1. ENGINEERINGTHERMODYNAMICST H I R D E D I T I O NSI Units VersionR. K. RajputE N G I N E E R I N G S E R I E S

2. ENGINEERING THERMODYNAMICS 3. Also available :DHARMM-thermTITLE.PM5 i iSTEAM TABLESandMOLLIER DIAGRAM(S.I. UNITS)Edited byR.K. RAJPUTPatiala 4. ENGINEERINGTHERMODYNAMICS[For Engineering Students of All Indian Universitiesand Competitive Examinations]S.I. UNITSByR.K. RAJPUTM.E. (Heat Power Engg.) Hons.Gold Medallist ; Grad. (Mech. Engg. & Elect. Engg.) ;M.I.E. (India) ; M.S.E.S.I. ; M.I.S.T.E. ; C.E. (India)Principal (Formerly)Punjab College of Information TechnologyPATIALA, Punjab 5. BANGALORECHENNAICOCHINGUWAHATIHYDERABADJALANDHARKOLKATALUCKNOWMUMBAIRANCHINEW DELHIBOSTON, USA 6. Published by :LAXMI PUBLICATIONS (P) LTD113, Golden House, Daryaganj,New Delhi-110002Phone : 011-43 53 25 00Fax : 011-43 53 25 28www.laxmipublications.cominfo@laxmipublications.com All rights reserved with the Publishers.No part of this publication may be reproduced, stored in a retrieval system, ortransmitted in any form or by any means, electronic, mechanical, photocopying,recording or otherwise without the prior written permission of the publisher.ISBN: 978-0-7637-8272-63678Price : Rs. 350.00 Only. First Edition : 1996Second Edition : 2003Third Edition : 2007Offices :India USA Bangalore (Phone : 080-26 61 15 61) Boston Chennai (Phone : 044-24 34 47 26) 11, Leavitt Street, Hingham, Cochin (Phone : 0484-239 70 04) MA 02043, USA Guwahati (Phones : 0361-254 36 69, 251 38 81) Phone : 781-740-4487 Hyderabad (Phone : 040-24 75 02 47) Jalandhar (Phone : 0181-222 12 72) Kolkata (Phones : 033-22 27 37 73, 22 27 52 47) Lucknow (Phone : 0522-220 95 78) Mumbai (Phones : 022-24 91 54 15, 24 92 78 69) Ranchi (Phone : 0651-230 77 64)EET-0556-350-ENGG THERMODYNAMICS C12751/06/07Typeset at : Goswami Printers, Delhi Printed at : Ajit Printers, Delhi 7. Preface to The Third EditionI am pleased to present the third edition of this book. The warm reception which theprevious editions and reprints of this book have enjoyed all over India and abroad has beena matter of great satisfaction to me.The entire book has been thoroughly revised ; a large number of solved examples (questionshaving been selected from various universities and competitive examinations) and ampleadditional text have been added.Any suggestions for the improvement of the book will be thankfully acknowledged andincorporated in the next edition.DHARMM-thermTITLE.PM5 vAuthorPreface to The First EditionSeveral books are available in the market on the subject of Engineering Thermo-dynamicsbut either they are too bulky or are miserly written and as such do not cover thesyllabii of various Indian Universities effectively. Hence a book is needed which shouldassimilate subject matter that should primarily satisfy the requirements of the students fromsyllabus/examination point of view ; these requirements are completely met by this book.The book entails the following features : The presentation of the subject matter is very systematic and language of the textis quite lucid and simple to understand. A number of figures have been added in each chapter to make the subject matterself speaking to a great extent. A large number of properly graded examples have been added in various chaptersto enable the students to attempt different types of questions in the examinationwithout any difficulty. Highlights, objective type questions, theoretical questions, and unsolved exampleshave been added at the end of each chapter to make the book a complete unit inall respects.The authors thanks are due to his wife Ramesh Rajput for rendering all assistanceduring preparation and proof reading of the book. The author is thankful to Mr. R.K. Syalfor drawing beautiful and well proportioned figures for the book.The author is grateful to M/s Laxmi Publications for taking lot of pains in bringing outthe book in time and pricing it moderately inspite of heavy cost of the printing.Constructive criticism is most welcome from the readers.Author 8. ContentsChapter PagesIntroduction to S.I. Units and Conversion Factors (xvi)(xx)Nomenclature (xxi)(xxii)1. INTRODUCTIONOUTLINE OF SOME DESCRIPTIVE SYSTEMS ... 1131.1. Steam Power Plant ... 11.1.1. Layout ... 11.1.2. Components of a modern steam power plant ... 21.2. Nuclear Power Plant ... 31.3. Internal Combustion Engines ... 41.3.1. Heat engines ... 41.3.2. Development of I.C. engines ... 41.3.3. Different parts of I.C. engines ... 41.3.4. Spark ignition (S.I.) engines ... 51.3.5. Compression ignition (C.I.) engines ... 71.4. Gas Turbines ... 71.4.1. General aspects ... 71.4.2. Classification of gas turbines ... 81.4.3. Merits and demerits of gas turbines ... 81.4.4. A simple gas turbine plant ... 91.4.5. Energy cycle for a simple-cycle gas turbine ... 101.5. Refrigeration Systems ... 10Highlights ... 12Theoretical Questions ... 132. BASIC CONCEPTS OF THERMODYNAMICS ... 14622.1. Introduction to Kinetic Theory of Gases ... 142.2. Definition of Thermodynamics ... 182.3. Thermodynamic Systems ... 182.3.1. System, boundary and surroundings ... 182.3.2. Closed system ... 182.3.3. Open system ... 192.3.4. Isolated system ... 192.3.5. Adiabatic system ... 192.3.6. Homogeneous system ... 192.3.7. Heterogeneous system ... 192.4. Macroscopic and Microscopic Points of View ... 192.5. Pure Substance ... 202.6. Thermodynamic Equilibrium ... 202.7. Properties of Systems ... 212.8. State ... 21 9. Chapter PagesDHARMM-thermTITLE.PM5 v i i( vii )2.9. Process ... 212.10. Cycle ... 222.11. Point Function ... 222.12. Path Function ... 222.13. Temperature ... 232.14. Zeroth Law of Thermodynamics ... 232.15. The Thermometer and Thermometric Property ... 242.15.1. Introduction ... 242.15.2. Measurement of temperature ... 242.15.3. The international practical temperature scale ... 312.15.4. Ideal gas ... 332.16. Pressure ... 332.16.1. Definition of pressure ... 332.16.2. Unit for pressure ... 342.16.3. Types of pressure measurement devices ... 342.16.4. Mechanical type instruments ... 342.17. Specific Volume ... 452.18. Reversible and Irreversible Processes ... 462.19. Energy, Work and Heat ... 462.19.1. Energy ... 462.19.2. Work and heat ... 462.20. Reversible Work ... 48Highlights ... 58Objective Type Questions ... 59Theoretical Questions ... 61Unsolved Examples ... 613. PROPERTIES OF PURE SUBSTANCES ... 631003.1. Definition of the Pure Substance ... 633.2. Phase Change of a Pure Substance ... 643.3. p-T (Pressure-temperature) Diagram for a Pure Substance ... 663.4. p-V-T (Pressure-Volume-Temperature) Surface ... 673.5. Phase Change Terminology and Definitions ... 673.6. Property Diagrams in Common Use ... 683.7. Formation of Steam ... 683.8. Important Terms Relating to Steam Formation ... 703.9. Thermodynamic Properties of Steam and Steam Tables ... 723.10. External Work Done During Evaporation ... 733.11. Internal Latent Heat ... 733.12. Internal Energy of Steam ... 733.13. Entropy of Water ... 733.14. Entropy of Evaporation ... 733.15. Entropy of Wet Steam ... 743.16. Entropy of Superheated Steam ... 743.17. Enthalpy-Entropy (h-s) Chart or Mollier Diagram ... 75 10. Chapter PagesDHARMM-thermTITLE.PM5 v i i i( viii )3.18. Determination of Dryness Fraction of Steam ... 893.18.1. Tank or bucket calorimeter ... 893.18.2. Throttling calorimeter ... 923.18.3. Separating and throttling calorimeter ... 93Highlights ... 96Objective Type Questions ... 97Theoretical Questions ... 99Unsolved Examples ... 994. FIRST LAW OF THERMODYNAMICS ... 1012264.1. Internal Energy ... 1014.2. Law of Conservation of Energy ... 1014.3. First Law of Thermodynamics ... 1014.4. Application of First Law to a Process ... 1034.5. EnergyA Property of System ... 1034.6. Perpetual Motion Machine of the First Kind-PMM1 ... 1044.7. Energy of an Isolated System ... 1054.8. The Perfect Gas ... 1054.8.1. The characteristic equation of state ... 1054.8.2. Specific heats ... 1064.8.3. Joules law ... 1074.8.4. Relationship between two specific heats ... 1074.8.5. Enthalpy ... 1084.8.6. Ratio of specific heats ... 1094.9. Application of First Law of Thermodynamics to Non-flow or ClosedSystem ... 1094.10. Application of First Law to Steady Flow Process ... 1504.11. Energy Relations for Flow Process ... 1524.12. Engineering Applications of Steady Flow Energy Equation (S.F.E.E.) ... 1554.12.1. Water turbine ... 1554.12.2. Steam or gas turbine ... 1564.12.3. Centrifugal water pump ... 1574.12.4. Centrifugal compressor ... 1574.12.5. Reciprocating compressor ... 1584.12.6. Boiler ... 1594.12.7. Condenser ... 1594.12.8. Evaporator ... 1604.12.9. Steam nozzle ... 1614.13. Throttling Process and Joule-Thompson Porous Plug Experiment ... 1624.14. Heating-Cooling and Expansion of Vapours ... 1834.15. Unsteady Flow Processes ... 210Highlights ... 215Objective Type Questions ... 216Theoretical Questions ... 219Unsolved Examples ... 219 11. Chapter PagesDHARMM-thermTITLE.PM5 i x( ix )5. SECOND LAW OF THERMODYNAMICS AND ENTROPY ... 2273055.1. Limitations of First Law of Thermodynamics and Introduction toSecond Law ... 2275.2. Performance of Heat Engines and Reversed Heat Engines ... 2275.3. Reversible Processes ... 2285.4. Statements of Second Law of Thermodynamics ... 2295.4.1. Clausius statement ... 2295.4.2. Kelvin-Planck statement ... 2295.4.3. Equivalence of Clausius statement to the Kelvin-Planckstatement ... 2295.5. Perpetual Motion Machine of the Second Kind ... 2305.6. Thermodynamic Temperature ... 2315.7. Clausius Inequality ... 2315.8. Carnot Cycle ... 2335.9. Carnots Theorem ... 2355.10. Corollary of Carnots Theorem ... 2375.11. Efficiency of the Reversible Heat Engine ... 2375.12. Entropy ... 2525.12.1. Introduction ... 2525.12.2. Entropya property of a system ... 2525.12.3. Change of entropy in a reversible process ... 2535.13. Entropy and Irreversibility ... 2545.14. Change in Entropy of the Universe ... 2555.15. Temperature Entropy Diagram ... 2575.16. Characteristics of Entropy ... 2575.17. Entropy Changes for a Closed System ... 2585.17.1. General case for change of entropy of a gas ... 2585.17.2. Heating a gas at constant volume ... 2595.17.3. Heating a gas at constant pressure ... 2605.17.4. Isothermal process ... 2605.17.5. Adiabatic process (reversible) ... 2615.17.6. Polytropic process ... 2625.17.7. Approximation for heat absorbed ... 2635.18. Entropy Changes for an Open System ... 2645.19. The Third Law of Thermodynamics ... 265Highlights ... 298Objective Type Questions ... 299Theoretical Questions ... 302Unsolved Examples ... 3026. AVAILABILITY AND IRREVERSIBILITY ... 3063406.1. Available and Unavailable Energy ... 3066.2. Available Energy Referred to a Cycle ... 3066.3. Decrease in Available Energy When Heat is Transferred Througha Finite Temperature Difference ... 3086.4. Availability in Non-flow Systems ... 310 12. Chapter PagesDHARMM-thermTITLE.PM5 x( x )6.5. Availability in Steady-flow Systems ... 3116.6. Helmholtz and Gibbs Functions ... 3116.7. Irreversibility ... 3126.8. Effectiveness ... 313Highlights ... 336Objective Type Questions ... 337Theoretical Questions ... 338Unsolved Examples ... 3387. THERMODYNAMIC RELATIONS ... 3413757.1. General Aspects ... 3417.2. Fundamentals of Partial Differentiation ... 