diagram The main features of a temperature–entropy diagram are shown in Fig. 6.2. For detailed figures for water in SI and English units, see Fig. A-7. Observe that lines of constant enthalpy are shown on these figures. Also note that in the superheated vapor region constant specific volume lines have a steeper slope than constant-pressure lines. Lines of constant quality are shown in the two-phase liquid–vapor region. On some figures, lines of constant quality are marked as percent moisture lines. The percent moisture is defined as the ratio of the mass of liquid to the total mass. In the superheated vapor region of the T–s diagram, constant specific enthalpy lines become nearly horizontal as pressure is reduced. These superheated vapor states are shown as the shaded area on Fig. 6.2. For states in this region of the diagram, the enthalpy is determined primarily by the temperature: h(T, p) < h(T). This is the

T–s diagram

Chapter 6 Using Entropy

h = constant

T

v = constant p = const ant p = constan t

248

h

p = constant

T = constant

Critical point

T = constant

p va

x = 0.9

or

p=

con sta nt con sta nt

vap or d ate

x = 0.2

p=

ur Sat

Satu rate d

v = constant

Sat u

rate

d li quid

p = constant

x= x=

0.96

0.9 0

Critical point s

s

Fig. 6.2 Temperature–entropy diagram.

Fig. 6.3 Enthalpy–entropy diagram.

region of the diagram where the ideal gas model provides a reasonable approximation. For superheated vapor states outside the shaded area, both temperature and pressure are required to evaluate enthalpy, and the ideal gas model is not suitable.

Enthalpy–Entropy Diagram Mollier diagram

The essential features of an enthalpy–entropy diagram, commonly known as a Mollier diagram, are shown in Fig. 6.3. For detailed figures for water in SI and English units, see Fig. A-8. Note the location of the critical point and the appearance of lines of constant temperature and constant pressure. Lines of constant quality are shown in the two-phase liquid–vapor region (some figures give lines of constant percent moisture). The figure is intended for evaluating properties at superheated vapor states and for two-phase liquid–vapor mixtures. Liquid data are seldom shown. In the superheated vapor region, constant-temperature lines become nearly horizontal as pressure is reduced. These superheated vapor states are shown, approximately, as the shaded area on Fig. 6.3. This area corresponds to the shaded area on the temperature–entropy diagram of Fig. 6.2, where the ideal gas model provides a reasonable approximation. to illustrate the use of the Mollier diagram in SI units, consider two states of water. At state 1, T 1 5 240°C, p1 5 0.10 MPa. The specific enthalpy and quality are required at state 2, where p2 5 0.01 MPa and s2 5 s1. Turning to Fig. A-8, state 1 is located in the superheated vapor region. Dropping a vertical line into the two-phase liquid–vapor region, state 2 is located. The quality and specific enthalpy at state 2 read from the figure agree closely with values obtained using Table A-3 and A-4: x2 5 0.98 and h2 5 2537 kJ/kg. b b b b b

6.3 Introducing the T dS Equations Although the change in entropy between two states can be determined in principle by using Eq. 6.2a, such evaluations are generally conducted using the T dS equations developed in this section. The T dS equations allow entropy changes to be evaluated from other more readily determined property data. The use of the T dS equations

6.3 Introducing the T dS Equations to evaluate entropy changes for an incompressible substance is illustrated in Sec. 6.4 and for ideal gases in Sec. 6.5. The importance of the T dS equations is greater than their role in assigning entropy values, however. In Chap. 11 they are used as a point of departure for deriving many important property relations for pure, simple compressible systems, including means for constructing the property tables giving u, h, and s. The T dS equations are developed by considering a pure, simple compressible system undergoing an internally reversible process. In the absence of overall system motion and the effects of gravity, an energy balance in differential form is (𝛿Q)int 5 dU 1 (𝛿W)int

rev

rev

(6.6)

By definition of simple compressible system (Sec. 3.1.2), the work is (𝛿W)int 5 p dV rev

(6.7a)

On rearrangement of Eq. 6.2b, the heat transfer is (𝛿Q)int 5 T dS rev

(6.7b)

Substituting Eqs. 6.7 into Eq. 6.6, the first T dS equation results T ds 5 dU 1 p dV

(6.8)

first T dS equation

The second T dS equation is obtained from Eq. 6.8 using H 5 U 1 pV. Forming the differential dH 5 dU 1 d(pV) 5 dU 1 p dV 1 V dp On rearrangement dU 1 p dV 5 dH 2 V dp Substituting this into Eq. 6.8 gives the second T dS equation T dS 5 dH 2 V dp

(6.9)

The T dS equations can be written on a unit mass basis as T ds 5 du 1 p d𝜐 T ds 5 dh 2 𝜐 dp

(6.10a) (6.10b)

T d s 5 du 1 p d𝜐 T ds 5 dh 2 𝜐 dp

(6.11a) (6.11b)

or on a per mole basis as

Although the T dS equations are derived by considering an internally reversible process, an entropy change obtained by integrating these equations is the change for any process, reversible or irreversible, between two equilibrium states of a system. Because entropy is a property, the change in entropy between two states is independent of the details of the process linking the states. To show the use of the T dS equations, consider a change in phase from saturated liquid to saturated vapor at constant temperature and pressure. Since pressure is constant, Eq. 6.10b reduces to give ds 5

dh T

second T dS equation

249

250

Chapter 6 Using Entropy Then, because temperature is also constant during the phase change sg 2 s f 5

hg 2 hf

(6.12)

T

This relationship shows how sg 2 sf is calculated for tabulation in property tables. consider Refrigerant 134a at 08C. From Table A-10, hg 2 hf 5 197.21 kJ/kg, so with Eq. 6.12 s g 2 sf 5

197.21 kJ/kg kJ 5 0.7220 273.15 K kg ? K

which is the value calculated using sf and sg from the table.

b b b b b

6.4 Entropy Change of an Incompressible Substance In this section, Eq. 6.10a of Sec. 6.3 is used to evaluate the entropy change between two states of an incompressible substance. The incompressible substance model introduced in Sec. 3.10.2 assumes that the specific volume (density) is constant and the specific internal energy depends solely on temperature. Thus, du 5 c(T )dT, where c denotes the specific heat of the substance, and Eq. 6.10a reduces to give 0

c(T)dT c(T)dT pd𝜐 1 5 T T T

ds 5

On integration, the change in specific entropy is s2 2 s1 5

#

T2

T1

c(T) dT T

When the specific heat is constant, this becomes

s2 2 s1 5 c ln

T2 (incompressible, constant c) T1

(6.13)

Equation 6.13, along with Eqs. 3.20 giving Du and Dh, respectively, are applicable to liquids and solids modeled as incompressible. Specific heats of some common liquids and solids are given in Table A-19. consider a system consisting of liquid water initially at T1 5 300 K, p1 5 2 bar undergoing a process to a final state at T2 5 323 K, p2 5 1 bar. There are two ways to evaluate the change in specific entropy in this case. The first approach is to use Eq. 6.5 together with saturated liquid data from Table A-2. That is, s1 < sf (T1) 5 0.3954 KJ/kg ? K and s2 < sf (T2) 5 0.7038 KJ/kg ? K, giving s2 2 s1 5 0.308 KJ/kg ? K. The second approach is to use the incompressible model. That is, with Eq. 6.13 and c 5 4.18 KJ/kg ? K from Table A-19, we get

6.5 Entropy Change of an Ideal Gas

s2 2 s1 5 c ln

T2 T1

5 a4.18

kJ 323 K b ln a b 5 0.309 KJ/kg ? K kg ? K 300 K

Comparing the values obtained for the change in specific entropy using the two approaches considered here, we see they are in agreement. b b b b b

6.5 Entropy Change of an Ideal Gas In this section, the T dS equations of Sec. 6.3, Eqs. 6.10, are used to evaluate the entropy change between two states of an ideal gas. For a quick review of ideal gas model relations, see Table 6.1. It is convenient to begin with Eqs. 6.10 expressed as p du 1 d𝜐 (6.14) T T dh 𝜐 (6.15) ds 5 2 dp T T For an ideal gas, du 5 c𝜐(T) dT, dh 5 cp(T) dT, and p𝜐 5 RT. With these relations, Eqs. 6.14 and 6.15 become, respectively, ds 5

ds 5 c𝜐(T)

dp dT d𝜐 dT 1 R and ds 5 cp(T) 2R 𝜐 p T T

(6.16)

On integration, Eqs. 6.16 give, respectively,

s(T 2, 𝜐 2) 2 s(T1, 𝜐 1) 5

#

s(T2, p2) 2 s(T1, p1) 5

#

T2

T1

c𝜐(T)

𝜐2 dT 1 R ln 𝜐 T 1

(6.17)

cp(T )

p2 dT 2 R ln p1 T

(6.18)

T2

T1

Since R is a constant, the last terms of Eqs. 6.16 can be integrated directly. However, because c𝜐 and cp are functions of temperature for ideal gases, it is necessary to have information about the functional relationships before the integration of the first term in these equations can be performed. Since the two specific heats are related by cp(T) 5 c𝜐(T) 1 R

(3.44)

where R is the gas constant, knowledge of either specific function suffices.

6.5.1

Using Ideal Gas Tables

As for internal energy and enthalpy changes of ideal gases, the evaluation of entropy changes for ideal gases can be reduced to a convenient tabular approach. We begin by introducing a new variable s8(T) as s°(T) 5

T

cp(T)

T¿

T

#

where T9 is an arbitrary reference temperature.

dT

(6.19)

251

252

Chapter 6 Using Entropy TABLE 6.1

Ideal Gas Model Review Equations of state: p𝜐 5 RT pV 5 mRT

(3.32) (3.33)

Changes in u and h: T2

# c (T )dT

u(T2 ) 2 u(T1 ) 5

(3.40)

𝜐

T1

T2

h(T2 ) 2 h(T1 ) 5

# c (T )dT

(3.43)

p

T1

Constant Specific Heats

Variable Specific Heats

u(T2 ) 2 u(T1 ) 5 c𝜐(T2 2 T1 ) h(T2 ) 2 h(T1 ) 5 cp(T2 2 T1 )

u(T ) and h(T ) are evaluated from Table A-22 for air (mass basis) and Table A-23 for several other gases (molar basis).

(3.50) (3.51)

See Tables A-20, 21 for c𝜐 and cp data.

The integral of Eq. 6.18 can be expressed in terms of s° as follows:

#

T2

cp

T1

dT 5 T

#

T2

#

T

1 dT dT 2 cp T T T¿ T¿ 5 s°(T2) 2 s°(T1)

cp

Thus, Eq. 6.18 can be written as s(T2, p2) 2 s(T1, p1) 5 s°(T2) 2 s°(T1) 2 R ln

p2 p1

(6.20a)

p2 p1

(6.20b)

or on a per mole basis as s(T2, p2) 2 s(T1, p1) 5 s °(T2) 2 s ° (T1) 2 R ln

Because s8 depends only on temperature, it can be tabulated versus temperature, like h and u. For air as an ideal gas, s8 with units of kJ/kg ? K is given in Table A-22. Values of s° for several other common gases are given in Table A-23 with units of kJ/kmol ? K. In passing, we note the arbitrary reference temperature T9 of Eq. 6.19 is specified differently in Table A-22 than in Table A-23. As discussed in Sec. 13.5.1, Table A-23 give absolute entropy values. Using Eqs. 6.20 and the tabulated values for s8 or s°, as appropriate, entropy changes can be determined that account explicitly for the variation of specific heat with temperature. let us evaluate the change in specific entropy, in kJ/kg ? K, of air modeled as an ideal gas from a state where T1 5 300 K and p1 5 1 bar to a state where T2 5 1000 K and p2 5 3 bar. Using Eq. 6.20a and data from Table A-22 s2 2 s1 5 s°(T2) 2 s°(T1) 2 R ln 5 (2.96770 2 1.70203) 5 0.9504 kJ/kg ? K

p2 p1

kJ 8.314 kJ 3 bar 2 ln kg ? K 28.97 kg ? K 1 bar

b b b b b

6.5 Entropy Change of an Ideal Gas If a table giving s8 (or s°) is not available for a particular gas of interest, the integrals of Eqs. 6.17 and 6.18 can be performed analytically or numerically using specific heat data such as provided in Tables A-20 and A-21.

6.5.2

Assuming Constant Specific Heats

When the specific heats c𝜐 and cp are taken as constants, Eqs. 6.17 and 6.18 reduce, respectively, to s(T2, 𝜐 2) 2 s(T1, 𝜐1). 5 c𝜐 ln

T2 𝜐2 1 R ln 𝜐1 T1

(6.21)

s(T 2, p2) 2 s(T 1, p1) 5 cp ln

p2 T2 2 R ln p1 T1

(6.22)

These equations, along with Eqs. 3.50 and 3.51 giving Du and Dh, respectively, are applicable when assuming the ideal gas model with constant specific heats. let us determine the change in specific entropy, in KJ/kg ? K, of air as an ideal gas undergoing a process from T1 5 300 K, p1 5 1 bar to T2 5 400 K, p2 5 5 bar. Because of the relatively small temperature range, we assume a constant value of cp evaluated at 350 K. Using Eq. 6.22 and cp 5 1.008 KJ/kg ? K from Table A-20 p2 T2 2 R ln p1 T1 kJ 400 K 8.314 kJ 5 bar 5 a1.008 b lna b2a b lna b kg ? K 300 K 28.97 kg ? K 1 bar 5 20.1719 kJ/kg ? K b b b b b

Ds 5 cp ln

6.5.3

Computer Retrieval

For gases modeled as ideal gases, IT directly returns s(T, p) using the following special form of Eq. 6.18: s(T, p) 2 s(Tref , pref) 5

#

T

Tref

cp(T) T

dT 2 R ln

p pref

and the following choice of reference state and reference value: Tref 5 0 K, pref 5 1 atm, and s(Tref, pref) 5 0, giving s(T, p) 5

#

T

0

cp(T) T

dT 2 R ln

p pref

(a)

Such reference state and reference value choices equip IT for use in combustion applications. See the discussion of absolute entropy in Sec. 13.5.1. Changes in specific entropy evaluated using IT agree with entropy changes evaluated using ideal gas tables. consider a process of air as an ideal gas from T1 5 300 K, p1 5 1 bar to T2 5 1000 K, p2 5 3 bar. The change in specific entropy, denoted as dels, is determined in SI units using IT as follows: p1 5 1//bar T1 5 300//K p2 5 3 T2 5 1000 s1 5 s_TP(“Air”,T1,p1) s2 5 s_TP(“Air”,T2,p2) dels 5 s2 2 s1

253

254

Chapter 6 Using Entropy The software returns values of s1 5 1.706, s2 5 2.656, and dels 5 0.9501, all in units of kJ/kg ? K. This value for Ds agrees with the value obtained using Table A-22: 0.9504 kJ/kg ? K, as shown in the concluding example of Sec. 6.5.1. b b b b b Note again that IT returns specific entropy directly using Eq. (a) above. IT does not use the special function s8.

6.6 Entropy Change in Internally Reversible Processes of Closed Systems In this section the relationship between entropy change and heat transfer for internally reversible processes is considered. The concepts introduced have important applications in subsequent sections of the book. The present discussion is limited to the case of closed systems. Similar considerations for control volumes are presented in Sec. 6.13. As a closed system undergoes an internally reversible process, its entropy can increase, decrease, or remain constant. This can be brought out using dS 5 a

𝛿Q b int T rev

(6.2b)

which indicates that when a closed system undergoing an internally reversible process receives energy by heat transfer, the system experiences an increase in entropy. Conversely, when energy is removed from the system by heat transfer, the entropy of the system decreases. This can be interpreted to mean that an entropy transfer accompanies heat transfer. The direction of the entropy transfer is the same as that of the heat transfer. In an adiabatic internally reversible process, entropy remains constant. A constant-entropy process is called an isentropic process.

entropy transfer

isentropic process

6.6.1

Area Representation of Heat Transfer

On rearrangement, Eq. 6.2b gives Qint = rev

T (δQ) = T dS int

2 1

(𝛿Q)int 5 T dS

T dS 2

rev

Integrating from an initial state 1 to a final state 2

rev

2

1

Qint 5 rev

S

Fig. 6.4 Area representation of heat transfer for an internally reversible process of a closed system.

(6.23)

1

From Eq. 6.23 it can be concluded that an energy transfer by heat to a closed system during an internally reversible process can be represented as an area on a temperature– entropy diagram. Figure 6.4 illustrates the area representation of heat transfer for an arbitrary internally reversible process in which temperature varies. Carefully note that temperature must be in kelvins or degrees Rankine, and the area is the entire area under the curve (shown shaded). Also note that the area representation of heat transfer is not valid for irreversible processes, as will be demonstrated later.

