Main Thermodynamics: An Engineering Approach 8th Edition
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Total pages: 1115
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Total pages: 1115
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THERMODYNAMICS AN ENGINEERING APPROACH EIGHTH EDITION This page intentionally left blank THERMODYNAMICS AN ENGINEERING APPROACH EIGHTH EDITION YUNUS A. ÇENGEL University of Nevada, Reno MICHAEL A. BOLES North Carolina State University THERMODYNAMICS: AN ENGINEERING APPROACH, EIGHTH EDITION Published by McGraw-Hill Education, 2 Penn Plaza, New York, NY 10121. Copyright © 2015 by McGraw-Hill Education. All rights reserved. Printed in the United States of America. Previous editions © 2011, 2008, 2006, and 2002. No part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written consent of McGraw-Hill Education, including, but not limited to, in any network or other electronic storage or transmission, or broadcast for distance learning. Some ancillaries, including electronic and print components, may not be available to customers outside the United States. This book is printed on acid-free paper. 1 2 3 4 5 6 7 8 9 0 DOW/DOW 1 0 9 8 7 6 5 4 ISBN 978-0-07-339817-4 MHID 0-07-339817-9 Senior Vice President, Products & Markets: Kurt L. Strand Vice President, General Manager: Marty Lange Vice President, Content Production & Technology Services: Kimberly Meriwether David Global Publisher: Raghothaman Srinivasan Executive Editor: Bill Stenquist Developmental Editor: Lorraine K. Buczek Marketing Manager: Heather Wagner Director, Content Production: Terri Schiesl Content Project Manager: Jolynn Kilburg Buyer: Jennifer Pickel Cover Designer: Studio Montage, St. Louis, MO. Cover Photo: Photo provided by Alstom. © 2007 Bryon Paul McCartney | www.photoworks312.com | all rights reserved. Compositor: RPK Editorial Services, Inc. Typeface: 10.5/12 Times LT Std Roman Printer: R. R. Donnelley About the Cover: A fully bladed GT26 gas turbine rotor at Alstom’s rotor factory in Birr, Switzerland. All credits appearing on page or at the end of the book are considered to be an extension of the copyright page. Library of Cong; ress Cataloging-in-Publication Data on File The Internet addresses listed in the text were accurate at the time of publication. The inclusion of a website does not indicate an endorsement by the authors or McGraw-Hill Education, and McGraw-Hill Education does not guarantee the accuracy of the information presented at these sites. www.mhhe.com Quotes on Ethics Without ethics, everything happens as if we were all five billion passengers on a big machinery and nobody is driving the machinery. And it’s going faster and faster, but we don’t know where. —Jacques Cousteau Because you’re able to do it and because you have the right to do it doesn’t mean it’s right to do it. —Laura Schlessinger A man without ethics is a wild beast loosed upon this world. —Manly Hall The concern for man and his destiny must always be the chief interest of all technical effort. Never forget it among your diagrams and equations. —Albert Einstein Cowardice asks the question, ‘Is it safe?’ Expediency asks the question, ‘Is it politic?’ Vanity asks the question, ‘Is it popular?’ But, conscience asks the question, ‘Is it right?’ And there comes a time when one must take a position that is neither safe, nor politic, nor popular but one must take it because one’s conscience tells one that it is right. —Martin Luther King, Jr To educate a man in mind and not in morals is to educate a menace to society. —Theodore Roosevelt Politics which revolves around benefit is savagery. —Said Nursi The true test of civilization is, not the census, nor the size of the cities, nor the crops, but the kind of man that the country turns out. —Ralph W. Emerson The measure of a man’s character is what he would do if he knew he never would be found out. —Thomas B. Macaulay About the Authors Yunus A. Çengel is Professor Emeritus of Mechanical Engineering at the University of Nevada, Reno. He received his B.S. in mechanical engineering from Istanbul Technical University and his M.S. and Ph.D. in mechanical engineering from North Carolina State University. His areas of interest are renewable energy, energy efficiency, energy policies, heat transfer enhancement, and engineering education. He served as the director of the Industrial Assessment Center (IAC) at the University of Nevada, Reno, from 1996 to 2000. He has led teams of engineering students to numerous manufacturing facilities in Northern Nevada and California to perform industrial assessments, and has prepared energy conservation, waste minimization, and productivity enhancement reports for them. He has also served as an advisor for various government organizations and corporations. Dr. Çengel is also the author or coauthor of the widely adopted textbooks Heat and Mass Transfer: Fundamentals and Applications (5th ed., 2015), Fluid Mechanics:Fundamentals and Applications (3rd ed., 2014), Fundamentals of Thermal-Fluid Sciences (4th ed., 2012), Introduction to Thermodynamics and Heat Transfer (2nd ed., 2008), and Differential Equations for Engineers and Scientists (1st ed., 2013), all published by McGraw-Hill. Some of his textbooks have been translated into Chinese, Japanese, Korean, Thai, Spanish, Portuguese, Turkish, Italian, Greek, and French. Dr. Çengel is the recipient of several outstanding teacher awards, and he has received the ASEE Meriam/Wiley Distinguished Author Award for excellence in authorship in 1992 and again in 2000. Dr. Çengel is a registered Professional Engineer in the State of Nevada, and is a member of the American Society of Mechanical Engineers (ASME) and the American Society for Engineering Education (ASEE). Michael A. Boles is Associate Professor of Mechanical and Aerospace Engineering at North Carolina State University, where he earned his Ph.D. in mechanical engineering and is an Alumni Distinguished Professor. Dr. Boles has received numerous awards and citations for excellence as an engineering educator. He is a past recipient of the SAE Ralph R. Teetor Education Award and has been twice elected to the NCSU Academy of Outstanding Teachers. The NCSU ASME student section has consistently recognized him as the outstanding teacher of the year and the faculty member having the most impact on mechanical engineering students. Dr. Boles specializes in heat transfer and has been involved in the analytical and numerical solution of phase change and drying of porous media. He is a member of the American Society of Mechanical Engineers (ASME), the American Society for Engineering Education (ASEE), and Sigma Xi. Dr. Boles received the ASEE Meriam /Wiley Distinguished Author Award in 1992 for excellence in authorship. Brief Contents chapter one INTRODUCTION AND BASIC CONCEPTS 1 chapter two ENERGY, ENERGY TRANSFER, AND GENERAL ENERGY ANALYSIS 51 chapter three PROPERTIES OF PURE SUBSTANCES 111 chapter four ENERGY ANALYSIS OF CLOSED SYSTEMS 163 chapter five MASS AND ENERGY ANALYSIS OF CONTROL VOLUMES 213 chapter six THE SECOND LAW OF THERMODYNAMICS 275 chapter seven ENTROPY 329 chapter eight EXERGY 421 chapter nine GAS POWER CYCLES 485 chapter ten VAPOR AND COMBINED POWER CYCLES 553 chapter eleven REFRIGERATION CYCLES 607 chapter twelve THERMODYNAMIC PROPERTY RELATIONS 655 chapter thirteen GAS MIXTURES 687 chapter fourteen GAS–VAPOR MIXTURES AND AIR-CONDITIONING 725 chapter fifteen CHEMICAL REACTIONS 759 chapter sixteen CHEMICAL AND PHASE EQUILIBRIUM 805 chapter seventeen COMPRESSIBLE FLOW 839 chapter eighteen (web chapter) RENEWABLE ENERGY viii THERMODYNAMICS appendix 1 PROPERTY TABLES AND CHARTS (SI UNITS) 897 appendix 2 PROPERTY TABLES AND CHARTS (ENGLISH UNITS) 947 Contents http://highered.mheducation.com/sites/0073398179/information_center_view0/index.html Preface xvii Engineering Equation Solver (EES) 37 A Remark on Significant Digits 39 Summary 40 References and Suggested Readings Problems 41 chapter one INTRODUCTION AND BASIC CONCEPTS 1–1 1–2 Thermodynamics and Energy 2 Application Areas of Thermodynamics 3 Importance of Dimensions and Units 1 chapter two 3 Some SI and English Units 6 Dimensional Homogeneity 8 Unity Conversion Ratios 9 1–3 1–4 12 Density and Specific Gravity State and Equilibrium The State Postulate 1–7 13 14 Processes and Cycles 2–3 2–4 2–5 16 2–6 22 Variation of Pressure with Depth Energy Transfer by Heat 60 Historical Background on Heat 61 Energy Transfer by Work 1–10 Pressure Measurement Devices 27 2–7 Step 1: Problem Statement 34 Step 2: Schematic 35 Step 3: Assumptions and Approximations 35 Step 4: Physical Laws 35 Step 5: Properties 35 Step 6: Calculations 35 Step 7: Reasoning, Verification, and Discussion 35 Engineering Software Packages 36 62 65 Mechanical Forms of Work 66 The First Law of Thermodynamics 70 Energy Conversion Efficiencies 78 Efficiencies of Mechanical and Electrical Devices 33 1–11 Problem-Solving Technique 34 55 Energy Balance 72 Energy Change of a System, DEsystem 72 Mechanisms of Energy Transfer, Ein and Eout 73 24 The Barometer 27 The Manometer 30 Other Pressure Measurement Devices 53 Shaft Work 66 Spring Work 67 Work Done on Elastic Solid Bars 67 Work Associated with the Stretching of a Liquid Film Work Done to Raise or to Accelerate a Body 68 Nonmechanical Forms of Work 70 Temperature and the Zeroth Law of Thermodynamics 17 Pressure 52 Forms of Energy Electrical Work 15 Temperature Scales 18 The International Temperature Scale of 1990 (ITS-90) 20 1–9 Introduction Some Physical Insight to Internal Energy More on Nuclear Energy 56 Mechanical Energy 58 15 The Steady-Flow Process 1–8 10 Properties of a System 12 Continuum 1–5 1–6 ENERGY, ENERGY TRANSFER, AND GENERAL ENERGY ANALYSIS 51 2–1 2–2 Systems and Control Volumes 41 2–8 Energy and Environment 85 Ozone and Smog 86 Acid Rain 87 The Greenhouse Effect: Global Warming and Climate Change 88 Topic of Special Interest: Mechanisms of Heat Transfer 91 Summary 96 References and Suggested Readings 97 Problems 97 82 68 x THERMODYNAMICS chapter three PROPERTIES OF PURE SUBSTANCES 3–1 3–2 3–3 Pure Substance 4–2 4–3 4–4 111 112 Phases of a Pure Substance 4–5 Phase-Change Processes of Pure Substances 113 120 The Ideal-Gas Equation of State 3–8 5–2 Compressibility Factor—A Measure of Deviation from Ideal-Gas Behavior 138 168 Flow Work and the Energy of a Flowing Fluid 221 Energy Analysis of Steady-Flow Systems 225 5–4 Some Steady-Flow Engineering Devices 228 1 2 3 4a 4b 5 5–5 chapter four Polytropic Process 214 5–3 Topic of Special Interest: Vapor Pressure and Phase Equilibrium 146 Summary 150 References and Suggested Readings 151 Problems 151 164 Conservation of Mass Total Energy of a Flowing Fluid 222 Energy Transport by Mass 223 Other Equations of State 141 Moving Boundary Work 184 Mass and Volume Flow Rates 214 Conservation of Mass Principle 216 Mass Balance for Steady-Flow Processes 218 Special Case: Incompressible Flow 219 van der Waals Equation of State 142 Beattie-Bridgeman Equation of State 142 Benedict-Webb-Rubin Equation of State 143 Virial Equation of State 144 4–1 Internal Energy, Enthalpy, and Specific Heats of Solids and Liquids 183 chapter five 134 ENERGY ANALYSIS OF CLOSED SYSTEMS 178 Topic of Special Interest: Thermodynamic Aspects of Biological Systems 187 Summary 195 References and Suggested Readings 195 Problems 196 5–1 Is Water Vapor an Ideal Gas? 137 3–7 Internal Energy, Enthalpy, and Specific Heats of Ideal Gases 176 MASS AND ENERGY ANALYSIS OF CONTROL VOLUMES 213 Property Tables 124 Enthalpy—A Combination Property 124 1a Saturated Liquid and Saturated Vapor States 125 1b Saturated Liquid–Vapor Mixture 127 2 Superheated Vapor 130 3 Compressed Liquid 131 Reference State and Reference Values 132 3–6 169 174 Internal Energy Changes Enthalpy Changes 184 Property Diagrams for Phase-Change Processes 118 1 The T-v Diagram 118 2 The P-v Diagram 120 Extending the Diagrams to Include the Solid Phase 3 The P-T Diagram 122 The P-v-T Surface 123 3–5 Specific Heats Specific Heat Relations of Ideal Gases 112 Compressed Liquid and Saturated Liquid 114 Saturated Vapor and Superheated Vapor 114 Saturation Temperature and Saturation Pressure 115 Some Consequences of Tsat and Psat Dependence 116 3–4 Energy Balance for Closed Systems 163 Nozzles and Diffusers 229 Turbines and Compressors 232 Throttling Valves 234 Mixing Chambers 236 Heat Exchangers 238 Pipe and Duct Flow 240 Energy Analysis of Unsteady-Flow Processes 242 Topic of Special Interest: General Energy Equation 247 Summary 251 References and Suggested Readings 252 Problems 252 xi CONTENTS chapter six THE SECOND LAW OF THERMODYNAMICS 6–1 6–2 6–3 Introduction to the Second Law Thermal Energy Reservoirs Heat Engines 275 276 7–7 7–8 7–9 277 278 Refrigerators and Heat Pumps Perpetual-Motion Machines 283 Irreversibilities 293 Internally and Externally Reversible Processes 6–7 The Carnot Cycle 294 352 364 307 Topic of Special Interest: Reducing the Cost of Compressed Air 386 Summary 395 References and Suggested Readings 396 Problems 397 chapter eight 330 EXERGY A Special Case: Internally Reversible Isothermal Heat Transfer Processes 7–3 The Entropy Change of Ideal Gases 349 Entropy Change of a System, DSsystem 374 Mechanisms of Entropy Transfer, Sin and Sout 374 1 Heat Transfer 374 2 Mass Flow 375 Entropy Generation, Sgen 376 Closed Systems 377 Control Volumes 378 Entropy Generation Associated with a Heat Transfer Process 385 303 329 333 8–1 The Increase of Entropy Principle 334 Some Remarks about Entropy Entropy Change of Liquids and Solids 7–13 Entropy Balance 373 chapter seven 7–2 347 Isentropic Efficiency of Turbines 367 Isentropic Efficiencies of Compressors and Pumps Isentropic Efficiency of Nozzles 371 Topic of Special Interest: Household Refrigerators Summary 311 References and Suggested Readings 312 Problems 312 Entropy 346 7–12 Isentropic Efficiencies of Steady-Flow Devices 367 6–11 The Carnot Refrigerator and Heat Pump 304 7–1 The T ds Relations Multistage Compression with Intercooling 292 6–8 The Carnot Principles 297 6–9 The Thermodynamic Temperature Scale 299 6–10 The Carnot Heat Engine 301 ENTROPY 343 7–11 Minimizing the Compressor Work 363 297 The Quality of Energy 302 Quantity versus Quality in Daily Life What Is Entropy? 