3417.3. Some General Thermodynamic Relations ... 3437.4. Entropy Equations (Tds Equations) ... 3447.5. Equations for Internal Energy and Enthalpy ... 3457.6. Measurable Quantities ... 3467.6.1. Equation of state ... 3467.6.2. Co-efficient of expansion and compressibility ... 3477.6.3. Specific heats ... 3487.6.4. Joule-Thomson co-efficient ... 3517.7. Clausius-Claperyon Equation ... 353Highlights ... 373Objective Type Questions ... 374Exercises ... 3758. IDEAL AND REAL GASES ... 3764108.1. Introduction ... 3768.2. The Equation of State for a Perfect Gas ... 3768.3. p-V-T Surface of an Ideal Gas ... 3798.4. Internal Energy and Enthalpy of a Perfect Gas ... 3798.5. Specific Heat Capacities of an Ideal Gas ... 3808.6. Real Gases ... 3818.7. Van der Waals Equation ... 3818.8. Virial Equation of State ... 3908.9. Beattie-Bridgeman Equation ... 3908.10. Reduced Properties ... 3918.11. Law of Corresponding States ... 3928.12. Compressibility Chart ... 392Highlights ... 407Objective Type Questions ... 408Theoretical Questions ... 408Unsolved Examples ... 4099. GASES AND VAPOUR MIXTURES ... 4114489.1. Introduction ... 411 13. Chapter PagesDHARMM-thermTITLE.PM5 x i( xi )9.2. Daltons Law and Gibbs-Dalton Law ... 4119.3. Volumetric Analysis of a Gas Mixture ... 4139.4. The Apparent Molecular Weight and Gas Constant ... 4149.5. Specific Heats of a Gas Mixture ... 4179.6. Adiabatic Mixing of Perfect Gases ... 4189.7. Gas and Vapour Mixtures ... 419Highlights ... 444Objective Type Questions ... 444Theoretical Questions ... 445Unsolved Examples ... 44510. PSYCHROMETRICS ... 44948610.1. Concept of Psychrometry and Psychrometrics ... 44910.2. Definitions ... 44910.3. Psychrometric Relations ... 45010.4. Psychrometers ... 45510.5. Psychrometric Charts ... 45610.6. Psychrometric Processes ... 45810.6.1. Mixing of air streams ... 45810.6.2. Sensible heating ... 45910.6.3. Sensible cooling ... 46010.6.4. Cooling and dehumidification ... 46110.6.5. Cooling and humidification ... 46210.6.6. Heating and dehumidification ... 46310.6.7. Heating and humidification ... 463Highlights ... 483Objective Type Questions ... 483Theoretical Questions ... 484Unsolved Examples ... 48511. CHEMICAL THERMODYNAMICS ... 48759211.1. Introduction ... 48711.2. Classification of Fuels ... 48711.3. Solid Fuels ... 48811.4. Liquid Fuels ... 48911.5. Gaseous Fuels ... 48911.6. Basic Chemistry ... 49011.7. Combustion Equations ... 49111.8. Theoretical Air and Excess Air ... 49311.9. Stoichiometric Air Fuel (A/F) Ratio ... 49311.10. Air-Fuel Ratio from Analysis of Products ... 49411.11. How to Convert Volumetric Analysis to Weight Analysis ... 49411.12. How to Convert Weight Analysis to Volumetric Analysis ... 49411.13. Weight of Carbon in Flue Gases ... 49411.14. Weight of Flue Gases per kg of Fuel Burnt ... 49511.15. Analysis of Exhaust and Flue Gas ... 495 14. Chapter PagesDHARMM-thermTITLE.PM5 x i i( xii )11.16. Internal Energy and Enthalpy of Reaction ... 49711.17. Enthalpy of Formation (Hf) ... 50011.18. Calorific or Heating Values of Fuels ... 50111.19. Determination of Calorific or Heating Values ... 50111.19.1. Solid and Liquid Fuels ... 50211.19.2. Gaseous Fuels ... 50411.20. Adiabatic Flame Temperature ... 50611.21. Chemical Equilibrium ... 50611.22. Actual Combustion Analysis ... 507Highlights ... 537Objective Type Questions ... 538Theoretical Questions ... 539Unsolved Examples ... 54012. VAPOUR POWER CYCLES ... 54360312.1. Carnot Cycle ... 54312.2. Rankine Cycle ... 54412.3. Modified Rankine Cycle ... 55712.4. Regenerative Cycle ... 56212.5. Reheat Cycle ... 57612.6. Binary Vapour Cycle ... 584Highlights ... 601Objective Type Questions ... 601Theoretical Questions ... 602Unsolved Examples ... 60313. GAS POWER CYCLES ... 60471213.1. Definition of a Cycle ... 60413.2. Air Standard Efficiency ... 60413.3. The Carnot Cycle ... 60513.4. Constant Volume or Otto Cycle ... 61313.5. Constant Pressure or Diesel Cycle ... 62913.6. Dual Combustion Cycle ... 63913.7. Comparison of Otto, Diesel and Dual Combustion Cycles ... 65513.7.1. Efficiency versus compression ratio ... 65513.7.2. For the same compression ratio and the same heat input ... 65513.7.3. For constant maximum pressure and heat supplied ... 65613.8. Atkinson Cycle ... 65713.9. Ericsson Cycle ... 66013.10. Gas Turbine Cycle-Brayton Cycle ... 66113.10.1. Ideal Brayton cycle ... 66113.10.2. Pressure ratio for maximum work ... 66313.10.3. Work ratio ... 66413.10.4. Open cycle gas turbine-actual brayton cycle ... 66513.10.5. Methods for improvement of thermal efficiency of open cyclegas turbine plant ... 667 15. Chapter PagesDHARMM-thermTITLE.PM5 x i i i( xiii )13.10.6. Effect of operating variables on thermal efficiency ... 67113.10.7. Closed cycle gas turbine ... 67413.10.8. Gas turbine fuels ... 679Highlights ... 706Theoretical Questions ... 707Objective Type Questions ... 707Unsolved Examples ... 70914. REFRIGERATION CYCLES ... 71377714.1. Fundamentals of Refrigeration ... 71314.1.1. Introduction ... 71314.1.2. Elements of refrigeration systems ... 71414.1.3. Refrigeration systems ... 71414.1.4. Co-efficient of performance (C.O.P.) ... 71414.1.5. Standard rating of a refrigeration machine ... 71514.2. Air Refrigeration System ... 71514.2.1. Introduction ... 71514.2.2. Reversed Carnot cycle ... 71614.2.3. Reversed Brayton cycle ... 72214.2.4. Merits and demerits of air refrigeration system ... 72414.3. Simple Vapour Compression System ... 73014.3.1. Introduction ... 73014.3.2. Simple vapour compression cycle ... 73014.3.3. Functions of parts of a simple vapour compression system ... 73114.3.4. Vapour compression cycle on temperature-entropy (T-s) diagram ... 73214.3.5. Pressure-enthalpy (p-h) chart ... 73414.3.6. Simple vapour compression cycle on p-h chart ... 73514.3.7. Factors affecting the performance of a vapour compressionsystem ... 73614.3.8. Actual vapour compression cycle ... 73714.3.9. Volumetric efficiency ... 73914.3.10. Mathematical analysis of vapour compression refrigeration ... 74014.4. Vapour Absorption System ... 74114.4.1. Introduction ... 74114.4.2. Simple vapour absorption system ... 74214.4.3. Practical vapour absorption system ... 74314.4.4. Comparison between vapour compression and vapourabsorption systems ... 74414.5. Refrigerants ... 76414.5.1. Classification of refrigerants ... 76414.5.2. Desirable properties of an ideal refrigerant ... 76614.5.3. Properties and uses of commonly used refrigerants ... 768Highlights ... 771Objective Type Questions ... 772Theoretical Questions ... 773Unsolved Examples ... 774 16. Chapter PagesDHARMM-thermTITLE.PM5 x i v( xiv )15. HEAT TRANSFER ... 77885615.1. Modes of Heat Transfer ... 77815.2. Heat Transmission by Conduction ... 77815.2.1. Fouriers law of conduction ... 77815.2.2. Thermal conductivity of materials ... 78015.2.3. Thermal resistance (Rth) ... 78215.2.4. General heat conduction equation in cartesian coordinates ... 78315.2.5. Heat conduction through plane and composite walls ... 78715.2.6. The overall heat transfer coefficient ... 79015.2.7. Heat conduction through hollow and composite cylinders ... 79915.2.8. Heat conduction through hollow and composite spheres ... 80515.2.9. Critical thickness of insulation ... 80815.3. Heat Transfer by Convection ... 81215.4. Heat Exchangers ... 81515.4.1. Introduction ... 81515.4.2. Types of heat exchangers ... 81515.4.3. Heat exchanger analysis ... 82015.4.4. Logarithmic temperature difference (LMTD) ... 82115.5. Heat Transfer by Radiation ... 83215.5.1. Introduction ... 83215.5.2. Surface emission properties ... 83315.5.3. Absorptivity, reflectivity and transmittivity ... 83415.5.4. Concept of a black body ... 83615.5.5. The Stefan-Boltzmann law ... 83615.5.6. Kirchhoff s law ... 83715.5.7. Plancks law ... 83715.5.8. Wiens displacement law ... 83915.5.9. Intensity of radiation and Lamberts cosine law ... 84015.5.10. Radiation exchange between black bodies separated by anon-absorbing medium ... 843Highlights ... 851Objective Type Questions ... 852Theoretical Questions ... 854Unsolved Examples ... 85416. COMPRESSIBLE FLOW ... 85790316.1. Introduction ... 85716.2. Basic Equations of Compressible Fluid Flow ... 85716.2.1. Continuity equation ... 85716.2.2. Momentum equation ... 85816.2.3. Bernoullis or energy equation ... 85816.3. Propagation of Disturbances in Fluid and Velocity of Sound ... 86216.3.1. Derivation of sonic velocity (velocity of sound) ... 86216.3.2. Sonic velocity in terms of bulk modulus ... 86416.3.3. Sonic velocity for isothermal process ... 86416.3.4. Sonic velocity for adiabatic process ... 865 17. Chapter Pages16.4. Mach Number ... 86516.5. Propagation of Disturbance in Compressible Fluid ... 86616.6. Stagnation Properties ... 86916.6.1. Expression for stagnation pressure (ps) in compressible flow ... 86916.6.2. Expression for stagnation density (s) ... 87216.6.3. Expression for stagnation temperature (Ts) ... 87216.7. AreaVelocity Relationship and Effect of Variation of Area forSubsonic, Sonic and Supersonic Flows ... 87616.8. Flow of Compressible Fluid Through a Convergent Nozzle ... 87816.9. Variables of Flow in Terms of Mach Number ... 88316.10. Flow Through Laval Nozzle (Convergent-divergent Nozzle) ... 88616.11. Shock Waves ... 89216.11.1. Normal shock wave ... 89216.11.2. Oblique shock wave ... 89516.11.3. Shock Strength ... 895Highlights ... 896Objective Type Questions ... 899Theoretical Questions ... 901Unsolved Examples ... 902 Competitive Examinations Questions with Answers ... 904919Index ... 920922 Steam Tables and Mollier Diagram ... (i)(xx)DHARMM-thermTITLE.PM5 x v( xv ) 18. Introduction to SI Units and Conversion FactorsA. INTRODUCTION TO SI UNITSSI, the international system of units are divided into three classes :1. Base units2. Derived units3. Supplementary units.From the scientific point of view division of SI units into these classes is to a certain extentarbitrary, because it is not essential to the physics of the subject. Nevertheless the General Confer-ence,considering the advantages of a single, practical, world-wide system for international rela-tions,for teaching and for scientific work, decided to base the international system on a choice ofsix well-defined units given in Table 1 below :Table 1. SI Base UnitsQuantity Name Symbollength metre mmass kilogram kgtime second selectric current ampere Athermodynamic temperature kelvin Kluminous intensity candela cdamount of substance mole molThe second class of SI units contains derived units, i.e., units which can be formed by com-biningbase units according to the algebraic relations linking the corresponding quantities. Severalof these algebraic expressions in terms of base units can be replaced by special names and symbolscan themselves be used to form other derived units.Derived units may, therefore, be classified under three headings. Some of them are given inTables 2, 3 and 4.