6.6.2

Carnot cycle

# T dS

Carnot Cycle Application

To provide an example illustrating both the entropy change that accompanies heat transfer and the area representation of heat transfer, consider Fig. 6.5a, which shows a Carnot power cycle (Sec. 5.10.1). The cycle consists of four internally reversible processes in series: two isothermal processes alternated with two adiabatic processes. In Process 2–3, heat transfer to the system occurs while the temperature of the system remains constant

6.6 Entropy Change in Internally Reversible Processes of Closed Systems

TH

2

3

T

TH

4

3

1

2

255

T TC

1

4

b

a

TC

b

a

S

S

(a)

(b)

Fig. 6.5 Carnot cycles on the temperature–entropy diagram. (a) Power cycle. (b) Refrigeration or heat pump cycle. at TH. The system entropy increases due to the accompanying entropy transfer. For this process, Eq. 6.23 gives Q23 5 TH(S3 2 S2), so area 2–3–a–b–2 on Fig. 6.5a represents the heat transfer during the process. Process 3–4 is an adiabatic and internally reversible process and thus is an isentropic (constant-entropy) process. Process 4–1 is an isothermal process at TC during which heat is transferred from the system. Since entropy transfer accompanies the heat transfer, system entropy decreases. For this process, Eq. 6.23 gives Q41 5 TC(S1 2 S4), which is negative in value. Area 4–1–b–a–4 on Fig. 6.5a represents the magnitude of the heat transfer Q41. Process 1–2, which completes the cycle, is adiabatic and internally reversible (isentropic). The net work of any cycle is equal to the net heat transfer, so enclosed area 1–2–3–4–1 represents the net work of the cycle. The thermal efficiency of the cycle also can be expressed in terms of areas: Wcycle area 122232421 𝜂5 5 Q23 area 2232a2b22 The numerator of this expression is (TH 2 TC)(S3 2 S2) and the denominator is TH(S3 2 S2), so the thermal efficiency can be given in terms of temperatures only as 𝜂 5 1 2 TC /TH. This of course agrees with Eq. 5.9. If the cycle were executed as shown in Fig. 6.5b, the result would be a Carnot refrigeration or heat pump cycle. In such a cycle, heat is transferred to the system while its temperature remains at TC, so entropy increases during Process 1–2. In Process 3–4 heat is transferred from the system while the temperature remains constant at TH and entropy decreases.

6.6.3

Work and Heat Transfer in an Internally Reversible Process of Water

To further illustrate concepts introduced in this section, Example 6.1 considers water undergoing an internally reversible process while contained in a piston–cylinder assembly. cccc

EXAMPLE 6.1 c

Evaluating Work and Heat Transfer for an Internally Reversible Process of Water Water, initially a saturated liquid at 1508C (423.15 K), is contained in a piston–cylinder assembly. The water undergoes a process to the corresponding saturated vapor state, during which the piston moves freely in the cylinder. If the change of state is brought about by heating the water as it undergoes an internally reversible process at constant pressure and temperature, determine the work and heat transfer per unit of mass, each in kJ/kg.

256

Chapter 6 Using Entropy

SOLUTION Known: Water contained in a piston–cylinder assembly undergoes an internally reversible process at 1508C from

saturated liquid to saturated vapor. Find: Determine the work and heat transfer per unit mass. Schematic and Given Data:

p

T 2

1 Water

System boundary

W –– m

2

1

150°C

Q –– m

150°C

s

v

Fig. E6.1 Engineering Model: 1. The water in the piston–cylinder assembly is a closed system. 2. The process is internally reversible. 3. Temperature and pressure are constant during the process. 4. There is no change in kinetic or potential energy between the two end states. Analysis: At constant pressure the work is

W 5 m

2

# p d𝜐 5 p(𝜐 2 𝜐 ) 2

1

1

With values from Table A-2 at 1508C m3 105 N/m2 1 kJ W ` ` ` 3 5 (4.758 bar)(0.3928 2 1.0905 3 1023) a b ` m kg 1 bar 10 N ? m 5 186.38 kJ/kg Since the process is internally reversible and at constant temperature, Eq. 6.23 gives Q5

#

1

2

2

T dS 5 m

# T ds 1

or Q 5 T(s2 2 s1) m With values from Table A-2 ➊

Q 5 (423.15 K)(6.8379 2 1.8418) kJ/kg ? K 5 2114.1 kJ/kg m

As shown in the accompanying figure, the work and heat transfer can be represented as areas on p–𝜐 and T–s diagrams, respectively.

6.7 Entropy Balance for Closed Systems

257

➊ The heat transfer can be evaluated alternatively from an energy balance written on a unit mass basis as u2 2 u1 5

✓ Skills Developed

Q W 2 m m

Ability to… ❑ evaluate work and heat

Introducing W/m 5 p(𝜐2 2 𝜐1) and solving Q 5 (u2 2 u1) 1 p(𝜐2 2 𝜐1) m 5 (u2 1 p𝜐2) 2 (u1 1 p𝜐1) 5 h 2 2 h1 From Table A-2 at 1508C, h2 2 h1 5 2114.3 kJ/kg, which agrees with the value for Q/m obtained in the solution.

transfer for an internally reversible process, and represent them as areas on p–𝜐 and T–s diagrams, respectively. ❑ retrieve entropy data for water.

If the initial and final states were saturation states at 1008C (373.15 K), determine the work and heat transfer per unit of mass, each in kJ/kg. Ans. 170 kJ/kg, 2257 kJ/kg.

6.7 Entropy Balance for Closed Systems In this section, we begin our study of the entropy balance. The entropy balance is an expression of the second law that is particularly effective for thermodynamic analysis. The current presentation is limited to closed systems. The entropy balance is extended to control volumes in Sec. 6.9. Just as mass and energy are accounted for by mass and energy balances, respectively, entropy is accounted for by an entropy balance. In Eq. 5.2, the entropy balance is introduced in words as

entropy balance

net amount of change in the amount of entropy transferred in amount of entropy produced entropy contained within ≥ ¥ 5 E across the system U 1 C within the system S the system during some boundary during the during the time interval time interval time interval

In symbols, the closed system entropy balance takes the form 2

S2 2 S1 5

@Q

#aTb 1

entropy change

entropy transfer

1 𝜎 b

(6.24) entropy production

where subscript b signals that the integrand is evaluated at the system boundary. For the development of Eq. 6.24, see the box. It is sometimes convenient to use the entropy balance expressed in differential form 𝛿Q b 1 𝛿𝜎 (6.25) T b Note that the differentials of the nonproperties Q and 𝜎 are shown, respectively, as 𝛿Q and 𝛿𝜎. When there are no internal irreversibilities, 𝛿𝜎 vanishes and Eq. 6.25 reduces to Eq. 6.2b. In each of its alternative forms the entropy balance can be regarded as a statement of the second law of thermodynamics. For the analysis of engineering systems, the entropy balance is a more effective means for applying the second law than the Clausius and Kelvin–Planck statements given in Chap. 5. dS 5 a

closed system entropy balance

258

Chapter 6 Using Entropy

Developing the Closed System Entropy Balance The entropy balance for closed systems can be developed using the Clausius inequality expressed by Eq. 5.13 (Sec. 5.11) and the defining equation for entropy change, Eq. 6.2a, as follows: 2 R

Shown in Fig. 6.6 is a cycle executed by a closed system. The cycle consists of process I, during which internal irreversibilities are present, followed by internally reversible process R. For this cycle, Eq. 5.13 takes the form 2

𝛿Q

1

b

1

1

Fig. 6.6 Cycle used to develop the entropy balance.

𝛿Q

#aTb1#aTb

I

2

int rev

5 2𝜎

(a)

where the first integral is for process I and the second is for process R. The subscript b in the first integral serves as a reminder that the integrand is evaluated at the system boundary. The subscript is not required in the second integral because temperature is uniform throughout the system at each intermediate state of an internally reversible process. Since no irreversibilities are associated with process R, the term 𝜎cycle of Eq. 5.13, which accounts for the effect of irreversibilities during the cycle, refers only to process I and is shown in Eq. (a) simply as 𝜎. Applying the definition of entropy change, Eq. 6.2a, we can express the second integral of Eq. (a) as 1

𝛿Q

#a T b 2

5 S1 2 S2

(b)

1 (S1 2 S2) 5 2𝜎

(c)

int rev

With this, Eq. (a) becomes 2

𝛿Q

#aTb 1

b

On rearrangement, Eq. (c) gives Eq. 6.24, the closed system entropy balance.

6.7.1

entropy transfer accompanying heat transfer

entropy production

Interpreting the Closed System Entropy Balance

If the end states are fixed, the entropy change on the left side of Eq. 6.24 can be evaluated independently of the details of the process. However, the two terms on the right side depend explicitly on the nature of the process and cannot be determined solely from knowledge of the end states. The first term on the right side of Eq. 6.24 is associated with heat transfer to or from the system during the process. This term can be interpreted as the entropy transfer accompanying heat transfer. The direction of entropy transfer is the same as the direction of the heat transfer, and the same sign convention applies as for heat transfer: A positive value means that entropy is transferred into the system, and a negative value means that entropy is transferred out. When there is no heat transfer, there is no entropy transfer. The entropy change of a system is not accounted for solely by the entropy transfer, but is due in part to the second term on the right side of Eq. 6.24 denoted by 𝜎. The term 𝜎 is positive when internal irreversibilities are present during the process and vanishes when no internal irreversibilities are present. This can be described by saying that entropy is produced (or generated) within the system by the action of irreversibilities. The second law of thermodynamics can be interpreted as requiring that entropy is produced by irreversibilities and conserved only in the limit as irreversibilities are reduced to zero. Since 𝜎 measures the effect of irreversibilities present within the system during a process, its value depends on the nature of the process and not solely on the end states. Entropy production is not a property.

6.7 Entropy Balance for Closed Systems

259

When applying the entropy balance to a closed system, it is essential to remember the requirements imposed by the second law on entropy production: The second law requires that entropy production be positive, or zero, in value: 𝜎: e

.0 50

irreversibilities present within the system no irreversibilities present within the system

(6.26)

The value of the entropy production cannot be negative. In contrast, the change in entropy of the system may be positive, negative, or zero: .0 S 2 2 S1 : c 5 0 ,0

(6.27)

Like other properties, entropy change for a process between two specified states can be determined without knowledge of the details of the process.

6.7.2

Entropy_Bal_Closed _Sys A.24 – All Tabs

A

Evaluating Entropy Production and Transfer

The objective in many applications of the entropy balance is to evaluate the entropy production term. However, the value of the entropy production for a given process of a system often does not have much significance by itself. The significance is normally determined through comparison. For example, the entropy production within a given component might be compared to the entropy production values of the other components included in an overall system formed by these components. By comparing entropy production values, the components where appreciable irreversibilities occur can be identified and rank ordered. This allows attention to be focused on the components that contribute most to inefficient operation of the overall system. To evaluate the entropy transfer term of the entropy balance requires information regarding both the heat transfer and the temperature on the boundary where the heat transfer occurs. The entropy transfer term is not always subject to direct evaluation, however, because the required information is either unknown or not defined, such as when the system passes through states sufficiently far from equilibrium. In such applications, it may be convenient, therefore, to enlarge the system to include enough of the immediate surroundings that the temperature on the boundary of the enlarged system corresponds to the temperature of the surroundings away from the immediate vicinity of the system, Tf. The entropy transfer term is then simply Q/Tf. However, as the irreversibilities present would not be just for the system of interest but for the enlarged system, the entropy production term would account for the effects of internal irreversibilities within the original system and external irreversibilities present within that portion of the surroundings included within the enlarged system.

Boundary of enlarged system T > Tf

Temperature variation

Tf

TAKE NOTE...

6.7.3

Applications of the Closed System Entropy Balance

The following examples illustrate the use of the energy and entropy balances for the analysis of closed systems. Property relations and property diagrams also contribute significantly in developing solutions. Example 6.2 reconsiders the system and end states of Example 6.1 to demonstrate that entropy is produced when internal irreversibilities are present and that the amount of entropy production is not a property. In Example 6.3, the entropy balance is used to determine the minimum theoretical compression work.

On property diagrams, solid lines are used for internally reversible processes. A dashed line signals only that a process has occurred between initial and final equilibrium states and does not define a path for the process.

260 cccc

Chapter 6 Using Entropy

EXAMPLE 6.2 c

Determining Work and Entropy Production for an Irreversible Process of Water Water initially a saturated liquid at 1508C is contained within a piston–cylinder assembly. The water undergoes a process to the corresponding saturated vapor state, during which the piston moves freely in the cylinder. There is no heat transfer with the surroundings. If the change of state is brought about by the action of a paddle wheel, determine the net work per unit mass, in kJ/kg, and the amount of entropy produced per unit mass, in kJ/kg ? K. SOLUTION Known: Water contained in a piston–cylinder assembly undergoes an adiabatic process from saturated liquid to saturated vapor at 1508C. During the process, the piston moves freely, and the water is rapidly stirred by a paddle wheel. Find: Determine the net work per unit mass and the entropy produced per unit mass. Schematic and Given Data: Engineering Model: p

T 1

➊

1. The water in the piston–

2 150°C Area is not work

1

2 Water

150°C v

Area is not heat

System boundary

cylinder assembly is a closed system. 2. There is no heat transfer with

the surroundings. 3. The system is at an equilibrium

s

state initially and finally. There is no change in kinetic or potential energy between these two states.

Fig. E6.2 Analysis: As the volume of the system increases during the process, there is an energy transfer by work from the system during the expansion, as well as an energy transfer by work to the system via the paddle wheel. The net work can be evaluated from an energy balance, which reduces with assumptions 2 and 3 to

DU 1 DKE 0 1 DPE 0 5 Q 0 2 W On a unit mass basis, the energy balance is then W 5 2(u2 2 u1) m With specific internal energy values from Table A-2 at 1508C, u1 5 631.68 kJ/kg, u2 5 2559.5 kJ/kg, we get W kJ 5 21927.82 m kg The minus sign indicates that the work input by stirring is greater in magnitude than the work done by the water as it expands. The amount of entropy produced is evaluated by applying the entropy balance Eq. 6.24. Since there is no heat transfer, the term accounting for entropy transfer vanishes DS 5

#

1

2

a

𝛿Q 0 b 1𝜎 T b

On a unit mass basis, this becomes on rearrangement 𝜎 5 s2 2 s1 m

6.7 Entropy Balance for Closed Systems

261

With specific entropy values from Table A-2 at 1508C, s1 5 1.8418 kJ/kg ? K, s2 5 6.8379 kJ/kg ? K, we get ➋

𝜎 kJ 5 4.9961 m kg ? K

➊ Although each end state is an equilibrium state at the same pressure and temperature, the pressure and temperature are not necessarily uniform throughout the system at intervening states, nor are they necessarily constant in value during the process. Accordingly, there is no well-defined “path” for the process. This is emphasized by the use of dashed lines to represent the process on these p–𝜐 and T–s diagrams. The dashed lines indicate only that a process has taken place, and no “area” should be associated with them. In particular, note that the process is adiabatic, so the “area” below the dashed line on the T–s diagram can have no significance as heat transfer. Similarly, the work cannot be associated with an area on the p–𝜐 diagram. ➋ The change of state is the same in the present example as in Example 6.1. However, in Example 6.1 the change of state is brought about by heat transfer while the system undergoes an internally reversible process. Accordingly, the value of entropy production for the process of Example 6.1 is zero. Here, fluid friction is present during the process and the entropy production is positive in value. Accordingly, different values of entropy production are obtained for two processes between the same end states. This demonstrates that entropy production is not a property.

✓ Skills Developed Ability to… ❑ apply the closed system

energy and entropy balances.

❑ retrieve property data for

water.

If the initial and final states were saturation states at 1008C, determine the net work, in kJ/kg, and the amount of entropy produced, in kJ/kg ? K. Ans. 22087.56 kJ/kg, 6.048 kJ/kg ? K. As an illustration of second law reasoning, minimum theoretical compression work is evaluated in Example 6.3 using the fact that the entropy production term of the entropy balance cannot be negative. cccc

EXAMPLE 6.3 c

Evaluating Minimum Theoretical Compression Work Refrigerant 134a is compressed adiabatically in a piston–cylinder assembly from saturated vapor at 08C to a final pressure of 0.7 MPa. Determine the minimum theoretical work input required per unit mass of refrigerant, in kJ/kg. SOLUTION Known: Refrigerant 134a is compressed without heat transfer from a specified initial state to a specified final pressure. Find: Determine the minimum theoretical work input required per unit of mass. Schematic and Given Data:

T

2 2s

Internally reversible compression

Accessible states Internal energy decreases

Engineering Model: 1. The Refrigerant 134a is a closed

system. 2. There is no heat transfer with the

Actual compression

1

Insulation

3. The initial and final states are R-134a

s

surroundings. equilibrium states. There is no change in kinetic or potential energy between these states. Fig. E6.3

262

Chapter 6 Using Entropy

Analysis: An expression for the work can be obtained from an energy balance. By applying assumptions 2 and 3,

we get DU 1 DKE 0 1 DPE 0 5 Q 0 2 W When written on a unit mass basis, the work input is then a2

W b 5 u2 2 u 1 m

The specific internal energy u1 can be obtained from Table A-10E as u1 5 227.06 kJ/kg. Since u1 is known, the value for the work input depends on the specific internal energy u2. The minimum work input corresponds to the smallest allowed value for u2 , determined using the second law as follows. Applying the entropy balance, Eq. 6.24, we get DS 5

#

1

2

a

𝛿Q 0 b 1𝜎 T b

where the entropy transfer term is set equal to zero because the process is adiabatic. Thus, the allowed final states must satisfy s2 2 s 1 5

𝜎 $0 m

The restriction indicated by the foregoing equation can be interpreted using the accompanying T–s diagram. Since 𝜎 cannot be negative, states with s2 , s1 are not accessible adiabatically. When irreversibilities are present during the compression, entropy is produced, so s2 . s1. The state labeled 2s on the diagram would be attained in the limit as irreversibilities are reduced to zero. This state corresponds to an isentropic compression. By inspection of Table A-12, we see that when pressure is fixed, the specific internal energy decreases as specific entropy decreases. Thus, the smallest allowed value for u2 corresponds to state 2s. Interpolating in Table A-12 at 0.7 MPa, with s2s 5 s1 5 0.9190 kJ/kg ? K, we find that u2s 5 244.32 kJ/kg, which corresponds to a temperature at state 2s of about 38.38C. Finally, the minimum work input is ➊

a2

W b 5 u2s 2 u1 5 244.32 2 227.06 5 17.26 kJ/kg m min

➊ The effect of irreversibilities exacts a penalty on the work input required: A greater work input is needed for the actual adiabatic compression process than for an internally reversible adiabatic process between the same initial state and the same final pressure. See the Quick Quiz to follow.

