Thermodynamics: An Engineering Approach 8th edition

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Thermodynamics, An Engineering Approach, eighth edition, covers the basic principles of thermodynamics while presenting a wealth of real-world engineering examples so students get a feel for how thermodynamics is applied in engineering practice. This text helps students develop an intuitive understanding by emphasizing the physics and physical arguments. Cengel and Boles explore the various facets of thermodynamics through careful explanations of concepts and use of numerous practical examples and figures, having students develop necessary skills to bridge the gap between knowledge and the confidence to properly apply their knowledge.

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THERMODYNAMICS AN ENGINEERING APPROACH EIGHTH EDITION cen98179_fm_i-xxvi.indd i cen98179_fm_i-xxvi.indd i 11/29/13 6:39 PM 11/29/13 6:39 PM

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THERMODYNAMICS AN ENGINEERING APPROACH EIGHTH EDITION YUNUS A. ÇENGEL University of Nevada Reno MICHAEL A. BOLES North Carolina State University cen98179_fm_i-xxvi.indd iii cen98179_fm_i-xxvi.indd iii 11/29/13 6:39 PM 11/29/13 6:39 PM

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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 Congress 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 cen98179_fm_i-xxvi.indd iv cen98179_fm_i-xxvi.indd iv 11/29/13 6:39 PM 11/29/13 6:39 PM

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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 posi- tion 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 cen98179_fm_i-xxvi.indd v cen98179_fm_i-xxvi.indd v 11/29/13 6:39 PM 11/29/13 6:39 PM

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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 edu- cation. 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 mini- mization 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 Engi- neer 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 Engi- neering at North Carolina State University where he earned his Ph.D. in mechani- cal 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 Edu cation 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 ana- lytical 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. About the Authors cen98179_fm_i-xxvi.indd vi cen98179_fm_i-xxvi.indd vi 11/29/13 6:39 PM 11/29/13 6:39 PM

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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 cen98179_fm_i-xxvi.indd vii cen98179_fm_i-xxvi.indd vii 11/29/13 6:39 PM 11/29/13 6:39 PM

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viii THERMODYNAMICS appendix 1 PROPERTY TABLES AND CHARTS SI UNITS 897 appendix 2 PROPERTY TABLES AND CHARTS ENGLISH UNITS 947 cen98179_fm_i-xxvi.indd viii cen98179_fm_i-xxvi.indd viii 11/29/13 6:39 PM 11/29/13 6:39 PM

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Preface xvii chapter one INTRODUCTION AND BASIC CONCEPTS 1 1–1 Thermodynamics and Energy 2 Application Areas of Thermodynamics 3 1–2 Importance of Dimensions and Units 3 Some SI and English Units 6 Dimensional Homogeneity 8 Unity Conversion Ratios 9 1–3 Systems and Control V olumes 10 1–4 Properties of a System 12 Continuum 12 1–5 Density and Specific Gravity 13 1–6 State and Equilibrium 14 The State Postulate 15 1–7 Processes and Cycles 15 The Steady-Flow Process 16 1–8 Temperature and the Zeroth Law of Thermodynamics 17 Temperature Scales 18 The International Temperature Scale of 1990 ITS-90 20 1–9 Pressure 22 Variation of Pressure with Depth 24 1–10 Pressure Measurement Devices 27 The Barometer 27 The Manometer 30 Other Pressure Measurement Devices 33 1–11 Problem-Solving Technique 34 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 Engineering Equation Solver EES 37 A Remark on Significant Digits 39 Summary 40 References and Suggested Readings 41 Problems 41 chapter two ENERGY ENERGY TRANSFER AND GENERAL ENERGY ANALYSIS 51 2–1 Introduction 52 2–2 Forms of Energy 53 Some Physical Insight to Internal Energy 55 More on Nuclear Energy 56 Mechanical Energy 58 2–3 Energy Transfer by Heat 60 Historical Background on Heat 61 2–4 Energy Transfer by Work 62 Electrical Work 65 2–5 Mechanical Forms of Work 66 Shaft Work 66 Spring Work 67 Work Done on Elastic Solid Bars 67 Work Associated with the Stretching of a Liquid Film 68 Work Done to Raise or to Accelerate a Body 68 Nonmechanical Forms of Work 70 2–6 The First Law of Thermodynamics 70 Energy Balance 72 Energy Change of a System DE system 72 Mechanisms of Energy Transfer E in and E out 73 2–7 Energy Conversion Efficiencies 78 Efficiencies of Mechanical and Electrical Devices 82 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 Contents cen98179_fm_i-xxvi.indd ix cen98179_fm_i-xxvi.indd ix 11/29/13 6:39 PM 11/29/13 6:39 PM

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x THERMODYNAMICS chapter three PROPERTIES OF PURE SUBSTANCES 111 3–1 Pure Substance 112 3–2 Phases of a Pure Substance 112 3–3 Phase-Change Processes of Pure Substances 113 Compressed Liquid and Saturated Liquid 114 Saturated Vapor and Superheated Vapor 114 Saturation Temperature and Saturation Pressure 115 Some Consequences of T sat and P sat Dependence 116 3–4 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 120 3 The P-T Diagram 122 The P-v-T Surface 123 3–5 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 The Ideal-Gas Equation of State 134 Is Water Vapor an Ideal Gas 137 3–7 Compressibility Factor—A Measure of Deviation from Ideal-Gas Behavior 138 3–8 Other Equations of State 141 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 Topic of Special Interest: Vapor Pressure and Phase Equilibrium 146 Summary 150 References and Suggested Readings 151 Problems 151 chapter four ENERGY ANALYSIS OF CLOSED SYSTEMS 163 4–1 Moving Boundary Work 164 Polytropic Process 168 4–2 Energy Balance for Closed Systems 169 4–3 Specific Heats 174 4–4 Internal Energy Enthalpy and Specific Heats of Ideal Gases 176 Specific Heat Relations of Ideal Gases 178 4–5 Internal Energy Enthalpy and Specific Heats of Solids and Liquids 183 Internal Energy Changes 184 Enthalpy Changes 184 Topic of Special Interest: Thermodynamic Aspects of Biological Systems 187 Summary 195 References and Suggested Readings 195 Problems 196 chapter five MASS AND ENERGY ANALYSIS OF CONTROL VOLUMES 213 5–1 Conservation of Mass 214 Mass and Volume Flow Rates 214 Conservation of Mass Principle 216 Mass Balance for Steady-Flow Processes 218 Special Case: Incompressible Flow 219 5–2 Flow Work and the Energy of a Flowing Fluid 221 Total Energy of a Flowing Fluid 222 Energy Transport by Mass 223 5–3 Energy Analysis of Steady-Flow Systems 225 5–4 Some Steady-Flow Engineering Devices 228 1 Nozzles and Diffusers 229 2 Turbines and Compressors 232 3 Throttling Valves 234 4a Mixing Chambers 236 4b Heat Exchangers 238 5 Pipe and Duct Flow 240 5–5 Energy Analysis of Unsteady-Flow Processes 242 Topic of Special Interest: General Energy Equation 247 Summary 251 References and Suggested Readings 252 Problems 252 cen98179_fm_i-xxvi.indd x cen98179_fm_i-xxvi.indd x 11/29/13 6:39 PM 11/29/13 6:39 PM