(xvi) 19. INTRODUCTION TO SI UNITS AND CONVERSION FACTORS (xvii)Table 2. Examples of SI Derived Units Expressed in terms of Base UnitsdharmM-thermth0-1SI UnitsQuantityName Symbolarea square metre m2volume cubic metre m3speed, velocity metre per second m/sacceleration metre per second squared m/s2wave number 1 per metre m1density, mass density kilogram per cubic metre kg/m3concentration (of amount of substance) mole per cubic metre mol/m3activity (radioactive) 1 per second s1specific volume cubic metre per kilogram m3/kgluminance candela per square metre cd/m2Table 3. SI Derived Units with Special NamesSI UnitsQuantity Name Symbol Expression Expressionin terms of in terms ofother SI baseunits unitsfrequency hertz Hz s1force newton N m.kg.s2pressure pascal Pa N/m2 m1.kg.s2energy, work, quantity of heat power joule J N.m m2.kg.s2radiant flux quantity of electricity watt W J/S m2.kg.s3electric charge coloumb C A.s s.Aelectric tension, electric potential volt V W/A m2.kg.s3.A1capacitance farad F C/V m2.kg1.s4electric resistance ohm V/A m2.kg.s3.A2conductance siemens S A/V m2.kg1.s3.A2magnetic flux weber Wb V.S. m2.kg.s2.A1magnetic flux density tesla T Wb/m2 kg.s2.A1inductance henry H Wb/A m2.kg.s2.A2luminous flux lumen lm cd.srilluminance lux lx m2.cd.sr 20. (xviii) ENGINEERING THERMODYNAMICSTable 4. Examples of SI Derived Units Expressed by means of Special NamesdharmM-thermth0-1SI UnitsQuantity Name Symbol Expressionin terms ofSI baseunitsdynamic viscosity pascal second Pa-s m1.kg.s1moment of force metre newton N.m m2.kg.s2surface tension newton per metre N/m kg.s2heat flux density, irradiance watt per square metre W/m2 kg.s2heat capacity, entropy joule per kelvin J/K m2.kg.s2.K1specific heat capacity, specific joule per kilogram kelvin J/(kg.K) m2.s2.K1entropyspecific energy joule per kilogram J/kg m2.s2thermal conductivity watt per metre kelvin W/(m.K) m.kg.s3.K1energy density joule per cubic metre J/m3 m1.kg.s2electric field strength volt per metre V/m m.kg.s3.A1electric charge density coloumb per cubic metre C/m3 m3.s.Aelectric flux density coloumb per square metre C/m2 m2.s.Apermitivity farad per metre F/m m3.kg1.s4.A4current density ampere per square metre A/m2 magnetic field strength ampere per metre A/m permeability henry per metre H/m m.kg.s2.A2molar energy joule per mole J/mol m2.kg.s2mol1molar heat capacity joule per mole kelvin J/(mol.K) m2.kg.s2.K1.mol1The SI units assigned to third class called Supplementary units may be regarded either asbase units or as derived units. Refer Table 5 and Table 6.Table 5. SI Supplementary UnitsSI UnitsQuantityName Symbolplane angle radian radsolid angle steradian sr 21. INTRODUCTION TO SI UNITS AND CONVERSION FACTORS (xix)Table 6. Examples of SI Derived Units Formed by Using Supplementary UnitsdharmM-thermth0-1SI UnitsQuantityName Symbolangular velocity radian per second rad/sangular acceleration radian per second squared rad/s2radiant intensity watt per steradian W/srradiance watt per square metre steradian W-m2.sr1Table 7. SI PrefixesFactor Prefix Symbol Factor Prefix Symbol1012 tera T 101 deci d109 giga G 102 centi c106 mega M 103 milli m103 kilo k 106 micro 102 hecto h 109 nano n101 deca da 1012 pico p1015 fasnto f1018 atto aB. CONVERSION FACTORS1. Force :1 newton = kg-m/sec2 = 0.012 kgf1 kgf = 9.81 N2. Pressure :1 bar = 750.06 mm Hg = 0.9869 atm = 105 N/m2 = 103 kg/m-sec21 N/m2 = 1 pascal = 105 bar = 102 kg/m-sec21 atm = 760 mm Hg = 1.03 kgf/cm2 = 1.01325 bar= 1.01325 105 N/m23. Work, Energy or Heat :1 joule = 1 newton metre = 1 watt-sec= 2.7778 107 kWh = 0.239 cal= 0.239 103 kcal1 cal = 4.184 joule = 1.1622 106 kWh1 kcal = 4.184 103 joule = 427 kgf-m= 1.1622 103 kWh1 kWh = 8.6042 105 cal = 860 kcal = 3.6 106 joule1 kgf-m =1427kcal = 9.81 joules 22. (xx) ENGINEERING THERMODYNAMICS4. Power :dharmM-thermth0-11 watt = 1 joule/sec = 0.860 kcal/h1 h.p. = 75 m kgf/sec = 0.1757 kcal/sec = 735.3 watt1 kW = 1000 watts = 860 kcal/h5. Specific heat :1 kcal/kg-K = 0.4184 joules/kg-K6. Thermal conductivity :1 watt/m-K = 0.8598 kcal/h-m-C1 kcal/h-m-C = 1.16123 watt/m-K = 1.16123 joules/s-m-K.7. Heat transfer co-efficient :1 watt/m2-K = 0.86 kcal/m2-h-C1 kcal/m2-h-C = 1.163 watt/m2-K.C. IMPORTANT ENGINEERING CONSTANTS AND EXPRESSIONSEngineering constants M.K.S. system SI Unitsand expressions1. Value of g0 9.81 kg-m/kgf-sec2 1 kg-m/N-sec22. Universal gas constant 848 kgf-m/kg mole-K 848 9.81 = 8314 J/kg-mole-K( 1 kgf-m = 9.81 joules)3. Gas constant (R) 29.27 kgf-m/kg-K831429= 287 joules/kg-Kfor air for air4. Specific heats (for air) cv = 0.17 kcal/kg-K cv = 0.17 4.184= 0.71128 kJ/kg-Kcp = 0.24 kcal/kg-K cp = 0.24 4.184= 1 kJ/kg-K5. Flow through nozzle-Exit 91.5 U , where U is in kcal 44.7 U , where U is in kJvelocity (C2)6. Refrigeration 1 ton = 50 kcal/min = 210 kJ/min7. Heat transferThe Stefan Boltzman Q = T4 kcal/m2-h Q = T4 watts/m2-hLaw is given by : when = 4.9 108 when = 5.67 108kcal/h-m2-K4 W/m2K4 23. INTRODUCTION TO SI UNITS AND CONVERSION FACTORS (xxi)dharmM-thermth0-1NomenclatureA areab steady-flow availability functionC velocityC temperature on the celsius (or centigrade) scalec specific heatcp specific heat at constant pressurecv specific heat at constant volumeCp molar heat at constant pressureCv molar heat at constant volumeD, d bore ; diameterE emissive power ; total energye base of natural logarithmsg gravitational accelerationH enthalpyh specific enthalpy ; heat transfer co-efficienthf specific enthalpy of saturated liquid (fluid)hfg latent heathg specific enthalpy of saturated vapour ; gasesK temperature on kelvin scale (i.e., celsius absolute, compressibility)k thermal conductivity, blade velocity co-efficientL strokeM molecular weightm massm rate of mass flowN rotational speedn polytropic index, number of moles ; number of cylindersP powerp absolute pressurepm mean effective pressurepi indicated mean effective pressurepb brake mean effective pressure, back pressure 24. (xxii) ENGINEERING THERMODYNAMICSQ heat, rate of heat transferq rate of heat transfer per unit areaR gas constant ; thermal resistance ; radius ; total expansion ratio in compounddharmM-thermth0-1steam enginesR0 universal gas constantr radius, expansion ratio, compression ratioS entropys specific entropyT absolute temperature ; torquet temperatureU internal energy ; overall heat transfer co-efficientu specific internal energyV volumev specific volumeW work ; rate of work transfer ; brake load ; weightw specific weight ; velocity of whirlx dryness fraction ; lengthGreek Symbols absorptivity ratio of specific heats, cp/cv emissivity ; effectiveness efficiency temperature difference, angle density Stefan-Boltzmann constant relative humidity, angle. 25. IntroductionOutline of Some Descriptive Systems1.1. Steam power plant : Layoutcomponents of a modern steam power plant. 1.2. Nuclearpower plant. 1.3. Internal combustion engines : Heat enginesdevelopment of I.C. enginesdifferent parts of I.C. enginesspark ignition enginescompression ignition engines.1.4. Gas turbines : General aspectsclassification of gas turbinesmerits and demerits ofgas turbinesa simple gas turbine plantenergy cycle for a simple-cycle gas turbine.1.5. Refrigeration systemsHighlightsTheoretical questions.11.1. STEAM POWER PLANT11.1.1. LayoutRefer to Fig. 1.1. The layout of a modern steam power plant comprises of the following fourcircuits :1. Coal and ash circuit.2. Air and gas circuit.3. Feed water and steam flow circuit.4. Cooling water circuit.Coal and Ash Circuit. Coal arrives at the storage yard and after necessary handling,passes on to the furnaces through the fuel feeding device. Ash resulting from combustion of coalcollects at the back of the boiler and is removed to the ash storage yard through ash handlingequipment.Air and Gas Circuit. Air is taken in from atmosphere through the action of a forced orinduced draught fan and passes on to the furnace through the air preheater, where it has beenheated by the heat of flue gases which pass to the chimney via the preheater. The flue gases afterpassing around boiler tubes and superheater tubes in the furnace pass through a dust catchingdevice or precipitator, then through the economiser, and finally through the air preheater beforebeing exhausted to the atmosphere.Feed Water and Steam Flow Circuit. In the water and steam circuit condensate leav-ingthe condenser is first heated in a closed feed water heater through extracted steam from thelowest pressure extraction point of the turbine. It then passes through the deaerator and a fewmore water heaters before going into the boiler through economiser.In the boiler drum and tubes, water circulates due to the difference between the density ofwater in the lower temperature and the higher temperature sections of the boiler. Wet steam fromthe drum is further heated up in the superheater for being supplied to the primemover. Afterexpanding in high pressure turbine steam is taken to the reheat boiler and brought to its originaldryness or superheat before being passed on to the low pressure turbine. From there it is exhaustedthrough the condenser into the hot well. The condensate is heated in the feed heaters using thesteam trapped (blow steam) from different points of turbine. 26. 2 ENGINEERING THERMODYNAMICSdharmM-thermTh1-1.pm5To atmosphereEcono-miserFluegasesSteamturbineGeneratorCooling towerPumpFeed waterpumpBoilerwithSuperheaterCoal/OilAir fromboilerAir preheaterChimneyCondenserFig. 1.1. Layout of a steam power plant.A part of steam and water is lost while passing through different components and this iscompensated by supplying additional feed water. This feed water should be purified before hand, toavoid the scaling of the tubes of the boiler.Cooling Water Circuit. The cooling water supply to the condenser helps in maintaininga low pressure in it. The water may be taken from a natural source such as river, lake or sea or thesame water may be cooled and circulated over again. In the latter case the cooling arrangement ismade through spray pond or cooling tower.1.1.2. Components of a Modern Steam Power PlantA modern steam power plant comprises of the following components :1. Boiler(i) Superheater (ii) Reheater(iii) Economiser (iv) Air-heater.2. Steam turbine 3. Generator4. Condenser 5. Cooling towers6. Circulating water pump 7. Boiler feed pump8. Wagon tippler 9. Crusher house10. Coal mill 11. Induced draught fans12. Ash precipitators 13. Boiler chimney14. Forced draught fans 15. Water treatment plant16. Control room 17. Switch yard.Functions of some important parts of a steam power plant :1. Boiler. Water is converted into wet steam.2. Superheater. It converts wet steam into superheated steam.3. Turbine. Steam at high pressure expands in the turbine and drives the generator. 27. INTRODUCTIONOUTLINE OF SOME DESCRIPTIVE SYSTEMS 34. Condenser. It condenses steam used by the steam turbine. The condensed steam (knownas condensate) is used as a feed water.5. Cooling tower. It cools the condenser circulating water. Condenser cooling water ab-sorbsheat from steam. This heat is discharged to atmosphere in cooling water.6. Condenser circulating water pump. It circulates water through the condenser andthe cooling tower.7. Feed water pump. It pumps water in the water tubes of boiler against boiler steampressure.8. Economiser. In economiser heat in flue gases is partially used to heat incoming feedwater.9. Air preheater. In air preheater heat in flue gases (the products of combustion) is par-tiallyused to heat incoming air.1.2. NUCLEAR POWER PLANTFig. 