✓ Skills Developed Ability to… ❑ apply the closed system

energy and entropy balances. retr ieve property data for ❑ Refrigerant 134a.

If the refrigerant were compressed adiabatically to a final state where p2 5 0.7 MPa, T2 5 498C, determine the work input, in kJ/kg, and the amount of entropy produced, in kJ/kg ? K. Ans. 39.9 kJ/kg, 36.43 kJ/kg ? K.

6.7.4

Closed System Entropy Rate Balance

If the temperature Tb is constant, Eq. 6.24 reads Q S2 2 S 1 5 1𝜎 Tb where Q/Tb represents the amount of entropy transferred through the portion of the # boundary at temperature Tb. Similarly, the quantity Q/Tj represents the time rate of

6.7 Entropy Balance for Closed Systems

263

entropy transfer through the portion of the boundary whose instantaneous temperature is Tj. This quantity appears in the closed system entropy rate balance considered next. On a time rate basis, the closed system entropy rate balance is # Qj dS # 5 a 1𝜎 dt j Tj

(6.28)

closed system entropy rate balance

# where dS/dt is the time rate of change of entropy of the system. The term Qj /Tj represents the time rate of entropy transfer through the portion of the boundary whose # instantaneous temperature is Tj. The term 𝜎 accounts for the time rate of entropy production due to irreversibilities within the system. To pinpoint the relative significance of the internal and external irreversibilities, Example 6.4 illustrates the application of the entropy rate balance to a system and to an enlarged system consisting of the system and a portion of its immediate surroundings.

cccc

EXAMPLE 6.4 c

Pinpointing Irreversibilities # Referring to Example 2.4, evaluate the rate of entropy production 𝜎, in kW/K, for (a) the gearbox as the system and (b) an enlarged system consisting of the gearbox and enough of its surroundings that heat transfer occurs at the temperature of the surroundings away from the immediate vicinity of the gearbox, Tf 5 293 K (208C). SOLUTION Known: A gearbox operates at steady state with known values for the power input through the high-speed shaft, power output through the low-speed shaft, and heat transfer rate. The temperature on the outer surface of the gearbox and the temperature of the surroundings away from the gearbox are also known. # Find: Evaluate the entropy production rate 𝜎 for each of the two specified systems shown in the schematic. Schematic and Given Data: At this boundary the temperature is Tf = 293 K

System boundary

Temperature variation Tb

Q = –1.2 kW 60 kW

60 kW

Tf

58.8 kW 58.8 kW

Tb = 300 K Gearbox (a)

(b)

Fig. E6.4

Engineering Model: 1. In part (a), the gearbox is taken as a closed system operating at steady state, as shown on the

accompanying sketch labeled with data from Example 2.4. 2. In part (b), the gearbox and a portion of its surroundings are taken as a closed system, as shown on the

accompanying sketch labeled with data from Example 2.4. 3. The temperature of the outer surface of the gearbox and the temperature of the surroundings do not vary.

264

Chapter 6 Using Entropy

Analysis: (a) To obtain an expression for the entropy production rate, begin with the entropy balance for a closed system on a time rate basis: Eq. 6.28. Since heat transfer takes place only at temperature Tb, the entropy rate balance reduces at steady state to # Q dS 0 # 5 1𝜎 dt Tb

Solving

# Q # 𝜎52 Tb # Introducing the known values for the heat transfer rate Q and the surface temperature Tb (21.2 kW) # 𝜎52 5 4 3 1023 kW/K (300 K) (b) Since heat transfer takes place at temperature Tf for the enlarged system, the entropy rate balance reduces

at steady state to

# Q dS 0 # 5 1𝜎 dt Tf

Solving

# Q # 𝜎52 Tf # Introducing the known values for the heat transfer rate Q and the temperature Tf (21.2 kW) # 𝜎52 5 4.1 3 1023 kW/K (293 K)

➊

➊ The value of the entropy production rate calculated in part (a) gauges the significance of irreversibilities associated with friction and heat transfer within the gearbox. In part (b), an additional source of irreversibility is included in the enlarged system, namely the irreversibility associated with the heat transfer from the outer surface of the gearbox at Tb to the surroundings at Tf. In this case, the irreversibilities within the gearbox are dominant, accounting for about 98% of the total rate of entropy production.

✓ Skills Developed Ability to… ❑ apply the closed system

entropy rate balance.

❑ develop an engineering model.

If the power delivered were 59.32 kW, evaluate the outer surface temperature, in K, and the rate of entropy production, in kW/K, for the gearbox as the system, keeping input power, h, and A from Example 2.4 the same. Ans. 297 K, 2.3 3 1023 kW/K.

6.8 Directionality of Processes Our study of the second law of thermodynamics began in Sec. 5.1 with a discussion of the directionality of processes. In this section we consider two related aspects for which there are significant applications: the increase in entropy principle and a statistical interpretation of entropy.

6.8.1

Increase of Entropy Principle

In the present discussion, we use the closed system energy and entropy balances to introduce the increase of entropy principle. Discussion centers on an enlarged system

6.8 Directionality of Processes

265

consisting of a system and that portion of the surroundings affected by the system as it undergoes a process. Since all energy and mass transfers taking place are included within the boundary of the enlarged system, the enlarged system is an isolated system. An energy balance for the isolated system reduces to DE]isol 5 0

(6.29a)

because no energy transfers take place across its boundary. Thus, the energy of the isolated system remains constant. Since energy is an extensive property, its value for the isolated system is the sum of its values for the system and surroundings, respectively, so Eq. 6.29a can be written as DE]system 1 DE]surr 5 0

(6.29b)

In either of these forms, the conservation of energy principle places a constraint on the processes that can occur. For a process to take place, it is necessary for the energy of the system plus the surroundings to remain constant. However, not all processes for which this constraint is satisfied can actually occur. Processes also must satisfy the second law, as discussed next. An entropy balance for the isolated system reduces to 2 𝛿Q 0 DS]isol 5 a b 1 𝜎isol T b 1

#

or DS]isol 5 𝜎isol

(6.30a)

where 𝜎isol is the total amount of entropy produced within the system and its surroundings. Since entropy is produced in all actual processes, the only processes that can occur are those for which the entropy of the isolated system increases. This is known as the increase of entropy principle. The increase of entropy principle is sometimes considered an alternative statement of the second law. Since entropy is an extensive property, its value for the isolated system is the sum of its values for the system and surroundings, respectively, so Eq. 6.30a can be written as DS]system 1 DS]surr 5 𝜎isol

increase of entropy principle

(6.30b)

Notice that this equation does not require the entropy change to be positive for both the system and surroundings but only that the sum of the changes is positive. In either of these forms, the increase of entropy principle dictates the direction in which any process can proceed: Processes occur only in such a direction that the total entropy of the system plus surroundings increases. We observed previously the tendency of systems left to themselves to undergo processes until a condition of equilibrium is attained (Sec. 5.1). The increase of entropy principle suggests that the entropy of an isolated system increases as the state of equilibrium is approached, with the equilibrium state being attained when the entropy reaches a maximum. This interpretation is considered again in Sec. 14.1, which deals with equilibrium criteria. Example 6.5 illustrates the increase of entropy principle.

cccc

EXAMPLE 6.5 c

Quenching a Hot Metal Bar A 0.3 kg metal bar initially at 1200 K is removed from an oven and quenched by immersing it in a closed tank containing 9 kg of water initially at 300 K. Each substance can be modeled as incompressible. An appropriate constant specific heat value for the water is cw 5 4.2 kJ/kg ? K, and an appropriate value for the metal is cm 5 0.42 kJ/kg ? K. Heat transfer from the tank contents can be neglected. Determine (a) the final equilibrium temperature of the metal bar and the water, in K, and (b) the amount of entropy produced, in kJ/K.

266

Chapter 6 Using Entropy

SOLUTION Known: A hot metal bar is quenched by immersing it in a tank containing water. Find: Determine the final equilibrium temperature of the metal bar and the water, and the amount of entropy

produced. Schematic and Given Data: System boundary

Engineering Model: 1. The metal bar and the water within the tank form a system, as

shown on the accompanying sketch. 2. There is no energy transfer by heat or work: The system is

isolated. 3. There is no change in kinetic or potential energy. 4. The water and metal bar are each modeled as incompressible Metal bar: Tmi = 1200 K cm = 0.42 kJ/kg.K mm = 0.3 kg

Water: Twi = 300 K cw = 4.2 kJ/kg.K mw = 9 kg

with known specific heats.

Fig. E6.5 Analysis: (a) The final equilibrium temperature can be evaluated from an energy balance for the isolated system 0

0

0

DKE 1 DPE 1 DU 5 Q 2 W

0

where the indicated terms vanish by assumptions 2 and 3. Since internal energy is an extensive property, its value for the isolated system is the sum of the values for the water and metal, respectively. Thus, the energy balance becomes DU]water 1 DU]metal 5 0 Using Eq. 3.20a to evaluate the internal energy changes of the water and metal in terms of the constant specific heats mwcw(Tf 2 Twi) 1 mmcm(Tf 2 Tmi) 5 0 where Tf is the final equilibrium temperature, and Twi and Tmi are the initial temperatures of the water and metal, respectively. Solving for Tf and inserting values Tf 5

m w(cw ycm)Twi 1 mmT mi m w(cw ycm) 1 m m

(9 kg)(10)(300 K) 1 (0.3 kg)(1200 K) (9 kg)(10) 1 (0.3 kg) 5 303 K

5

(b) The amount of entropy production can be evaluated from an entropy balance. Since no heat transfer occurs

between the isolated system and its surroundings, there is no accompanying entropy transfer, and an entropy balance for the isolated system reduces to DS 5

#

1

2

0

𝛿Q a b 1𝜎 T b

Entropy is an extensive property, so its value for the isolated system is the sum of its values for the water and the metal, respectively, and the entropy balance becomes DS]water 1 DS]metal 5 𝜎 Evaluating the entropy changes using Eq. 6.13 for incompressible substances, the foregoing equation can be written as 𝜎 5 m w cw ln

Tf Tf 1 m m c m ln T wi T mi

6.8 Directionality of Processes

267

Inserting values ➊ ➋

kJ 303 kJ 303 b ln 1 (0.3 kg) a0.42 b ln kg ? K 300 kg ? K 1200 kJ kJ 5 a0.3761 b 1 a20.1734 b 5 0.2027 kJ/K K K

𝜎 5 (9 kg) a4.2

➊ The metal bar experiences a decrease in entropy. The entropy of the water increases. In accord with the increase of entropy principle, the entropy of the isolated system increases. ➋ The value of 𝜎 is sensitive to roundoff in the value of Tf.

✓ Skills Developed Ability to… ❑ apply the closed system

energy and entropy balances.

❑ apply the incompressible

substance model.

If the mass of the metal bar were 0.2 kg, determine the final equilibrium temperature, in K, and the amount of entropy produced, in kJ/K, keeping all other given data the same. Ans. 296 K, 0.1049 kJ/K.

6.8.2

Statistical Interpretation of Entropy

Building on the increase of entropy principle, in this section we Insulation Partition introduce an interpretation of entropy from a microscopic perspective based on probability. In statistical thermodynamics, entropy is associated with the notion Initially of microscopic disorder. From previous considerations we know that Evacuated in a spontaneous process of an isolated system, the system moves toward equilibrium and the entropy increases. From the microscopic viewpoint, this is equivalent to saying that as an isolated system moves (a) toward equilibrium our knowledge of the condition of individual particles making up the system decreases, which corresponds to an increase in microscopic disorder and a related increase in entropy. We use an elementary thought experiment to bring out some basic ideas needed to understand this view of entropy. Actual microscopic analysis of systems is more complicated than the discussion given here, but the essential concepts are the same. Consider N molecules initially contained in one-half of the box (b) shown in Fig. 6.7a. The entire box is considered an isolated system. We assume that the ideal gas model applies. In the initial condition, Fig. 6.7 N molecules a box. the gas appears to be at equilibrium in terms of temperature, pressure, and other properties. But, on the microscopic level the molecules are moving about randomly. We do know for sure, though, that initially all molecules are on the right side of the vessel. Suppose we remove the partition and wait until equilibrium is reached again, as in Fig. 6.7b. Because the system is isolated, the internal energy U does not change: U2 5 U1. Further, because the internal energy of an ideal gas depends on temperature alone, the temperature is unchanged: T2 5 T1. Still, at the final state a given molecule has twice the volume in which to move: V2 5 2V1. Just like a coin toss, the probability that the molecule is on one side or the other is now ½, which is the same as the volume ratio V1/V2. In the final condition, we have less knowledge about where each molecule is than we did originally. We can evaluate the change in entropy for the process of Fig. 6.7 by applying Eq. 6.17, expressed in terms of volumes and on a molar basis. The entropy change for

U1, T1, V1, S1

U2 = U1 T2 = T1 V2 = 2V1 S2 > S1

268

Chapter 6 Using Entropy

“Breaking” the Second Law Has Implications for Nanotechnology Some 135 years ago, renowned nineteenthcentury physicist J. C. Maxwell wrote, “. . . the second law is . . . a statistical . . . truth, for it depends on the fact that the bodies we deal with consist of millions of molecules . . . [Still] the second law is continually being violated . . . in any sufficiently small group of molecules belonging to a real body.” Although Maxwell’s view was bolstered by theorists over the years, experimental confirmation proved elusive. Then, in 2002, experimenters reported they had demonstrated violations of the second law: at the micron scale over time intervals of up to 2 seconds, entropy was consumed, not produced [see Phys. Rev. Lett. 89, 050601 (2002)].

While few were surprised that experimental confirmation had at last been achieved, some were surprised by implications of the research for the twenty-first-century field of nanotechnology: The experimental results suggest inherent limitations on nanomachines. These tiny devices—only a few molecules in size—may not behave simply as miniaturized versions of their larger counterparts; and the smaller the device, the more likely its motion and operation could be disrupted unpredictably. Occasionally and uncontrollably, nanomachines may not perform as designed, perhaps even capriciously running backward. Still, designers of these machines will applaud the experimental results if they lead to deeper understanding of behavior at the nanoscale.

the constant-temperature process is (S2 2 S1)/n 5 R ln(V2 /V1)

microstates thermodynamic probability

(6.31)

where n is the amount of substance on a molar basis (Eq. 1.8). Next, we consider how the entropy change would be evaluated from a microscopic point of view. Through more complete molecular modeling and statistical analysis, the total number of positions and velocities—microstates—available to a single molecule can be calculated. This total is called the thermodynamic probability, w. For a system of N molecules, the thermodynamic probability is wN. In statistical thermodynamics, entropy is considered to be proportional to ln(w)N. That is, S ~ N ln(w). This gives the Boltzmann relation

Boltzmann relation

S/N 5 k ln w

(6.32)

where the proportionality factor, k, is called Boltzmann’s constant. Applying Eq. 6.32 to the process of Fig. 6.7, we get (S2 2 S1)/N 5 k ln(w2) 2 k ln(w1) 5 k ln(w2 /w1)

disorder

(6.33)

Comparing Eqs. 6.31 and 6.33, the expressions for entropy change coincide when k 5 nR/N and w2/w1 5 V2/V1. The first of these expressions allows Boltzmann’s constant to be evaluated, giving k 5 1.3806 3 10223 J/K. Also, since V2 . V1 and w2 . w1, Eqs. 6.31 and 6.33 each predict an increase of entropy owing to entropy production during the irreversible adiabatic expansion in this example. From Eq. 6.33, we see that any process that increases the number of possible microstates of a system increases its entropy and conversely. Hence, for an isolated system, processes occur only in such a direction that the number of microstates available to the system increases, resulting in our having less knowledge about the condition of individual particles. Because of this concept of decreased knowledge, entropy reflects the microscopic disorder of the system. We can then say that the only processes an isolated system can undergo are those that increase the disorder of the system. This interpretation is consistent with the idea of directionality of processes discussed previously. The notion of entropy as a measure of disorder is sometimes used in fields other than thermodynamics. The concept is employed in information theory, statistics, biology, and even in some economic and social modeling. In these applications, the term entropy is used as a measure of disorder without the physical aspects of the thought experiment used here necessarily being implied.

6.9 Entropy Rate Balance for Control Volumes

269

BIOCONNECTIONS Do living things violate the second law of thermodynamics because they seem to create order from disorder? Living things are not isolated systems as considered in the previous discussion of entropy and disorder. Living things interact with their surroundings and are influenced by their surroundings. For instance, plants grow into highly ordered cellular structures synthesized from atoms and molecules originating in the earth and its atmosphere. Through interactions with their surroundings, plants exist in highly organized states and are able to produce within themselves even more organized, lower entropy states. In keeping with the second law, states of lower entropy can be realized within a system as long as the total entropy of the system and its surroundings increases. The self-organizing tendency of living things is widely observed and fully in accord with the second law.