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CONTENTS xi chapter six THE SECOND LAW OF THERMODYNAMICS 275 6–1 Introduction to the Second Law 276 6–2 Thermal Energy Reservoirs 277 6–3 Heat Engines 278 Thermal Efficiency 279 Can We Save Q out 281 The Second Law of Thermodynamics: Kelvin–Planck Statement 283 6–4 Refrigerators and Heat Pumps 283 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 Perpetual-Motion Machines 290 6–6 Reversible and Irreversible Processes 292 Irreversibilities 293 Internally and Externally Reversible Processes 294 6–7 The Carnot Cycle 295 The Reversed Carnot Cycle 297 6–8 The Carnot Principles 297 6–9 The Thermodynamic Temperature Scale 299 6–10 The Carnot Heat Engine 301 The Quality of Energy 302 Quantity versus Quality in Daily Life 303 6–11 The Carnot Refrigerator and Heat Pump 304 Topic of Special Interest: Household Refrigerators 307 Summary 311 References and Suggested Readings 312 Problems 312 chapter seven ENTROPY 329 7–1 Entropy 330 A Special Case: Internally Reversible Isothermal Heat Transfer Processes 333 7–2 The Increase of Entropy Principle 334 Some Remarks about Entropy 336 7–3 Entropy Change of Pure Substances 337 7–4 Isentropic Processes 340 7–5 Property Diagrams Involving Entropy 342 7–6 What Is Entropy 343 Entropy and Entropy Generation in Daily Life 346 7–7 The T ds Relations 347 7–8 Entropy Change of Liquids and Solids 349 7–9 The Entropy Change of Ideal Gases 352 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 7–10 Reversible Steady-Flow Work 359 Proof that Steady-Flow Devices Deliver the Most and Consume the Least Work When the Process is Reversible 362 7–11 Minimizing the Compressor Work 363 Multistage Compression with Intercooling 364 7–12 Isentropic Efficiencies of Steady-Flow Devices 367 Isentropic Efficiency of Turbines 367 Isentropic Efficiencies of Compressors and Pumps 369 Isentropic Efficiency of Nozzles 371 7–13 Entropy Balance 373 Entropy Change of a System DS system 374 Mechanisms of Entropy Transfer S in and S out 374 1 Heat Transfer 374 2 Mass Flow 375 Entropy Generation S gen 376 Closed Systems 377 Control Volumes 378 Entropy Generation Associated with a Heat Transfer Process 385 Topic of Special Interest: Reducing the Cost of Compressed Air 386 Summary 395 References and Suggested Readings 396 Problems 397 chapter eight EXERGY 421 8–1 Exergy: Work Potential of Energy 422 Exergy Work Potential Associated with Kinetic and Potential Energy 423 8–2 Reversible Work and Irreversibility 425 cen98179_fm_i-xxvi.indd xi cen98179_fm_i-xxvi.indd xi 11/29/13 6:39 PM 11/29/13 6:39 PM

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xii THERMODYNAMICS 8–3 Second-Law Efficiency 430 8–4 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 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 442 8–7 Exergy Balance: Closed Systems 443 8–8 Exergy Balance: Control V olumes 454 Exergy Balance for Steady-Flow Systems 455 Reversible Work 456 Second-Law Efficiency of Steady-Flow Devices 456 Topic of Special Interest: Second-Law Aspects of Daily Life 463 Summary 467 References and Suggested Readings 468 Problems 468 chapter nine 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 Air-Standard Assumptions 490 9–4 An Overview of Reciprocating Engines 490 9–5 Otto Cycle: The Ideal Cycle for Spark-Ignition Engines 492 9–6 Diesel Cycle: The Ideal Cycle for Compression-Ignition Engines 499 9–7 Stirling and Ericsson Cycles 502 9–8 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 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 Modifications to Turbojet Engines 524 9–12 Second-Law Analysis of Gas Power Cycles 526 Topic of Special Interest: Saving Fuel and Money by Driving Sensibly 530 Summary 536 References and Suggested Readings 538 Problems 538 chapter ten VAPOR AND COMBINED POWER CYCLES 553 10–1 The Carnot Vapor Cycle 554 10–2 Rankine Cycle: The Ideal Cycle for Vapor Power Cycles 555 Energy Analysis of the Ideal Rankine Cycle 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 Lowering the Condenser Pressure Lowers T lowavg 561 Superheating the Steam to High Temperatures Increases T highavg 562 Increasing the Boiler Pressure Increases T highavg 562 10–5 The Ideal Reheat Rankine Cycle 565 10–6 The Ideal Regenerative Rankine Cycle 569 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 cen98179_fm_i-xxvi.indd xii cen98179_fm_i-xxvi.indd xii 11/29/13 6:39 PM 11/29/13 6:39 PM

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CONTENTS xiii chapter eleven 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 11–9 Gas Refrigeration Cycles 630 11–10 Absorption Refrigeration Systems 633 Topic of Special Interest: Thermoelectric Power Generation and Refrigeration Systems 636 Summary 638 References and Suggested Readings 639 Problems 639 chapter twelve THERMODYNAMIC PROPERTY RELATIONS 655 12–1 A Little Math—Partial Derivatives and Associated Relations 656 Partial Differentials 657 Partial Differential Relations 659 12–2 The Maxwell Relations 661 12–3 The Clapeyron Equation 662 12–4 General Relations For du dh ds c v and c p 665 Internal Energy Changes 666 Enthalpy Changes 666 Entropy Changes 667 Specific Heats c v and c p 668 12–5 The Joule-Thomson Coefficient 672 12–6 The Dh Du and Ds of Real Gases 674 Enthalpy Changes of Real Gases 674 Internal Energy Changes of Real Gases 675 Entropy Changes of Real Gases 676 Summary 679 References and Suggested Readings 680 Problems 680 chapter thirteen GAS MIXTURES 687 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 13–3 Properties of Gas Mixtures: Ideal and Real Gases 695 Ideal-Gas Mixtures 696 Real-Gas Mixtures 700 Topic of Special Interest: Chemical Potential and the Separation Work of Mixtures 704 Summary 714 References and Suggested Readings 715 Problems 716 chapter fourteen GAS–VAPOR MIXTURES AND AIR-CONDITIONING 725 14–1 Dry and Atmospheric Air 726 14–2 Specific and Relative Humidity of Air 727 14–3 Dew-Point Temperature 729 14–4 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 738 Heating with Humidification 739 Cooling with Dehumidification 740 Evaporative Cooling 742 cen98179_fm_i-xxvi.indd xiii cen98179_fm_i-xxvi.indd xiii 11/29/13 6:39 PM 11/29/13 6:39 PM