342 Proof that Steady-Flow Devices Deliver the Most and Consume the Least Work When the Process is Reversible 362 295 The Reversed Carnot Cycle Property Diagrams Involving Entropy 7–10 Reversible Steady-Flow Work 359 290 Reversible and Irreversible Processes 340 Constant Specific Heats (Approximate Analysis) 353 Variable Specific Heats (Exact Analysis) 353 Isentropic Processes of Ideal Gases 355 Constant Specific Heats (Approximate Analysis) 355 Variable Specific Heats (Exact Analysis) 356 Relative Pressure and Relative Specific Volume 356 Coefficient of Performance 284 Heat Pumps 285 Performance of Refrigerators, Air-Conditioners, and Heat Pumps 286 The Second Law of Thermodynamics: Clausius Statement 288 Equivalence of the Two Statements 289 6–5 6–6 Isentropic Processes Entropy and Entropy Generation in Daily Life Thermal Efficiency 279 Can We Save Qout? 281 The Second Law of Thermodynamics: Kelvin–Planck Statement 283 6–4 7–4 7–5 7–6 337 Exergy: Work Potential of Energy Exergy (Work Potential) Associated with Kinetic and Potential Energy 336 Entropy Change of Pure Substances 421 8–2 422 423 Reversible Work and Irreversibility 425 369 xii THERMODYNAMICS 8–3 8–4 Second-Law Efficiency 9–9 The Brayton Cycle with Regeneration 513 9–10 The Brayton Cycle with Intercooling, Reheating, and Regeneration 516 9–11 Ideal Jet-Propulsion Cycles 520 430 Exergy Change of a System 433 Exergy of a Fixed Mass: Nonflow (or Closed System) Exergy 433 Exergy of a Flow Stream: Flow (or Stream) Exergy 436 8–5 Modifications to Turbojet Engines 9–12 Second-Law Analysis of Gas Power Cycles 526 Exergy Transfer by Heat, Work, And Mass 438 Exergy by Heat Transfer, Q 439 Exergy Transfer by Work, W 440 Exergy Transfer by Mass, m 440 8–6 The Decrease of Exergy Principle and Exergy Destruction 441 Exergy Destruction 8–7 8–8 524 Topic of Special Interest: Saving Fuel and Money by Driving Sensibly 530 Summary 536 References and Suggested Readings 538 Problems 538 442 Exergy Balance: Closed Systems 443 Exergy Balance: Control Volumes Exergy Balance for Steady-Flow Systems Reversible Work 456 Second-Law Efficiency of Steady-Flow Devices 456 chapter ten 454 VAPOR AND COMBINED POWER CYCLES 455 10–1 The Carnot Vapor Cycle 554 10–2 Rankine Cycle: The Ideal Cycle for Vapor Power Cycles 555 Topic of Special Interest: Second-Law Aspects of Daily Life 463 Summary 467 References and Suggested Readings 468 Problems 468 Energy Analysis of the Ideal Rankine Cycle GAS POWER CYCLES 485 9–1 Basic Considerations in the Analysis of Power Cycles 486 9–2 The Carnot Cycle and its Value in Engineering 488 9–3 9–4 9–5 Air-Standard Assumptions 9–6 Diesel Cycle: The Ideal Cycle for Compression-Ignition Engines 499 9–7 9–8 Stirling and Ericsson Cycles 490 Otto Cycle: The Ideal Cycle for Spark-Ignition Engines 492 Brayton Cycle: The Ideal Cycle for Gas-Turbine Engines 506 Development of Gas Turbines 509 Deviation of Actual Gas-Turbine Cycles from Idealized Ones 512 Lowering the Condenser Pressure (Lowers Tlow,avg) 561 Superheating the Steam to High Temperatures (Increases Thigh,avg) 562 Increasing the Boiler Pressure (Increases Thigh,avg) 562 10–5 The Ideal Reheat Rankine Cycle 565 10–6 The Ideal Regenerative Rankine Cycle 569 490 502 555 10–3 Deviation of Actual Vapor Power Cycles from Idealized Ones 558 10–4 How Can We Increase the Efficiency of the Rankine Cycle? 561 chapter nine An Overview of Reciprocating Engines 553 Open Feedwater Heaters 569 Closed Feedwater Heaters 571 10–7 Second-Law Analysis of Vapor Power Cycles 577 10–8 Cogeneration 579 10–9 Combined Gas–Vapor Power Cycles 584 Topic of Special Interest: Binary Vapor Cycles 587 Summary 589 References and Suggested Readings 590 Problems 590 xiii CONTENTS chapter eleven 12–5 The Joule-Thomson Coefficient 672 12–6 The Dh, Du, and Ds of Real Gases 674 REFRIGERATION CYCLES 607 11–1 Refrigerators and Heat Pumps 608 11–2 The Reversed Carnot Cycle 609 11–3 The Ideal Vapor-Compression Refrigeration Cycle 610 11–4 Actual Vapor-Compression Refrigeration Cycle 613 11–5 Second-Law Analysis of Vapor-Compression Refrigeration Cycle 615 11–6 Selecting the Right Refrigerant 620 11–7 Heat Pump Systems 622 11–8 Innovative Vapor-Compression Refrigeration Systems 623 Cascade Refrigeration Systems 624 Multistage Compression Refrigeration Systems 626 Multipurpose Refrigeration Systems with a Single Compressor 628 Liquefaction of Gases 629 Enthalpy Changes of Real Gases 674 Internal Energy Changes of Real Gases Entropy Changes of Real Gases 676 Summary 679 References and Suggested Readings Problems 680 GAS MIXTURES 13–1 Composition of a Gas Mixture: Mass and Mole Fractions 688 13–2 P-v-T Behavior of Gas Mixtures: Ideal and Real Gases 690 Ideal-Gas Mixtures 691 Real-Gas Mixtures 692 Ideal-Gas Mixtures 696 Real-Gas Mixtures 700 Topic of Special Interest: Chemical Potential and the Separation Work of Mixtures 704 659 12–2 The Maxwell Relations 661 12–3 The Clapeyron Equation 662 12–4 General Relations For du, dh, ds, cv , and cp 665 Internal Energy Changes 666 Enthalpy Changes 666 Entropy Changes 667 Specific Heats cv and cp 668 Summary 714 References and Suggested Readings Problems 716 715 chapter fourteen GAS–VAPOR MIXTURES AND AIR-CONDITIONING 725 chapter twelve Partial Differentials 657 Partial Differential Relations 687 13–3 Properties of Gas Mixtures: Ideal and Real Gases 695 Topic of Special Interest: Thermoelectric Power Generation and Refrigeration Systems 636 Summary 638 References and Suggested Readings 639 Problems 639 12–1 A Little Math—Partial Derivatives and Associated Relations 656 680 chapter thirteen 11–9 Gas Refrigeration Cycles 630 11–10 Absorption Refrigeration Systems 633 THERMODYNAMIC PROPERTY RELATIONS 675 655 14–1 14–2 14–3 14–4 Dry and Atmospheric Air 726 Specific and Relative Humidity of Air 727 Dew-Point Temperature 729 Adiabatic Saturation and Wet-Bulb Temperatures 731 14– 5 The Psychrometric Chart 734 14–6 Human Comfort and Air-Conditioning 735 14–7 Air-Conditioning Processes 737 Simple Heating and Cooling (v 5 constant) Heating with Humidification 739 Cooling with Dehumidification 740 Evaporative Cooling 742 738 xiv THERMODYNAMICS Adiabatic Mixing of Airstreams Wet Cooling Towers 745 743 Summary 828 References and Suggested Readings Problems 829 Summary 747 References and Suggested Readings 748 Problems 749 829 chapter seventeen chapter fifteen COMPRESSIBLE FLOW CHEMICAL REACTIONS 17–1 Stagnation Properties 840 17–2 Speed of Sound and Mach Number 843 17–3 One-Dimensional Isentropic Flow 845 759 15–1 Fuels and Combustion 760 15–2 Theoretical and Actual Combustion Processes 764 15–3 Enthalpy of Formation and Enthalpy of Combustion 771 15–4 First-Law Analysis of Reacting Systems 774 Variation of Fluid Velocity with Flow Area 847 Property Relations for Isentropic Flow of Ideal Gases 849 17–4 Isentropic Flow Through Nozzles 851 Converging Nozzles 852 Converging–Diverging Nozzles 856 17–5 Shock Waves and Expansion Waves 860 Steady-Flow Systems 775 Closed Systems 776 15–5 Adiabatic Flame Temperature 780 15–6 Entropy Change of Reacting Systems 782 15–7 Second-Law Analysis of Reacting Systems 784 Topic of Special Interest: Fuel Cells 790 Summary 792 References and Suggested Readings 793 Problems 793 Normal Shocks 860 Oblique Shocks 866 Prandtl–Meyer Expansion Waves 870 17–6 Duct Flow with Heat Transfer and Negligible Friction (Rayleigh Flow) 875 Property Relations for Rayleigh Flow 881 Choked Rayleigh Flow 882 17–7 Steam Nozzles 884 Summary 887 References and Suggested Readings Problems 889 chapter sixteen CHEMICAL AND PHASE EQUILIBRIUM 839 805 16–1 Criterion for Chemical Equilibrium 806 16–2 The Equilibrium Constant for Ideal-Gas Mixtures 808 16–3 Some Remarks about the Kp of Ideal-Gas Mixtures 812 16–4 Chemical Equilibrium for Simultaneous Reactions 816 16–5 Variation of Kp with Temperature 818 16–6 Phase Equilibrium 820 Phase Equilibrium for a Single-Component System 820 The Phase Rule 822 Phase Equilibrium for a Multicomponent System 822 888 chapter eighteen (web chapter) RENEWABLE ENERGY 18–1 Introduction 18-2 Solar Energy Solar Radiation Flat-Plate Solar Collector Concentrating Solar Collector Linear Concentrating Solar Power Collector Solar-Power Tower Plant Solar Pond Photovoltaic Cell Passive Solar Applications Solar Heat Gain through Windows xv CONTENTS 18-3 Wind Energy Wind Turbine Types and Power Performance Curve Wind Power Potential Wind Power Density Wind Turbine Efficiency Betz Limit for Wind Turbine Efficiency 18-4 Hydropower Analysis of Hydroelectric Power Plant Turbine Types 18–5 Geothermal Energy Geothermal Power Production 18–6 Biomass Energy Biomass Resources Conversion of Biomass to Biofuel Biomass Products Electricity and Heat Production by Biomass Solid Municipality Waste Summary References and Suggested Readings Problems Figure A–14 Figure A–15 Table A–16 Table A–17 Table A–18 Table A–19 Table A–20 Table A–21 Table A–22 Table A–23 appendix one Table A–24 PROPERTY TABLES AND CHARTS (SI UNITS) 897 Table A–25 Table A–26 Table A–1 Table A–2 Table A–3 Table A–4 Table A–5 Table A–6 Table A–7 Table A–8 Figure A–9 Figure A–10 Table A–11 Table A–12 Table A–13 Molar mass, gas constant, and criticalpoint properties 898 Ideal-gas specific heats of various common gases 899 Properties of common liquids, solids, and foods 902 Saturated water—Temperature table 904 Saturated water—Pressure table 906 Superheated water 908 Compressed liquid water 912 Saturated ice–water vapor 913 T-s diagram for water 914 Mollier diagram for water 915 Saturated refrigerant-134a— Temperature table 916 Saturated refrigerant-134a— Pressure table 918 Superheated refrigerant-134a 919 Table A–27 Table A–28 Figure A–29 Figure A–30 Figure A–31 Table A–32 Table A–33 Table A–34 P-h diagram for refrigerant-134a 921 Nelson–Obert generalized compressibility chart 922 Properties of the atmosphere at high altitude 923 Ideal-gas properties of air 924 Ideal-gas properties of nitrogen, N2 926 Ideal-gas properties of oxygen, O2 928 Ideal-gas properties of carbon dioxide, CO2 930 Ideal-gas properties of carbon monoxide, CO 932 Ideal-gas properties of hydrogen, H2 934 Ideal-gas properties of water vapor, H2O 935 Ideal-gas properties of monatomic oxygen, O 937 Ideal-gas properties of hydroxyl, OH 937 Enthalpy of formation, Gibbs function of formation, and absolute entropy at 258C, 1 atm 938 Properties of some common fuels and hydrocarbons 939 Natural logarithms of the equilibrium constant Kp 940 Generalized enthalpy departure chart 941 Generalized entropy departure chart 942 Psychrometric chart at 1 atm total pressure 943 One-dimensional isentropic compressible-flow functions for an ideal gas with k 5 1.4 944 One-dimensional normal-shock functions for an ideal gas with k 5 1.4 945 Rayleigh flow functions for an ideal gas with k 5 1.4 946 xvi THERMODYNAMICS appendix two PROPERTY TABLES AND CHARTS (ENGLISH UNITS) 947 Table A–1E Molar mass, gas constant, and criticalpoint properties 948 Table A–2E Ideal-gas specific heats of various common gases 949 Table A–3E Properties of common