6.9 Entropy Rate Balance for Control Volumes Thus far the discussion of the entropy balance concept has been restricted to the case of closed systems. In the present section the entropy balance is extended to control volumes. Like mass and energy, entropy is an extensive property, so it too can be transferred into or out of a control volume by streams of matter. Since this is the principal difference between the closed system and control volume forms, the control volume entropy rate balance can be obtained by modifying Eq. 6.28 to account for these entropy transfers. The result is # Qj dScv # # # 5 a 1 a misi 2 a me se 1 𝜎cv (6.34) dt j Tj i e rate of rates of rate of entropy entropy entropy change transfer production where dScv/dt# represents # the time rate of change of entropy within the control volume. The terms mi si and me se account, respectively, for rates of entropy transfer accom# panying mass flow into and out of the control volume. The term Qj represents the time rate of heat transfer at the # location on the boundary where the instantaneous temperature is Tj. The ratio Qj /Tj accounts for the accompanying rate of entropy # transfer. The term 𝜎cv denotes the time rate of entropy production due to irreversibilities within the control volume.

Integral Form of the Entropy Rate Balance As for the cases of the control volume mass and energy rate balances, the entropy rate balance can be expressed in terms of local properties to obtain forms that are more generally applicable. Thus, the term Scv(t), representing the total entropy associated with the control volume at time t, can be written as a volume integral Scv(t) 5

# 𝜌s dV V

where 𝜌 and s denote, respectively, the local density and specific entropy. The rate of entropy transfer accompanying heat transfer can be expressed more generally as an integral over the surface of the control volume time rate of entropy £ transfer accompanying § 5 heat transfer

# q a b dA A T b

#

control volume entropy rate balance

entropy transfer accompanying mass flow Entropy_Rate_Bal_CV A.25 – Tabs a & b

A

270

Chapter 6 Using Entropy # where q is the heat flux, the time rate of heat transfer per unit of surface area, through the location on the boundary where the instantaneous temperature is T. The subscript “b” is added as a reminder that the integrand is evaluated on the boundary of the control volume. In addition, the terms accounting for entropy transfer accompanying mass flow can be expressed as integrals over the inlet and exit flow areas, resulting in the following form of the entropy rate balance # q d # (6.35) 𝜌s dV 5 a b dA 1 a a s𝜌Vn dAb 2 a a s𝜌Vn dAb 1 𝜎cv dt V A T b A A i i e e

#

#

#

#

where Vn denotes the normal component in the direction of flow of the velocity relative to the flow area. In some cases, it is also convenient to express the entropy production rate as a volume integral of the local volumetric rate of entropy production within the control volume. The study of Eq. 6.35 brings out the assumptions underlying Eq. 6.34. Finally, note that for a closed system the sums accounting for entropy transfer at inlets and exits drop out, and Eq. 6.35 reduces to give a more general form of Eq. 6.28.

6.10

Rate Balances for Control Volumes at Steady State

Since a great many engineering analyses involve control volumes at steady state, it is instructive to list steady-state forms of the balances developed for mass, energy, and entropy. At steady state, the conservation of mass principle takes the form # # (4.6) a mi 5 a me i

e

The energy rate balance at steady state is # # V 2i Ve2 # # 0 5 Qcv 2 Wcv 1 a mi ahi 1 1 gzib 2 a me ahe 1 1 gzeb 2 2 i e

(4.18)

Finally, the steady-state form of the entropy rate balance is obtained by reducing Eq. 6.34 to give steady-state entropy rate balance

# Qj

# # # 05 a 1 a mi si 2 a me se 1 𝜎cv j Tj i e

(6.36)

These equations often must be solved simultaneously, together with appropriate property relations. Mass and energy are conserved quantities, but entropy is not conserved. Equation 4.6 indicates that at steady state the total rate of mass flow into the control volume equals the total rate of mass flow out of the control volume. Similarly, Eq. 4.18 indicates that the total rate of energy transfer into the control volume equals the total rate of energy transfer out of the control volume. However, Eq. 6.36 requires that the rate at which entropy is transferred out must exceed the rate at which entropy enters, the difference being the rate of entropy production within the control volume owing to irreversibilities.

6.10.1 One-Inlet, One-Exit Control Volumes at Steady State Since many applications involve one-inlet, one-exit control volumes at steady state, let us also list the form of the entropy rate balance for this important case. Thus, Eq. 6.36 reduces to read

6.10 Rate Balances for Control Volumes at Steady State # Qj

05a j Tj

# # 1 m(s1 2 s2) 1 𝜎cv

# Or, on dividing by the mass flow rate m and rearranging # # Qj 𝜎cv 1 s2 2 s 1 5 # a a b 1 # m j Tj m

(6.37)

271

Entropy_Rate_Bal_CV A.25 – Tab c

A

Turbine A.19 – Tab d

A

(6.38)

The two terms on the right side of Eq. 6.38 denote, respectively, the rate of entropy transfer accompanying heat transfer and the rate of entropy production within the control volume, each per unit of mass flowing through the control volume. From Eq. 6.38 it can be concluded that the entropy of a unit of mass passing from inlet to exit can increase, decrease, or remain the same. Furthermore, because the value of the second term on the right can never be negative, a decrease in the specific entropy from inlet to exit can be realized only when more entropy is transferred out of the control volume accompanying heat transfer than is produced by irreversibilities within the control volume. When the value of this entropy transfer term is positive, the specific entropy at the exit is greater than the specific entropy at the inlet whether internal irreversibilities are present or not. In the special case where there is no entropy transfer accompanying heat transfer, Eq. 6.38 reduces to # 𝜎cv (6.39) s2 2 s 1 5 # m Accordingly, when irreversibilities are present within the control volume, the entropy of a unit of mass increases as it passes from inlet to exit. In the limiting case in which no irreversibilities are present, the unit mass passes through the control volume with no change in its entropy—that is, isentropically.

6.10.2

Applications of the Rate Balances to Control Volumes at Steady State

The following examples illustrate the use of the mass, energy, and entropy balances for the analysis of control volumes at steady state. Carefully note that property relations and property diagrams also play important roles in arriving at solutions. In Example 6.6, we evaluate the rate of entropy production within a turbine operating at steady state when there is heat transfer from the turbine.

cccc

EXAMPLE 6.6 c

Determining Entropy Production in a Steam Turbine Steam enters a turbine with a pressure of 30 bar, a temperature of 4008C, and a velocity of 160 m/s. Saturated vapor at 1008C exits with a velocity of 100 m/s. At steady state, the turbine develops work equal to 540 kJ per kg of steam flowing through the turbine. Heat transfer between the turbine and its surroundings occurs at an average outer surface temperature of 350 K. Determine the rate at which entropy is produced within the turbine per kg of steam flowing, in kJ/kg ? K. Neglect the change in potential energy between inlet and exit. SOLUTION Known: Steam expands through a turbine at steady state for which data are provided. Find: Determine the rate of entropy production per kg of steam flowing.

272

Chapter 6 Using Entropy

Schematic and Given Data:

p1 = 30 bar T1 = 400°C V1 = 160 m/s 1

Engineering Model:

30 bar

T

1

400°C

1. The control volume shown

on the accompanying sketch is at steady state.

Surroundings at 293 K Wcv ––– = 540 kJ/kg m

2. Heat transfer from the 100°C

Tb = 350 K

2

turbine to the surroundings occurs at a specified average outer surface temperature.

2

T2 = 100°C Saturated vapor V2 = 100 m/s

s

3. The change in potential

energy between inlet and exit can be neglected.

Fig. E6.6

Analysis: To determine the entropy production per unit mass flowing through the turbine, begin with mass and entropy rate balances for the one-inlet, one-exit control volume at steady state: # # m1 5 m2 # Qj # # # 05 a 1 m 1s1 2 m2 s2 1 𝜎cv T j j

Since # heat transfer occurs only at Tb 5 350 K, the first term on the right side of the entropy rate balance reduces to Qcv /Tb. Combining the mass and entropy rate balances # Qcv # # 05 1 m(s1 2 s2) 1 𝜎cv Tb # # # where m is the mass flow rate. Solving for 𝜎cv /m # # # Qcv /m 𝜎cv 1 (s1 2 s2) # 52 Tb m # # The heat transfer rate, Qcv/m, required by this expression is evaluated next. Reduction of the mass and energy rate balances results in # # Qcv Wcv V22 2 V 21 b # 5 # 1 (h2 2 h1) 1 a 2 m m where the potential energy change from inlet to exit is dropped by assumption 3. From Table A-4 at 30 bar, 4008C, h1 5 3230.9 kJ/kg, and from Table A-2, h2 5 hg(1008C) 5 2676.1 kJ/kg. Thus # (100)2 2 (160)2 m2 Qcv kJ kJ 1N 1 kJ 5 540 ` 1 (2676.1 2 3230.9)a b 1 c da 2 b ` ` ` 3 # 2 kg kg 2 s 1 kg ? m/s 10 N?m m 5 540 2 554.8 2 7.8 5 222.6 kJ/kg Ski lls Dev elo ped From Table A-2, s 5 7.3549 kJ/kg ? K, and from Table A-4, s 5 6.9212 kJ/ 2

1

kg ? K. Inserting values into the expression for entropy production # (222.6 kJ/kg) 𝜎cv kJ 1 (7.3549 2 6.9212)a b # 52 350 K kg ?K m 5 0.0646 1 0.4337 5 0.498 kJ/kg ? K

If the boundary were located to include the turbine and a portion of the immediate surroundings so heat transfer occurs at the temperature of the surroundings, 293 K, determine the rate at which entropy is produced within the enlarged control volume, in kJ/K per kg of steam flowing, keeping all other given data the same. Ans. 0.511 kJ/kg ? K.

✓

Ability to… ❑ apply the control volume

mass, energy, and entropy rate balances. ❑ retrieve property data for water.

6.10 Rate Balances for Control Volumes at Steady State In Example 6.7, the mass, energy, and entropy rate balances are used to evaluate a performance claim for a device producing hot and cold streams of air from a single stream of air at an intermediate temperature.

cccc

Heat_Exchanger A.22 – Tab d

273

A

EXAMPLE 6.7 c

Evaluating a Performance Claim An inventor claims to have developed a device requiring no energy transfer by work or heat transfer, yet able to produce hot and cold streams of air from a single stream of air at an intermediate temperature. The inventor provides steady-state test data indicating that when air enters at a temperature of 21°C and a pressure of 5 bar, separate streams of air exit at temperatures of 218°C and 79°C, respectively, and each at a pressure of 1 bar. Sixty percent of the mass entering the device exits at the lower temperature. Evaluate the inventor’s claim, employing the ideal gas model for air and ignoring changes in the kinetic and potential energies of the streams from inlet to exit. SOLUTION Known: Data are provided for a device that at steady state produces hot and cold streams of air from a single stream of air at an intermediate temperature without energy transfers by work or heat. Find: Evaluate whether the device can operate as claimed. Schematic and Given Data: 1 2

Engineering Model:

T1 = 21°C p1 = 5 bar

1. The control volume shown on the accompanying sketch is at steady state. # # 2. For the control volume, Wcv 5 0 and Qcv 5 0.

Inlet

T2 = 79°C p2 = 1 bar

3. Changes in the kinetic and potential energies from inlet to exit can be ignored.

Hot outlet 3 Cold outlet

➊ 4. The air is modeled as an ideal gas with constant cp 5 1.0 kJ/kg ? K.

T3 = –18°C p3 = 1 bar

Fig. E6.7 Analysis: For the device to operate as claimed, the conservation of mass and energy principles must be satisfied. The second law of thermodynamics also must be satisfied; and in particular the rate of entropy production cannot be negative. Accordingly, the mass, energy and entropy rate balances are considered in turn. With assumptions 1–3, the mass and energy rate balances reduce, respectively, to # # # m1 5 m2 1 m3 # # # 0 5 m 1 h 1 2 m 2 h2 2 m 3 h3 # # # # Since m3 5 0.6m 1, it follows from the mass rate balance that m2 5 0.4m 1. By combining the mass and energy rate balances and evaluating changes in specific enthalpy using constant cp, the energy rate balance is also satisfied. That is # # # # 0 5 (m2 1 m3) h1 2 m2 h2 2 m 3 h3 # # 5 m2(h1 2 h2) 1 m3(h1 2 h3) # # 5 0.4m1[cp(T1 2 T2)] 1 0.6m1[cp(T1 2 T3)] ➋ 5 0.4(T 1 2 T 2 ) 1 0.6(T 1 2 T 3) 5 0.4(2105) 1 0.6(70) 50

274

Chapter 6 Using Entropy

Accordingly, with the given data the conservation of mass and energy principles are satisfied. Since no significant heat transfer occurs, the entropy rate balance at steady state reads

05a j

# 0 Qj Tj

# # # # 1 m1s1 2 m2 s2 2 m3 s3 1 𝜎cv

Combining the mass and entropy rate balances # # # # # 0 5 (m2 1 m3)s1 2 m2 s2 2 m3 s3 1 𝜎cv # # # 5 m2(s1 2 s2) 1 m3(s1 2 s3) 1 𝜎cv # # # 5 0.4m1(s1 2 s2) 1 0.6m1(s1 2 s3) 1 𝜎cv # # Solving for 𝜎cv /m1 and using Eq. 6.22 to evaluate changes in specific entropy # p2 p3 T3 𝜎cv T2 # 5 0.4 c cp ln 2 R ln d 1 0.6 c cp ln 2 R ln d p1 p1 m1 T1 T1 kJ 352 8.314 kJ 1 5 0.4 c a1.0 b ln 2a b ln d kg ? K 294 28.97 kg ? K 5.0 kJ 255 8.314 kJ 1 + 0.6 c a1.0 b ln 2a b ln d kg ? K 294 28.97 kg ? K 5.0 5 0.454 kJ/kg ? K

➌

➍

Thus, the second law of thermodynamics is also satisfied. ➎ On the basis of this evaluation, the inventor’s claim does not violate principles of thermodynamics. ➊ Since the specific heat cp of air varies little over the temperature interval from 0 to 79°C, cp can be taken as constant. From Table A-20, cp 5 1.0 kJ/kg ? K. ➋ Since temperature differences are involved in this calculation, the temperatures can be either in K or 8C.

✓ Skills Developed

➌ In this calculation involving temperature ratios, the temperatures are in K. Temperatures in 8C should not be used.

Ability to… ❑ apply the control volume

➍ If the value of the rate of entropy production had been negative or zero, the claim would be rejected. A negative value is impossible by the second law and a zero value would indicate operation without irreversibilities.

mass, energy, and entropy rate balances. ❑ apply the ideal gas model with constant cp.

➎ Such devices do exist. They are known as vortex tubes and are used in industry for spot cooling.

If the inventor would claim that the hot and cold streams exit the device at 5 bar, evaluate the revised claim, keeping all other given data the same. Ans. Claim invalid.

A

Compressor A.20 – Tab d Throttling_Dev A.23 – Tab d

In Example 6.8, we evaluate and compare the rates of entropy production for three components of a heat pump system. Heat pumps are considered in detail in Chap. 10.

6.10 Rate Balances for Control Volumes at Steady State cccc

275

EXAMPLE 6.8 c

Determining Entropy Production in Heat Pump Components Components of a heat pump for supplying heated air to a dwelling are shown in the schematic below. At steady state, Refrigerant 22 enters the compressor at 258C, 3.5 bar and is compressed adiabatically to 758C, 14 bar. From the compressor, the refrigerant passes through the condenser, where it condenses to liquid at 288C, 14 bar. The refrigerant then expands through a throttling valve to 3.5 bar. The states of the refrigerant are shown on the accompanying T–s diagram. Return air from the dwelling enters the condenser at 208C, 1 bar with a volumetric flow rate of 0.42 m3/s and exits at 508C with a negligible change in pressure. Using the ideal gas model for the air and neglecting kinetic and potential energy effects, (a) determine the rates of entropy production, in kW/K, for control volumes enclosing the condenser, compressor, and expansion valve, respectively. (b) Discuss the sources of irreversibility in the components considered in part (a). SOLUTION Known: Refrigerant 22 is compressed adiabatically, condensed by heat transfer to air passing through a heat exchanger, and then expanded through a throttling valve. Steady-state operating data are known. Find: Determine the entropy production rates for control volumes enclosing the condenser, compressor, and

expansion valve, respectively, and discuss the sources of irreversibility in these components. Schematic and Given Data: Supply air T = 50°C 6 p 6 = 1 bar 6

Indoor return air T5 = 20°C 5 p5 = 1 bar (AV)5 = 0.42 m3/s 3 p3 = 14 bar T3 = 28°C Expansion valve

T Condenser

2

p2 = 14 bar T2 = 75°C

2

75°C

14 bar 3

Compressor

p4 = 3.5 bar 4

1

28°C

T1 = –5°C p1 = 3.5 bar

3.5 bar

–5°C

1

4

s

Outdoor air Evaporator

Fig. E6.8 Engineering Model: 1. Each component is analyzed as a control volume at steady state. 2. The compressor operates adiabatically, and the expansion across the valve is a throttling process.

#

#

3. For the control volume enclosing the condenser, Wcv 5 0 and Qcv 5 0. 4. Kinetic and potential energy effects can be neglected.

➊ 5. The air is modeled as an ideal gas with constant cp 5 1.005 kJ/kg ? K. Analysis: (a) Let us begin by obtaining property data at each of the principal refrigerant states located on the accompanying schematic and T–s diagram. At the inlet to the compressor, the refrigerant is a superheated vapor at 258C, 3.5 bar, so from Table A-9, s1 5 0.9572 kJ/kg ? K. Similarly, at state 2, the refrigerant is a superheated vapor at 758C, 14 bar, so interpolating in Table A-9 gives s2 5 0.98225 kJ/kg ? K and h2 5 294.17 kJ/kg.