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xiv THERMODYNAMICS Adiabatic Mixing of Airstreams 743 Wet Cooling Towers 745 Summary 747 References and Suggested Readings 748 Problems 749 chapter fifteen CHEMICAL REACTIONS 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 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 chapter sixteen CHEMICAL AND PHASE EQUILIBRIUM 805 16–1 Criterion for Chemical Equilibrium 806 16–2 The Equilibrium Constant for Ideal-Gas Mixtures 808 16–3 Some Remarks about the K p of Ideal-Gas Mixtures 812 16–4 Chemical Equilibrium for Simultaneous Reactions 816 16–5 Variation of K p 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 Summary 828 References and Suggested Readings 829 Problems 829 chapter seventeen COMPRESSIBLE FLOW 839 17–1 Stagnation Properties 840 17–2 Speed of Sound and Mach Number 843 17–3 One-Dimensional Isentropic Flow 845 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 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 888 Problems 889 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 cen98179_fm_i-xxvi.indd xiv cen98179_fm_i-xxvi.indd xiv 11/29/13 6:39 PM 11/29/13 6:39 PM

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CONTENTS xv Figure A–14 P-h diagram for refrigerant-134a 921 Figure A–15 Nelson–Obert generalized compressibility chart 922 Table A–16 Properties of the atmosphere at high altitude 923 Table A–17 Ideal-gas properties of air 924 Table A–18 Ideal-gas properties of nitrogen N 2 926 Table A–19 Ideal-gas properties of oxygen O 2 928 Table A–20 Ideal-gas properties of carbon dioxide CO 2 930 Table A–21 Ideal-gas properties of carbon monoxide CO 932 Table A–22 Ideal-gas properties of hydrogen H 2 934 Table A–23 Ideal-gas properties of water vapor H 2 O 935 Table A–24 Ideal-gas properties of monatomic oxygen O 937 Table A–25 Ideal-gas properties of hydroxyl OH 937 Table A–26 Enthalpy of formation Gibbs function of formation and absolute entropy at 258C 1 atm 938 Table A–27 Properties of some common fuels and hydrocarbons 939 Table A–28 Natural logarithms of the equilibrium constant K p 940 Figure A–29 Generalized enthalpy departure chart 941 Figure A–30 Generalized entropy departure chart 942 Figure A–31 Psychrometric chart at 1 atm total pressure 943 Table A–32 One-dimensional isentropic compressible-flow functions for an ideal gas with k 5 1.4 944 Table A–33 One-dimensional normal-shock functions for an ideal gas with k 5 1.4 945 Table A–34 Rayleigh flow functions for an ideal gas with k 5 1.4 946 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 appendix one PROPERTY TABLES AND CHARTS SI UNITS 897 Table A–1 Molar mass gas constant and critical- point properties 898 Table A–2 Ideal-gas specific heats of various common gases 899 Table A–3 Properties of common liquids solids and foods 902 Table A–4 Saturated water—Temperature table 904 Table A–5 Saturated water—Pressure table 906 Table A–6 Superheated water 908 Table A–7 Compressed liquid water 912 Table A–8 Saturated ice–water vapor 913 Figure A–9 T-s diagram for water 914 Figure A–10 Mollier diagram for water 915 Table A–11 Saturated refrigerant-134a— Temperature table 916 Table A–12 Saturated refrigerant-134a— Pressure table 918 Table A–13 Superheated refrigerant-134a 919 cen98179_fm_i-xxvi.indd xv cen98179_fm_i-xxvi.indd xv 11/29/13 6:39 PM 11/29/13 6:39 PM

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xvi THERMODYNAMICS appendix two PROPERTY TABLES AND CHARTS ENGLISH UNITS 947 Table A–1E Molar mass gas constant and critical- point 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 N 2 974 Table A–19E Ideal-gas properties of oxygen O 2 976 Table A–20E Ideal-gas properties of carbon dioxide CO 2 978 Table A–21E Ideal-gas properties of carbon monoxide CO 980 Table A–22E Ideal-gas properties of hydrogen H 2 982 Table A–23E Ideal-gas properties of water vapor H 2 O 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 cen98179_fm_i-xxvi.indd xvi cen98179_fm_i-xxvi.indd xvi 11/29/13 6:39 PM 11/29/13 6:39 PM

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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 stu- dents 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 emphasiz- ing 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 understand- ing 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 thermo- dynamics. 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 Preface cen98179_fm_i-xxvi.indd xvii cen98179_fm_i-xxvi.indd xvii 11/29/13 6:39 PM 11/29/13 6:39 PM

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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 prob- lems formulate them and interpret the results. A conscious effort is made to emphasize these basic principles while also providing students with a per- spective 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 cen98179_fm_i-xxvi.indd xviii cen98179_fm_i-xxvi.indd xviii 11/29/13 6:39 PM 11/29/13 6:39 PM

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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 fa mil iar 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 mas- tery 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 physi- cal 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 self- instructive. 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. Not- ing 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 understand- ing. Further a large number of figures have been upgraded to become three- dimensional 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. cen98179_fm_i-xxvi.indd xix cen98179_fm_i-xxvi.indd xix 11/29/13 6:39 PM 11/29/13 6:39 PM

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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 rela- tions 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 proper- ties 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 dis- cussed 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 prob- lem 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 Prob- lems 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 indepen- dent exploration of topics of interest and to communicate their findings in a professional manner. Problems designated by an “E” are in English units and SI users can ignore them. Problems with the are solved using EES and complete solutions together with parametric studies are included on the textbook’s website. Problems with the are comprehensive in nature and 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 conve- nience 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 multiple- choice 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 engag- ing approach is adopted for interactions instead of a mechanical approach. cen98179_fm_i-xxvi.indd xx cen98179_fm_i-xxvi.indd xx 11/29/13 6:39 PM 11/29/13 6:39 PM

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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 formu- las 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 Mass balance: m in 2 m out 5 Dm system Energy balance: E in 2 E out    5  DE system Net energy transfer Change in internal kinetic by heat work and mass potential etc. energies Entropy balance: S in 2 S out   1 S gen   5 DS system Net entropy transfer Entropy Change by heat and mass generation in entropy Exergy balance: X in 2 X out   2 X destroyed   5 DX system Net exergy transfer Exergy Change by heat work and mass destruction 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 Ther- modynamic Aspects of Biological Systems in Chapter 4 Household Refrigera- tors 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. cen98179_fm_i-xxvi.indd xxi cen98179_fm_i-xxvi.indd xxi 11/29/13 6:39 PM 11/29/13 6:39 PM