1.2 shows schematically a nuclear power plant.dharmM-thermTh1-1.pm5Steamturbine GeneratorSteamCoolingwaterSteamgeneratorSteamWaterWaterFeed pumpCoolant pumpCoolantHot coolantReactorcoreReactorFig. 1.2. Nuclear power plant.The main components of a nuclear power plant are :1. Nuclear reactor2. Heat exchanger (steam generator)3. Steam turbine4. Condenser5. Electric generator.In a nuclear power plant the reactor performs the same function as that of the furnace ofsteam power plant (i.e., produces heat). The heat liberated in the reactor as a result of the nuclearfission of the fuel is taken up by the coolants circulating through the reactor core. Hot coolantleaves the reactor at the top and then flows through the tubes of steam generator and passes on itsheat to the feed water. The steam so produced expands in the steam turbine, producing work, andthereafter is condensed in the condenser. The steam turbine in turn runs an electric generatorthereby producing electrical energy. In order to maintain the flow of coolant, condensate and feedwater pumps are provided as shown in Fig. 1.2. 28. 4 ENGINEERING THERMODYNAMICS1.3. INTERNAL COMBUSTION ENGINES1.3.1. Heat EnginesAny type of engine or machine which derives heat energy from the combustion of fuel orany other source and converts this energy into mechanical work is termed as a heat engine.Heat engines may be classified into two main classes as follows :1. External Combustion Engine.2. Internal Combustion Engine.1. External Combustion Engines (E.C. Engines)In this case, combustion of fuel takes place outside the cylinder as in case of steam engineswhere the heat of combustion is employed to generate steam which is used to move a piston in acylinder. Other examples of external combustion engines are hot air engines, steam turbine andclosed cycle gas turbine. These engines are generally needed for driving locomotives, ships, gen-erationof electric power etc.2. Internal Combustion Engines (I.C. Engines)In this case combustion of the fuel with oxygen of the air occurs within the cylinder of theengine. The internal combustion engines group includes engines employing mixtures of combusti-blegases and air, known as gas engines, those using lighter liquid fuel or spirit known as petrolengines and those using heavier liquid fuels, known as oil compression ignition or diesel engines.1.3.2. Development of I.C. EnginesMany experimental engines were constructed around 1878. The first really successful enginedid not appear, however until 1879, when a German engineer Dr. Otto built his famous Otto gasengine. The operating cycle of this engine was based upon principles first laid down in 1860 by aFrench engineer named Bea de Rochas. The majority of modern I.C. engines operate according tothese principles.The development of the well known Diesel engine began about 1883 by Rudoff Diesel. Al-thoughthis differs in many important respects from the otto engine, the operating cycle of modernhigh speed Diesel engines is thermodynamically very similar to the Otto cycle.1.3.3. Different parts of I.C. EnginesA cross-section of an air-cooled I.C. engines with principal parts is shown in Fig. 1.3.A. Parts common to both petrol and diesel engines1. Cylinder 2. Cylinder head 3. Piston4. Piston rings 5. Gudgeon pin 6. Connecting rod7. Crankshaft 8. Crank 9. Engine bearing10. Crank case 11. Flywheel 12. Governor13. Valves and valve operating mechanism.B. Parts for petrol engines only1. Spark plugs 2. Carburettor 3. Fuel pump.C. Parts for Diesel engine only1. Fuel pump. 2. Injector.dharmM-thermTh1-1.pm5 29. INTRODUCTIONOUTLINE OF SOME DESCRIPTIVE SYSTEMS 5dharmM-thermTh1-1.pm5Exhaust valveRocker arm PetroltankEnginethrottlePetrolsupply pipePush rodInletmanifoldInletvalvePistonCarburettorConnecting rodCrankRollerIntercamCrankshaftSilencerExhaustSparkplugPiston ringHigh tensioncableMagnetJetGear exhaust CrankcasecamCoolingfinsAir inletOil pumpFig. 1.3. An air-cooled four-stroke petrol engine.1.3.4. Spark Ignition (S.I.) EnginesThese engines may work on either four stroke cycle or two stroke cycle, majority of them, ofcourse, operate on four stroke cycle.Four stroke petrol engine :Fig. 1.4 illustrates the various strokes/series of operations which take place in a four strokepetrol (Otto cycle) engine.Suction stroke. During suction stroke a mixture of air and fuel (petrol) is sucked throughthe inlet valve (I.V.). The exhaust valve remains closed during this operation.Compression stroke. During compression stroke, both the valves remain closed, and thepressure and temperature of the mixture increase. Near the end of compression stroke, the fuel isignited by means of an electric spark in the spark plug, causing combustion of fuel at the instantof ignition.Working stroke. Next is the working (also called power or expansion) stroke. During thisstroke, both the valves remain closed. Near the end of the expansion stroke, only the exhaust valveopens and the pressure in the cylinder at this stage forces most of the gases to leave the cylinder.Exhaust stroke. Next follows the exhaust stroke, when all the remaining gases are drivenaway from the cylinder, while the inlet valve remains closed and the piston returns to the top deadcentre. The cycle is then repeated. 30. 6 ENGINEERING THERMODYNAMICSS.P.I.V. E.V.Air-fuelmixturedharmM-thermTh1-1.pm5S.P. S.P. S.P.I.V. E.V.E.C.C.R.CExhaustgasesSuctionstrokeCompressionstrokeWorkingstrokeExhauststrokeI.V = Intel valve, E.V. = Exhaust valve, E.C. = Engine cylinder,C.R. = Connecting rod, C = Crank, S.P. = Spark plug.GasesFig. 1.4. Four stroke otto cycle engine.Two stroke petrol engine :In 1878, Dugald-clerk, a British engineer introduced a cycle which could be completed intwo strokes of piston rather than four strokes as is the case with the four stroke cycle engines. Theengines using this cycle were called two stroke cycle engines. In this engine suction and exhauststrokes are eliminated. Here instead of valves, ports are used. The exhaust gases are driven outfrom engine cylinder by the fresh change of fuel entering the cylinder nearly at the end of theworking stroke.Fig. 1.5 shows a two stroke petrol engine (used in scooters, motor cycles etc.). The cylinderL is connected to a closed crank chamber C.C. During the upward stroke of the piston M, thegases in L are compressed and at the same time fresh air and fuel (petrol) mixture enters thecrank chamber through the valve V. When the piston moves downwards, V closes and the mixturein the crank chamber is compressed. Refer Fig. 1.5 (i) the piston is moving upwards and iscompressing an explosive change which has previously been supplied to L. Ignition takes place atthe end of the stroke. The piston then travels downwards due to expansion of the gases [Fig. 1.5 (ii)]and near the end of this stroke the piston uncovers the exhaust port (E.P.) and the burnt exhaustgases escape through this port [Fig. 1.5 (iii)]. The transfer port (T.P.) then is uncovered immediately,and the compressed charge from the crank chamber flows into the cylinder and is deflected upwardsby the hump provided on the head of the piston. It may be noted that the incoming air petrolmixture helps the removal of gases from the engine-cylinder ; if, in case these exhaust gases do notleave the cylinder, the fresh charge gets diluted and efficiency of the engine will decrease. Thepiston then again starts moving from bottom dead centre (B.D.C.) to top dead centre (T.D.C.) and 31. INTRODUCTIONOUTLINE OF SOME DESCRIPTIVE SYSTEMS 7the charge gets compressed when E.P. (exhaust port) and T.P. are covered by the piston ; thus thecycle is repeated.dharmM-thermTh1-1.pm5LE.P. MVLE.P. M T.P.VC.C.LE.P.MT.P.VC.C.T.P.C.C.Sparkplug(i) (ii) (iii)L = Cylinder ; E.P. = Exhaust port ; T.P. = Transfer port ; V = Valve ; C.C. = Crank chamber(i) (ii) (iii)Fig. 1.5. Two-stroke petrol engine.The power obtained from a two-stroke cycle engine is theoretically twice the power obtain-ablefrom a four-stroke cycle engine.1.3.5. Compression Ignition (C.I.) EnginesThe operation of C.I. engines (or diesel engines) is practically the same as those of S.I.engines. The cycle in both the types, consists of suction, compression, ignition, expansion andexhaust. However, the combustion process in a C.I. engine is different from that of a S.I. engine asgiven below :In C.I. engine, only air is sucked during the stroke and the fuel is injected in the cylindernear the end of the compression stroke. Since the compression ratio is very high (between 14 : 1 to22 : 1), the temperature of the air after compression is quite high. So when fuel is injected in theform of a spray at this stage, it ignites and burns almost as soon as it is introduced. The burntgases are expanded and exhausted in the same way as is done in a S.I. engine.1.4. GAS TURBINES1.4.1. General AspectsProbably a wind-mill was the first turbine to produce useful work, wherein there is noprecompression and no combustion. The characteristic features of a gas turbine as we think of thename today include a compression process and an heat addition (or combustion) process. The gas 32. 8 ENGINEERING THERMODYNAMICSturbine represents perhaps the most satisfactory way of producing very large quantities of powerin a self-contained and compact unit. The gas turbine may have a future use in conjunction withthe oil engine. For smaller gas turbine units, the inefficiencies in compression and expansionprocesses become greater and to improve the thermal efficiency it is necessary to use a heatexchanger. In order that a small gas turbine may compete for economy with the small oil engine orpetrol engine it is necessary that a compact effective heat exchanger be used in the gas turbinecycle. The thermal efficiency of the gas turbine alone is still quite modest 20 to 30% compared withthat of a modern steam turbine plant 38 to 40%. It is possible to construct combined plants whoseefficiencies are of order of 45% or more. Higher efficiencies might be attained in future.The following are the major fields of application of gas turbines :1. Aviation2. Power generation3. Oil and gas industry4. Marine propulsion.The efficiency of a gas turbine is not the criteria for the choice of this plant. A gas turbine isused in aviation and marine fields because it is self-contained, light weight, not requiring coolingwater and generally fits into the overall shape of the structure. It is selected for power generationbecause of its simplicity, lack of cooling water, needs quick installation and quick starting. It isused in oil and gas industry because of cheaper supply of fuel and low installation cost.The gas turbines have the following limitations : (i) They are not self-starting ; (ii) Lowefficiencies at part loads ; (iii) Non-reversibility ; (iv) Higher rotor speeds ; and (v) Overall effi-ciencyof the plant is low.1.4.2. Classification of Gas TurbinesThe gas turbines are mainly divided into two groups :1. Constant pressure combustion gas turbine :(a) Open cycle constant pressure gas turbine(b) Closed cycle constant pressure gas turbine.2. Constant volume combustion gas turbine.In almost all the fields open cycle gas turbine plants are used. Closed cycle plants wereintroduced at one stage because of their ability to burn cheap fuel. In between