liquids, solids, and foods 952 Table A–4E Saturated water—Temperature table 954 Table A–5E Saturated water—Pressure table 956 Table A–6E Superheated water 958 Table A–7E Compressed liquid water 962 Table A–8E Saturated ice–water vapor 963 Figure A–9E T-s diagram for water 964 Figure A–10E Mollier diagram for water 965 Table A–11E Saturated refrigerant-134a— Temperature table 966 Table A–12E Saturated refrigerant-134a—Pressure table 967 Table A–13E Superheated refrigerant-134a 968 Figure A–14E P-h diagram for refrigerant-134a 970 Table A–16E Properties of the atmosphere at high altitude 971 Table A–17E Ideal-gas properties of air 972 Table A–18E Ideal-gas properties of nitrogen, N2 974 Table A–19E Ideal-gas properties of oxygen, O2 976 Table A–20E Ideal-gas properties of carbon dioxide, CO2 978 Table A–21E Ideal-gas properties of carbon monoxide, CO 980 Table A–22E Ideal-gas properties of hydrogen, H2 982 Table A–23E Ideal-gas properties of water vapor, H2O 983 Table A–26E Enthalpy of formation, Gibbs function of formation, and absolute entropy at 778C, 1 atm 985 Table A–27E Properties of some common fuels and hydrocarbons 986 Figure A–31E Psychrometric chart at 1 atm total pressure 987 INDEX 989 Preface BACKGROUND Thermodynamics is an exciting and fascinating subject that deals with energy, and thermodynamics has long been an essential part of engineering curricula all over the world. It has a broad application area ranging from microscopic organisms to common household appliances, transportation vehicles, power generation systems, and even philosophy. This introductory book contains sufficient material for two sequential courses in thermodynamics. Students are assumed to have an adequate background in calculus and physics. OBJECTIVES This book is intended for use as a textbook by undergraduate engineering students in their sophomore or junior year, and as a reference book for practicing engineers. The objectives of this text are • To cover the basic principles of thermodynamics. • To present a wealth of real-world engineering examples to give students a feel for how thermodynamics is applied in engineering practice. • To develop an intuitive understanding of thermodynamics by emphasizing the physics and physical arguments that underpin the theory. It is our hope that this book, through its careful explanations of concepts and its use of numerous practical examples and figures, helps students develop the necessary skills to bridge the gap between knowledge and the confidence to properly apply knowledge. PHILOSOPHY AND GOAL The philosophy that contributed to the overwhelming popularity of the prior editions of this book has remained unchanged in this edition. Namely, our goal has been to offer an engineering textbook that • Communicates directly to the minds of tomorrow’s engineers in a simple yet precise manner. • Leads students toward a clear understanding and firm grasp of the basic principles of thermodynamics. • Encourages creative thinking and development of a deeper understanding and intuitive feel for thermodynamics. • Is read by students with interest and enthusiasm rather than being used as an aid to solve problems. Special effort has been made to appeal to students’ natural curiosity and to help them explore the various facets of the exciting subject area of thermodynamics. The enthusiastic responses we have received from users of prior editions—from small colleges to large universities all over the world—and the continued translations into new languages indicate that our objectives xviii THERMODYNAMICS have largely been achieved. It is our philosophy that the best way to learn is by practice. Therefore, special effort is made throughout the book to reinforce material that was presented earlier. Yesterday’s engineer spent a major portion of his or her time substituting values into the formulas and obtaining numerical results. However, formula manipulations and number crunching are now being left mainly to computers. Tomorrow’s engineer will need a clear understanding and a firm grasp of the basic principles so that he or she can understand even the most complex problems, formulate them, and interpret the results. A conscious effort is made to emphasize these basic principles while also providing students with a perspective of how computational tools are used in engineering practice. The traditional classical, or macroscopic, approach is used throughout the text, with microscopic arguments serving in a supporting role as appropriate. This approach is more in line with students’ intuition and makes learning the subject matter much easier. NEW IN THIS EDITION The primary change in this eighth edition of the text is the effective use of full color to enhance the learning experience of students and to make it more enjoyable. Another significant change is the addition of a new web chapter on Renewable Energy available via the Online Learning Center. The third important change is the update of the R-134a tables to make property values consistent with those from the latest version of EES. All the solved examples and end-of-chapter problems dealing with R-134a are modified to reflect this change. This edition includes numerous new problems with a variety of applications. Problems, whose solutions require parametric investigations and thus the use of a computer, are identified by a computer-EES icon, as before. Some existing problems from previous editions have been removed, and other updates and changes for clarity and readability have been made throughout the text. The eighth edition also includes McGraw-Hill’s Connect® Engineering. This online homework management tool allows assignment of algorithmic problems for homework, quizzes and tests. It connects students with the tools and resources they’ll need to achieve success. To learn more, visit www.mcgrawhillconnect.com. McGraw-Hill LearnSmart™ is also available as an integrated feature of McGraw-Hill Connect® Engineering. It is an adaptive learning system designed to help students learn faster, study more efficiently, and retain more knowledge for greater success. LearnSmart assesses a student’s knowledge of course content through a series of adaptive questions. It pinpoints concepts the student does not understand and maps out a personalized study plan for success. Visit the following site for a demonstration: www.mhlearnsmart.com. LEARNING TOOLS EARLY INTRODUCTION OF THE FIRST LAW OF THERMODYNAMICS The first law of thermodynamics is introduced early in Chapter 2, “Energy, Energy Transfer, and General Energy Analysis.” This introductory chapter xix PREFACE sets the framework of establishing a general understanding of various forms of energy, mechanisms of energy transfer, the concept of energy balance, thermo-economics, energy conversion, and conversion efficiency using familiar settings that involve mostly electrical and mechanical forms of energy. It also exposes students to some exciting real-world applications of thermodynamics early in the course, and helps them establish a sense of the monetary value of energy. There is special emphasis on the utilization of renewable energy such as wind power and hydraulic energy, and the efficient use of existing resources. EMPHASIS ON PHYSICS A distinctive feature of this book is its emphasis on the physical aspects of the subject matter in addition to mathematical representations and manipulations. The authors believe that the emphasis in undergraduate education should remain on developing a sense of underlying physical mechanisms and a mastery of solving practical problems that an engineer is likely to face in the real world. Developing an intuitive understanding should also make the course a more motivating and worthwhile experience for students. EFFECTIVE USE OF ASSOCIATION An observant mind should have no difficulty understanding engineering sciences. After all, the principles of engineering sciences are based on our everyday experiences and experimental observations. Therefore, a physical, intuitive approach is used throughout this text. Frequently, parallels are drawn between the subject matter and students’ everyday experiences so that they can relate the subject matter to what they already know. The process of cooking, for example, serves as an excellent vehicle to demonstrate the basic principles of thermodynamics. SELF-INSTRUCTING The material in the text is introduced at a level that an average student can follow comfortably. It speaks to students, not over students. In fact, it is selfinstructive. The order of coverage is from simple to general. That is, it starts with the simplest case and adds complexities gradually. In this way, the basic principles are repeatedly applied to different systems, and students master how to apply the principles instead of how to simplify a general formula. Noting that the principles of sciences are based on experimental observations, all the derivations in this text are based on physical arguments, and thus they are easy to follow and understand. EXTENSIVE USE OF ARTWORK Figures are important learning tools that help students “get the picture,” and the text makes very effective use of graphics. This edition of Thermodynamics: An Engineering Approach, Eighth Edition features an enhanced art program done in four colors to provide more realism and pedagogical understanding. Further, a large number of figures have been upgraded to become threedimensional and thus more real-life. Figures attract attention and stimulate curiosity and interest. Most of the figures in this text are intended to serve as a means of emphasizing some key concepts that would otherwise go unnoticed; some serve as page summaries. xx THERMODYNAMICS LEARNING OBJECTIVES AND SUMMARIES Each chapter begins with an overview of the material to be covered and chapter-specific learning objectives. A summary is included at the end of each chapter, providing a quick review of basic concepts and important relations, and pointing out the relevance of the material. NUMEROUS WORKED-OUT EXAMPLES WITH A SYSTEMATIC SOLUTIONS PROCEDURE Each chapter contains several worked-out examples that clarify the material and illustrate the use of the basic principles. An intuitive and systematic approach is used in the solution of the example problems, while maintaining an informal conversational style. The problem is first stated, and the objectives are identified. The assumptions are then stated, together with their justifications. The properties needed to solve the problem are listed separately if appropriate. Numerical values are used together with their units to emphasize that numbers without units are meaningless, and that unit manipulations are as important as manipulating the numerical values with a calculator. The significance of the findings is discussed following the solutions. This approach is also used consistently in the solutions presented in the instructor’s solutions manual. A WEALTH OF REAL-WORLD END-OF-CHAPTER PROBLEMS The end-of-chapter problems are grouped under specific topics to make problem selection easier for both instructors and students. Within each group of problems are Concept Questions, indicated by “C,” to check the students’ level of understanding of basic concepts. The problems under Review Problems are more comprehensive in nature and are not directly tied to any specific section of a chapter—in some cases they require review of material learned in previous chapters. Problems designated as Design and Essay are intended to encourage students to make engineering judgments, to conduct independent exploration of topics of interest, and to communicate their findings in a professional manner. Problems designated by an “E” are in English units, are solved using EES, and SI users can ignore them. Problems with the and complete solutions together with parametric studies are included on the are comprehensive in nature and textbook’s website. Problems with the are intended to be solved with a computer, possibly using the EES software. Several economics- and safety-related problems are incorporated throughout to promote cost and safety awareness among engineering students. Answers to selected problems are listed immediately following the problem for convenience to students. In addition, to prepare students for the Fundamentals of Engineering Exam (that is becoming more important for the outcome-based ABET 2000 criteria) and to facilitate multiple-choice tests, over 200 multiplechoice problems are included in the end-of-chapter problem sets. They are placed under the title Fundamentals of Engineering (FE) Exam Problems for easy recognition. These