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xxii THERMODYNAMICS GLOSSARY OF THERMODYNAMIC TERMS Throughout the chapters when an important key term or concept is intro- duced and defined it appears in boldface type. Fundamental thermo dynamic 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 plot- ting 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 engineer- ing 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 text- book 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/ cen98179_fm_i-xxvi.indd xxii cen98179_fm_i-xxvi.indd xxii 11/29/13 6:39 PM 11/29/13 6:39 PM

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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 apprecia- tion 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 cen98179_fm_i-xxvi.indd xxiii cen98179_fm_i-xxvi.indd xxiii 11/29/13 6:39 PM 11/29/13 6:39 PM

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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 course- work 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 quiz- zes 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. Online Resources for Students and Instructors cen98179_fm_i-xxvi.indd xxv cen98179_fm_i-xxvi.indd xxv 11/29/13 6:39 PM 11/29/13 6:39 PM

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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. cen98179_fm_i-xxvi.indd xxvi cen98179_fm_i-xxvi.indd xxvi 11/29/13 6:39 PM 11/29/13 6:39 PM

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1 INTRODUCTION AND BASIC CONCEPTS E very science has a unique vocabulary associated with it and thermo- dynamics 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 ther- modynamics and the unit systems and continue with a discussion of some basic concepts such as system state state postulate equilibrium and pro- cess. 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 Tem- perature 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. 1 1 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 thermody- namics. ■ 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.      CHAPTER cen98179_ch01_001-050.indd 1 cen98179_ch01_001-050.indd 1 11/28/13 3:14 PM 11/28/13 3:14 PM

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2 INTRODUCTION AND BASIC CONCEPTS 1–1 ■ THERMODYNAMICS AND ENERGY Thermodynamics can be defined as the science of energy. Although every- body has a feeling of what energy is it is difficult to give a precise defini- tion 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 back- bone 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 E in 2 E out 5 DE. The first law of thermodynamics is simply an expression of the con- servation 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 trans- ferred 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 thermody- namics 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 FIGURE 1–1 Energy cannot be created or destroyed it can only change forms the first law. Potential energy Kinetic energy PE 7 units KE 3 units PE 10 units KE 0 FIGURE 1–2 Conservation of energy principle for the human body. Energy out 4 units Energy in 5 units Energy storage 1 unit FIGURE 1–3 Heat flows in the direction of decreasing temperature. Heat Cool environment 20°C Hot coffee 70°C cen98179_ch01_001-050.indd 2 cen98179_ch01_001-050.indd 2 11/28/13 3:14 PM 11/28/13 3:14 PM

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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 elabo- rate 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 thermodynam- ics 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 ordi- nary house is in some respects an exhibition hall filled with wonders of thermodynamics Fig. 1–4. Many ordinary household utensils and appli- ances are designed in whole or in part by using the principles of thermo- dynamics. 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 ordi- nary 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 FIGURE 1–4 The design of many engineering systems such as this solar hot water system involves thermodynamics. Solar collectors Hot water Heat exchanger Pump Shower Cold water Hot water tank cen98179_ch01_001-050.indd 3 cen98179_ch01_001-050.indd 3 11/28/13 3:14 PM 11/28/13 3:14 PM

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4 INTRODUCTION AND BASIC CONCEPTS Customary System USCS and the metric SI from Le Système Interna- tional 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 sys- tem 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 FIGURE 1–5 Some application areas of thermodynamics. Cars © Mark Evans/Getty Images RF Power plants © Malcolm Fife/Getty Images RF Human body © Ryan McVay/Getty Images RF Aircraft and spacecraft © PhotoLink/Getty Images RF Refrigerator © McGraw-Hill Education Jill Braaten Boats © Doug Menuez/Getty Images RF A piping network in an industrial facility. Courtesy of UMDE Engineering Contracting and Trading. Used by permission Wind turbines © F . Schussler/PhotoLink/Getty Images RF Food processing Glow Images RF cen98179_ch01_001-050.indd 4 cen98179_ch01_001-050.indd 4 11/28/13 3:14 PM 11/28/13 3:14 PM

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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 Con- ference 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 sen- tence. 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 convert- ing 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 con- version 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 transi- tion 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 meter m Mass kilogram kg Time second s Temperature kelvin K Electric current ampere A Amount of light candela cd Amount of matter mole mol TABLE 1–2 Standard prefixes in SI units Multiple Prefix 10 24 yotta Y 10 21 zetta Z 10 18 exa E 10 15 peta P 10 12 tera T 10 9 giga G 10 6 mega M 10 3 kilo k 10 2 hecto h 10 1 deka da 10 21 deci d 10 22 centi c 10 23 milli m 10 26 micro m 10 29 nano n 10 212 pico p 10 215 femto f 10 218 atto a 10 221 zepto z 10 224 yocto y FIGURE 1–6 The SI unit prefixes are used in all branches of engineering. 1 kg 200 mL 0.2 L 10 3 g 1 M 10 6 cen98179_ch01_001-050.indd 5 cen98179_ch01_001-050.indd 5 11/28/13 3:14 PM 11/28/13 3:14 PM

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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 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 g c 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 MassAcceleration or F 5 ma 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/s 2 . 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/s 2 Fig. 1–7. That is 1 N 5 1 kg·m/s 2 1 lbf 5 32.174 lbm·ft/s 2 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 m total 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 gravi- tational force applied to a body and its magnitude is determined from Newton’s second law W 5 mg N 1–2 where m is the mass of the body and g is the local gravitational acceleration g is 9.807 m/s 2 or 32.174 ft/s 2 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 FIGURE 1–7 The definition of the force units. m 1 kg m 32.174 lbm a 1 m/s 2 a 1 ft/s 2 F 1 lbf F 1 N FIGURE 1–8 The relative magnitudes of the force units newton N kilogram-force kgf and pound-force lbf. 1 kgf 10 apples m 1 kg 4 apples m 1 lbm 1 lbf 1 apple m 102 g 1 N cen98179_ch01_001-050.indd 6 cen98179_ch01_001-050.indd 6 11/28/13 3:14 PM 11/28/13 3:14 PM