their progressremained slow because of availability of cheap oil and natural gas. Because of rising oil prices, nowagain, the attention is being paid to closed cycle plants.1.4.3. Merits and Demerits of Gas TurbinesMerits over I.C. engines :1. The mechanical efficiency of a gas turbine (95%) is quite high as compared with I.C.engine (85%) since the I.C. engine has a large many sliding parts.2. A gas turbine does not require a flywheel as the torque on the shaft is continuous anduniform. Whereas a flywheel is a must in case of an I.C. engine.3. The weight of gas turbine per H.P. developed is less than that of an I.C. engine.4. The gas turbine can be driven at a very high speeds (40,000 r.p.m.) whereas this is notpossible with I.C. engines.5. The work developed by a gas turbine per kg of air is more as compared to an I.C. engine.This is due to the fact that gases can be expanded upto atmospheric pressure in case ofa gas turbine whereas in an I.C. engine expansion upto atmospheric pressure is notpossible.dharmM-thermTh1-1.pm5 33. INTRODUCTIONOUTLINE OF SOME DESCRIPTIVE SYSTEMS 96. The components of the gas turbine can be made lighter since the pressures used in it arevery low, say 5 bar compared with I.C. engine, say 60 bar.7. In the gas turbine the ignition and lubrication systems are much simpler as comparedwith I.C. engines.8. Cheaper fuels such as paraffine type, residue oils or powdered coal can be used whereasspecial grade fuels are employed in petrol engine to check knocking or pinking.9. The exhaust from gas turbine is less polluting comparatively since excess air is used forcombustion.10. Because of low specific weight the gas turbines are particularly suitable for use in aircrafts.Demerits of gas turbines1. The thermal efficiency of a simple turbine cycle is low (15 to 20%) as compared with I.C.engines (25 to 30%).2. With wide operating speeds the fuel control is comparatively difficult.3. Due to higher operating speeds of the turbine, it is imperative to have a speed reductiondevice.4. It is difficult to start a gas turbine as compared to an I.C. engine.5. The gas turbine blades need a special cooling system.1.4.4. A Simple Gas Turbine PlantA gas turbine plant may be defined as one in which the principal prime-mover is of theturbine type and the working medium is a permanent gas.Refer to Fig. 1.6. A simple gas turbine plant consists of the following :1. Turbine.2. A compressor mounted on the same shaft or coupled to the turbine.3. The combustor.4. Auxiliaries such as starting device, auxiliary lubrication pump, fuel system, oil systemand the duct system etc.dharmM-thermTh1-1.pm5FuelCondenserC TAir in ExhaustGeneratorC = CompressureT = TurbineFig. 1.6. Simple gas turbine plant.A modified plant may have in addition to above an intercooler, regenerator, a reheater etc.The working fluid is compressed in a compressor which is generally rotary, multistagetype. Heat energy is added to the compressed fluid in the combustion chamber. This high energyfluid, at high temperature and pressure, then expands in the turbine unit thereby generatingpower. Part of the power generated is consumed in driving the generating compressor and accessories 34. 10 ENGINEERING THERMODYNAMICSand the rest is utilised in electrical energy. The gas turbines work on open cycle, semiclosed cycleor closed cycle. In order to improve efficiency, compression and expansion of working fluid iscarried out in multistages.1.4.5. Energy Cycle for a Simple-Cycle Gas TurbineFig. 1.7 shows an energy-flow diagram for a simple-cycle gas turbine, the description ofwhich is given below :dharmM-thermTh1-1.pm5Fuel inCompressedairCompressor TurbineAir in ExhaustPowergasCombustorFig. 1.7. Energy flow diagram for gas-turbine unit. The air brings in minute amount of energy (measured above 0C). Compressor adds considerable amount of energy. Fuel carries major input to cycle. Sum of fuel and compressed-air energy leaves combustor to enter turbine. In turbine smallest part of entering energy goes to useful output, largest part leaves inexhaust.Shaft energy to drive compressor is about twice as much as the useful shaft output.Actually the shaft energy keeps circulating in the cycle as long as the turbine runs. Theimportant comparison is the size of the output with the fuel input. For the simple-cycle gas tur-binethe output may run about 20% of the fuel input for certain pressure and temperature condi-tionsat turbine inlet. This means 80% of the fuel energy is wasted. While the 20% thermalefficiency is not too bad, it can be improved by including additional heat recovery apparatus.1.5. REFRIGERATION SYSTEMSRefrigeration means the cooling of or removal of heat from a system. Refrigerators workmainly on two processes :1. Vapour compression, and2. Vapour absorption.Simple Vapour Compression System :In a simple vapour compression system the following fundamental processes are completedin one cycle :1. Expansion 2. Vapourisation 3. Compression 4. Condensation. 35. INTRODUCTIONOUTLINE OF SOME DESCRIPTIVE SYSTEMS 11The flow diagram of such a cycle is shown in Fig. 1.8.dharmM-thermTh1-1.pm5ExpansionvalveCondenserSReceiverNMCompressorEvaporatorLFig. 1.8. Simple vapour compression cycle.The vapour at low temperature and pressure (state M) enters the compressor where it iscompressed isoentroprically and subsequently its temperature and pressure increase considerably(state N). This vapour after leaving the compressor enters the condenser where it is condensedinto high pressure liquid (state S) and is collected in a receiver. From receiver it passes throughthe expansion valve, here it is throttled down to a lower pressure and has a low temperature(state L). After finding its way through expansion valve it finally passes on to evaporator whereit extracts heat from the surroundings and vapourises to low pressure vapour (state M).Domestic Refrigerator :Refrigerators, these days, are becoming the common item for house hold use, vendorsshop, hotels, motels, offices, laboratories, hospitals, chemists and druggists shops, studios etc.They are manufactured in different size to meet the needs of various groups of people. They areusually rated with internal gross volume and the freezer volume. The freezer space is meant topreserve perishable products at a temperature much below 0C such as fish, meat, chicken etc.and to produce ice and icecream as well. The refrigerators in India are available in different sizesof various makes, i.e., 90, 100, 140, 160, 200, 250, 380 litres of gross volume. The freezers areusually provided at top portion of the refrigerator space occupying around one-tenth to one-third ofthe refrigerator volume. In some refrigerators, freezers are provided at the bottom.A domestic refrigerator consists of the following two main parts :1. The refrigeration system.2. The insulated cabinet.Fig. 1.9 shows a flow diagram of a typical refrigeration system used in a domestic refrigera-tor.A simple domestic refrigerator consists of a hermetic compressor placed in the cabinet base.The condenser is installed at the back and the evaporator is placed inside the cabinet at the top.The working of the refrigerator is as follows : The low pressure and low temperature refrigerant vapour (usually R12) is drawn throughthe suction line to the compressor. The accumulator provided between the suction lineand the evaporator collects liquid refrigerant coming out of the evaporator due to incom-pleteevaporation, if any, prevents it from entering the compressor. The compressorthen compresses the refrigerant vapour to a high pressure and high temperature. Thecompressed vapour flows through the discharge line into condenser (vertical naturaldraft, wire-tube type). In the condenser the vapour refrigerant at high pressure and at high temperature iscondensed to the liquid refrigerant at high pressure and low temperature. 36. 12 ENGINEERING THERMODYNAMICSdharmM-thermTh1-1.pm5Condenser (wire-tube type)High pressure gasCompre-ssorFilterDischarge line Suction lineExpansion device(Capillary tube)High pressureliquidLow pressure gas AccumulatorEvaporatorLow pressureliquidSound deadnerFig. 1.9. Domestic refrigerator. The high pressure liquid refrigerant then flows through the filter and then enters thecapillary tube (expansion device). The capillary tube is attached to the suction line asshown in Fig. 1.9. The warm refrigerant passing through the capillary tube gives someof its heat to cold suction line vapour. This increases the heat absorbing quality of theliquid refrigerant slightly and increases the superheat of vapour entering the compressor.The capillary tube expands the liquid refrigerant at high pressure to the liquid refrigerantat low pressure so that a measured quantity of liquid refrigerant is passed into the evaporator. In the evaporator the liquid refrigerant gets evaporated by absorbing heat from thecontainer/articles placed in the evaporative chamber and is sucked back into the com-pressorand the cycle is repeated.1. The layout of a modern steam power plant comprises of the following four circuits :(i) Coal and ash circuit(ii) Air and gas circuit(iii) Feed water and steam flow circuit(iv) Cooling water circuit.2. Any type of engine or machine which derives heat energy from the combustion of fuel or any other sourceand converts this energy into mechanical work is termed as a heat engine.3. The major fields of application of gas turbines are :(i) Aviation (ii) Power generation(iii) Oil and gas industry and (iv) Marine propulsion.4. A simple gas turbine plant consists of the following : Turbine Compressor 37. INTRODUCTIONOUTLINE OF SOME DESCRIPTIVE SYSTEMS 13 Combustor Auxiliaries such as starting device, auxiliary lubrication pump, fuel system, oil system and the ductsystem etc.5. Refrigeration means the cooling or removal of heat from a system. Refrigerators work mainly on twoprocesses(i) Vapour compression and(ii) Vapour absorption.dharmM-thermTh1-1.pm5THEORETICAL QUESTIONS1. Give the layout of a modern steam power plant and explain its various circuits.2. List the components of a nuclear power plant.3. Draw the cross-section of an air cooled I.C. engine and label its various parts.4. Explain with neat sketches the working of a four stroke petrol engine.5. How are gas turbines classified ?6. What are the major fields of application of gas turbines ?7. With the help of a neat diagram explain the working of a simple gas turbine plant.8. Draw the energy cycle for a simple-cycle gas turbine.9. Explain with a neat sketch the working of a simple vapour compression system.10. Draw the neat diagram of a domestic refrigerator, showing its various parts. Explain its working also. 38. 2Basic Concepts of Thermodynamics2.1. Introduction to kinetic theory of gases. 2.2. Definition of thermodynamics.2.3. Thermodynamic systemssystem, boundary and surroundingsclosed systemopensystemisolated systemadiabatic systemhomogeneous systemheterogeneoussystem. 2.4. Macroscopic and microscopic points of view. 2.5. Pure substance.2.6. Thermodynamic equilibrium. 2.7. Properties of systems. 2.8 State. 2.9. Process.2.10. Cycle. 2.11. Point function. 2.12. Path function. 2.13. Temperature. 2.14. Zeroth law ofthermodynamics. 