276

Chapter 6 Using Entropy

State 3 is compressed liquid at 288C, 14 bar. From Table A-7, s3 < sf (288C) 5 0.2936 kJ/kg ? K and h3 < hf(288C) 5 79.05 kJ/kg. The expansion through the valve is a throttling process, so h3 5 h4. Using data from Table A-8, the quality at state 4 is x4 5

(h4 2 h f4) (79.05 2 33.09) 5 5 0.216 (hfg)4 (212.91)

and the specific entropy is s4 5 sf4 1 x4(sg4 2 sf4) 5 0.1328 1 0.216(0.9431 2 0.1328) 5 0.3078 kJ/kg ? K Condenser

Consider the control volume enclosing the condenser. With assumptions 1 and 3, the entropy rate balance reduces to # # # 0 5 mref(s2 2 s3) 1 mair (s5 2 s6) 1 𝜎cond # # # To evaluate 𝜎cond requires the two mass flow rates, mair and mref , and the change in specific entropy for the air. These are obtained next. Evaluating the mass flow rate of air using the ideal gas model and the known volumetric flow rate (AV)5 p5 # mair 5 5 (AV)5 𝜐5 RT5 (1 bar) 105 N/m2 m3 1 kJ ` 5 0.5 kg/s ` 5 a0.42 b `` 3 s 1 bar 8.314 kJ 10 N ? m a b(293 K) 28.97 kg ? K The refrigerant mass flow rate is determined using an energy balance for the control volume enclosing the condenser together with assumptions 1, 3, and 4 to obtain # mair (h6 2 h5) # mref 5 (h2 2 h3) With assumption 5, h6 2 h5 5 cp(T6 2 T5). Inserting values

➋

# m ref 5

a0.5

kg kJ b(323 2 293)K b a1.005 s kg ? K (294.17 2 79.05) kJ/kg

5 0.07 kg/s

Using Eq. 6.22, the change in specific entropy of the air is p6 T6 2 R ln p5 T5 0 kJ 323 1.0 5 a1.005 b ln a b 2 R ln a b 5 0.098 kJ/kg ? K kg ? K 293 1.0 # Finally, solving the entropy balance for 𝜎cond and inserting values # # # 𝜎cond 5 m ref (s3 2 s2) 1 mair (s6 2 s5) s6 2 s5 5 cp ln

kg kJ 1 kW b (0.2936 2 0.98225) 1 (0.5)(0.098) d ` ` s kg ? K 1 kJ/s kW 5 7.95 3 1024 K 5 c a0.07

Compressor

For the control volume enclosing the compressor, the entropy rate balance reduces with assumptions 1 and 3 to # # 0 5 mref (s1 2 s2) 1 𝜎comp

6.11 Isentropic Processes or

277

# # 𝜎comp 5 mref (s2 2 s1) kg kJ 1 kW b` ` 5 a0.07 b(0.98225 2 0.9572) a s kg ? K 1 kJ/s 5 17.5 3 1024 kW/K

Valve

Finally, for the control volume enclosing the throttling valve, the entropy rate balance reduces to # # 0 5 mref (s3 2 s4) 1 𝜎valve # Solving for 𝜎valve and inserting values kg # 1 kW kJ # 𝜎valve 5 mref (s4 2 s3) 5 a0.07 b (0.3078 2 0.2936) a b` ` s kg ? K 1 kJ/s 5 9.94 3 1024 kW/K (b) The following table summarizes, in rank order, the calculated entropy production rates:

➌

Component

# 𝜎cv (kW/K)

compressor valve condenser

17.5 3 1024 9.94 3 1024 7.95 3 1024

Entropy production in the compressor is due to fluid friction, mechanical friction of the moving parts, and internal heat transfer. For the valve, the irreversibility is primarily due to fluid friction accompanying the expansion across the valve. The principal source of irreversibility in the condenser is the temperature difference between the air and refrigerant streams. In this example, there are no pressure drops for either stream passing through the condenser, but slight pressure drops due to fluid friction would normally contribute to the irreversibility of condensers. The evaporator shown in Fig. E6.8 has not been analyzed. ➊ Due to the relatively small temperature change of the air, the specific heat cp can be taken as constant at the average of the inlet and exit air temperatures. # ➋ Temperatures in K are used to evaluate mref , but since a temperature difference is involved the same result would be obtained if temperatures in °C were used. Temperatures in K, and not °C, are required when a temperature ratio is involved, as in Eq. 6.22 used to evaluate s6 2 s5. ➌ By focusing attention on reducing irreversibilities at the sites with the highest entropy production rates, thermodynamic improvements may be possible. However, costs and other constraints must be considered, and can be overriding.

If the compressor operated adiabatically and without internal irreversibilities, determine the temperature of the refrigerant at the compressor exit, in °C, keeping the compressor inlet state and exit pressure the same. Ans. 65°C.

6.11

Isentropic Processes

The term isentropic means constant entropy. Isentropic processes are encountered in many subsequent discussions. The object of the present section is to show how properties are related at any two states of a process in which there is no change in specific entropy.

✓ Skills Developed Ability to… ❑ apply the control volume

mass, energy, and entropy rate balances. ❑ develop an engineering model. ❑ retrieve property data for Refrigerant 22. ❑ apply the ideal gas model with constant cp.

278

Chapter 6 Using Entropy T

h

p1

1

1

T1

p1 T1 p2

2

2

p2 T2

T3 p3

3

p3

T2

3

T3 s

s

Fig. 6.8 T–s and h–s diagrams showing states having the same value of specific entropy.

6.11.1

General Considerations

The properties at states having the same specific entropy can be related using the graphical and tabular property data discussed in Sec. 6.2. For example, as illustrated by Fig. 6.8, temperature–entropy and enthalpy–entropy diagrams are particularly convenient for determining properties at states having the same value of specific entropy. All states on a vertical line passing through a given state have the same entropy. If state 1 on Fig. 6.8 is fixed by pressure p1 and temperature T1, states 2 and 3 are readily located once one additional property, such as pressure or temperature, is specified. The values of several other properties at states 2 and 3 can then be read directly from the figures. Tabular data also can be used to relate two states having the same specific entropy. For the case shown in Fig. 6.8, the specific entropy at state 1 could be determined from the superheated vapor table. Then, with s2 5 s1 and one other property value, such as p2 or T2, state 2 could be located in the superheated vapor table. The values of the properties 𝜐, u, and h at state 2 can then be read from the table. (An illustration of this procedure is given in Sec. 6.2.1.) Note that state 3 falls in the two-phase liquid–vapor regions of Fig. 6.8. Since s3 5 s1, the quality at state 3 could be determined using Eq. 6.4. With the quality known, other properties such as 𝜐, u, and h could then be evaluated. Computer retrieval of entropy data provides an alternative to tabular data.

6.11.2 T 2

1

v2 p2

Using the Ideal Gas Model

Figure 6.9 shows two states of an ideal gas having the same value of specific entropy. Let us consider relations among pressure, specific volume, and temperature at these states, first using the ideal gas tables and then assuming specific heats are constant.

T2

v1 p1

Ideal Gas Tables

T1

For two states having the same specific entropy, Eq. 6.20a reduces to

s

Fig. 6.9 Two states of an ideal gas where s2 = s1.

0 5 s°(T2) 2 s°(T1) 2 R ln

p2 p1

(6.40a)

Equation 6.40a involves four property values: p1, T1, p2, and T2. If any three are known, the fourth can be determined. If, for example, the temperature at state 1 and

6.11 Isentropic Processes

279

the pressure ratio p2/p1 are known, the temperature at state 2 can be determined from s°(T2) 5 s°(T1) 1 R ln

p2 p1

(6.40b)

Since T1 is known, s8(T1) would be obtained from the appropriate table, the value of s8(T2) would be calculated, and temperature T2 would then be determined by interpolation. If p1, T1, and T2 are specified and the pressure at state 2 is the unknown, Eq. 6.40a would be solved to obtain p2 5 p1 exp c

s°(T2) 2 s°(T1) d R

(6.40c)

Equations 6.40 can be used when s8 (or s° ) data are known, as for the gases of Tables A-22 and A-23.

AIR. For the special case of air modeled as an ideal gas, Eq. 6.40c provides the basis for an alternative tabular approach for relating the temperatures and pressures at two states having the same specific entropy. To introduce this, rewrite the equation as exp[s°(T2)/R] p2 5 p1 exp[s°(T1)/R] The quantity exp[s8(T)/R] appearing in this expression is solely a function of temperature, and is given the symbol pr(T). A tabulation of pr versus temperature for air is provided in Table A-22.1 In terms of the function pr, the last equation becomes p2 pr2 5 (s1 5 s2, air only) p1 pr1

(6.41)

where pr1 5 pr(T1) and pr2 5 pr(T2). The function pr is sometimes called the relative pressure. Observe that pr is not truly a pressure, so the name relative pressure has no physical significance. Also, be careful not to confuse pr with the reduced pressure of the compressibility diagram. A relation between specific volumes and temperatures for two states of air having the same specific entropy can also be developed. With the ideal gas equation of state, 𝜐 5 RT/p, the ratio of the specific volumes is p1 𝜐2 RT2 5a ba b 𝜐1 p2 RT1 Then, since the two states have the same specific entropy, Eq. 6.41 can be introduced to give pr(T1) 𝜐2 RT2 5 c d dc 𝜐1 pr(T2) RT1 The ratio RT/pr(T) appearing on the right side of the last equation is solely a function of temperature, and is given the symbol 𝜐r(T). Values of 𝜐r for air are tabulated

1

The values of pr determined with this definition are inconveniently large, so they are divided by a scale factor before tabulating to give a convenient range of numbers.

TAKE NOTE...

When applying the software IT to relate two states of an ideal gas having the same value of specific entropy, IT returns specific entropy directly and does not employ the special functions s°, pr , and vr .

280

Chapter 6 Using Entropy versus temperature in Table A-22. In terms of the function 𝜐r, the last equation becomes 𝜐2 𝜐r2 5 (s1 5 s2, air only) 𝜐1 𝜐r1

(6.42)

where 𝜐r1 5 𝜐r(T1) and 𝜐r2 5 𝜐r(T2). The function 𝜐r is sometimes called the relative volume. Despite the name given to it, 𝜐r(T) is not truly a volume. Also, be careful not to confuse it with the pseudoreduced specific volume of the compressibility diagram.

Assuming Constant Specific Heats Let us consider next how properties are related for isentropic processes of an ideal gas when the specific heats are constants. For any such case, Eqs. 6.21 and 6.22 reduce to the equations p2 T2 2 R ln p1 T1 T2 𝜐2 0 5 c𝜐 ln 1 R ln 𝜐1 T1

0 5 cp ln

Introducing the ideal gas relations cp 5

kR R , c𝜐 5 k21 k21

(3.47)

where k is the specific heat ratio and R is the gas constant, these equations can be solved, respectively, to give p2 (k21) /k T2 5a b (s1 5 s2, constant k) p1 T1

(6.43)

T2 𝜐1 k21 5 a b (s1 5 s2, constant k) 𝜐2 T1

(6.44)

The following relation can be obtained by eliminating the temperature ratio from Eqs. 6.43 and 6.44: p2 𝜐1 k 5 a b (s1 5 s2, constant k) 𝜐2 p1

(6.45)

Previously, we have identified an internally reversible process described by p𝜐n 5 constant, where n is a constant, as a polytropic process. From the form of Eq. 6.45, it can be concluded that the polytropic process p𝜐k 5 constant of an ideal gas with constant specific heat ratio k is an isentropic process. We observed in Sec. 3.15 that a polytropic process of an ideal gas for which n 5 1 is an isothermal (constanttemperature) process. For any fluid, n 5 0 corresponds to an isobaric (constant-pressure) process and n 5 6` corresponds to an isometric (constant-volume) process. Polytropic processes corresponding to these values of n are shown in Fig. 6.10 on p–𝜐 and T–s diagrams.

6.11.3

Illustrations: Isentropic Processes of Air

Means for evaluating data for isentropic processes of air modeled as an ideal gas are illustrated in the next two examples. In Example 6.9, we consider three alternative methods.

6.11 Isentropic Processes p

T

n=k

n=±∞ n = –1 n=0 t n a ns t co

=

co ns

tan

t

n = –1

281

T

=

s=

co n

ns co

t tan

n=±∞

sta

v

n=0

p=

n=1

nt n=1 n=k v

s

Fig. 6.10 Polytropic processes on p–𝝊 and T–s diagrams.

cccc

EXAMPLE 6.9 c

Isentropic Process of Air Air undergoes an isentropic process from p1 5 1 bar, T1 5 300 K to a final state where the temperature is T2 5 650 K. Employing the ideal gas model, determine the final pressure p2, in bar. Solve using (a) pr data from Table A-22, (b) Interactive Thermodynamics: IT, and (c) a constant specific heat ratio k evaluated at the mean temperature, 475 K, from Table A-20. SOLUTION Known: Air undergoes an isentropic process from a state where pressure and temperature are known to a state where the temperature is specified. Find: Determine the final pressure using (a) pr data, (b) IT, and (c) a constant value for the specific heat ratio k. Schematic and Given Data: Engineering Model:

T 2

1

p2 = ?

1. A quantity of air as the system undergoes an isentropic process.

T2 = 650 K

2. The air can be modeled as an ideal gas. 3. In part (c), the specific heat ratio is constant.

p1 = 1 bar T1 = 300 K

s

Fig. E6.9

Analysis: (a) The pressures and temperatures at two states of an ideal gas having the same specific entropy are related by Eq. 6.41

p2 pr2 5 p1 pr1 Solving p2 5 p 1

pr2 pr1

282

Chapter 6 Using Entropy

With p r values from Table A-22 p 5 (1 bar)

21.18 5 15.77 bar 1.3860

(b) The IT solution follows:

T1 5 300 // K p1 5 1 // bar T2 5 650 // K ➊ s_TP(“Air”, T1,p1) 5 s_TP(“Air”,T2,p2) // Result: p2 5 15.77 bar (c) When the specific heat ratio k is assumed constant, the temperatures and pressures at two states of an ideal

gas having the same specific entropy are related by Eq. 6.43. Thus p2 5 p 1 a

T2 ky(k21) b T1

From Table A-20 at the mean temperature, 202°C (475 K), k 5 1.39. Inserting values into the above expression ➋

p2 5 (1 bar) a

650 1.39y0.39 b 5 15.81 bar 300

➊ IT returns a value for p2 even though it is an implicit variable in the specific entropy function. Also note that IT returns values for specific entropy directly and does not employ special functions such as s°, pr , and 𝜐r. ➋ The close agreement between the answer obtained in part (c) and that of parts (a), (b) can be attributed to the use of an appropriate value for the specific heat ratio k.

✓ Skills Developed Ability to… ❑ analyze an isentropic process

using Table A-22 data,

❑ Interactive Thermodynamics,

and ❑ a constant specific heat

ratio k.

Determine the final pressure, in bar, using a constant specific heat ratio k evaluated at T1 5 300 K. Expressed as a percent, how much does this pressure value differ from that of part (c)? Ans. 14.72 bar, 25%

Another illustration of an isentropic process of an ideal gas is provided in Example 6.10 dealing with air leaking from a tank.

cccc

EXAMPLE 6.10 c

Considering Air Leaking from a Tank A rigid, well-insulated tank is filled initially with 5 kg of air at a pressure of 5 bar and a temperature of 500 K. A leak develops, and air slowly escapes until the pressure of the air remaining in the tank is 1 bar. Employing the ideal gas model, determine the amount of mass remaining in the tank and its temperature. SOLUTION Known: A leak develops in a rigid, insulated tank initially containing air at a known state. Air slowly escapes until the pressure in the tank is reduced to a specified value. Find: Determine the amount of mass remaining in the tank and its temperature.

6.11 Isentropic Processes

283

Schematic and Given Data: Engineering Model: 1. As shown on the accompanying sketch, the closed system is the

mass initially in the tank that remains in the tank.

System boundary Slow leak Mass initially in the tank that remains in the tank

2. There is no significant heat transfer between the system and its

surroundings. 3. Irreversibilities within the tank can be ignored as the air slowly

Mass initially in the tank that escapes

escapes. 4. The air is modeled as an ideal gas.

Initial condition of tank

Fig. E6.10 Analysis: With the ideal gas equation of state, the mass initially in the tank that remains in the tank at the end

of the process is m2 5

p2 V (R/M)T2

where p2 and T2 are the final pressure and temperature, respectively. Similarly, the initial amount of mass within the tank, m1, is p1 V m1 5 (R/M)T1 where p1 and T1 are the initial pressure and temperature, respectively. Eliminating volume between these two expressions, the mass of the system is m2 5 a

p2 T1 b a bm p1 T2 1

Except for the final temperature of the air remaining in the tank, T2, all required values are known. The remainder of the solution mainly concerns the evaluation of T2. For the closed system under consideration, there are no significant irreversibilities (assumption 3), and no heat transfer occurs (assumption 2). Accordingly, the entropy balance reduces to DS 5

#

2

1

a

𝛿Q 0 b 1 𝜎0 5 0 T b

Since the system mass remains constant, DS 5 m2 Ds, so Ds 5 0 That is, the initial and final states of the system have the same value of specific entropy. Using Eq. 6.41 pr2 5 a

p2 bp p1 r1

✓ Skills Developed

where p1 5 5 bar and p2 5 1 bar. With pr1 5 8.411 from Table A-22 at 500 K, the previous equation gives pr2 5 1.6822. Using this to interpolate in Table A-22, T2 5 317 K. Finally, inserting values into the expression for system mass

Ability to… ❑ develop an engineering model. ❑ apply the closed system

1 bar 500 K m2 5 a ba b (5 kg) 5 1.58 kg 5 bar 317 K

❑ analyze an isentropic process.