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7 CHAPTER 1 with altitude. On the surface of the moon an astronaut weighs about one- sixth 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 mag- nitudes of the masses and inversely proportional to the square of the dis- tance 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/s 2 at the poles 9.789 at the equator to 7.322 m/s 2 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/s 2 is less than 1 percent. Therefore for most practical purposes the gravitational acceleration can be assumed to be constant at 9.807 m/s 2 often rounded to 9.81 m/s 2 . 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 dis- tance 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 1–3 A more common unit for energy in SI is the kilojoule 1 kJ 5 10 3 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 approxi- mately 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 1–9 A body weighing 150 lbf on earth will weigh only 25 lbf on the moon. FIGURE 1–10 The weight of a unit mass at sea level. g 9.807 m/s 2 W 9.807 kg·m/s 2 9.807 N 1 kgf W 32.174 lbm·ft/s 2 1 lbf g 32.174 ft/s 2 kg lbm FIGURE 1–11 A typical match yields about one Btu or one kJ of energy if completely burned. Photo by John M. Cimbala cen98179_ch01_001-050.indd 7 cen98179_ch01_001-050.indd 7 11/28/13 3:14 PM 11/28/13 3:14 PM

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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 tur- bine with a rated power of 50 kW will generate 120000 kWh of electricity per year.” Dimensional Homogeneity We all know that apples and oranges do not add. But we somehow man- age 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 posi- tion 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 timeTime interval 5 30 kW2200 h 5 66000 kWh The money saved per year is the monetary value of this energy determined as Money saved 5 Total energyUnit cost of energy 5 66000 kWh0.12/kWh 5 7920 Discussion The annual electric energy production also could be determined in kJ by unit manipulations as Total energy 5 30 kW2200 ha 3600 s 1 h ba 1 kJ/s 1 kW b 5 2.38 3 10 8 kJ which is equivalent to 66000 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. FIGURE 1–12 A wind turbine as discussed in Example 1–1. ©Bear Dancer Studios/Mark Dierker RF cen98179_ch01_001-050.indd 8 cen98179_ch01_001-050.indd 8 11/28/13 3:14 PM 11/28/13 3:14 PM

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9 CHAPTER 1 EXAMPLE 1–2 Obtaining Formulas from Unit Considerations A tank is filled with oil whose density is r 5 850 kg/m 3 . If the volume of the tank is V 5 2 m 3 determine the amount of mass m in the tank. 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/m 3   and  V 5 2 m 3 It is obvious that we can eliminate m 3 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/m 3 2 m 3 5 1700 kg Discussion  Note that this approach may not work for more complicated for- mulas. 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 homo- geneous is definitely wrong Fig. 1–14 but a dimensionally homogeneous formula is not necessarily right. Unity Conversion Ratios Just as all nonprimary dimensions can be formed by suitable combina- tions 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 s 2   and  1 lbf 5 32.174 lbm ft s 2 They can also be expressed more conveniently as unity conversion ratios as 1 N 1 kg·m / s 2 5 1  and   1 lbf 32.174 lbm·ft / s 2 5 1 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 g c defined as g c 5 32.174 lbm·ft/lbf·s 2 5 1 kg·m/N·s 2 5 1 into equations in order to force FIGURE 1–13 Schematic for Example 1–2. Oil 2 m 3 m ρ 850 kg/m 3 FIGURE 1–14 Always check the units in your calculations. 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. 0.3048 m 1 ft 1 min 60 s 1 lbm 0.45359 kg 32.174 lbmft/s 2 1 lbf 1 kgm/s 2 1 N 1 kPa 1000 N/m 2 1 kJ 1000 Nm 1 W 1 J/s cen98179_ch01_001-050.indd 9 cen98179_ch01_001-050.indd 9 11/28/13 3:14 PM 11/28/13 3:14 PM

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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. 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. 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/s 2 . 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 lbm32.174 ft /s 2 a 1 lbf 32.174 lbm·ft /s 2 b 5 1.00 lbf Discussion The quantity in large parentheses in this equation is a unity con- version 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 g9.81 m/s 2 a 1 N 1 kg·m/s 2 ba 1 kg 1000 g b 5 4.49 N 1–3 ■ 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 surround- ings 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 bound- ary 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 boun- dary. That is no mass can enter or leave a closed system as shown in FIGURE 1–16 A mass of 1 lbm weighs 1 lbf on earth. lbm FIGURE 1–17 A quirk in the metric system of units. Net weight: One pound 454 grams FIGURE 1–18 System surroundings and boundary. Surroundings Boundary System cen98179_ch01_001-050.indd 10 cen98179_ch01_001-050.indd 10 11/28/13 3:14 PM 11/28/13 3:14 PM

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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 prop- erly 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 con- trol 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 selec- tion 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. FIGURE 1–19 Mass cannot cross the boundaries of a closed system but energy can. Closed system Yes m constant Energy No Mass FIGURE 1–20 A closed system with a moving boundary. Gas 2 kg 1.5 m 3 Gas 2 kg 1 m 3 Moving boundary Fixed boundary FIGURE 1–21 A control volume can involve fixed moving real and imaginary boundaries. Real boundary a A control volume CV with real and imaginary boundaries Imaginary boundary CV a nozzle CV Moving boundary Fixed boundary b A control volume CV with fxed and moving boundaries as well as real and imaginary boundaries cen98179_ch01_001-050.indd 11 cen98179_ch01_001-050.indd 11 11/28/13 3:14 PM 11/28/13 3:14 PM

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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 analy- sis. 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 care- fully. 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. 1–4 ■ PROPERTIES OF A SYSTEM Any characteristic of a system is called a property. Some familiar proper- ties are pressure P temperature T volume V and mass m. The list can be extended to include less familiar ones such as viscosity thermal conductiv- ity 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 vol- ume 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 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. FIGURE 1–22 An open system a control volume with one inlet and one exit. © McGraw-Hill Education Christopher Kerrigan FIGURE 1–23 Criterion to differentiate intensive and extensive properties. cen98179_ch01_001-050.indd 12 cen98179_ch01_001-050.indd 12 11/28/13 3:14 PM 11/28/13 3:14 PM

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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 discon- tinuities. 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 con- sider a container filled with oxygen at atmospheric conditions. The diameter of the oxygen molecule is about 3 3 10 210 m and its mass is 5.3 3 10 226 kg. Also the mean free path of oxygen at 1 atm pressure and 20°C is 6.3 3 10 28 m. That is an oxygen molecule travels on average a distance of 6.3 3 10 28 m about 200 times of its diameter before it col- lides with another molecule. Also there are about 3 3 10 16 molecules of oxygen in the tiny volume of 1 mm 3 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 ■ DENSITY AND SPECIFIC GRAVITY Density is defined as mass per unit volume Fig. 1–25. Density: r 5 m V   kg/m 3 1–4 The reciprocal of density is the specific volume v which is defined as vol- ume per unit mass. That is v 5 V m 5 1 r 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 pres- sure. 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/m 3 at 1 atm to 1003 kg/m 3 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/m 3 at 20°C to 975 kg/m 3 at 75°C a change of 2.3 percent which can still be neglected in many engineering analyses. 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. VOID 1 atm 20°C O 2 3 ´ 10 16 molecules/mm 3 FIGURE 1–25 Density is mass per unit volume specific volume is volume per unit mass. 3 12 m 12 m m 3 kg 3 kg 3 3 /kg /kg 0.25 kg/m 0.25 kg/m 4 m 4 m 1 v V r r – cen98179_ch01_001-050.indd 13 cen98179_ch01_001-050.indd 13 11/28/13 3:14 PM 11/28/13 3:14 PM