2.15. The thermometer and thermometric propertyintroductionmeasurement of temperaturethe international practical temperature scaleideal gas.2.16. Pressuredefinition of pressureunit for pressuretypes of pressure measurementdevicesmechanical-type instrumentsliquid manometersimportant types of pressuregauges. 2.17. Specific volume. 2.18. Reversible and irreversible processes. 2.19. Energy,work and heatenergywork and heat. 2.20. Reversible workHighlightsObjective TypeQuestionsTheoretical Questions Unsolved Examples.2.1. INTRODUCTION TO KINETIC THEORY OF GASESThe kinetic theory of gases deals with the behaviour of molecules constituting the gas.According to this theory, the molecules of all gases are in continuous motion. As a result of thisthey possess kinetic energy which is transferred from molecule to molecule during their collision.The energy so transferred produces a change in the velocity of individual molecules.The complete phenomenon of molecular behaviour is quite complex. The assumptions aretherefore made to simplify the application of theory of an ideal gas.Assumptions :1. The molecules of gases are assumed to be rigid, perfectly elastic solid spheres, identicalin all respects such as mass, form etc.2. The mean distance between molecules is very large compared to their own dimensions.3. The molecules are in state of random motion moving in all directions with all possiblevelocities and gas is said to be in state of molecular chaos.4. The collisions between the molecules are perfectly elastic and there are no intermolecu-larforces of attraction or repulsion. This means that energy of gas is all kinetic.5. The number of molecules in a small volume is very large.6. The time spent in collision is negligible, compared to the time during which the mol-eculesare moving independently.7. Between collisions, the molecules move in a straight line with uniform velocity becauseof frictionless motion between molecules. The distance between two collisions is calledfree path of the molecule, the average distance travelled by a molecule between succes-sivecollision is known as mean free path.8. The volume of molecule is so small that it is negligible compared to total volume of the gas.14 39. BASIC CONCEPTS OF THERMODYNAMICS 15Pressure exerted by an Ideal Gas :Let us consider a quantity of gas to be contained in a cubical vessel of side l with perfectlyelastic wall and N represent the very large number of molecules in the vessel. Now let us considera molecule which may be assumed to have a velocity C1 in a certain direction. The velocity can beresolved into three components u1, v1, w1 parallel to three co-ordinate axes X, Y and Z which areagain assumed parallel to the sides of the cube as shown in Fig. 2.1.dharmM-therm/th2-1.pm5Fig. 2.12 = u1Thus, C12 + v12 + w12 .Let this molecule having mass m strike wall surface ABCD of the cube with velocity u1.Since the collision is perfectly elastic, the molecule will rebound from this surface with the samevelocity u1. Therefore,The momentum of the molecule before it strikes the face ABCD = mu1The momentum of the molecule after impact = mu1.Hence change of momentum at each impact in direction normal to the surfaceABCD = mu1 ( mu1) = 2mu1After striking the surface ABCD, the molecule rebounds and travels back to the face EFGH,collides with it and travels back again to the face ABCD covering 2l distance. This means moleculecovers 2l distance to hit the same face again. Hence the time taken by the same molecule to strikethe same face ABCD again is 2lu1.Therefore, the rate of change of momentum for one molecule of the gasmu 2lu221= =1mul1 40. 16 ENGINEERING THERMODYNAMICSAccording to Newtons second law of motion the rate of change of momentum is the force.If F1 is the force due to one molecule, thendharmM-therm/th2-1.pm5F mu1 l21=Similarly, then force F2 due to the impact of another molecule having velocity C2 whosecomponents are u2, v2, w2 is given byFmu2 l22=Hence total force Fx on the face ABCD due to impact of N molecules is given byF ml22 2x = (u1 + u 2+ ......uN )Since the pressure (p) is the force per unit area, hence pressure exerted on the wall ABCD isgiven bypFlmlx( 22 ...... 2)= = + + N 2 3 1x u u 2uSimilarly, if py and pz represent the pressures on other faces which are perpendicular to theY and Z axis respectively, we havepml( 22 ...... 2)y = v + v 3 12+ vN and pml( 22 ...... 2)z = w + w 3 12+ wN Since pressure exerted by the gas is the same in all directions, i.e., px = py = pz the averagepressure p of the gas is given byp+ +3px py pz =or pml[( 222) ( 22) ......( 2 2 2)]= u + v + w + u 3 1112+ v + w + uN + vN + wN 2222= ( 2+ 2+ 2)But C1 u 1v 1w12= ( 2+ 2+ 2) and so onl3 = V = volume of gas (m3)C2 u 2v 2w2 p= 1m+ + + 3 v1( 22 ...... 2)C C C CN223= 1 NC3or p mv2 ...(2.1)22 2= + + + where C C C C C...... known as mean square velocity2 1 NN22322 ...... 2= 1 + + + Nor C C C C CN223where C is called the root mean square velocity of the molecules and equal to the square root ofthe mean of square of velocities of individual molecules which is evidently not the same as mean ofvelocities of different molecules......= + + +. ., Ni e CC C C Cmean N 41. 1 2 3 42. BASIC CONCEPTS OF THERMODYNAMICS 17or pV = m NC 1dharmM-therm/th2-1.pm532 ...(2.2)This equation is the fundamental equation of kinetic theory of gases and is often referred toas kinetic equation of gases.Equation (2.2) may be written aspV = 2/3 1/2 m NC2where 12mNC2 is the average transmission or linear kinetic energy of the system of particles.Equation (2.1) can be written asp = 1/3 C2 ...(2.3)where is the density. = 43. mNV, i.e.,Total massTotal volumeThis equation expresses the pressure which any volume of gas exerts in terms of its densityunder the prevailing conditions and its mean square molecular speed.From equations (2.2) and (2.3),C p pV= = mN 3 3Kinetic interpretation of Temperature :If Vmol is the volume occupied by a gram molecule of a gas and N0 is the number of moles inone gram molecule of gas,M = molecular weight = mN0. ...(i)Since p Vmol = R0T ......Molar gas equation ...(ii)From equations (2.2) and (ii),1/3 m N0 C = R0T R0 = Universal gas constantor 2/3 12 m N0 C2 = R0T N0 = Avogadros numberor 12 mC2 = 3/2 KT ...(2.4)RN00= K (Boltzmans constant)(i.e., K.E. per molecule = 3/2 KT)or C =3KTmor C =3R0TM KmRN mRM= 0 =00or C = 3RT ...(2.5)R RM = 0where R is characteristic gas constant.From equation (2.4) it is seen that temperature is a measure of the average kinetic energyof translation possessed by molecule. It is known as the kinetic interpretation of temperature.Hence, the absolute temperature of a gas is proportional to the mean translational kinetic energyof the molecules it consists. If the temperature is fixed, then the average K.E. of the moleculesremains constant despite encounters. 44. 18 ENGINEERING THERMODYNAMICS2.2. DEFINITION OF THERMODYNAMICSThermodynamics may be defined as follows : Thermodynamics is an axiomatic science which deals with the relations among heat,work and properties of system which are in equilibrium. It describes state and changesin state of physical systems.Surroundings BoundarydharmM-therm/th2-1.pm5OrThermodynamics is the science of the regularities governing processes of energyconversion.OrThermodynamics is the science that deals with the interaction between energy andmaterial systems.Thermodynamics, basically entails four laws or axioms known as Zeroth, First, Second andThird law of thermodynamics. The First law throws light on concept of internal energy. The Zeroth law deals with thermal equilibrium and establishes a concept of temperature. The Second law indicates the limit of converting heat into work and introduces theprinciple of increase of entropy. The Third law defines the absolute zero of entropy.These laws are based on experimental observations and have no mathematical proof. Likeall physical laws, these laws are based on logical reasoning.2.3. THERMODYNAMIC SYSTEMS2.3.1. System, Boundary and SurroundingsSystem. A system is a finite quantity of matter or a prescribed region of space (Refer Fig. 2.2)Boundary. The actual or hypothetical envelope enclosing the system is the boundary ofthe system. The boundary may be fixed or it may move, as and when a system containing a gas iscompressed or expanded. The boundary may be real or imaginary. It is not difficult to envisage areal boundary but an example of imaginary boundary would be one drawn around a system con-sistingof the fresh mixture about to enter the cylinder of an I.C. engine together with the remanantsof the last cylinder charge after the exhaust process (Refer Fig. 2.3).SystemSurroundingsSystemPistonPistonCylinderRealboundaryConvenientimaginaryboundarySystemFig. 2.2. The system. Fig. 2.3. The real and imaginary boundaries.2.3.2. Closed SystemRefer to Fig. 2.4. If the boundary of the system is impervious to the flow of matter, it iscalled a closed system. An example of this system is mass of gas or vapour contained in an enginecylinder, the boundary of which is drawn by the cylinder walls, the cylinder head and pistoncrown. Here the boundary is continuous and no matter may enter or leave. 45. BASIC CONCEPTS OF THERMODYNAMICS 19dharmM-therm/th2-1.pm5Mass non-necessarilyconstantMass remains constantregardless variation ofboundariesOutGasBoundaryBoundaryInGasFig. 2.4. Closed system. Fig. 2.5. Open system.2.3.3. Open SystemRefer to Fig. 2.5. An open system is one in which matter flows into or out of the system.Most of the engineering systems are open.2.3.4. Isolated SystemAn isolated system is that system which exchanges neither energy nor matter with anyother system or with environment.2.3.5. Adiabatic SystemAn adiabatic system is one which is thermally insulated from its surroundings. It can,however, exchange work with its surroundings. If it does not, it becomes an isolated system.Phase. A phase is a quantity of matter which is homogeneous throughout in chemicalcomposition and physical structure.2.3.6. Homogeneous SystemA system which consists of a single phase is termed as homogeneous system. Examples :Mixture of air and water vapour, water plus nitric acid and octane plus heptane.2.3.7. Heterogeneous SystemA system which consists of two or more phases is called a heterogeneous system. Examples :Water plus steam, ice plus water and water plus oil.2.4. MACROSCOPIC AND MICROSCOPIC POINTS OF VIEWThermodynamic studies are undertaken by the following two different approaches.1. Macroscopic approach(Macro mean big or total)2. Microscopic approach(Micro means small) 46. 