problems are intended to check the understanding of fundamentals and to help readers avoid common pitfalls. RELAXED SIGN CONVENTION The use of a formal sign convention for heat and work is abandoned as it often becomes counterproductive. A physically meaningful and engaging approach is adopted for interactions instead of a mechanical approach. xxi PREFACE Subscripts “in” and “out,” rather than the plus and minus signs, are used to indicate the directions of interactions. PHYSICALLY MEANINGFUL FORMULAS The physically meaningful forms of the balance equations rather than formulas are used to foster deeper understanding and to avoid a cookbook approach. The mass, energy, entropy, and exergy balances for any system undergoing any process are expressed as min 2 mout 5 Dmsystem Mass balance: Energy balance: Ein 2 Eout 5 Net energy transfer by heat, work, and mass Entropy balance: Sin 2 Sout 1 Net entropy transfer by heat and mass Exergy balance: Xin 2 Xout Net exergy transfer by heat, work, and mass 2 Sgen Change in internal, kinetic, potential, etc., energies 5 Entropy generation Xdestroyed Exergy destruction DEsystem DSsystem Change in entropy 5 DXsystem Change in exergy These relations reinforce the fundamental principles that during an actual process mass and energy are conserved, entropy is generated, and exergy is destroyed. Students are encouraged to use these forms of balances in early chapters after they specify the system, and to simplify them for the particular problem. A more relaxed approach is used in later chapters as students gain mastery. A CHOICE OF SI ALONE OR SI/ENGLISH UNITS In recognition of the fact that English units are still widely used in some industries, both SI and English units are used in this text, with an emphasis on SI. The material in this text can be covered using combined SI/English units or SI units alone, depending on the preference of the instructor. The property tables and charts in the appendices are presented in both units, except the ones that involve dimensionless quantities. Problems, tables, and charts in English units are designated by “E” after the number for easy recognition, and they can be ignored by SI users. TOPICS OF SPECIAL INTEREST Most chapters contain a section called “Topic of Special Interest” where interesting aspects of thermodynamics are discussed. Examples include Thermodynamic Aspects of Biological Systems in Chapter 4, Household Refrigerators in Chapter 6, Second-Law Aspects of Daily Life in Chapter 8, and Saving Fuel and Money by Driving Sensibly in Chapter 9. The topics selected for these sections provide intriguing extensions to thermodynamics, but they can be ignored if desired without a loss in continuity. xxii THERMODYNAMICS GLOSSARY OF THERMODYNAMIC TERMS Throughout the chapters, when an important key term or concept is introduced and defined, it appears in boldface type. Fundamental thermodynamic terms and concepts also appear in a glossary located on our accompanying website (www.mhhe.com/cengel). This unique glossary helps to reinforce key terminology and is an excellent learning and review tool for students as they move forward in their study of thermodynamics. In addition, students can test their knowledge of these fundamental terms by using the flash cards and other interactive resources. CONVERSION FACTORS Frequently used conversion factors and physical constants are listed on the inner cover pages of the text for easy reference. SUPPLEMENTS The following supplements are available to users of the book. ENGINEERING EQUATION SOLVER (EES) Developed by Sanford Klein and William Beckman from the University of Wisconsin—Madison, this software combines equation-solving capability and engineering property data. EES can do optimization, parametric analysis, and linear and nonlinear regression, and provides publication-quality plotting capabilities. Thermodynamics and transport properties for air, water, and many other fluids are built in, and EES allows the user to enter property data or functional relationships. EES is a powerful equation solver with built-in functions and property tables for thermodynamic and transport properties as well as automatic unit checking capability. It requires less time than a calculator for data entry and allows more time for thinking critically about modeling and solving engineering problems. Look for the EES icons in the homework problems sections of the text. The Limited Academic Version of EES is available for departmental license upon adoption of the Eighth Edition of Thermodynamics: An Engineering Approach (meaning that the text is required for students in the course). You may load this software onto your institution’s computer system, for use by students and faculty related to the course, as long as the arrangement between McGraw-Hill Education and F-Chart is in effect. There are minimum order requirements stipulated by F-Chart to qualify. PROPERTIES TABLE BOOKLET (ISBN 0-07-762477-7) This booklet provides students with an easy reference to the most important property tables and charts, many of which are found at the back of the textbook in both the SI and English units. COSMOS McGraw-Hill’s COSMOS (Complete Online Solutions Manual Organization System) allows instructors to streamline the creation of assignments, quizzes, and tests by using problems and solutions from the textbook, as well as their own custom material. COSMOS is now available online at http://cosmos.mhhe.com/ xxiii PREFACE ACKNOWLEDGMENTS The authors would like to acknowledge with appreciation the numerous and valuable comments, suggestions, constructive criticisms, and praise from the following evaluators and reviewers: Edward Anderson Texas Tech University John Biddle Cal Poly Pomona University Gianfranco DiGiuseppe Kettering University Shoeleh Di Julio California State University-Northridge Afshin Ghajar Oklahoma State University Harry Hardee New Mexico State University Kevin Lyons North Carolina State University Kevin Macfarlan John Brown University Saeed Manafzadeh University of Illinois-Chicago Alex Moutsoglou South Dakota State University Rishi Raj The City College of New York Maria Sanchez California State University-Fresno Kalyan Srinivasan Mississippi State University Robert Stiger Gonzaga University Their suggestions have greatly helped to improve the quality of this text. In particular we would like to express our gratitude to Mehmet Kanoglu of the University of Gaziantep, Turkey, for his valuable contributions, his critical review of the manuscript, and for his special attention to accuracy and detail. We also would like to thank our students, who provided plenty of feedback from students’ perspectives. Finally, we would like to express our appreciation to our wives, Zehra Çengel and Sylvia Boles, and to our children for their continued patience, understanding, and support throughout the preparation of this text. Yunus A. Çengel Michael A. Boles This page intentionally left blank Online Resources for Students and Instructors MCGRAW-HILL CONNECT® ENGINEERING McGraw-Hill Connect Engineering is a web-based assignment and assessment platform that gives students the means to better connect with their coursework, with their instructors, and with the important concepts that they will need to know for success now and in the future. With Connect Engineering, instructors can deliver assignments, quizzes, and tests easily online. Students can practice important skills at their own pace and on their own schedule. Connect Engineering for Thermodynamics: An Engineering Approach, Eighth Edition is available via the text website at www.mhhe.com/cengel COSMOS McGraw-Hill’s COSMOS (Complete Online Solutions Manual Organization System) allows instructors to streamline the creation of assignments, quizzes, and tests by using problems and solutions from the textbook, as well as their own custom material. COSMOS is now available online at http://cosmos. mhhe.com/ WWW.MHHE.COM/CENGEL This site offers resources for students and instructors. The following resources are available for students: • Glossary of Key Terms in Thermodynamics—Bolded terms in the text are defined in this accessible glossary. Organized at the chapter level or available as one large file. • Student Study Guide—This resource outlines the fundamental concepts of the text and is a helpful guide that allows students to focus on the most important concepts. The guide can also serve as a lecture outline for instructors. • Learning Objectives—The chapter learning objectives are outlined here. Organized by chapter and tied to ABET objectives. • Self-Quizzing—Students can test their knowledge using multiple-choice quizzing. These self-tests provide immediate feedback and are an excellent learning tool. • Flashcards—Interactive flashcards test student understanding of the text terms and their definitions. The program also allows students to flag terms that require further understanding. • Crossword Puzzles—An interactive, timed puzzle that provides hints as well as a notes section. • Errata—If errors should be found in the text, they will be reported here. xxvi THERMODYNAMICS The following resources are available for instructors under password protection: • Instructor Testbank—Additional problems prepared for instructors to assign to students. Solutions are given, and use of EES is recommended to verify accuracy. • Correlation Guide—New users of this text will appreciate this resource. The guide provides a smooth transition for instructors not currently using the Çengel/Boles text. • Image Library—The electronic version of the figures are supplied for easy integration into course presentations, exams, and assignments. • Instructor’s Guide—Provides instructors with helpful tools such as sample syllabi and exams, an ABET conversion guide, a thermodynamics glossary, and chapter objectives. • Errata—If errors should be found in the solutions manual, they will be reported here. • Solutions Manual—The detailed solutions to all text homework problems are provided in PDF form. • EES Solutions Manual—The entire solutions manual is also available in EES. Any problem in the text can be modified and the solution of the modified problem can readily be obtained by copying and pasting the given EES solution on a blank EES screen and hitting the solve button. • PP slides—Powerpoint presentation slides for all chapters in the text are available for use in lectures • Appendices—These are provided in PDF form for ease of use. CHAPTER 1 INTRODUCTION AND BASIC CONCEPTS E very science has a unique vocabulary associated with it, and thermodynamics is no exception. Precise definition of basic concepts forms a sound foundation for the development of a science and prevents possible misunderstandings. We start this chapter with an overview of thermodynamics and the unit systems, and continue with a discussion of some basic concepts such as system, state, state postulate, equilibrium, and process. We discuss intensive and extensive properties of a system and define density, specific gravity, and specific weight. We also discuss temperature and temperature scales with particular emphasis on the International Temperature Scale of 1990. We then present pressure, which is the normal force exerted by a fluid per unit area and discuss absolute and gage pressures, the variation of pressure with depth, and pressure measurement devices, such as manometers and barometers. Careful study of these concepts is essential for a good understanding of the topics in the following chapters. Finally, we present an intuitive systematic problem-solving technique that can be used as a model in solving engineering problems. OBJECTIVES The objectives of Chapter 1 are to: ■ Identify the unique vocabulary associated with thermodynamics through the precise definition of basic concepts to form a sound foundation for the development of the principles of thermodynamics. ■ ■ ■ ■ ■ Review the metric SI and the English unit systems that will be used throughout the text. Explain the basic concepts of thermodynamics such as system, state, state postulate, equilibrium, process, and cycle. Discuss properties of a system and define density, specific gravity, and specific weight. Review concepts of temperature, temperature scales, pressure, and absolute and gage pressure. Introduce an intuitive systematic problem-solving technique. 1 2 INTRODUCTION AND BASIC CONCEPTS PE = 10 units KE = 0 PE = 7 units KE = 3 units Potential energy Kinetic energy FIGURE 1–1 Energy cannot be created or destroyed; it can only change forms (the first law). Energy storage (1 unit) Energy in (5 units) Energy out (4 units) FIGURE 1–2 Conservation of energy principle for the human body. Cool environment 20°C Hot coffee 70°C Heat FIGURE 1–3 Heat flows in the direction of decreasing temperature. 