Evaluate the tank volume, in m3. Ans. 1.43 m3.

entropy balance.

284

Chapter 6 Using Entropy

6.12

Isentropic Efficiencies of Turbines, Nozzles, Compressors, and Pumps

Engineers make frequent use of efficiencies and many different efficiency definitions are employed. In the present section, isentropic efficiencies for turbines, nozzles, compressors, and pumps are introduced. Isentropic efficiencies involve a comparison between the actual performance of a device and the performance that would be achieved under idealized circumstances for the same inlet state and the same exit pressure. These efficiencies are frequently used in subsequent sections of the book.

6.12.1

Isentropic Turbine Efficiency

To introduce the isentropic turbine efficiency, refer to Fig. 6.11, which shows a turbine expansion on a Mollier diagram. The state of the matter entering the turbine and the exit pressure are fixed. Heat transfer between the turbine and its surroundings is ignored, as are kinetic and potential energy effects. With these assumptions, the mass and energy rate balances reduce, at steady state, to give the work developed per unit of mass flowing through the turbine # Wcv # 5 h1 2 h 2 m Since state 1 is fixed, the specific enthalpy h1 is known. Accordingly, the value of the work depends on the specific enthalpy h2 only, and increases as h2 is reduced. The maximum value for the turbine work corresponds to the smallest allowed value for the specific enthalpy at the turbine exit. This can be determined using the second law as follows. Since there is no heat transfer, the allowed exit states are constrained by Eq. 6.39 # 𝜎cv # 5 s2 2 s1 $ 0 m # # Because the entropy production 𝜎cv /m cannot be negative, states with s2 , s1 are not accessible in an adiabatic expansion. The only states that actually can be attained adiabatically are those with s2 . s1. The state labeled “2s”on Fig. 6.11 would be attained only in the limit of no internal irreversibilities. This corresponds p1 h T1

1

h1 – h2s

Actual expansion

h1 – h2

2

Isentropic expansion

2s Accessible states p2

Fig. 6.11 Comparison of actual and isentropic expansions through a turbine.

s

6.12 Isentropic Efficiencies of Turbines, Nozzles, Compressors, and Pumps to an isentropic expansion through the turbine. For fixed exit pressure, the specific enthalpy h2 decreases as the specific entropy s2 decreases. Therefore, the smallest allowed value for h2 corresponds to state 2s, and the maximum value for the turbine work is # Wcv a # b 5 h1 2 h2s m s In an actual expansion through the turbine h2 . h2s, and thus less work than the maximum would be developed. This difference can be gauged by the isentropic turbine efficiency defined by # # Wcv /m h1 2 h2 # 𝜂t 5 # 5 h (Wcv /m)s 1 2 h2s

TAKE NOTE...

The subscript s denotes a quantity evaluated for an isentropic process from a specified inlet state to a specified exit pressure.

isentropic turbine efficiency

(6.46)

Both the numerator and denominator of this expression are evaluated for the same inlet state and the same exit pressure. The value of 𝜂t is typically 0.7 to 0.9 (70–90%). The two examples to follow illustrate the isentropic turbine efficiency concept. In Example 6.11 the isentropic efficiency of a steam turbine is known and the objective is to determine the turbine work.

cccc

285

Turbine A.19 – Tab e

A

EXAMPLE 6.11 c

Determining Turbine Work Using the Isentropic Efficiency A steam turbine operates at steady state with inlet conditions of p1 5 5 bar, T1 5 3208C. Steam leaves the turbine at a pressure of 1 bar. There is no significant heat transfer between the turbine and its surroundings, and kinetic and potential energy changes between inlet and exit are negligible. If the isentropic turbine efficiency is 75%, determine the work developed per unit mass of steam flowing through the turbine, in kJ/kg. SOLUTION Known: Steam expands through a turbine operating at steady state from a specified inlet state to a specified exit pressure. The turbine efficiency is known. Find: Determine the work developed per unit mass of steam flowing through the turbine. Schematic and Given Data: p1 = 5 bar

Engineering Model:

h

1. A control volume enclosing the turbine is at steady state. T1

1

h1 – h2s

2. The expansion is adiabatic and changes in kinetic and poten-

tial energy between the inlet and exit can be neglected.

Actual expansion Isentropic expansion

h1 – h2

2 2s Accessible states

p2 = 1 bar s

Fig. E6.11

286

Chapter 6 Using Entropy

Analysis: The work developed can be determined using the isentropic turbine efficiency, Eq. 6.46, which on rearrangement gives # # Wcv Wcv # 5 𝜂t a # b 5 𝜂t(h1 2 h2s) m m s

From Table A-4, h1 5 3105.6 kJ/kg and s1 5 7.5308 kJ/kg ? K. The exit state for an isentropic expansion is ➊ fixed by p2 5 1 and s2s 5 s1. Interpolating with specific entropy in Table A-4 at 1 bar gives h2s 5 2743.0 kJ/kg. Substituting values # Wcv ➋ # 5 0.75(3105.6 2 2743.0) 5 271.95 kJ/kg m

✓ Skills Developed

➊ At 2s, the temperature is about 1338C. ➋ The effect of irreversibilities is to exact a penalty on the work output of the turbine. The work is only 75% of what it would be for an isentropic expansion between the given inlet state and the turbine exhaust pressure. This is clearly illustrated in terms of enthalpy differences on the accompanying h–s diagram.

Ability to… ❑ apply the isentropic turbine

efficiency, Eq. 6.46.

❑ retrieve steam table data.

Determine the temperature of the steam at the turbine exit, in 8C. Ans. 1798C. Example 6.12 is similar to Example 6.11, but here the working substance is air as an ideal gas. Moreover, in this case the turbine work is known and the objective is to determine the isentropic turbine efficiency. cccc

EXAMPLE 6.12 c

Evaluating Isentropic Turbine Efficiency A turbine operating at steady state receives air at a pressure of p1 5 3.0 bar and a temperature of T1 5 390 K. Air exits the turbine at a pressure of p2 5 1.0 bar. The work developed is measured as 74 kJ per kg of air flowing through the turbine. The turbine operates adiabatically, and changes in kinetic and potential energy between inlet and exit can be neglected. Using the ideal gas model for air, determine the isentropic turbine efficiency. SOLUTION Known: Air expands adiabatically through a turbine at steady state from a specified inlet state to a specified exit pressure. The work developed per kg of air flowing through the turbine is known. Find: Determine the turbine efficiency. Schematic and Given Data: Air turbine p1 = 3.0 bar T1 = 390 K 1

3.0 bar

T

1

Wcv ––– = 74 kJ/kg m p2 = 1.0 bar 2

T1 = 390 K Actual expansion

Isentropic expansion

1.0 bar 2 2s

s

Fig. E6.12

6.12 Isentropic Efficiencies of Turbines, Nozzles, Compressors, and Pumps

287

Engineering Model: 1. The control volume shown on the accompanying sketch is at steady state. 2. The expansion is adiabatic and changes in kinetic and potential energy between inlet and exit can be

neglected. 3. The air is modeled as an ideal gas. Analysis: The numerator of the isentropic turbine efficiency, Eq. 6.46, is known. The denominator is evaluated as follows. The work developed in an isentropic expansion from the given inlet state to the specified exit pressure is # Wcv a # b 5 h1 2 h2s m s

From Table A-22 at 390 K, h1 5 390.88 kJ/kg. To determine h2s, use Eq. 6.41 pr (T2s) 5 a

p2 b p (T ) p1 r 1

With p1 5 3.0 bar, p2 5 1.0 bar, and pr1 5 3.481 from Table A-22 at 390 K pr (T2s) 5 a

1.0 b (3.481) 5 1.1603 3.0

Interpolation in Table A-22 gives h2s 5 285.27 kJ/kg. Thus # Wcv a # b 5 390.88 2 285.27 5 105.6 kJ/kg m s

✓ Skills Developed Ability to… ❑ apply the isentropic turbine

Substituting values into Eq. 6.46 # # 74 kJ/kg Wcv /m 𝜂t 5 # 5 0.70(70%) # 5 105.6 kJ/kg (Wcv /m)s

efficiency, Eq. 6.46.

❑ retrieve data for air as an

ideal gas.

Determine the rate of entropy production, in kJ/K per kg of air flowing through the turbine. Ans. 0.105 kJ/kg ? K.

6.12.2

Isentropic Nozzle Efficiency

A similar approach to that for turbines can be used to introduce the isentropic efficiency of nozzles operating at steady state. The isentropic nozzle efficiency is defined as the ratio of the actual specific kinetic energy of the gas leaving the nozzle, V22 y2, to the kinetic energy at the exit that would be achieved in an isentropic expansion between the same inlet state and the same exit pressure, (V22 y2)s . That is, 𝜂nozzle 5

V22 y2 (V22 y2)s

(6.47)

Nozzle efficiencies of 95% or more are common, indicating that well-designed nozzles are nearly free of internal irreversibilities. In Example 6.13, the objective is to determine the isentropic efficiency of a steam nozzle.

isentropic nozzle efficiency

Nozzle A.17 – Tab e

A

288 cccc

Chapter 6 Using Entropy

EXAMPLE 6.13 c

Evaluating the Isentropic Nozzle Efficiency Steam enters a nozzle operating at steady state at p1 5 9.7 bar and T1 5 316°C with a velocity of 30 m/s. The pressure and temperature at the exit are p2 5 2.8 bar and T2 5 177°C. There is no significant heat transfer between the nozzle and its surroundings, and changes in potential energy between inlet and exit can be neglected. Determine the nozzle efficiency. SOLUTION Known: Steam expands through a nozzle at steady state from a specified inlet state to a specified exit state. The velocity at the inlet is known. Find: Determine the nozzle efficiency. Schematic and Given Data:

h p1 = 9.7 bar T1 = 316°C V1 = 30 m/s

Isentropic expansion

2.8 bar 1 316°C Actual expansion

p2 = 2.8 bar T2 = 177°C

2.8 bar 2

Steam nozzle

177°C

2s 2

1

s

Fig. E6.13

Engineering Model: 1. The control volume shown on the accompanying sketch operates adiabatically at steady state.

#

2. For the control volume, Wcv 5 0 and the change in potential energy between inlet and exit can be neglected. Analysis: The nozzle efficiency given by Eq. 6.47 requires the actual specific kinetic energy at the nozzle exit

and the specific kinetic energy that would be achieved at the exit in an isentropic expansion from the given inlet state to the given exit pressure. The energy rate balance for a one-inlet, one-exit control volume at steady state (Eq. 4.20b) reduces to give V22 V21 5 h1 2 h2 1 2 2 This equation applies for both the actual expansion and the isentropic expansion. From Table A-4 at T1 5 320°C and p1 5 1 MPa, h1 5 3093.9 kJ/kg, s1 5 7.1962 kJ/kg ? K. Also, with T2 5 180°C and p2 5 0.3 MPa, h2 5 2823.9 kJ/kg. Thus, the actual specific kinetic energy at the exit in kJ/kg is (30)2 m 2 V22 kJ IN 1 kJ b 5 (3093.9 2 2823.9) 1 a ba ba s 2 kg 2 1 kg ? m/s2 103 N ? m kJ 5 270.45 kg Interpolating in Table A-4 at 0.3 MPa, with s2s 5 s1 5 7.1962 kJ/kg ? K, results in h2s 5 2813.3 kJ/kg. Accordingly, the specific kinetic energy at the exit for an isentropic expansion is a

(30)2 V22 5 281.05 kJ/kg b 5 3093.9 2 2813.3 1 2 s 2(103)

6.12 Isentropic Efficiencies of Turbines, Nozzles, Compressors, and Pumps Substituting values into Eq. 6.47 𝜂nozzle

➊

(V22 y2)

270.45 5 5 5 0.962 (96.2%) 2 281.05 (V2 y2)s

➊ The principal irreversibility in nozzles is friction between the flowing gas or liquid and the nozzle wall. The effect of friction is that a smaller exit kinetic energy, and thus a smaller exit velocity, is realized than would have been obtained in an isentropic expansion to the same pressure.

289

✓ Skills Developed Ability to… ❑ apply the control volume

energy rate balance.

❑ apply the isentropic nozzle

efficiency, Eq. 6.47.

❑ retrieve steam table data.

Determine the temperature, in 8C, corresponding to state 2s in Fig. E6.13. Ans. 1668C.

6.12.3

Isentropic Compressor and Pump Efficiencies

The form of the isentropic efficiency for compressors and pumps is taken up next. Refer to Fig. 6.12, which shows a compression process on a Mollier diagram. The state of the matter entering the compressor and the exit pressure are fixed. For negligible heat transfer with the surroundings and no appreciable kinetic and potential energy effects, the work input per unit of mass flowing through the compressor is # Wcv a2 # b 5 h2 2 h1 m Since state 1 is fixed, the specific enthalpy h1 is known. Accordingly, the value of the work input depends on the specific enthalpy at the exit, h2. The above expression shows that the magnitude of the work input decreases as h2 decreases. The minimum work input corresponds to the smallest allowed value for the specific enthalpy at the compressor exit. With similar reasoning as for the turbine, the smallest allowed enthalpy at the exit state would be achieved in an isentropic compression from the specified inlet state to the specified exit pressure. The minimum work input is given, therefore, by # Wcv a2 # b 5 h2s 2 h1 m s Accessible states p2

h

2 2s Actual compression Isentropic compression

h2 – h1 h2s – h1

p1 1

s

Fig. 6.12 Comparison of actual and isentropic compressions.

Compressor A.20 – Tab e Pump A.21 – Tab e

A

290

Chapter 6 Using Entropy In an actual compression, h2 . h2s, and thus more work than the minimum would be required. This difference can be gauged by the isentropic compressor efficiency defined by # # (2Wcv ym)s h2s 2 h1 # 𝜂c 5 (6.48) # 5 h2 2 h1 (2Wcv ym)

isentropic compressor efficiency

Both the numerator and denominator of this expression are evaluated for the same inlet state and the same exit pressure. The value of 𝜂c is typically 75 to 85% for compressors. An isentropic pump efficiency, 𝜂p, is defined similarly. In Example 6.14, the isentropic efficiency of a refrigerant compressor is evaluated, first using data from property tables and then using IT.

isentropic pump efficiency

cccc

EXAMPLE 6.14 c

Evaluating Isentropic Compressor Efficiency For the compressor of the heat pump system in Example 6.8, determine the power, in kW, and the isentropic efficiency using (a) data from property tables, (b) Interactive Thermodynamics: IT. SOLUTION Known: Refrigerant 22 is compressed adiabatically at steady state from a specified inlet state to a specified exit state. The mass flow rate is known. Find: Determine the compressor power and the isentropic efficiency using (a) property tables, (b) IT. Schematic and Given Data: 2s

T

Engineering Model:

2 T2 = 75°C

14 bar

1. A control volume enclosing the compressor is at steady state. 2. The compression is adiabatic, and changes in kinetic and potential energy

between the inlet and the exit can be neglected. 3.5 bar

1 T1 = –5°C s

Fig. E6.14

Analysis: (a) By assumptions 1 and 2, the mass and energy rate balances reduce to give

# # Wcv 5 m(h1 2 h2)

From Table A-9, h1 5 249.75 kJ/kg and h2 5 294.17 kJ/kg. Thus, with the mass flow rate determined in Example 6.8 # 1 kW Wcv 5 (0.07 kg/s)(249.75 2 294.17) kJ/kg ` ` 5 23.11 kW 1 kJ/s The isentropic compressor efficiency is determined using Eq. 6.48 # # (2Wcv ym)s (h2s 2 h1) # 𝜂c 5 # 5 (h2 2 h1) (2Wcv ym) In this expression, the denominator represents the work input per unit mass of refrigerant flowing for the actual compression process, as considered above. The numerator is the work input for an isentropic compression between the initial state and the same exit pressure. The isentropic exit state is denoted as state 2s on the accompanying T–s diagram. From Table A-9, s1 5 0.9572 kJ/kg ? K. With s2s 5 s1, interpolation in Table A-9 at 14 bar gives h2s 5 285.58 kJ/kg. Substituting values 𝜂c 5

(285.58 2 249.75) 5 0.81(81%) (294.17 2 249.75)

6.13 Heat Transfer and Work in Internally Reversible, Steady-State Flow Processes #

291

#

(b) The IT program follows. In the program, Wcv is denoted as Wdot, m as mdot, and 𝜂c as eta_c.

// Given Data: T1 5 25 // 8C p1 5 3.5 // bar T2 5 75 // 8C p2 5 14 // bar mdot 5 0.07 // kg/s // Determine the specific enthalpies. h1 5 h_PT(“R22”,p1,T1) h2 5 h_PT(“R22”,p2,T2) // Calculate the power. Wdot 5 mdot * (h1 2 h2) // Find h2s: s1 5 s_PT(“R22”,p1,T1) ➊ s2s 5 s_Ph(“R22”,p2,h2s) s2s 5 s1

✓ Skills Developed

// Determine the isentropic compressor efficiency. eta_c 5 (h2s 2 h1)/(h2 2 h1) # Use the Solve button to obtain: Wcv 5 23.111 kW and 𝜂c 5 80.58%, which, as expected, agree closely with the values obtained above.

Ability to… ❑ apply the control volume

energy rate balance.

❑ apply the isentropic com-

pressor efficiency, Eq. 6.48.