20 ENGINEERING THERMODYNAMICSThese approaches are discussed (in a comparative way) below :S. No. Macroscopic approach Microscopic approach1. In this approach a certain quantity of matter isconsidered without taking into account the eventsoccurring at molecular level. In other words thisapproach to thermodynamics is concerned withgross or overall behaviour. This is known asclassical thermodynamics.dharmM-therm/th2-1.pm5The approach considers that the system is madeup of a very large number of discrete particlesknown as molecules. These molecules havedifferent velocities and energies. The values ofthese energies are constantly changing with time.This approach to thermodynamics which isconcerned directly with the structure of thematter is known as statistical thermodynamics.2. The analysis of macroscopic system requiressimple mathematical formulae.The behaviour of the system is found by usingstatistical methods as the number of molecules isvery large. So advanced statistical and mathe-maticalmethods are needed to explain thechanges in the system.3. The values of the properties of the system aretheir average values. For example, consider asample of a gas in a closed container. The pressureof the gas is the average value of the pressureexerted by millions of individual molecules.Similarly the temperature of this gas is the averagevalue of translational kinetic energies of millionsof individual molecules. These properties likepressure and temperature can be measured veryeasily. The changes in properties can be felt byour senses.The properties like velocity, momentum, impulse,kinetic energy, force of impact etc. which describethe molecule cannot be easily measured byinstruments. Our senses cannot feel them.4. In order to describe a system only a few propertiesare needed.Large number of variables are needed to describea system. So the approach is complicated.Note. Although the macroscopic approach seems to be different from microscopic one, there exists arelation between them. Hence when both the methods are applied to a particular system, they give the sameresult.2.5. PURE SUBSTANCEA pure substance is one that has a homogeneous and invariable chemical composition eventhough there is a change of phase. In other words, it is a system which is (a) homogeneous incomposition, (b) homogeneous in chemical aggregation. Examples : Liquid, water, mixture of liquidwater and steam, mixture of ice and water. The mixture of liquid air and gaseous air is not a puresubstance.2.6. THERMODYNAMIC EQUILIBRIUMA system is in thermodynamic equilibrium if the temperature and pressure at all pointsare same ; there should be no velocity gradient ; the chemical equilibrium is also necessary.Systems under temperature and pressure equilibrium but not under chemical equilibrium aresometimes said to be in metastable equilibrium conditions. It is only under thermodynamic equi-libriumconditions that the properties of a system can be fixed.Thus for attaining a state of thermodynamic equilibrium the following three types of equi-libriumstates must be achieved : 47. BASIC CONCEPTS OF THERMODYNAMICS 211. Thermal equilibrium. The temperature of the system does not change with time andhas same value at all points of the system.2. Mechanical equilibrium. There are no unbalanced forces within the system or betweenthe surroundings. The pressure in the system is same at all points and does not change withrespect to time.3. Chemical equilibrium. No chemical reaction takes place in the system and the chemi-calcomposition which is same throughout the system does not vary with time.2.7. PROPERTIES OF SYSTEMSA property of a system is a characteristic of the system which depends upon its state, butnot upon how the state is reached. There are two sorts of property :1. Intensive properties. These properties do not depend on the mass of the system.Examples : Temperature and pressure.2. Extensive properties. These properties depend on the mass of the system. Example :Volume. Extensive properties are often divided by mass associated with them to obtain the inten-siveproperties. For example, if the volume of a system of mass m is V, then the specific volume ofmatter within the system is VdharmM-therm/th2-1.pm5m = v which is an intensive property.2.8. STATEState is the condition of the system at an instant of time as described or measured by itsproperties. Or each unique condition of a system is called a state.It follows from the definition of state that each property has a single value at each state.Stated differently, all properties are state or point functions. Therefore, all properties are identicalfor identical states.On the basis of the above discussion, we can determine if a given variable is property or notby applying the following tests : A variable is a property, if and only if, it has a single value at each equilibrium state. A variable is a property, if and only if, the change in its value between any two pre-scribedequilibrium states is single-valued.Therefore, any variable whose change is fixed by the end states is a property.2.9. PROCESSA process occurs when the system undergoes a change in a state or an energy transfer at asteady state. A process may be non-flow in which a fixed mass within the defined boundary isundergoing a change of state. Example : A substance which is being heated in a closed cylinderundergoes a non-flow process (Fig. 2.4). Closed systems undergo non-flow processes. A processmay be a flow process in which mass is entering and leaving through the boundary of an opensystem. In a steady flow process (Fig. 2.5) mass is crossing the boundary from surroundings atentry, and an equal mass is crossing the boundary at the exit so that the total mass of the systemremains constant. In an open system it is necessary to take account of the work delivered from thesurroundings to the system at entry to cause the mass to enter, and also of the work delivered fromthe system at surroundings to cause the mass to leave, as well as any heat or work crossing theboundary of the system.Quasi-static process. Quasi means almost. A quasi-static process is also called a re-versibleprocess. This process is a succession of equilibrium states and infinite slowness is itscharacteristic feature. 48. 22 ENGINEERING THERMODYNAMICS2.10. CYCLEAny process or series of processes whose end states are identical is termed a cycle. Theprocesses through which the system has passed can be shown on a state diagram, but a completesection of the path requires in addition a statement of the heat and work crossing the boundary ofthe system. Fig. 2.6 shows such a cycle in which a system commencing at condition 1 changes inpressure and volume through a path 123 and returns to its initial condition 1.2 Q2 Q1 and is shown as 1Q2 or Q12dharmM-therm/th2-1.pm5123p (Pressure)V (Volume)Fig. 2.6. Cycle of operations.2.11. POINT FUNCTIONWhen two properties locate a point on the graph (co-ordinate axes) then those propertiesare called as point function.Examples. Pressure, temperature, volume etc.2dV = V2 V11(an exact differential).2.12. PATH FUNCTIONThere are certain quantities which cannot be located on a graph by a point but are given bythe area or so, on that graph. In that case, the area on the graph, pertaining to the particularprocess, is a function of the path of the process. Such quantities are called path functions.Examples. Heat, work etc.Heat and work are inexact differentials. Their change cannot be written as difference be-tweentheir end states.Thus Q12 W2 W1, and is shown as 1W2 or W12Similarly W1Note. The operator is used to denote inexact differentials and operator d is used to denote exactdifferentials. 49. BASIC CONCEPTS OF THERMODYNAMICS 232.13. TEMPERATURE The temperature is a thermal state of a body which distinguishes a hot body from acold body. The temperature of a body is proportional to the stored molecular energyi.e., the average molecular kinetic energy of the molecules in a system. (A particularmolecule does not hhave a temperature, it has energy. The gas as a system has tempera-ture). Instruments for measuring ordinary temperatures are known as thermometers andthose for measuring high temperatures are known as pyrometers. It has been found that a gas will not occupy any volume at a certain temperature. Thistemperature is known as absolute zero temperature. The temperatures measured withabsolute zero as basis are called absolute temperatures. Absolute temperature is statedin degrees centigrade. The point of absolute temperature is found to occur at 273.15Cbelow the freezing point of water.Then : Absolute temperature = Thermometer reading in C + 273.15.Absolute temperature is degree centigrade is known as degrees kelvin, denoted by K (SI unit).2.14. ZEROTH LAW OF THERMODYNAMICS Zeroth law of thermodynamics states that if two systems are each equal in tem-peraturedharmM-therm/th2-1.pm5to a third, they are equal in temperature to each other.Fig. 2.7. Zeroth law of thermodynamics.Example. Refer Fig. 2.7. System 1 may consist of a mass of gas enclosed in a rigid vesselfitted with a pressure gauge. If there is no change of pressure when this system is brought intocontact with system 2 a block of iron, then the two systems are equal in temperature (assumingthat the systems 1 and 2 do not react each other chemically or electrically). Experiment revealsthat if system 1 is brought into contact with a third system 3 again with no change of propertiesthen systems 2 and 3 will show no change in their properties when brought into contact providedthey do not react with each other chemically or electrically. Therefore, 2 and 3 must be inequilibrium. This law was enunciated by R.H. Fowler in the year 1931. However, since the first andsecond laws already existed at that time, it was designated as zeroth law so that itprecedes the first and second laws to form a logical sequence. 50. 24 ENGINEERING THERMODYNAMICS2.15. THE THERMOMETER AND THERMOMETRIC PROPERTY2.15.1. Introduction The zeroth law of thermodynamics provides the basis for the measurement of tempera-ture.dharmM-therm/th2-1.pm5It enables us to compare temperatures of two bodies 1 and 2 with the help of athird body 3 and say that the temperature of 1 is the same as the temperature of 2without actually bringing 1 and 2 in thermal contact. In practice, body 3 in thezeroth law is called the thermometer. It is brought into thermal equilibrium with a setof standard temperature of a body 2, and is thus calibrated. Later, when any other body1 is brought in thermal communication with the thermometer, we say that the body 1has attained equality of temperature with the thermometer, and hence with body 2.This way, the body 1 has the temperature of body 2 given for example by, say theheight of mercury column in the thermometer 3. The height of mercury column in a thermometer, therefore, becomes a thermometricproperty.There are other methods of temperature measurement which utilize various other proper-tiesof materials, that are functions of temperature, as thermometric properties.Six different kinds of thermometers, and the names of the corresponding thermometricproperties employed are given below :Thermometer Thermometric property1. Constant volumes gas Pressure (p)2. Constant pressure gas Volume (V)3. Alcohol or mercury-in-glass Length (L)4. Electric resistance Resistance (R)5. Thermocouple Electromotive force (E)6. Radiation (pyrometer) Intensity of radiation (I or J)2.15.2. Measurement of TemperatureTemperature can be depicted as a thermal state which depends upon the internal ormolecular energy of the body.2.15.2.1. Temperature Measuring InstrumentsThese instruments may be classified in two broad categories :1. Non-electrical methods :(i) By using change in volume of a liquid when its temperature is changed.(ii) By using change in pressure of a gas when its temperature is changed.