1–1 ■ THERMODYNAMICS AND ENERGY Thermodynamics can be defined as the science of energy. Although everybody has a feeling of what energy is, it is difficult to give a precise definition for it. Energy can be viewed as the ability to cause changes. The name thermodynamics stems from the Greek words therme (heat) and dynamis (power), which is most descriptive of the early efforts to convert heat into power. Today the same name is broadly interpreted to include all aspects of energy and energy transformations including power generation, refrigeration, and relationships among the properties of matter. One of the most fundamental laws of nature is the conservation of energy principle. It simply states that during an interaction, energy can change from one form to another but the total amount of energy remains constant. That is, energy cannot be created or destroyed. A rock falling off a cliff, for example, picks up speed as a result of its potential energy being converted to kinetic energy (Fig. 1–1). The conservation of energy principle also forms the backbone of the diet industry: A person who has a greater energy input (food) than energy output (exercise) will gain weight (store energy in the form of fat), and a person who has a smaller energy input than output will lose weight (Fig. 1–2). The change in the energy content of a body or any other system is equal to the difference between the energy input and the energy output, and the energy balance is expressed as Ein 2 Eout 5 DE. The first law of thermodynamics is simply an expression of the conservation of energy principle, and it asserts that energy is a thermodynamic property. The second law of thermodynamics asserts that energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy. For example, a cup of hot coffee left on a table eventually cools, but a cup of cool coffee in the same room never gets hot by itself (Fig. 1–3). The high-temperature energy of the coffee is degraded (transformed into a less useful form at a lower temperature) once it is transferred to the surrounding air. Although the principles of thermodynamics have been in existence since the creation of the universe, thermodynamics did not emerge as a science until the construction of the first successful atmospheric steam engines in England by Thomas Savery in 1697 and Thomas Newcomen in 1712. These engines were very slow and inefficient, but they opened the way for the development of a new science. The first and second laws of thermodynamics emerged simultaneously in the 1850s, primarily out of the works of William Rankine, Rudolph Clausius, and Lord Kelvin (formerly William Thomson). The term thermodynamics was first used in a publication by Lord Kelvin in 1849. The first thermodynamics textbook was written in 1859 by William Rankine, a professor at the University of Glasgow. It is well-known that a substance consists of a large number of particles called molecules. The properties of the substance naturally depend on the behavior of these particles. For example, the pressure of a gas in a container is the result of momentum transfer between the molecules and the walls of the container. However, one does not need to know the behavior of the gas particles to determine the pressure in the container. It would be sufficient to attach a pressure gage to the container. This macroscopic approach to the 3 CHAPTER 1 study of thermodynamics that does not require a knowledge of the behavior of individual particles is called classical thermodynamics. It provides a direct and easy way to the solution of engineering problems. A more elaborate approach, based on the average behavior of large groups of individual particles, is called statistical thermodynamics. This microscopic approach is rather involved and is used in this text only in the supporting role. Application Areas of Thermodynamics All activities in nature involve some interaction between energy and matter; thus, it is hard to imagine an area that does not relate to thermodynamics in some manner. Therefore, developing a good understanding of basic principles of thermodynamics has long been an essential part of engineering education. Thermodynamics is commonly encountered in many engineering systems and other aspects of life, and one does not need to go very far to see some application areas of it. In fact, one does not need to go anywhere. The heart is constantly pumping blood to all parts of the human body, various energy conversions occur in trillions of body cells, and the body heat generated is constantly rejected to the environment. The human comfort is closely tied to the rate of this metabolic heat rejection. We try to control this heat transfer rate by adjusting our clothing to the environmental conditions. Other applications of thermodynamics are right where one lives. An ordinary house is, in some respects, an exhibition hall filled with wonders of thermodynamics (Fig. 1–4). Many ordinary household utensils and appliances are designed, in whole or in part, by using the principles of thermodynamics. Some examples include the electric or gas range, the heating and air-conditioning systems, the refrigerator, the humidifier, the pressure cooker, the water heater, the shower, the iron, and even the computer and the TV. On a larger scale, thermodynamics plays a major part in the design and analysis of automotive engines, rockets, jet engines, and conventional or nuclear power plants, solar collectors, and the design of vehicles from ordinary cars to airplanes (Fig. 1–5). The energy-efficient home that you may be living in, for example, is designed on the basis of minimizing heat loss in winter and heat gain in summer. The size, location, and the power input of the fan of your computer is also selected after an analysis that involves thermodynamics. 1–2 ■ IMPORTANCE OF DIMENSIONS AND UNITS Any physical quantity can be characterized by dimensions. The magnitudes assigned to the dimensions are called units. Some basic dimensions such as mass m, length L, time t, and temperature T are selected as primary or fundamental dimensions, while others such as velocity V, energy E, and volume V are expressed in terms of the primary dimensions and are called secondary dimensions, or derived dimensions. A number of unit systems have been developed over the years. Despite strong efforts in the scientific and engineering community to unify the world with a single unit system, two sets of units are still in common use today: the English system, which is also known as the United States Solar collectors Shower Hot water Hot water tank Cold water Heat exchanger Pump FIGURE 1–4 The design of many engineering systems, such as this solar hot water system, involves thermodynamics. 4 INTRODUCTION AND BASIC CONCEPTS Refrigerator Boats Aircraft and spacecraft © McGraw-Hill Education, Jill Braaten © Doug Menuez/Getty Images RF © PhotoLink/Getty Images RF Power plants Human body Cars © Malcolm Fife/Getty Images RF © Ryan McVay/Getty Images RF © Mark Evans/Getty Images RF Wind turbines Food processing A piping network in an industrial facility. © F. Schussler/PhotoLink/Getty Images RF Glow Images RF Courtesy of UMDE Engineering Contracting and Trading. Used by permission FIGURE 1–5 Some application areas of thermodynamics. Customary System (USCS), and the metric SI (from Le Système International d’ Unités), which is also known as the International System. The SI is a simple and logical system based on a decimal relationship between the various units, and it is being used for scientific and engineering work in most of the industrialized nations, including England. The English system, however, has no apparent systematic numerical base, and various units in this system are related to each other rather arbitrarily (12 in 5 1 ft, 1 mile 5 5280 ft, 4 qt 5 1 gal, etc.), which makes it confusing and difficult to learn. The United States is the only industrialized country that has not yet fully converted to the metric system. The systematic efforts to develop a universally acceptable system of units dates back to 1790 when the French National Assembly charged the French Academy of Sciences to come up with such a unit system. An early version of the metric system was soon developed in France, but it did not 5 CHAPTER 1 find universal acceptance until 1875 when The Metric Convention Treaty was prepared and signed by 17 nations, including the United States. In this international treaty, meter and gram were established as the metric units for length and mass, respectively, and a General Conference of Weights and Measures (CGPM) was established that was to meet every six years. In 1960, the CGPM produced the SI, which was based on six fundamental quantities, and their units were adopted in 1954 at the Tenth General Conference of Weights and Measures: meter (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, degree Kelvin (°K) for temperature, and candela (cd) for luminous intensity (amount of light). In 1971, the CGPM added a seventh fundamental quantity and unit: mole (mol) for the amount of matter. Based on the notational scheme introduced in 1967, the degree symbol was officially dropped from the absolute temperature unit, and all unit names were to be written without capitalization even if they were derived from proper names (Table 1–1). However, the abbreviation of a unit was to be capitalized if the unit was derived from a proper name. For example, the SI unit of force, which is named after Sir Isaac Newton (1647–1723), is newton (not Newton), and it is abbreviated as N. Also, the full name of a unit may be pluralized, but its abbreviation cannot. For example, the length of an object can be 5 m or 5 meters, not 5 ms or 5 meter. Finally, no period is to be used in unit abbreviations unless they appear at the end of a sentence. For example, the proper abbreviation of meter is m (not m.). The recent move toward the metric system in the United States seems to have started in 1968 when Congress, in response to what was happening in the rest of the world, passed a Metric Study Act. Congress continued to promote a voluntary switch to the metric system by passing the Metric Conversion Act in 1975. A trade bill passed by Congress in 1988 set a September 1992 deadline for all federal agencies to convert to the metric system. However, the deadlines were relaxed later with no clear plans for the future. The industries that are heavily involved in international trade (such as the automotive, soft drink, and liquor industries) have been quick in converting to the metric system for economic reasons (having a single worldwide design, fewer sizes, smaller inventories, etc.). Today, nearly all the cars manufactured in the United States are metric. Most car owners probably do not realize this until they try an English socket wrench on a metric bolt. Most industries, however, resisted the change, thus slowing down the conversion process. Presently the United States is a dual-system society, and it will stay that way until the transition to the metric system is completed. This puts an extra burden on today’s engineering students, since they are expected to retain their understanding of the English system while learning, thinking, and working in terms of the SI. Given the position of the engineers in the transition period, both unit systems are used in this text, with particular emphasis on SI units. As pointed out, the SI is based on a decimal relationship between units. The prefixes used to express the multiples of the various units are listed in Table 1–2. They are standard for all units, and the student is encouraged to memorize them because of their widespread use (Fig. 1–6). TABLE 1–1 The seven fundamental (or primary) dimensions and their units in SI Dimension Unit Length Mass Time Temperature Electric current Amount of light Amount of matter meter (m) kilogram (kg) second (s) kelvin (K) ampere (A) candela (cd) mole (mol) TABLE 1–2 Standard prefixes in SI units Multiple Prefix 24 yotta, Y zetta, Z exa, E peta, P tera, T giga, G mega, M kilo, k hecto, h deka, da deci, d centi, c milli, m micro, m nano, n pico, p femto, f atto, a zepto, z yocto, y 10 1021 1018 1015 1012 109 106 103 102 101 1021 1022 1023 1026 1029 10212 10215 10218 10221 10224 200 mL (0.2 L) 1 kg (103 g) 1 M⍀ (10 6 ⍀) FIGURE 1–6 The SI unit prefixes are used in all branches of engineering. 