❑ retrieve data for Refriger-

➊ Note that IT solves for the value of h2s even though it is an implicit variable in the specific entropy function. Determine the minimum theoretical work input, in kJ per kg flowing, for an adiabatic compression from state 1 to the exit pressure of 14 bar. Ans. 35.83 kJ/kg.

6.13

Heat Transfer and Work in Internally Reversible, Steady-State Flow Processes

This section concerns one-inlet, one-exit control volumes at steady state. The objective is to introduce expressions for the heat transfer and the work in the absence of internal irreversibilities. The resulting expressions have several important applications.

6.13.1

Heat Transfer

For a control volume at steady state in which the flow is both isothermal at temperature T and internally reversible, the appropriate form of the entropy rate balance is ?

Qcv # # 05 1 m(s1 2 s2) 1 𝜎cv0 T # where 1 and 2 denote the inlet and exit, respectively, and m is the mass flow rate. Solving this equation, the heat transfer per unit of mass passing through the control volume is ?

Qcv # 5 T(s2 2 s1) m

ant 22.

292

Chapter 6 Using Entropy · Qcv ––– m·

( )

T

int rev

=

2 1

More generally, temperature varies as the gas or liquid flows through the control volume. We can consider such a temperature variation to consist of a series of infinitesimal steps. Then, the heat transfer per unit of mass is given as

T ds 2

1

?

Qcv a # b 5 m int rev

s

Fig. 6.13 Area representation of heat transfer for an internally reversible flow process.

2

# T ds

(6.49)

1

The subscript “int rev” serves to remind us that the expression applies only to control volumes in which there are no internal irreversibilities. The integral of Eq. 6.49 is performed from inlet to exit. When the states visited by a unit mass as it passes reversibly from inlet to exit are described by a curve on a T–s diagram, the magnitude of the heat transfer per unit of mass flowing can be represented as the area under the curve, as shown in Fig. 6.13.

6.13.2

Work

The work per unit of mass passing through a one-inlet, one-exit control volume can be found from an energy rate balance, which reduces at steady state to give # ? Qcv Wcv V12 2 V 22 5 b 1 g(z1 2 z2) # # 1 (h1 2 h2) 1 a 2 m m This equation is a statement of the conservation of energy principle that applies when irreversibilities are present within the control volume as well as when they are absent. However, if consideration is restricted to the internally reversible case, Eq. 6.49 can be introduced to obtain # 2 Wcv V12 2 V22 b 1 g(z1 2 z2) (6.50) a # b 5 T ds 1 (h1 2 h2) 1 a int 2 m rev 1

#

where the subscript “int rev” has the same significance as before. Since internal irreversibilities are absent, a unit of mass traverses a sequence of equilibrium states as it passes from inlet to exit. Entropy, enthalpy, and pressure changes are therefore related by Eq. 6.10b T ds 5 dh 2 𝜐 dp which on integration gives

#

2

2

T ds 5 (h2 2 h1) 2

1

# 𝜐 dp 1

Introducing this relation, Eq. 6.50 becomes # 2 Wcv V12 2 V 22 a # b 5 2 𝜐 dp 1 a b 1 g(z1 2 z2) int 2 m rev 1

#

p

When the states visited by a unit of mass as it passes reversibly from inlet to exit are described by a curve on a p–𝜐 diagram as shown in Fig. 6.14, the magnitude of the integral e 𝜐 dp is represented by the shaded area behind the curve. Equation 6.51a is applicable to devices such as turbines, compressors, and pumps. In many of these cases, there is no significant change in kinetic or potential energy from inlet to exit, so

2 2 1

(6.51a)

vdp 1 v

Fig. 6.14 Area representation of e 𝝊 dp. 2 1

# Wcv a # b 52 int m rev

2

# 𝜐 dp (Dke 5 Dpe 5 0)

(6.51b)

1

This expression shows that the work value is related to the magnitude of the specific volume of the gas or liquid as it flows from inlet to exit.

6.13 Heat Transfer and Work in Internally Reversible, Steady-State Flow Processes consider two devices: a pump through which liquid water passes and a compressor through which water vapor passes. For the same pressure rise, the pump requires a much smaller work input per unit of mass flowing than the compressor because the liquid specific volume is much smaller than that of vapor. This conclusion is also qualitatively correct for actual pumps and compressors, where irreversibilities are present during operation. b b b b b If the specific volume remains approximately constant, as in many applications with liquids, Eq. 6.51b becomes # Wcv a # b 5 2𝜐(p2 2 p1) (𝜐 5 constant, Dke 5 Dpe 5 0) int m rev

(6.51c)

Equation 6.51a also can# be applied to study the performance of control volumes at steady state in which Wcv is zero, as in the case of nozzles and diffusers. For any such case, the equation becomes

#

2

𝜐 dp 1 a

1

V 22 2 V12 b 1 g(z2 2 z1) 5 0 2

(6.52)

which is a form of the Bernoulli equation frequently used in fluid mechanics.

BIOCONNECTIONS Bats, the only mammals that can fly, play several important ecological roles, including feeding on crop-damaging insects. Currently, nearly one-quarter of bat species is listed as endangered or threatened. For unknown reasons, bats are attracted to large wind turbines, where some perish by impact and others from hemorrhaging. Near rapidly moving turbine blades there is a drop in air pressure that expands the lungs of bats, causing fine capillaries to burst and their lungs to fill with fluid, killing them. The relationship between air velocity and pressure in these instances is captured by the following differential form of Eq. 6.52, the Bernoulli equation: 𝜐 dp 5 2V dV which shows that as the local velocity V increases, the local pressure p decreases. The pressure reduction near the moving turbine blades is the source of peril to bats. Some say migrating bats experience most of the fatalities, so the harm may be decreased at existing wind farms by reducing turbine operation during peak migration periods. New wind farms should be located away from known migratory routes.

6.13.3 Work in Polytropic Processes We have identified an internally reversible process described by p𝜐n 5 constant, where n is a constant, as a polytropic process (see Sec. 3.15 and the discussion of Fig. 6.10). If each unit of mass passing through a one-inlet, one-exit control volume undergoes a polytropic process, then introducing p𝜐n 5 constant in Eq. 6.51b, and performing the integration, gives the work per unit of mass in the absence of internal irreversibilities and significant changes in kinetic and potential energy. That is, ?

Wcv a # b 52 int m rev 52

2

# 𝜐 dp 5 2(constant) # 1yn

1

1

2

dp p1yn

n (p2 𝜐 2 2 p1𝜐1) (polytropic, n ? 1) n21

(6.53)

Bernoulli equation

293

294

Chapter 6 Using Entropy Equation 6.53 applies for any value of n except n 5 1. When n 5 1, p𝜐 5 constant, and the work is ?

2 2 dp Wcv a # b 5 2 𝜐 dp 5 2constant int m rev 1 1 p 5 2(p1𝜐1) ln(p2 /p1) (polytropic, n 5 1)

#

#

(6.54)

Equations 6.53 and 6.54 apply generally to polytropic processes of any gas (or liquid).

IDEAL GAS CASE. For the special case of an ideal gas, Eq. 6.53 becomes # Wcv nR a # b int 5 2 (T2 2 T1) n21 m rev

(ideal

gas, n ? 1)

(6.55a)

For a polytropic process of an ideal gas, Eq. 3.56 applies: p2 (n21)/n T2 5a b p1 T1 Thus, Eq. 6.55a can be expressed alternatively as # Wcv nRT1 p2 (n21)/n a # b int 5 2 ca b 2 1 d (ideal gas, n ? 1) n 2 1 p1 m rev For the case of an ideal gas, Eq. 6.54 becomes # Wcv a # b int 5 2RT ln (p2 /p1) (ideal gas, n 5 1) m rev

(6.55b)

(6.56)

In Example 6.15, we consider air modeled as an ideal gas undergoing a polytropic compression process at steady state.

cccc

EXAMPLE 6.15 c

Determining Work and Heat Transfer for a Polytropic Compression of Air An air compressor operates at steady state with air entering at p1 5 1 bar, T1 5 208C, and exiting at p2 5 5 bar. Determine the work and heat transfer per unit of mass passing through the device, in kJ/kg, if the air undergoes a polytropic process with n 5 1.3. Neglect changes in kinetic and potential energy between the inlet and the exit. Use the ideal gas model for air. SOLUTION Known: Air is compressed in a polytropic process from a specified inlet state to a specified exit pressure. Find: Determine the work and heat transfer per unit of mass passing through the device. Schematic and Given Data: p

2

Engineering Model: 5 bar

1. A control volume enclosing the compressor is at steady state. 2. The air undergoes a polytropic process with n 5 1.3.

pv1.3 = constant

3. The air behaves as an ideal gas.

➊

4. Changes in kinetic and potential energy from inlet to exit

1

1 bar Shaded area = magnitude of (Wcv/m) int

can be neglected.

rev

v

Fig. E6.15

Chapter Summary and Study Guide

295

Analysis: The work is obtained using Eq. 6.55a, which requires the temperature at the exit, T2. The temperature

T2 can be found using Eq. 3.56 p2 (n21)/n 5 (1.321)/1.3 T2 5 T1 a b 5 293a b 5 425 K p1 1 Substituting known values into Eq. 6.55a then gives ?

Wcv nR 1.3 8.314 kJ (T2 2 T1) 5 2 a b(425 2 293) K # 52 n21 1.3 2 1 28.97 kg ? K m 5 2164.2 kJ/kg The heat transfer is evaluated by reducing the mass and energy rate balances with the appropriate assumptions to obtain ?

?

Qcv Wcv # 5 # 1 h2 2 h 1 m m Using the temperatures Tl and T2, the required specific enthalpy values are obtained from Table A-22 as h1 5 293.17 kJ/kg and h2 5 426.35 kJ/kg. Thus ?

Qcv # 5 2164.15 1 (426.35 2 293.17) 5 231 kJ/kg m ➊ The states visited in the polytropic compression process are shown by the curve on the accompanying p–𝜐 diagram. The magnitude of the work per unit of mass passing through the compressor is represented by the shaded area behind the curve.

✓ Skills Developed Ability to… ❑ analyze a polytropic

process of an ideal gas.

❑ apply the control volume

energy rate balance.

If the air were to undergo a polytropic process with n 5 1.0, determine the work and heat transfer, each in kJ per kg of air flowing, keeping all other given data the same. Ans. 2135.3 kJ/kg.

c CHAPTER SUMMARY AND STUDY GUIDE In this chapter, we have introduced the property entropy and illustrated its use for thermodynamic analysis. Like mass and energy, entropy is an extensive property that can be transferred across system boundaries. Entropy transfer accompanies both heat transfer and mass flow. Unlike mass and energy, entropy is not conserved but is produced within systems whenever internal irreversibilities are present. The use of entropy balances is featured in this chapter. Entropy balances are expressions of the second law that account for the entropy of systems in terms of entropy transfers and entropy production. For processes of closed systems, the entropy balance is Eq. 6.24, and a corresponding rate form is Eq. 6.28. For control volumes, rate forms include Eq. 6.34 and the companion steady-state expression given by Eq. 6.36. The following checklist provides a study guide for this chapter. When your study of the text and end-of-chapter exercises has been completed, you should be able to

c write out meanings of the terms listed in the margins

throughout the chapter and understand each of the related concepts. The subset of key concepts listed below is particularly important in subsequent chapters. c apply entropy balances in each of several alternative forms, modeling the case at hand, correctly observing sign conventions. c use entropy data appropriately, to include –retrieving data from Tables A-2 through A-18, using Eq. 6.4 to evaluate the specific entropy of two-phase liquid– vapor mixtures, sketching T–s and h–s diagrams and locating states on such diagrams, and appropriately using Eqs. 6.5 and 6.13. –determining Ds of ideal gases using Eq. 6.20 for variable specific heats together with Tables A-22 and A-23, and using Eqs. 6.21 and 6.22 for constant specific heats.

296

Chapter 6 Using Entropy

–evaluating isentropic efficiencies for turbines, nozzles, compressors, and pumps from Eqs. 6.46, 6.47, and 6.48, respectively, including for ideal gases the appropriate use of Eqs. 6.41–6.42 for variable specific heats and Eqs. 6.43– 6.45 for constant specific heats.

c apply Eq. 6.23 for closed systems and Eqs. 6.49 and

6.51 for one-inlet, one-exit control volumes at steady state, correctly observing the restriction to internally reversible processes.

c KEY ENGINEERING CONCEPTS entropy change, p. 244 T–s diagram, p. 247 Mollier diagram, p. 248 T ds equations, p. 249

isentropic process, p. 254 entropy transfer, pp. 254, 258 entropy balance, p. 257 entropy production, p. 258

entropy rate balance, pp. 263, 269 increase of entropy principle, p. 265 isentropic efficiencies, pp. 285, 287, 290

c KEY EQUATIONS 𝛿Q

2

S2 2 S1 5

#aTb 1

1𝜎

(6.24) p. 257

Closed system entropy balance.

(6.28) p. 263

Closed system entropy rate balance.

(6.34) p. 269

Control volume entropy rate balance.

(6.36) p. 270

Steady-state control volume entropy rate balance.

(6.46) p. 285

Isentropic turbine efficiency.

(6.47) p. 287

Isentropic nozzle efficiency.

(6.48) p. 290

Isentropic compressor (and pump) efficiency.

b

?

Qj dS # 5 a 1𝜎 dt T j j ?

Qj dScv # # # 5 a 1 a mi si 2 a me se 1 𝜎cv dt j Tj i e ?

Qj

# # # 05 a 1 a mi si 2 a me se 1 𝜎cv T j j i e # # Wcv /m h1 2 h2 𝜂t 5 # # 5h 2h 1 2s (Wcv /m)s 𝜂 nozzle 5

V22 y2 (V22 y2)s

# # (2Wcv ym)s h 2s 2 h1 # 𝜂c 5 # 5 h 2h 2 1 (2Wcv ym)

Ideal Gas Model Relations s(T2, 𝜐 2) 2 s(T1, 𝜐1) 5

#

T2

c𝜐(T)

T1

s(T2, 𝜐 2) 2 s(T1, 𝜐 1) 5 c𝜐 ln

s(T2, p2) 2 s(T1, p1) 5

#

T2 𝜐2 1 R ln 𝜐1 T1

T2

cp(T)

T1

𝜐2 dT 1 R ln 𝜐1 T

p2 dT 2 R ln p1 T

s(T2, p2) 2 s(T1, p1) 5 s°(T2) 2 s°(T1) 2 R ln s(T2, p2) 2 s(T1, p1) 5 cp ln

p2 T2 2 R ln p T1 1

p2 p1

(6.17) p. 251

Change in specific entropy; general form for T and 𝝊 as independent properties.

(6.21) p. 253

Constant specific heat, c𝝊.

(6.18) p. 251

Change in specific entropy; general form for T and p as independent properties.

(6.20a) p. 252

s8 for air from Table A-22. (s8 for other gases from Table A-23).

(6.22) p. 253

Constant specific heat, cp.

Problems: Developing Engineering Skills p2 pr2 5 p1 pr1

(6.41) p. 279

𝜐2 𝜐r2 5 𝜐1 𝜐r1

(6.42) p. 280

p2 (k21)/k T2 5a b p1 T1

(6.43) p. 280

T2 𝜐1 k21 5a b 𝜐2 T1

(6.44) p. 280

p2 𝜐1 k 5a b 𝜐2 p1

(6.45) p. 280

297

s1 5 s2 (air only), pr and 𝝊r from Table A-22.

s1 5 s2, constant specific heat ratio k.

c EXERCISES: THINGS ENGINEERS THINK ABOUT 1. Of the extensive properties mass, energy, and entropy, which are conserved? Which are produced?

the net work developed by these cycles. Which cycle has the greater thermal efficiency?

2. Is it possible for entropy to be negative? For entropy change to be negative? For entropy production to be negative?

T

2

3

1

4

T

2

3

3. By what means can entropy be transferred across the boundary of a closed system? Across the boundary of a control volume? 4. Is it possible for the entropy of both a closed system and its surroundings to decrease during a process? Both to increase during a process? 5. What happens to the entropy produced within an insulated, one-inlet, one-exit control volume operating at steady state?

1 S

S

10. Sketch the T–s diagram for the Carnot vapor cycle of Fig. 5.15.

6. If a closed system would undergo an internally reversible process and an irreversible process between the same end states, how would the changes in entropy for the two processes compare? How would the amounts of entropy produced compare?

11. Sketch T–s and p–v diagrams of an ideal gas executing a power cycle consisting of four internally reversible processes in series: constant specific volume, constant pressure, isentropic, isothermal.

7. Referring to Example 3.1, the entropy of the system changes. Why? Repeat for Example 3.2.

12. Could friction be less for a slurry of pulverized coal and water flowing through a pipeline than for water alone? Explain.

8. Referring to Fig. 2.3, if systems A and B operate adiabatically, does the entropy of each increase, decrease, or remain the same? 9. The two power cycles shown to the same scale in the figure are composed of internally reversible processes. Compare

13. Reducing irreversibilities within a system can improve its thermodynamic performance, but steps taken in this direction are usually constrained by other considerations. What are some of these?

c PROBLEMS: DEVELOPING ENGINEERING SKILLS 6.1 Use the tables for water to determine the specific entropy at the following states: (a) P 5 10 MPa, h 5 2300 kJ/kg (b) P 5 10 MPa, T 5 450°C (c) P 5 10 MPa, T 5 120°C Also, locate these states on a sketch of the T– s diagram. 6.2 Using the appropriate table, determine the change in specific entropy between the specified states, in kJ/kg ? K. (a) water, p 1 5 10 MPa, T 1 5 400°C, p 2 5 10 MPa, T 2 5 100°C.