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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 den- sity and is defined as the ratio of the density of a substance to the den- sity of some standard substance at a specified temperature usually water at 4°C for which r H 2 O 5 1000 kg/m 3 . That is Specific gravity: SG 5 r r H 2 O 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 sub- stance is exactly equal to its density in g/cm 3 or kg/L or 0.001 times the density in kg/m 3 since the density of water at 4°C is 1 g/cm 3 5 1 kg/L 5 1000 kg/m 3 . The specific gravity of mercury at 0°C for example is 13.6. Therefore its density at 0°C is 13.6 g/cm 3 5 13.6 kg/L 5 13600 kg/m 3 . 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: g s 5 rg  N/m 3 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. 1–6 ■ STATE AND EQUILIBRIUM Consider a system not undergoing any change. At this point all the prop- erties 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 thermody- namic equilibrium unless the conditions of all the relevant types of equi- librium 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 pres- sure 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. TABLE 1–3 Specific gravities of some substances at 0°C Substance SG Water 1.0 Blood 1.05 Seawater 1.025 Gasoline 0.7 Ethyl alcohol 0.79 Mercury 13.6 Wood 0.3–0.9 Gold 19.2 Bones 1.7–2.0 Ice 0.92 Air at 1 atm 0.0013 FIGURE 1–26 A system at two different states. m 2 kg T 2 20°C V 2 2.5 m 3 a State 1 m 2 kg T 1 20°C V 1 1.5 m 3 b State 2 FIGURE 1–27 A closed system reaching thermal equilibrium. 20°C a Before b After 23°C 35°C 40°C 30°C 42°C 32°C 32°C 32°C 32°C 32°C 32°C cen98179_ch01_001-050.indd 14 cen98179_ch01_001-050.indd 14 11/28/13 3:14 PM 11/28/13 3:14 PM

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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 equi- librium 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 speci- fied 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 elec- trical magnetic gravitational motion and surface tension effects. These effects are due to external force fields and are negligible for most engineer- ing problems. Otherwise an additional property needs to be specified for each effect that is significant. If the gravitational effects are to be consid- ered 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 indepen- dent to fix the state. Two properties are independent if one property can be varied while the other one is held constant. Temperature and specific vol- ume 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 fP 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 ■ 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. FIGURE 1–28 The state of nitrogen is fixed by two independent intensive properties. Nitrogen T 25°C v 0.9 m 3 /kg FIGURE 1–29 A process between states 1 and 2 and the process path. State 1 State 2 Process path Property B Property A cen98179_ch01_001-050.indd 15 cen98179_ch01_001-050.indd 15 11/28/13 3:14 PM 11/28/13 3:14 PM

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16 INTRODUCTION AND BASIC CONCEPTS When a process proceeds in such a manner that the system remains infini- tesimally 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 equilib- rium and this makes the entire process nonquasi-equilibrium. However if the piston is moved slowly the molecules will have sufficient time to redis- tribute 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 ideal- ized 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 quasi- equilibrium processes for two reasons. First they are easy to analyze sec- ond 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 quasi- equilibrium 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 FIGURE 1–30 Quasi-equilibrium and nonquasi- equilibrium compression processes. a Slow compression quasi-equilibrium b Very fast compression nonquasi-equilibrium FIGURE 1–31 The P-V diagram of a compression process. Initial state Final state Process path 2 1 P V 2 V 1 V 2 System 1 cen98179_ch01_001-050.indd 16 cen98179_ch01_001-050.indd 16 11/28/13 3:14 PM 11/28/13 3:14 PM

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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 steady- flow process Fig. 1–33. Steady-flow conditions can be closely approximated by devices that are intended for continuous operation such as turbines pumps boilers con- densers and heat exchangers or power plants or refrigeration systems. Some cyclic devices such as reciprocating engines or compressors do not sat- isfy 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 exam- ple 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 thermo- meter 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 even- tually 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 tempera- ture 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. FIGURE 1–32 During a steady-flow process fluid properties within the control volume may change with position but not with time. 300°C 250°C 200°C 150°C Control volume 225°C Mass in Time: 1 PM Mass out 300°C 250°C 200°C 150°C Control volume 225°C Mass in Time: 3 PM Mass out FIGURE 1–33 Under steady-flow conditions the mass and energy contents of a control volume remain constant. Control volume m CV const. E CV const. Mass in Mass out FIGURE 1–34 Two bodies reaching thermal equilibrium after being brought into contact in an isolated enclosure. 150°C Iron 20°C Copper 60°C Iron 60°C Copper cen98179_ch01_001-050.indd 17 cen98179_ch01_001-050.indd 17 11/28/13 3:14 PM 11/28/13 3:14 PM

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18 INTRODUCTION AND BASIC CONCEPTS The zeroth law of thermodynamics states that if two bodies are in ther- mal 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 for temperature mea- surements and several have been introduced throughout history. All tem- perature scales are based on some easily reproducible states such as the freezing and boiling points of water which are also called the ice point and the steam point respectively. A mixture of ice and water that is in equilib- rium with air saturated with vapor at 1 atm pressure is said to be at the ice point and a mixture of liquid water and water vapor with no air in equilib- rium at 1 atm pressure is said to be at the steam point. The temperature scales used in the SI and in the English system today are the Celsius scale formerly called the centigrade scale in 1948 it was renamed after the Swedish astronomer A. Celsius 1702–1744 who devised it and the Fahrenheit scale named after the German instrument maker G. Fahrenheit 1686–1736 respectively. On the Celsius scale the ice and steam points were originally assigned the values of 0 and 100°C respec- tively. The corresponding values on the Fahrenheit scale are 32 and 212°F. These are often referred to as two-point scales since temperature values are assigned at two different points. In thermodynamics it is very desirable to have a temperature scale that is independent of the properties of any substance or substances. Such a temperature scale is called a thermodynamic temperature scale which is developed later in conjunction with the second law of thermodynamics. The thermodynamic temperature scale in the SI is the Kelvin scale named after Lord Kelvin 1824–1907. The temperature unit on this scale is the kelvin which is designated by K not °K the degree symbol was officially dropped from kelvin in 1967. The lowest temperature on the Kelvin scale is absolute zero or 0 K. Then it follows that only one nonzero reference point needs to be assigned to establish the slope of this linear scale. Using nonconventional refrigeration techniques scientists have approached abso- lute zero kelvin they achieved 0.000000002 K in 1989. The thermodynamic temperature scale in the English system is the Rankine scale named after William Rankine 1820–1872. The tempera- ture unit on this scale is the rankine which is designated by R. A temperature scale that turns out to be nearly identical to the Kelvin scale is the ideal-gas temperature scale. The temperatures on this scale are cen98179_ch01_001-050.indd 18 cen98179_ch01_001-050.indd 18 11/28/13 3:14 PM 11/28/13 3:14 PM