6 INTRODUCTION AND BASIC CONCEPTS Some SI and English Units In SI, the units of mass, length, and time are the kilogram (kg), meter (m), and second (s), respectively. The respective units in the English system are the pound-mass (lbm), foot (ft), and second (s). The pound symbol lb is actually the abbreviation of libra, which was the ancient Roman unit of weight. The English retained this symbol even after the end of the Roman occupation of Britain in 410. The mass and length units in the two systems are related to each other by 1 lbm 5 0.45359 kg 1 ft 5 0.3048 m a = 1 m/s 2 m = 1 kg F=1N In the English system, force is usually considered to be one of the primary dimensions and is assigned a nonderived unit. This is a source of confusion and error that necessitates the use of a dimensional constant (gc) in many formulas. To avoid this nuisance, we consider force to be a secondary dimension whose unit is derived from Newton’s second law, that is, Force 5 (Mass)(Acceleration) a = 1 ft/s 2 m = 32.174 lbm F = 1 lbf or F 5 ma FIGURE 1–7 The definition of the force units. (1–1) In SI, the force unit is the newton (N), and it is defined as the force required to accelerate a mass of 1 kg at a rate of 1 m/s2. In the English system, the force unit is the pound-force (lbf) and is defined as the force required to accelerate a mass of 32.174 lbm (1 slug) at a rate of 1 ft/s2 (Fig. 1–7). That is, 1 kgf 1 N 5 1 kg·m/s2 10 apples m ⬇ 1 kg 1 apple m ⬇ 102 g 1N 1 lbf 5 32.174 lbm·ft/s2 4 apples m ⬇ 1 lbm 1 lbf A force of 1 N is roughly equivalent to the weight of a small apple (m 5 102 g), whereas a force of 1 lbf is roughly equivalent to the weight of four medium apples (mtotal 5 454 g), as shown in Fig. 1–8. Another force unit in common use in many European countries is the kilogram-force (kgf), which is the weight of 1 kg mass at sea level (1 kgf 5 9.807 N). The term weight is often incorrectly used to express mass, particularly by the “weight watchers.” Unlike mass, weight W is a force. It is the gravitational force applied to a body, and its magnitude is determined from Newton’s second law, W 5 mg (N) FIGURE 1–8 The relative magnitudes of the force units newton (N), kilogram-force (kgf), and pound-force (lbf). (1–2) where m is the mass of the body, and g is the local gravitational acceleration (g is 9.807 m/s2 or 32.174 ft/s2 at sea level and 45° latitude). An ordinary bathroom scale measures the gravitational force acting on a body. The mass of a body remains the same regardless of its location in the universe. Its weight, however, changes with a change in gravitational acceleration. A body weighs less on top of a mountain since g decreases 7 CHAPTER 1 with altitude. On the surface of the moon, an astronaut weighs about onesixth of what she or he normally weighs on earth (Fig. 1–9). At sea level a mass of 1 kg weighs 9.807 N, as illustrated in Fig. 1–10. A mass of 1 lbm, however, weighs 1 lbf, which misleads people to believe that pound-mass and pound-force can be used interchangeably as pound (lb), which is a major source of error in the English system. It should be noted that the gravity force acting on a mass is due to the attraction between the masses, and thus it is proportional to the magnitudes of the masses and inversely proportional to the square of the distance between them. Therefore, the gravitational acceleration g at a location depends on the local density of the earth’s crust, the distance to the center of the earth, and to a lesser extent, the positions of the moon and the sun. The value of g varies with location from 9.832 m/s2 at the poles (9.789 at the equator) to 7.322 m/s2 at 1000 km above sea level. However, at altitudes up to 30 km, the variation of g from the sea-level value of 9.807 m/s2 is less than 1 percent. Therefore, for most practical purposes, the gravitational acceleration can be assumed to be constant at 9.807 m/s2, often rounded to 9.81 m/s2. It is interesting to note that at locations below sea level, the value of g increases with distance from the sea level, reaches a maximum at about 4500 m, and then starts decreasing. (What do you think the value of g is at the center of the earth?) The primary cause of confusion between mass and weight is that mass is usually measured indirectly by measuring the gravity force it exerts. This approach also assumes that the forces exerted by other effects such as air buoyancy and fluid motion are negligible. This is like measuring the distance to a star by measuring its red shift, or measuring the altitude of an airplane by measuring barometric pressure. Both of these are also indirect measurements. The correct direct way of measuring mass is to compare it to a known mass. This is cumbersome, however, and it is mostly used for calibration and measuring precious metals. Work, which is a form of energy, can simply be defined as force times distance; therefore, it has the unit “newton-meter (N·m),” which is called a joule (J). That is, 1 J 5 1 N·m FIGURE 1–9 A body weighing 150 lbf on earth will weigh only 25 lbf on the moon. kg lbm g = 9.807 m/s2 W = 9.807 kg·m/s2 = 9.807 N = 1 kgf g = 32.174 ft/s2 W = 32.174 lbm·ft/s2 = 1 lbf FIGURE 1–10 The weight of a unit mass at sea level. (1–3) A more common unit for energy in SI is the kilojoule (1 kJ 5 103 J). In the English system, the energy unit is the Btu (British thermal unit), which is defined as the energy required to raise the temperature of 1 lbm of water at 68°F by 1°F. In the metric system, the amount of energy needed to raise the temperature of 1 g of water at 14.5°C by 1°C is defined as 1 calorie (cal), and 1 cal 5 4.1868 J. The magnitudes of the kilojoule and Btu are almost identical (1 Btu 5 1.0551 kJ). Here is a good way to get a feel for these units: If you light a typical match and let it burn itself out, it yields approximately one Btu (or one kJ) of energy (Fig. 1–11). The unit for time rate of energy is joule per second (J/s), which is called a watt (W). In the case of work, the time rate of energy is called power. A commonly used unit of power is horsepower (hp), which is equivalent to 746 W. Electrical energy typically is expressed in the unit kilowatt-hour (kWh), which is equivalent to 3600 kJ. An electric appliance with a rated power of 1 kW consumes 1 kWh of electricity when running continuously FIGURE 1–11 A typical match yields about one Btu (or one kJ) of energy if completely burned. Photo by John M. Cimbala 8 INTRODUCTION AND BASIC CONCEPTS for one hour. When dealing with electric power generation, the units kW and kWh are often confused. Note that kW or kJ/s is a unit of power, whereas kWh is a unit of energy. Therefore, statements like “the new wind turbine will generate 50 kW of electricity per year” are meaningless and incorrect. A correct statement should be something like “the new wind turbine with a rated power of 50 kW will generate 120,000 kWh of electricity per year.” Dimensional Homogeneity We all know that apples and oranges do not add. But we somehow manage to do it (by mistake, of course). In engineering, all equations must be dimensionally homogeneous. That is, every term in an equation must have the same unit. If, at some stage of an analysis, we find ourselves in a position to add two quantities that have different units, it is a clear indication that we have made an error at an earlier stage. So checking dimensions can serve as a valuable tool to spot errors. EXAMPLE 1–1 Electric Power Generation by a Wind Turbine A school is paying $0.12/kWh for electric power. To reduce its power bill, the school installs a wind turbine (Fig. 1–12) with a rated power of 30 kW. If the turbine operates 2200 hours per year at the rated power, determine the amount of electric power generated by the wind turbine and the money saved by the school per year. SOLUTION A wind turbine is installed to generate electricity. The amount of electric energy generated and the money saved per year are to be determined. Analysis The wind turbine generates electric energy at a rate of 30 kW or 30 kJ/s. Then the total amount of electric energy generated per year becomes Total energy 5 (Energy per unit time)(Time interval) 5 (30 kW)(2200 h) 5 66,000 kWh The money saved per year is the monetary value of this energy determined as Money saved 5 (Total energy)(Unit cost of energy) 5 (66,000 kWh)($0.12/kWh) 5 $7920 Discussion The annual electric energy production also could be determined in kJ by unit manipulations as FIGURE 1–12 A wind turbine, as discussed in Example 1–1. ©Bear Dancer Studios/Mark Dierker RF Total energy 5 (30 kW)(2200 h)a 3600 s 1 kJ/s ba b 5 2.38 3 108 kJ 1h 1 kW which is equivalent to 66,000 kWh (1 kWh = 3600 kJ). We all know from experience that units can give terrible headaches if they are not used carefully in solving a problem. However, with some attention and skill, units can be used to our advantage. They can be used to check formulas; sometimes they can even be used to derive formulas, as explained in the following example. 9 CHAPTER 1 EXAMPLE 1–2 Obtaining Formulas from Unit Considerations A tank is filled with oil whose density is r 5 850 kg/m3. If the volume of the tank is V 5 2 m3, determine the amount of mass m in the tank. Oil = 2 m3 ρ = 850 kg/m3 m=? SOLUTION The volume of an oil tank is given. The mass of oil is to be determined. Assumptions Oil is a nearly incompressible substance and thus its density is constant. Analysis A sketch of the system just described is given in Fig. 1–13. Suppose we forgot the formula that relates mass to density and volume. However, we know that mass has the unit of kilograms. That is, whatever calculations we do, we should end up with the unit of kilograms. Putting the given information into perspective, we have r 5 850 kg/m3 and FIGURE 1–13 Schematic for Example 1–2. V 5 2 m3 It is obvious that we can eliminate m3 and end up with kg by multiplying these two quantities. Therefore, the formula we are looking for should be m 5 rV Thus, m 5 (850 kg/m3)(2 m3) 5 1700 kg Discussion Note that this approach may not work for more complicated formulas. Nondimensional constants also may be present in the formulas, and these cannot be derived from unit considerations alone. You should keep in mind that a formula that is not dimensionally homogeneous is definitely wrong (Fig. 1–14), but a dimensionally homogeneous formula is not necessarily right. Unity Conversion Ratios FIGURE 1–14 Always check the units in your calculations. Just as all nonprimary dimensions can be formed by suitable combinations of primary dimensions, all nonprimary units (secondary units) can be formed by combinations of primary units. Force units, for example, can be expressed as 1 N 5 1 kg m s2 and 1 lbf 5 32.174 lbm ft s2 They can also be expressed more conveniently as unity conversion ratios as 1N 51 1 kg·m /s2 and 1 lbf 51 32.174 lbm·ft /s2 Unity conversion ratios are identically equal to 1 and are unitless, and thus such ratios (or their inverses) can be inserted conveniently into any calculation to properly convert units (Fig. 1–15). You are encouraged to always use unity conversion ratios such as those given here when converting units. Some textbooks insert the archaic gravitational constant gc defined as gc 5 32.174 lbm·ft/lbf·s2 5 1 kg·m/N·s2 5 1 into equations in order to force 32.174 lbm?ft/s2 1 kg?m/s2 1 lbf 1N 1W 1 J/s 1 kJ 1000 N?m 0.3048 m 1 ft 1 min 60 s 1 kPa 1000 N/m2 1 lbm 0.45359 kg FIGURE 1–15 Every unity conversion ratio (as well as its inverse) is exactly equal to one. Shown here are a few commonly used unity conversion ratios. 10 INTRODUCTION AND BASIC CONCEPTS units to match. This practice leads to unnecessary confusion and is strongly discouraged by the present authors. We recommend that you instead use unity conversion ratios. lbm EXAMPLE 1–3 The Weight of One Pound-Mass Using unity conversion ratios, show that 1.00 lbm weighs 1.00 lbf on earth (Fig. 1–16). FIGURE 1–16 A mass of 1 lbm weighs 1 lbf on earth. SOLUTION A mass of 1.00 lbm is subjected to standard earth gravity. Its weight in lbf is to be determined. Assumptions Standard sea-level conditions are assumed. Properties The gravitational constant is g 5 32.174 ft/s2. Analysis We apply Newton’s second law to calculate the weight (force) that corresponds to the known mass and acceleration. The weight of any object is equal to its mass times the local value of gravitational acceleration. Thus, W 5 mg 5 (1.00 lbm)(32.174 ft /s2)a Net weight: One pound (454 grams) 1 lbf b 5 1.00 lbf 32.174 lbm·ft /s2 Discussion The quantity in large parentheses in this equation is a unity conversion ratio. Mass is the same regardless of its location. However, on some other planet with a different value of gravitational acceleration, the weight of 1 lbm would differ from that calculated here. When you buy a box of breakfast cereal, the printing may say “Net weight: One pound (454 grams).” (See Fig. 1–17.) Technically, this means that the cereal inside the box weighs 1.00 lbf on earth and has a mass of 453.6 g (0.4536 kg). Using Newton’s second law, the actual weight of the cereal on earth is W 5 mg 5 (453.6 g)(9.81 m/s2)a FIGURE 1–17 A quirk in the metric system of units. Surroundings System Boundary FIGURE 1–18 System, surroundings, and boundary. 