(b) Refrigerant 134a, h1 5 111.44 kJ/kg, T1 5 240°C, saturated vapor at p2 5 5 bar. (c) air as an ideal gas, T1 5 7°C, p1 5 2 bar, T2 5 327°C, p2 5 1 bar. (d) hydrogen (H2) as an ideal gas, T1 5 727°C, p1 5 1 bar, T2 5 25°C, p2 5 3 bar. 6.3 Calculate the specific entropy of water with specific enthalpy of 2300 kJ/kg at 6 MPa pressure. 6.4 Using IT, determine the change in specific entropy between the specified states. Compare with results obtained from the appropriate table.

298

Chapter 6 Using Entropy

(a) Specific entropy change, in kJ/kg ? K, for the cases of Problem 6.3. (b) Specific entropy change, in kJ/kg ? K, for the cases of Problem 6.4. 6.5 Using steam table data, determine the indicated property data for a process in which there is no change in specific entropy between state 1 and state 2. In each case, locate the states on a sketch of the T–s diagram. (a) Dh, (b) Du,

T1 5 40°C, x1 5 100%, p2 5 150 kPa. Find T2, in °C, and in kJ/kg. T1 5 10°C, x1 5 75%, p2 5 1 MPa. Find T2, in °C, and in kJ/kg.

6.13 Steam enters a turbine operating at steady state at 1.5 MPa, 240°C and exits at 45°C with a quality of 85%. Stray heat transfer and kinetic and potential energy effects are negligible. Determine (a) the power developed by the turbine, in kJ per kg of steam flowing, (b) the change in specific entropy from inlet to exit, in kJ/K per kg of steam flowing. 6.14 A closed system consists of an ideal gas with constant specific heat ratio k. (a) The gas undergoes a process in which temperature increases from T1 to T2. Show that the entropy change for the process is greater if the change in state occurs at constant pressure than if it occurs at constant volume. Sketch the processes on p–v and T–s coordinates. (b) Using the results of (a), show on T–s coordinates that a line of constant specific volume passing through a state has a greater slope than a line of constant pressure passing through that state. (c) The gas undergoes a process in which pressure increases from p1 to p2. Show that the ratio of the entropy change for an isothermal process to the entropy change for a constantvolume process is (1 2 k). Sketch the processes on p–v and T– s coordinates.

6.6 At 1.20 MPa pressure and 250°C temperature steam enters a steam turbine operating at steady state. Steam exits the turbine at 50°C with a quality of 85%. Neglect the effect of kinetic and potential energies, also neglect strong heat transfer. Determine (a) the change in specific entropy of steam firm inlet to exit. (b) the power developed by the turbine per kg of steam flowing. 6.7 Propane undergoes a process from state 1, where p1 5 1.6 MPa, T1 5 70°C, to state 2, where p2 5 1.2 MPa, during which the change in specific entropy is s2 2 s1 5 20.047 kJ/kg ? K. At state 2, determine the temperature, in °C, and the specific enthalpy, in kJ/kg. 6.8 Air in a piston–cylinder assembly undergoes a process from state 1, where T1 5 400 K, p1 5 200 kPa, to state 2, where T2 5 600 K, p2 5 750 kPa. Using the ideal gas model for air, determine the change in specific entropy between these states, in kJ/kg ? K, if the process occurs (a) without internal irreversibilities, (b) with internal irreversibilities.

Analyzing Internally Reversible Processes 6.15 One kg of water in a piston-cylinder assembly undergoes the two internally reversible processes in series shown in Fig. P6.15. For each process, determine, in kJ, the heat transfer and the work. T T = constant

3

p2 = 0.3 MPa s = constant

6.9 Methane gas (CH4) enters a compressor at 298 K, 1 bar and exits at 2 bar and temperature T. Employing the ideal gas model, determine T, in K, if there is no change in specific entropy from inlet to exit. 6.10 Two kg of water contained in a piston–cylinder assembly, initially at 200°C, 300 kPa, undergo an isothermal compression process to saturated liquid. For the process, W 5 2481.5 kJ. Determine for the process, (a) the heat transfer, in kJ. (b) the change in entropy, in kJ/K. Show the process on a sketch of the T–s diagram. 6.11 One-fifth kmol of carbon monoxide (CO) in a pistoncylinder assembly undergoes a process from p1 5 100 kPa, T1 5 298 K to p2 5 400 kPa, T2 5 360 K. For the process, W 5 2250 kJ. Employing the ideal gas model, determine (a) the heat transfer, in kJ. (b) the change in entropy, in kJ/K. Show the process on a sketch of the T–s diagram. 6.12 Argon in a piston–cylinder assembly is compressed from state 1, where T1 5 500 K, V1 5 2 m3, to state 2, where T2 5 400 K. If the change in specific entropy is s2 2 s1 5 20.30 kJ/kg ? K, determine the final volume, in m3. Assume the ideal gas model with k 5 1.67.

2

p3 = 1 MPa

1

p1 = 0.05 MPa T1 = 100°C

Fig. P6.15

s

6.16 Two kg of water in a piston–cylinder assembly undergoe the two internally reversible processes in series shown in Fig. P6.16. For each process, determine, in kJ, the heat transfer and the work. 3 p3 = 1.5 MPa T3 = 500°C

T

p = constant 2 s = constant 1

p1 = 0.15 MPa T1 = 120°C s

Fig. P6.16

6.17 Water undergoes a constant volume process in which the temperature of water rises from 30°C to 35°C. There is no heat transfer. Determine the change in the entropy of water.

Problems: Developing Engineering Skills 6.18 One kg mass of water initially a saturated liquid at 100 kPa undergoes a constant-pressure, internally reversible expansion to x 5 80%. Determine the work and heat transfer, each in kJ. Sketch the process on p–v and T– s coordinates. Associate the work and heat transfer with areas on these diagrams. 6.19 A gas initially at 14 bar and 60°C expands to a final pressure of 2.8 bar in an isothermal, internally reversible process. Determine the heat transfer and the work, each in kJ per kg of gas, if the gas is (a) Refrigerant 134a, (b) air as an ideal gas. Sketch the processes on p–v and T–s coordinates. 6.20 Reconsider the data of Problem 6.24, but now suppose the gas expands to 2.8 bar isentropically. Determine the work, in kJ per kg of gas, if the gas is (a) Refrigerant 134a, (b) air as an ideal gas. Sketch the processes on p–v and T–s coordinates. 6.21 Nitrogen (N2) initially occupying 0.5 m3 at 1.0 bar, 20°C, undergoes an internally reversible compression during which pV1.30 5 constant to a final state where the temperature is 200°C. Assuming the ideal gas model, determine (a) the pressure at the final state, in bar. (b) the work and heat transfer, each in kJ. (c) the entropy change, in kJ/K. 6.22 Air is compressed isentropically in a piston-cylinder assembly from an initial state where temperature T 1 5 400 K such that final pressure is 1.5 times of that of initial pressure. Determine the value of temperature at state 2 that is T2 in K. Also, determine the work in kJ/kg. Assume that air behaves as an ideal gas. 6.23 Air in a piston–cylinder assembly undergoes a thermodynamic cycle consisting of three internally reversible processes in series. Process 1–2: Constant-volume heating from T1 5 288 K, p1 5 0.1 MPa to p2 5 0.42 MPa. Process 2–3: Constant-pressure cooling.

(b) the net work developed per cycle, in kJ. (c) the thermal efficiency. 6.26 In a piston–cylinder assembly, water is contained initially at 200°C as a saturated liquid. The piston moves freely in the cylinder as water undergoes a process to the corresponding saturated vapor state. This change of state occurs at constant pressure and temperature by heating the water. The process is internally reversible. Determine the heat transfer per unit mass. 6.27 Water in a piston–cylinder assembly undergoes a Carnot power cycle. At the beginning of the isothermal expansion, the temperature is 225°C and the quality is 75%. The isothermal expansion continues until the pressure is 1.8 MPa. The adiabatic expansion then occurs to a final temperature of 150°C. (a) Sketch the cycle on T–s coordinates. (b) Determine the heat transfer and work, in kJ/kg, for each process. (c) Evaluate the thermal efficiency. 6.28 A Carnot power cycle operates at steady state as shown in Fig. 5.15 with water as the working fluid. The boiler pressure is 1.4 MPa, with saturated liquid entering and saturated vapor exiting. The condenser pressure is 138 kPa. (a) Sketch the cycle on T–s coordinates. (b) Determine the heat transfer and work for each process, in kJ per kg of water flowing. (c) Evaluate the thermal efficiency. 6.29 Figure P6.29 gives the schematic of a vapor power plant in which water steadily circulates through the four components shown. The water flows through the boiler and condenser at constant pressure, and flows through the turbine and pump adiabatically. (a) Sketch the cycle on T–s coordinates. (b) Determine the thermal efficiency and compare with the thermal efficiency of a Carnot cycle operating between the same maximum and minimum temperatures.

Process 3–1: Isothermal heating to the initial state. Employing the ideal gas model with cp 5 1 kJ/kg ? K, evaluate the change in specific entropy, in kJ/kg ? K, for each process. Sketch the cycle on p–v and T–s coordinates. For each process, associate work and heat transfer with areas on these diagrams, respectively. 6.24 One-fifth kg of a gas in a piston–cylinder assembly undergoes a Carnot power cycle for which the isothermal expansion occurs at 100 K. The change in specific entropy of the gas during the isothermal compression, which occurs at 500 K, is 250 kJ/kg ? K. Determine (a) the net work developed per cycle, in kJ, and (b) the thermal efficiency. 6.25 One kg mass of air as an ideal gas contained within a piston–cylinder assembly undergoes a Carnot power cycle. At the beginning of the isothermal expansion, the temperature is 890 K and the pressure is 8.27 MPa. The isothermal compression occurs at 278 K and the heat added per cycle is 42.2 kJ. Assuming the ideal gas model for the air, determine (a) the pressures at the end of the isothermal expansion, the adiabatic expansion, and the isothermal compression, each in kPa.

299

Hot reservoir · QH Saturated 4 liquid at 10 bar

Pump

Work 0.2 bar, x = 18%

3

1

Boiler

Turbine

Condenser

2

Saturated vapor at 10 bar Work 0.2 bar, x = 88%

· QC Cold reservoir

Fig. P6.29 Applying the Entropy Balance: Closed Systems 6.30 A fixed mass of water m, initially a saturated liquid, is brought to a saturated vapor condition while its pressure and

300

Chapter 6 Using Entropy

temperature remain constant. Volume change is the only work mode. (a) Derive expressions for the work and heat transfer in terms of the mass m and properties that can be obtained directly from the steam tables. (b) Demonstrate that this process is internally reversible. 6.31 Seven kg of water contained in a piston-cylinder assembly expand from an initial state where T1 5 500°C, p1 5 500 kPa to a final state where T2 5 240°C, p2 5 150 kPa, with no significant effects of kinetic and potential energy. The accompanying table provides additional data at the two states. It is claimed that the water undergoes an adiabatic process between these states, while developing work. Evaluate this claim. State T(8C)

1 2

500 240

p(kPa) 𝝊(m3/kg) u(kJ/kg)

500 150

0.7109 1.570

3128.4 2717.2

h(kJ/kg) s(kJ/kg

3483.9 2952.7

?

K)

8.0873 7.8052

6.32 Five m3 of air in a rigid, insulated container fitted with a paddle wheel is initially at 300 K, 300 kPa. The air receives 720 kJ by work from the paddle wheel. Assuming the ideal gas model with cv 5 0.80 kJ/kg ? K, determine for the air (a) the mass, in kg, (b) final temperature, in K, and (c) the amount of entropy produced, in kJ/K. 6.33 In a system, air is contained at 298 K and 1 bar. The temperature is raised by stirring the air with a paddle wheel adiabatically to a state where temperature becomes 450 K. The volume of air is kept constant during the process. Determine the amount of entropy produced in kJ/kg ? K. 6.34 One kg of air contained in a piston–cylinder assembly is initially at 1 bar and 450 K. Can a final state at 2 bar and 350 K be attained in an adiabatic process? 6.35 One kg mass of Refrigerant 134a contained within a piston–cylinder assembly undergoes a process from a state where the pressure is 0.8 MPa and the quality is 40% to a state where the temperature is 108C and the refrigerant is saturated liquid. Determine the change in specific entropy of the refrigerant, in kJ/kg ? K. Can this process be accomplished adiabatically? 6.36 One-half kg of ice at 24°C is exposed to the atmosphere which is at 18°C. The ice melts and comes into thermal equilibrium with the atmosphere. Determine the entropy change of ice as it melts into water. Take the value of latent heat of fusion of ice equal to 333.3 kJ/kg and the value of specific heat at constant pressure (cp) equal to 2093 kJ/kg ? K. 6.37 Two kg of air contained in a piston–cylinder assembly undergoes a process from an initial state where T1 5 320 K, v1 5 1.2 m3/kg to a final state where T2 5 440 K, v2 5 0.4 m3/kg. Can this process occur adiabatically? If yes, determine the work, in kJ, for an adiabatic process between these states. If no, determine the direction of the heat transfer. Assume the ideal gas model for air. 6.38 In a piston–cylinder assembly, 1 kg mass of Refrigerant 134a undergoes a process with initial pressure of 1 MPa and quality of 45% to a final temperature of 6°C at which the refrigerant is saturated liquid. Determine the change in specific entropy of the refrigerant. Is it possible to accomplish this process adiabatically.

6.39 A piston–cylinder assembly initially contains 0.04 m3 of water at 1.0 MPa, 320°C. The water expands adiabatically to a final pressure of 0.1 MPa. Develop a plot of the work done by the water, in kJ, versus the amount of entropy produced, in kJ/K. 6.40 Two kg mass of steam contained in a piston–cylinder assembly expands adiabatically from state 1, where p1 5 0.7 MPa, T1 5 260°C, to a final pressure of 70 kPa. Develop a plot of the work done by the water, in kJ, versus the amount of entropy produced, in kJ/K. 6.41 One-tenth kg of water, initially at 300 kPa, 200°C, is compressed in a piston–cylinder assembly to 1500 kPa, 210°C. If heat transfer from the water occurs at an average temperature of 205°C, determine the minimum theoretical work of compression, in kJ. 6.42 One-half kg of propane initially at 4 bar, 30°C undergoes a process to 14 bar, 100°C while being rapidly compressed in a piston–cylinder assembly. Heat transfer with the surroundings at 20°C occurs through a thin wall. The net work is measured as 272.5 kJ. Kinetic and potential energy effects can be ignored. Determine whether it is possible for the work measurement to be correct. 6.43 One kg mass of carbon dioxide (CO2) in a piston-cylinder assembly, initially at 21°C and 1.24 MPa, expands isothermally to a final pressure of 0.1 MPa while receiving energy by heat transfer through a wall separating the carbon dioxide from a thermal energy reservoir at 49°C. (a) For the carbon dioxide as the system, evaluate the work and heat transfer, each in kJ, and the amount of entropy produced, in kJ/K. Model the carbon dioxide as an ideal gas. (b) Evaluate the entropy production for an enlarged system that includes the carbon dioxide and the wall, assuming the state of the wall remains unchanged. Compare with the entropy production of part (a) and comment on the difference. 6.44 Two kg of Refrigerant 134a initially at 1.4 bar, 60°C, are compressed to saturated vapor at 60°C. During this process, the temperature of the refrigerant departs by no more than 0.01°C from 60°C. Determine the minimum theoretical heat transfer from the refrigerant during the process, in kJ. 6.45 An inventor claims that the electricity-generating unit shown in Fig. P6.45 receives a heat transfer at the rate of 270 kJ/s at a temperature of 280 K, a second heat transfer at the rate of 380 kJ/s at 400 K, and a third at the rate of 530 kJ at 560 K. For operation at steady state, evaluate this claim. · Q1 = 270 kJ/s T1 = 280 K

· Q2 = 380 kJ/s

+ –

T2 = 400 K T3 = 560 K

· Q3 = 530 kJ/s

Fig. P6.45

Problems: Developing Engineering Skills 6.46 One kg of water at 280 K is brought into contact with a heat reservoir at 350 K. Calculate the entropy change of water, the heat reservoir, and of the universe, when the temperature of water has reached 350 K. 6.47 At steady state, the 50-W curling iron has an outer surface temperature of 95°C. For the curling iron, determine the rate of heat transfer, in kJ/h, and the rate of entropy production, in kJ/h ? K. 6.48 A thermally insulated 30-ohm resistor receives a current of 6 amps. The mass of the resistor is 0.1 kg, its specific heat is 0.8 kJ/kg ? K, and its initial temperature is 21°C. For the resistor, develop plots of the temperature, in K, and the amount of entropy produced, in kJ/K, versus time ranging from 0 to 3 s. 6.49 An electric water heater having a 200 liter capacity employs an electric resistor to heat water from 23 to 55°C. The outer surface of the resistor remains at an average temperature of 80°C. Heat transfer from the outside of the water heater is negligible and the states of the resistor and the tank holding the water do not change significantly. Modeling the water as incompressible, determine the amount of entropy produced, in kJ/K, for (a) the water as the system. (b) the overall water heater including the resistor. Compare the results of parts (a) and (b), and discuss. 6.50 One kg of water is heated from 280 K to 350 K by first bringing it into contact with a reservoir at 310 K and then with a reservoir at 350 K. Calculate the entropy change of the universe. 6.51 An electric motor operating at steady state draws a current of 10 amp with a voltage of 220 V. The output shaft rotates at 1000 RPM with a torque of 16 N ? m applied to an external lo