(iii) By using changes in the vapour pressure when the temperature is changed.2. Electrical method :(i) By thermocouples.(ii) By change in resistance of material with change in temperature.(iii) By comparing the colours of filament and the object whose temperature is to be found out.(iv) By ascertaining the energy received by radiation.The thermometers may also be classified as follows :1. Expansion thermometers(i) Liquid-in-glass thermometers (ii) Bimetallic thermometers.2. Pressure thermometers(i) Vapour pressure thermometers (ii) Liquid-filled thermometers(iii) Gas-filled thermometers. 51. BASIC CONCEPTS OF THERMODYNAMICS 253. Thermocouple thermometers4. Resistance thermometers5. Radiation pyrometers6. Optical pyrometers.1. Expansion Thermometers :The expansion thermometers make use of the differential expan-sionof two different substances. Thus in liquid-in-glass thermometers,it is the difference in expansion of liquid and the containing glass. Andin bimetallic thermometers, the indication is due to the difference inexpansion of the two solids. These thermometers are discussed below :(i) Liquid-in-glass thermometer. This is a very familiar typeof thermometer. The mercury or other liquid fills the glassbulb and extends into the bore of the glass stem. Mercury isthe most suitable liquid and is used from 38.9C (meltingpoint) to about 600C. The thermometers employed in thelaboratory have the scale engraved directly on the glass stem.A usual type of mercury-in-glass thermometer is shown inFig. 2.8. An expansion bulb is usually provided at the top ofthe stem to allow room for expansion of mercury, in case thethermometer is subjected to temperature above its range. Theupper limit for mercury-in-glass thermometers is about 600C.As the upper limit is far above the boiling point of mercury,some inert gas i.e., nitrogen is introduced above the mercuryto prevent boiling.Pentane, ethyl alcohol and toluene are the other liquids whichFig. 2.8. Mercury-in-glassthermometer.can be used for liquid-in-glass thermometers. Since these liquids arenormally colourless a dye is added to facilitate reading. These liquids have a low freezing point asshown below and are suitable for low temperature thermometers.dharmM-therm/th2-1.pm5Liquid Boiling point Freezing pointPentane 36C 130CEthyl alcohol 78C 100CToluene 110C 92C(ii) Bimetallic thermometers. In a bimetallic thermometer differential expansion ofbimetallic strips is used to indicate the temperature. It has the advantage over theliquid-in-glass thermometer, that it is less fragile and is easier to read. In this type ofthermometer two flat strips of different metals are placed side by side and are weldedtogether. Many different metals can be used for this purpose. Generally one is a lowexpanding metal and the other is high expanding metal. The bimetal strip is coiled inthe form of a spiral or helix. Due to rise in temperature, the curvature of the stripchanges. The differential expansion of a strip causes the pointer to move on the dial ofthe thermometer.2. Pressure Thermometers :In pressure thermometers liquids, gases and vapours can all be used. The principle onwhich they work is quite simple. The fluid is confined in a closed system. In this case the pressureis a function of the temperature, so that when the fluid is heated, the pressure will rise. And thetemperature can be indicated by Bourdon type pressure gauge. In general, the thermometer consistsof a bulb which contains bulk of the fluid. The bulb is placed in the region whose temperature isrequired. A capillary tube connects the bulb to a Bourdon tube, which is graduated with atemperature scale. 52. 26 ENGINEERING THERMODYNAMICSPressure thermometers are discussed below :(i) Vapour pressure thermometer. A schematic diagram of a vapour pressure ther-mometeris shown in Fig. 2.9. When the bulb containing the fluid is installed in the region whosetemperature is required, some of the fluid vapourizes, and increases the vapour pressure. Thischange of pressure is indicated on the Bourdon tube. The relation between temperature and vapourpressure of a volatile liquid is of the exponential form. Therefore, the scale of a vapour pressurethermometer will not be linear.dharmM-therm/th2-1.pm5Pressuremeasuring deviceCapillarytubingCapillarytubingBourdonspringLiquidBulbBulbVapourLiquidFig. 2.9. Vapour pressure thermometer. Fig. 2.10. Liquid-filled thermometer.(ii) Liquid-filled thermometer. A liquid-filled thermometer is shown in Fig. 2.10. In thiscase, the expansion of the liquid causes the pointer to move in the dial. Therefore liquids havinghigh co-efficient of expansion should be used. In practice many liquids e.g., mercury, alcohol,toluene and glycerine have been successfully used. The operating pressure varies from about 3 to100 bar. These type of thermometers could be used for a temperature upto 650C in which mercurycould be used as the liquid.In actual design, the internal diameter of the capillary tube and Bourdon tube is, mademuch smaller than that of the bulb. This is because the capillary tube is subjected to a tempera-turewhich is quite different from that of the bulb. Therefore, to minimise the effect of variation intemperature to which the capillary tube is subjected, the volume of the bulb is made as large aspossible as compared with the volume of the capillary. However, large volume of bulb tends toincrease time lag, therefore, a compensating device is usually built into the recording or indicat-ingmechanism, which compensates the variations in temperature of the capillary and Bourdontubes.(iii) Gas-filled thermometers. The temperature range for gas thermometer is practicallythe same as that of liquid filled thermometer. The gases used in the gas thermometers are nitrogenand helium. Both these gases are chemically inert, have good values for their co-efficient of expansionand have low specific heats. The construction of this type of thermometer is more or less the sameas mercury-thermometer in which Bourdon spring is used. The errors are also compensated likewise.The only difference in this case is that bulb is made much larger than used in liquid-filled 53. BASIC CONCEPTS OF THERMODYNAMICS 27thermometers. For good performance the volume of the bulb should be made at least 8 times thanthat of the rest of the system.These thermometers are generally used for pressures below 35 bar.3. Thermocouple Thermometers :For higher range of temperature i.e., above 650C, filled thermometers are unsuitable. Forhigher range of temperature, thermocouples and pyrometers are used.dharmM-therm/th2-1.pm5Millivoltmeter ortemperature recorderCopperleadsReferencejunctionMetal-1Metal-2MeasuringjunctionHot bodyFig. 2.11. Thermocouple.In its simplest form a thermocouple consists of two dissimilar metals or alloys which develope.m.f. when the reference and measuring junctions are at different temperatures. The referencejunction or cold junction is usually maintained at some constant temperature, such as 0C. Fig.2.11, shows a simple circuit of a thermocouple and the temperature measuring device. In manyindustrial installations the instruments are equipped with automatic compensating devices fortemperature changes of the reference junction, thus eliminating the necessity of maintaining thisjunction at constant temperature.Table 2.1 gives the composition, useful temperatures range and temperature versus e.m.f.relationship for some commercial thermocouples.Table 2.1. Composition, useful temperature range ande.m.f. produced for some thermocouplesTemperature (C) ThermoelectricS.No. Thermocouple Composition powerUseful Max. C Millivolt Remarksrange1. Platinum vs Pure platinum 400 to 1700 0 0.0 Used for highPlatinum- vs Pt + 10 or 1450 500 4.219 temperaturerhodium 13% Rh 1000 9.569 measurements1500 15.4982. Chromel vs 90% Ni + 10% 200 to 1450 200 5.75 High resistancealumel Cr vs 95% 1200 0 0.0 to oxidationNi + 5% 300 12.21(Al + Sn) Mn 600 24.90900 37.361200 48.85 54. 28 ENGINEERING THERMODYNAMICS3. Iron vs Pure iron vs 200 to 1000 200 8.27constantan 45-60% Cu + 750 0 0.0dharmM-therm/th2-1.pm555-40% Ni 300 16.59 600 33.27900 52.294. Copper vs con- Pure copper vs 200 to 600 200 5.539 Not suitable in airstantan Cu-Ni 350 0 0.0 due to excessiveconstantan 200 9.285 oxidation400 20.8654. Resistance thermometers :The fact that the electrical resistance of the metals increases with temperature is made useof in resistance thermometers which are purely electrical in nature. A resistance thermometeris used for precision measurements below 150C.A simple resistance thermometer consists of a resistance element or bulb, electrical loadsand a resistance measuring or recording instrument. The resistance element (temperature sensitiveelement) is usually supplied by the manufacturers with its protecting tube and is ready for electricalconnections. The resistance of the metal used as resistance element should be reproducible at anygiven temperature. The resistance is reproducible if the composition or physical properties of themetal do not change with temperature. For this purpose platinum is preferred. A platinum resistancethermometer can measure temperatures to within 0.01C. However, because of high cost ofplatinum, nickel and copper are used as resistance elements for industrial purposes for lowtemperatures. The fine resistance wire is wound in a spiral form on a mica frame. The delicate coilis then enclosed in a porcelain or quartz tube. The change of resistance of this unit can be measuredby instruments such as Wheatstone bridge, potentiometer or galvanometer.Advantages :The resistance thermometers possess the following advantages over other devices :1. A resistance thermometer is very accurate for low ranges below 150C.2. It requires no reference junction like thermocouples and as such is more effective atroom temperature.3. The distance between the resistance element and the recording element can be mademuch larger than is possible with pressure thermometers.4. It resists corrosion and is physically stable.Disadvantages :1. The resistance thermometers cost more.2. They suffer from time lag.5. Radiation pyrometers :A device which measures the total intensity of radiation emitted from a body is calledradiation pyrometer.The elements of a total radiation pyrometer are illustrated in Fig. 2.12. It collects theradiation from an object (hot body) whose temperature is required. A mirror is used to focus thisradiation on a thermocouple. This energy which is concentrated on the thermocouple raises itstemperature, and in turn generates an e.m.f. This e.m.f. is then measured either by the galvanometeror potentiometer method. Thus rise of temperature is a function of the amount of radiation emittedfrom the object. 55. BASIC CONCEPTS OF THERMODYNAMICS 29dharmM-therm/th2-1.pm5Fig. 2.12. A schematic diagram of radiation pyrometer.Advantages of the pyrometers1. The temperatures of moving objects can be measured.2. A higher temperature measurement is possible than that possible by thermocouples etc.3. The average temperatures of the extended surface can be measured.4. The temperature of the objects which are not easily accessible can be measured.6. Optical pyrometer :An optical pyrometer works on the principle that matters glow