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19 CHAPTER 1 measured using a constant-volume gas thermometer which is basically a rigid vessel filled with a gas usually hydrogen or helium at low pressure. This thermometer is based on the principle that at low pressures the tem- perature of a gas is proportional to its pressure at constant volume. That is the temperature of a gas of fixed volume varies linearly with pressure at suf- ficiently low pressures. Then the relationship between the temperature and the pressure of the gas in the vessel can be expressed as T 5 a 1 bP 1–8 where the values of the constants a and b for a gas thermometer are deter- mined experimentally. Once a and b are known the temperature of a medium can be calculated from this relation by immersing the rigid vessel of the gas thermometer into the medium and measuring the gas pressure when thermal equilibrium is established between the medium and the gas in the vessel whose volume is held constant. An ideal-gas temperature scale can be developed by measuring the pres- sures of the gas in the vessel at two reproducible points such as the ice and the steam points and assigning suitable values to temperatures at those two points. Considering that only one straight line passes through two fixed points on a plane these two measurements are sufficient to determine the constants a and b in Eq. 1–8. Then the unknown tempera- ture T of a medium corresponding to a pressure reading P can be deter- mined from that equation by a simple calculation. The values of the con- stants will be different for each thermometer depending on the type and the amount of the gas in the vessel and the temperature values assigned at the two reference points. If the ice and steam points are assigned the values 0°C and 100°C respectively then the gas temperature scale will be identical to the Celsius scale. In this case the value of the constant a which corresponds to an absolute pressure of zero is determined to be 2273.15°C regardless of the type and the amount of the gas in the vessel of the gas thermometer. That is on a P-T diagram all the straight lines passing through the data points in this case will intersect the temperature axis at 2273.15°C when extrapolated as shown in Fig. 1–35. This is the lowest temperature that can be obtained by a gas thermometer and thus we can obtain an absolute gas temperature scale by assigning a value of zero to the constant a in Eq. 1–8. In that case Eq. 1–8 reduces to T 5 bP and thus we need to specify the temperature at only one point to define an absolute gas temperature scale. It should be noted that the absolute gas temperature scale is not a thermo- dynamic temperature scale since it cannot be used at very low temperatures due to condensation and at very high temperatures due to dissociation and ionization. However absolute gas temperature is identical to the thermody- namic temperature in the temperature range in which the gas thermometer can be used. Thus we can view the thermodynamic temperature scale at this point as an absolute gas temperature scale that utilizes an “ideal” or “imaginary” gas that always acts as a low-pressure gas regardless of the temperature. If such a gas thermometer existed it would read zero kelvin at absolute zero pressure which corresponds to 2273.15°C on the Celsius scale Fig. 1–36. FIGURE 1–35 P versus T plots of the experimental data obtained from a constant-volume gas thermometer using four different gases at different but low pressures. Measured data points P Gas A Gas B Gas C Gas D 0 –273.15 Extrapolation T °C FIGURE 1–36 A constant-volume gas thermometer would read 2273.15°C at absolute zero pressure. Absolute vacuum V constant T °C T K 0 0 – 273.15 P kPa –275 –250 –225 –200 0 25 50 75 0 40 80 120 cen98179_ch01_001-050.indd 19 cen98179_ch01_001-050.indd 19 11/28/13 3:14 PM 11/28/13 3:14 PM

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20 INTRODUCTION AND BASIC CONCEPTS The Kelvin scale is related to the Celsius scale by TK 5 T8C 1 273.15 1–9 The Rankine scale is related to the Fahrenheit scale by TR 5 T8F 1 459.67 1–10 It is common practice to round the constant in Eq. 1–9 to 273 and that in Eq. 1–10 to 460. The temperature scales in the two unit systems are related by TR 5 1.8TK 1–11 T8F 5 1.8T8C 1 32 1–12 A comparison of various temperature scales is given in Fig. 1–37. The reference temperature chosen in the original Kelvin scale was 273.15 K or 0°C which is the temperature at which water freezes or ice melts and water exists as a solid–liquid mixture in equilibrium under stan- dard atmospheric pressure the ice point. At the Tenth General Conference on Weights and Measures in 1954 the reference point was changed to a much more precisely reproducible point the triple point of water the state at which all three phases of water coexist in equilibrium which is assigned the value 273.16 K. The Celsius scale was also redefined at this conference in terms of the ideal-gas temperature scale and a single fixed point which is again the triple point of water with an assigned value of 0.01°C. The boil- ing temperature of water the steam point was experimentally determined to be again 100.00°C and thus the new and old Celsius scales were in good agreement. The International Temperature Scale of 1990 ITS-90 The International Temperature Scale of 1990 which supersedes the International Practical Temperature Scale of 1968 IPTS-68 1948 ITPS-48 and 1927 ITS-27 was adopted by the International Commit- tee of Weights and Measures at its meeting in 1989 at the request of the Eighteenth General Conference on Weights and Measures. The ITS-90 is similar to its predecessors except that it is more refined with updated values of fixed temperatures has an extended range and conforms more closely to the thermodynamic temperature scale. On this scale the unit of thermodynamic temperature T is again the kelvin K defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water which is sole defining fixed point of both the ITS-90 and the Kelvin scale and is the most important thermometric fixed point used in the cali- bration of thermometers to ITS-90. The unit of Celsius temperature is the degree Celsius °C which is by definition equal in magnitude to the kelvin K. A temperature difference FIGURE 1–37 Comparison of temperature scales. –273.15 °C 0 273.16 0.01 K –459.67 °F 0 491.69 32.02 R Triple point of water Absolute zero cen98179_ch01_001-050.indd 20 cen98179_ch01_001-050.indd 20 11/28/13 3:14 PM 11/28/13 3:14 PM