1–3 ■ 1 kg 1N ba b 5 4.49 N 1 kg·m/s2 1000 g SYSTEMS AND CONTROL VOLUMES A system is defined as a quantity of matter or a region in space chosen for study. The mass or region outside the system is called the surroundings. The real or imaginary surface that separates the system from its surroundings is called the boundary (Fig. 1–18). The boundary of a system can be fixed or movable. Note that the boundary is the contact surface shared by both the system and the surroundings. Mathematically speaking, the boundary has zero thickness, and thus it can neither contain any mass nor occupy any volume in space. Systems may be considered to be closed or open, depending on whether a fixed mass or a fixed volume in space is chosen for study. A closed system (also known as a control mass or just system when the context makes it clear) consists of a fixed amount of mass, and no mass can cross its boundary. That is, no mass can enter or leave a closed system, as shown in 11 CHAPTER 1 Fig. 1–19. But energy, in the form of heat or work, can cross the boundary; and the volume of a closed system does not have to be fixed. If, as a special case, even energy is not allowed to cross the boundary, that system is called an isolated system. Consider the piston-cylinder device shown in Fig. 1–20. Let us say that we would like to find out what happens to the enclosed gas when it is heated. Since we are focusing our attention on the gas, it is our system. The inner surfaces of the piston and the cylinder form the boundary, and since no mass is crossing this boundary, it is a closed system. Notice that energy may cross the boundary, and part of the boundary (the inner surface of the piston, in this case) may move. Everything outside the gas, including the piston and the cylinder, is the surroundings. An open system, or a control volume, as it is often called, is a properly selected region in space. It usually encloses a device that involves mass flow such as a compressor, turbine, or nozzle. Flow through these devices is best studied by selecting the region within the device as the control volume. Both mass and energy can cross the boundary of a control volume. A large number of engineering problems involve mass flow in and out of a system and, therefore, are modeled as control volumes. A water heater, a car radiator, a turbine, and a compressor all involve mass flow and should be analyzed as control volumes (open systems) instead of as control masses (closed systems). In general, any arbitrary region in space can be selected as a control volume. There are no concrete rules for the selection of control volumes, but the proper choice certainly makes the analysis much easier. If we were to analyze the flow of air through a nozzle, for example, a good choice for the control volume would be the region within the nozzle. The boundaries of a control volume are called a control surface, and they can be real or imaginary. In the case of a nozzle, the inner surface of the nozzle forms the real part of the boundary, and the entrance and exit areas form the imaginary part, since there are no physical surfaces there (Fig. 1–21a). Imaginary boundary Closed system Mass No m = constant Energy Yes FIGURE 1–19 Mass cannot cross the boundaries of a closed system, but energy can. Moving boundary Gas 2 kg 1.5 m3 Gas 2 kg 1 m3 Fixed boundary FIGURE 1–20 A closed system with a moving boundary. Real boundary Moving boundary CV (a nozzle) (a) A control volume (CV) with real and imaginary boundaries CV Fixed boundary (b) A control volume (CV) with fixed and moving boundaries as well as real and imaginary boundaries FIGURE 1–21 A control volume can involve fixed, moving, real, and imaginary boundaries. 12 INTRODUCTION AND BASIC CONCEPTS A control volume can be fixed in size and shape, as in the case of a nozzle, or it may involve a moving boundary, as shown in Fig. 1–21b. Most control volumes, however, have fixed boundaries and thus do not involve any moving boundaries. A control volume can also involve heat and work interactions just as a closed system, in addition to mass interaction. As an example of an open system, consider the water heater shown in Fig. 1–22. Let us say that we would like to determine how much heat we must transfer to the water in the tank in order to supply a steady stream of hot water. Since hot water will leave the tank and be replaced by cold water, it is not convenient to choose a fixed mass as our system for the analysis. Instead, we can concentrate our attention on the volume formed by the interior surfaces of the tank and consider the hot and cold water streams as mass leaving and entering the control volume. The interior surfaces of the tank form the control surface for this case, and mass is crossing the control surface at two locations. In an engineering analysis, the system under study must be defined carefully. In most cases, the system investigated is quite simple and obvious, and defining the system may seem like a tedious and unnecessary task. In other cases, however, the system under study may be rather involved, and a proper choice of the system may greatly simplify the analysis. FIGURE 1–22 An open system (a control volume) with one inlet and one exit. © McGraw-Hill Education, Christopher Kerrigan 1–4 ■ PROPERTIES OF A SYSTEM Any characteristic of a system is called a property. Some familiar properties are pressure P, temperature T, volume V, and mass m. The list can be extended to include less familiar ones such as viscosity, thermal conductivity, modulus of elasticity, thermal expansion coefficient, electric resistivity, and even velocity and elevation. Properties are considered to be either intensive or extensive. Intensive properties are those that are independent of the mass of a system, such as temperature, pressure, and density. Extensive properties are those whose values depend on the size—or extent—of the system. Total mass, total volume, and total momentum are some examples of extensive properties. An easy way to determine whether a property is intensive or extensive is to divide the system into two equal parts with an imaginary partition, as shown in Fig. 1–23. Each part will have the same value of intensive properties as the original system, but half the value of the extensive properties. Generally, uppercase letters are used to denote extensive properties (with mass m being a major exception), and lowercase letters are used for intensive properties (with pressure P and temperature T being the obvious exceptions). Extensive properties per unit mass are called specific properties. Some examples of specific properties are specific volume (v 5 V/m) and specific total energy (e 5 E/m). Continuum FIGURE 1–23 Criterion to differentiate intensive and extensive properties. Matter is made up of atoms that are widely spaced in the gas phase. Yet it is very convenient to disregard the atomic nature of a substance and view it as a continuous, homogeneous matter with no holes, that is, a continuum. 13 CHAPTER 1 The continuum idealization allows us to treat properties as point functions and to assume the properties vary continually in space with no jump discontinuities. This idealization is valid as long as the size of the system we deal with is large relative to the space between the molecules. This is the case in practically all problems, except some specialized ones. The continuum idealization is implicit in many statements we make, such as “the density of water in a glass is the same at any point.” To have a sense of the distance involved at the molecular level, consider a container filled with oxygen at atmospheric conditions. The diameter of the oxygen molecule is about 3 3 10210 m and its mass is 5.3 3 10226 kg. Also, the mean free path of oxygen at 1 atm pressure and 20°C is 6.3 3 1028 m. That is, an oxygen molecule travels, on average, a distance of 6.3 3 1028 m (about 200 times of its diameter) before it collides with another molecule. Also, there are about 3 3 1016 molecules of oxygen in the tiny volume of 1 mm3 at 1 atm pressure and 20°C (Fig. 1–24). The continuum model is applicable as long as the characteristic length of the system (such as its diameter) is much larger than the mean free path of the molecules. At very high vacuums or very high elevations, the mean free path may become large (for example, it is about 0.1 m for atmospheric air at an elevation of 100 km). For such cases the rarefied gas flow theory should be used, and the impact of individual molecules should be considered. In this text we will limit our consideration to substances that can be modeled as a continuum. 1–5 ■ O2 1 atm, 20°C 3 ´ 1016 molecules/mm3 VOID FIGURE 1–24 Despite the relatively large gaps between molecules, a gas can usually be treated as a continuum because of the very large number of molecules even in an extremely small volume. DENSITY AND SPECIFIC GRAVITY Density is defined as mass per unit volume (Fig. 1–25). Density: r5 m V (kg/m3) (1–4) The reciprocal of density is the specific volume v, which is defined as volume per unit mass. That is, v5 V 1 5 r m V = 12 m 3 m = 3 kg r = 0.25 kg/m 3 1 3 v=– r = 4 m /kg (1–5) For a differential volume element of mass dm and volume dV, density can be expressed as r 5 dm/dV. The density of a substance, in general, depends on temperature and pressure. The density of most gases is proportional to pressure and inversely proportional to temperature. Liquids and solids, on the other hand, are essentially incompressible substances, and the variation of their density with pressure is usually negligible. At 20°C, for example, the density of water changes from 998 kg/m3 at 1 atm to 1003 kg/m3 at 100 atm, a change of just 0.5 percent. The density of liquids and solids depends more strongly on temperature than it does on pressure. At 1 atm, for example, the density of water changes from 998 kg/m3 at 20°C to 975 kg/m3 at 75°C, a change of 2.3 percent, which can still be neglected in many engineering analyses. FIGURE 1–25 Density is mass per unit volume; specific volume is volume per unit mass. 14 INTRODUCTION AND BASIC CONCEPTS Sometimes the density of a substance is given relative to the density of a well-known substance. Then it is called specific gravity, or relative density, and is defined as the ratio of the density of a substance to the density of some standard substance at a specified temperature (usually water at 4°C, for which rH2O 5 1000 kg/m3). That is, TABLE 1–3 Specific gravities of some substances at 0°C Substance SG Water Blood Seawater Gasoline Ethyl alcohol Mercury Wood Gold Bones Ice Air (at 1 atm) 1.0 1.05 1.025 0.7 0.79 13.6 0.3–0.9 19.2 1.7–2.0 0.92 0.0013 Specific gravity: SG 5 r rH2O (1–6) Note that the specific gravity of a substance is a dimensionless quantity. However, in SI units, the numerical value of the specific gravity of a substance is exactly equal to its density in g/cm3 or kg/L (or 0.001 times the density in kg/m3) since the density of water at 4°C is 1 g/cm3 5 1 kg/L 5 1000 kg/m3. The specific gravity of mercury at 0°C, for example, is 13.6. Therefore, its density at 0°C is 13.6 g/cm3 5 13.6 kg/L 5 13,600 kg/m3. The specific gravities of some substances at 0°C are given in Table 1–3. Note that substances with specific gravities less than 1 are lighter than water, and thus they would float on water. The weight of a unit volume of a substance is called specific weight and is expressed as Specific weight: gs 5 rg (N/m3) (1–7) where g is the gravitational acceleration. The densities of liquids are essentially constant, and thus they can often be approximated as being incompressible substances during most processes without sacrificing much in accuracy. m = 2 kg T1 = 20°C V1 = 1.5 m3 (a) State 1 m = 2 kg T2 = 20°C V2 = 2.5 m3 (b) State 2 FIGURE 1–26 A system at two different states. 20°C 23°C 30°C 35°C 40°C 42°C (a) Before 32°C 32°C 32°C 32°C 32°C 32°C (b) After FIGURE 1–27 A closed system reaching thermal equilibrium. 1–6 ■ STATE AND EQUILIBRIUM Consider a system not undergoing any change. At this point, all the properties can be measured or calculated throughout the entire system, which gives us a set of properties that completely describes the condition, or the state, of the system. At a given state, all the properties of a system have fixed values. If the value of even one property changes, the state will change to a different one. In Fig. 1–26 a system is shown at two different states. Thermodynamics deals with equilibrium states. The word equilibrium implies a state of balance. In an equilibrium state there are no unbalanced potentials (or driving forces) within the system. A system in equilibrium experiences no changes when it is isolated from its surroundings. There are many types of equilibrium, and a system is not in thermodynamic equilibrium unless the conditions of all the relevant types of equilibrium are satisfied. For example, a system is in thermal equilibrium if the temperature is the same throughout the entire system, as shown in Fig. 1–27. That is, the system involves no temperature differential, which is the driving force for heat flow. Mechanical equilibrium is related to pressure, and a system is in mechanical equilibrium if there is no change in pressure at any point of the system with time. However, the pressure may vary within the system with elevation as a result of gravitational effects. 15 CHAPTER 1 For example, the higher pressure at a bottom layer is balanced by the extra weight it must carry, and, therefore, there is no imbalance of forces. The variation of pressure as a result of gravity in most thermodynamic systems is relatively small and usually disregarded. If a system involves two phases, it is in phase equilibrium when the mass of each phase reaches an equilibrium level and stays there. Finally, a system is in chemical equilibrium if its chemical composition does not change with time, that is, no chemical reactions occur. A system will not be in equilibrium unless all the relevant equilibrium criteria are satisfied. The State Postulate As noted earlier, the state of a system is described by its properties. But we know from experience that we do not need to specify all the properties in order to fix a state. Once a sufficient number of properties are specified, the rest of the properties assume certain values automatically. That is, specifying a certain number of properties is sufficient to fix a state. The number of properties required to fix the state of a system is given by the state postulate: The state of a simple compressible system is completely specified by two independent, intensive properties. A system is called a simple compressible system in the absence of electrical, magnetic, gravitational, motion, and surface tension effects. These effects are due to external force fields and are negligible for most engineering problems. Otherwise, an additional property needs to be specified for each effect that is significant. If the gravitational effects are to be considered, for example, the elevation z needs to be specified in addition to the two properties necessary to fix the state. The state postulate requires that the two properties specified be independent to fix the state. Two properties are independent if one property can be varied while the other one is held constant. Temperature and specific volume, for example, are always independent properties, and together they can fix the state of a simple compressible system (Fig. 1–28). Temperature and pressure, however, are independent properties for single-phase systems, but are dependent properties for multiphase systems. At sea level (P 5 1 atm), water boils at 100°C, but on a mountaintop where the pressure is lower, water boils at a lower temperature. That is, T 5 f(P) during a phase-change process; thus, temperature and pressure are not sufficient to fix the state of a two-phase system. Phase-change processes are discussed in detail in Chap. 3. 1–7 ■ Nitrogen T = 25°C v = 0.9 m3/kg FIGURE 1–28 The state of nitrogen is fixed by two independent, intensive properties. Property A State 2 Process path PROCESSES AND CYCLES Any change that a system undergoes from one equilibrium state to another is called a process, and the series of states through which a system passes during a process is called the path of the process (Fig. 1–29). To describe a process completely, one should specify the initial and final states of the process, as well as the path it follows, and the interactions with the surroundings. State 1 Property B FIGURE 1–29 A process between states 1 and 2 and the process path. 16 INTRODUCTION AND BASIC CONCEPTS (a) Slow compression (quasi-equilibrium) (b) Very fast compression (nonquasi-equilibrium) FIGURE 1–30 Quasi-equilibrium and nonquasiequilibrium compression processes. P Final state 2 Process path Initial state 1 V2 V1 V System (2) (1) FIGURE 1–31 The P-V diagram of a compression process. When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times, it is called a quasi-static, or quasi-equilibrium, process. A quasi-equilibrium process can be viewed as a sufficiently slow process that allows the system to adjust itself internally so that properties in one part of the system do not change any faster than those at other parts. This is illustrated in Fig. 1–30. When a gas in a piston-cylinder device is compressed suddenly, the molecules near the face of the piston will not have enough time to escape and they will have to pile up in a small region in front of the piston, thus creating a high-pressure region there. Because of this pressure difference, the system can no longer be said to be in equilibrium, and this makes the entire process nonquasi-equilibrium. However, if the piston is moved slowly, the molecules will have sufficient time to redistribute and there will not be a molecule pileup in front of the piston. As a result, the pressure inside the cylinder will always be nearly uniform and will rise at the same rate at all locations. Since equilibrium is maintained at all times, this is a quasi-equilibrium process. It should be pointed out that a quasi-equilibrium process is an idealized process and is not a true representation of an actual process. But many actual processes closely approximate it, and they can be modeled as quasi-equilibrium with negligible error. Engineers are interested in quasiequilibrium processes for two reasons. First, they are easy to analyze; second, work-producing devices deliver the most work when they operate on quasi-equilibrium processes. Therefore, quasi-equilibrium processes serve as standards to which actual processes can be compared. Process diagrams plotted by employing thermodynamic properties as coordinates are very useful in visualizing the processes. Some common properties that are used as coordinates are temperature T, pressure P, and volume V (or specific volume v). Figure 1–31 shows the P-V diagram of a compression process of a gas. Note that the process path indicates a series of equilibrium states through which the system passes during a process and has significance for quasiequilibrium processes only. For nonquasi-equilibrium processes, we are not able to characterize the entire system by a single state, and thus we cannot speak of a process path for a system as a whole. A nonquasi-equilibrium process is denoted by a dashed line between the initial and final states instead of a solid line. The prefix iso- is often used to designate a process for which a particular property remains constant. An isothermal process, for example, is a process during which the temperature T remains constant; an isobaric process is a process during which the pressure P remains constant; and an isochoric (or isometric) process is a process during which the specific volume v remains constant. A system is said to have undergone a cycle if it returns to its initial state at the end of the process. That is, for a cycle the initial and final states are identical. The Steady-Flow Process The terms steady and uniform are used frequently in engineering, and thus it is important to have a clear understanding of their meanings. The term 17 CHAPTER 1 steady implies no change with time. The opposite of steady is unsteady, or transient. The term uniform, however, implies no change with location over a specified region. These meanings are consistent with their everyday use (steady girlfriend, uniform properties, etc.). A large number of engineering devices operate for long periods of time under the same conditions, and they are classified as steady-flow devices. Processes involving such devices can be represented reasonably well by a somewhat idealized process, called the steady-flow process, which can be defined as a process during which a fluid flows through a control volume steadily (Fig. 1–32). That is, the fluid properties can change from point to point within the control volume, but at any fixed point they remain the same during the entire process. Therefore, the volume V, the mass m, and the total energy content E of the control volume remain constant during a steadyflow process (Fig. 1–33). Steady-flow conditions can be closely approximated by devices that are intended for continuous operation such as turbines, pumps, boilers, condensers, and heat exchangers or power plants or refrigeration systems. Some cyclic devices, such as reciprocating engines or compressors, do not satisfy any of the conditions stated above since the flow at the inlets and the exits will be pulsating and not steady. However, the fluid properties vary with time in a periodic manner, and the flow through these devices can still be analyzed as a steady-flow process by using time-averaged values for the properties. 1–8 ■ TEMPERATURE AND THE ZEROTH LAW OF THERMODYNAMICS Although we are familiar with temperature as a measure of “hotness” or “coldness,” it is not easy to give an exact definition for it. Based on our physiological sensations, we express the level of temperature qualitatively with words like freezing cold, cold, warm, hot, and red-hot. However, we cannot assign numerical values to temperatures based on our sensations alone. Furthermore, our senses may be misleading. A metal chair, for example, will feel much colder than a wooden one even when both are at the same temperature. Fortunately, several properties of materials change with temperature in a repeatable and predictable way, and this forms the basis for accurate temperature measurement. The commonly used mercury-in-glass thermometer, for example, is based on the expansion of mercury with temperature. Temperature is also measured by using several other temperature-dependent properties. It is a common experience that a cup of hot coffee left on the table eventually cools off and a cold drink eventually warms up. That is, when a body is brought into contact with another body that is at a different temperature, heat is transferred from the body at higher temperature to the one at lower temperature until both bodies attain the same temperature (Fig. 1–34). At that point, the heat transfer stops, and the two bodies are said to have reached thermal equilibrium. The equality of temperature is the only requirement for thermal equilibrium. Mass in 300°C 250°C Control volume 225°C 200°C Mass out 150°C Time: 1 PM Mass in 300°C 250°C Control volume 225°C 200°C Mass out 150°C Time: 3 PM FIGURE 1–32 During a steady-flow process, fluid properties within the control volume may change with position but not with time. Mass in Control volume mCV = const. Mass out ECV = const. FIGURE 1–33 Under steady-flow conditions, the mass and energy contents of a control volume remain constant. Iron Iron 150°C 60°C Copper Copper 20°C 60°C FIGURE 1–34 Two bodies reaching thermal equilibrium after being brought into contact in an isolated enclosure. 18 INTRODUCTION AND BASIC CONCEPTS The zeroth law of thermodynamics states that if two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. It may seem silly that such an obvious fact is called one of the basic laws of thermodynamics. However, it cannot be concluded from the other laws of thermodynamics, and it serves as a basis for the validity of temperature measurement. By replacing the third body with a thermometer, the zeroth law can be restated as two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact. The zeroth law was first formulated and labeled by R. H. Fowler in 1931. As the name suggests, its value as a fundamental physical principle was recognized more than half a century after the formulation of the first and the second laws of thermodynamics. It was named the zeroth law since it should have preceded the first and the second laws of thermodynamics. Temperature Scales Temperature scales enable us to use a common basis fo