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21 CHAPTER 1 may be expressed in kelvins or degrees Celsius. The ice point remains the same at 0°C 273.15 K in both ITS-90 and ITPS-68 but the steam point is 99.975°C in ITS-90 with an uncertainty of 60.005°C whereas it was 100.000°C in IPTS-68. The change is due to precise measurements made by gas thermometry by paying particular attention to the effect of sorption the impurities in a gas absorbed by the walls of the bulb at the reference temperature being desorbed at higher temperatures causing the measured gas pressure to increase. The ITS-90 extends upward from 0.65 K to the highest temperature prac- tically measurable in terms of the Planck radiation law using monochro- matic radiation. It is based on specifying definite temperature values on a number of fixed and easily reproducible points to serve as benchmarks and expressing the variation of temperature in a number of ranges and subranges in functional form. In ITS-90 the temperature scale is considered in four ranges. In the range of 0.65 to 5 K the temperature scale is defined in terms of the vapor pressure—temperature relations for 3 He and 4 He. Between 3 and 24.5561 K the triple point of neon it is defined by means of a prop- erly calibrated helium gas thermometer. From 13.8033 K the triple point of hydrogen to 1234.93 K the freezing point of silver it is defined by means of platinum resistance thermometers calibrated at specified sets of defining fixed points. Above 1234.93 K it is defined in terms of the Planck radiation law and a suitable defining fixed point such as the freez- ing point of gold 1337.33 K. We emphasize that the magnitudes of each division of 1 K and 1°C are identical Fig. 1–38. Therefore when we are dealing with temperature dif- ferences DT the temperature interval on both scales is the same. Raising the temperature of a substance by 10°C is the same as raising it by 10 K. That is DTK 5 DT8C 1–13 DTR 5 DT8F 1–14 Some thermodynamic relations involve the temperature T and often the question arises of whether it is in K or °C. If the relation involves tempera- ture differences such as a 5 bDT it makes no difference and either can be used. However if the relation involves temperatures only instead of tem- perature differences such as a 5 bT then K must be used. When in doubt it is always safe to use K because there are virtually no situations in which the use of K is incorrect but there are many thermodynamic relations that will yield an erroneous result if °C is used. EXAMPLE 1– 4 Expressing Temperature Rise in Different Units During a heating process the temperature of a system rises by 10°C. Express this rise in temperature in K °F and R. SOLUTION The temperature rise of a system is to be expressed in different units. FIGURE 1–38 Comparison of magnitudes of various temperature units. 1°C 1 K 1.8°F 1.8 R cen98179_ch01_001-050.indd 21 cen98179_ch01_001-050.indd 21 11/28/13 3:14 PM 11/28/13 3:14 PM

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22 INTRODUCTION AND BASIC CONCEPTS Analysis This problem deals with temperature changes which are identical in Kelvin and Celsius scales. Then DTK 5 DT8C 5 10 K The temperature changes in Fahrenheit and Rankine scales are also iden- tical and are related to the changes in Celsius and Kelvin scales through Eqs. 1–11 and 1–14: DTR 5 1.8 DTK 5 1.810 5 18 R and DT8F 5 DTR 5 188F Discussion Note that the units °C and K are interchangeable when dealing with temperature differences. 1–9 ■ PRESSURE Pressure is defined as a normal force exerted by a fluid per unit area. Normally we speak of pressure when we deal with a gas or a liquid. The counterpart of pressure in solids is normal stress. Note however that pres- sure is a scaler quantity while stress is a tensor. Since pressure is defined as force per unit area it has the unit of newtons per square meter N/m 2 which is called a pascal Pa. That is 1 Pa 5 1 N/m 2 The pressure unit pascal is too small for most pressures encountered in practice. Therefore its multiples kilopascal 1 kPa 5 10 3 Pa and megapas- cal 1 MPa 5 10 6 Pa are commonly used. Three other pressure units com- monly used in practice especially in Europe are bar standard atmosphere and kilogram-force per square centimeter: 1 bar 5 10 5 Pa 5 0.1 MPa 5 100 kPa 1 atm 5 101325 Pa 5 101.325 kPa 5 1.01325 bars 1 kgf / c m 2 5 9.807 N/cm 2 5 9.807 3 10 4 N/m 2 5 9.807 3 10 4 Pa 5 0.9807 bar 5 0.9679 atm Note the pressure units bar atm and kgf/cm 2 are almost equivalent to each other. In the English system the pressure unit is pound-force per square inch lbf/in 2 or psi and 1 atm 5 14.696 psi. The pressure units kgf/cm 2 and lbf/in 2 are also denoted by kg/cm 2 and lb/in 2 respectively and they are commonly used in tire gages. It can be shown that 1 kgf/cm 2 5 14.223 psi. Pressure is also used on solid surfaces as synonymous to normal stress which is the force acting perpendicular to the surface per unit area. For example a 150-pound person with a total foot imprint area of 50 in 2 cen98179_ch01_001-050.indd 22 cen98179_ch01_001-050.indd 22 11/28/13 3:14 PM 11/28/13 3:14 PM

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23 CHAPTER 1 exerts a pressure of 150 lbf/50 in 2 5 3.0 psi on the floor Fig. 1–39. If the person stands on one foot the pressure doubles. If the person gains exces- sive weight he or she is likely to encounter foot discomfort because of the increased pressure on the foot the size of the bottom of the foot does not change with weight gain. This also explains how a person can walk on fresh snow without sinking by wearing large snowshoes and how a person cuts with little effort when using a sharp knife. The actual pressure at a given position is called the absolute pressure and it is measured relative to absolute vacuum i.e. absolute zero pressure. Most pressure-measuring devices however are calibrated to read zero in the atmosphere Fig. 1–40 and so they indicate the difference between the absolute pressure and the local atmospheric pressure. This difference is called the gage pressure. P gage can be positive or negative but pressures below atmospheric pressure are sometimes called vacuum pressures and are measured by vacuum gages that indicate the difference between the atmospheric pressure and the absolute pressure. Absolute gage and vacuum pressures are related to each other by P gage 5 P abs 2 P atm 1–15 P vac 5 P atm 2 P abs 1–16 This is illustrated in Fig. 1–41. Like other pressure gages the gage used to measure the air pres- sure in an automobile tire reads the gage pressure. Therefore the com- mon reading of 32.0 psi 2.25 kgf/cm 2 indicates a pressure of 32.0 psi above the atmospheric pressure. At a location where the atmospheric pressure is 14.3 psi for example the absolute pressure in the tire is 32.0 1 14.3 5 46.3 psi. In thermodynamic relations and tables absolute pressure is almost always used. Throughout this text the pressure P will denote absolute pressure unless specified otherwise. Often the letters “a” for absolute pressure and “g” for gage pressure are added to pressure units such as psia and psig to clarify what is meant. FIGURE 1–39 The normal stress or “pressure” on the feet of a chubby person is much greater than on the feet of a slim person. 150 pounds A feet 50 in 2 P 3 psi P 6 psi 300 pounds W –––– A feet 150 lbf –––––– 50 in 2 P n 3 psi s FIGURE 1–40 Some basic pressure gages. Dresser Instruments Dresser Inc. Used by permission FIGURE 1–41 Absolute gage and vacuum pressures. Absolute vacuum Absolute vacuum P abs P vac P atm P atm P atm P gage P abs P abs 0 cen98179_ch01_001-050.indd 23 cen98179_ch01_001-050.indd 23 11/28/13 3:14 PM 11/28/13 3:14 PM

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