Understanding Psychrometrics Third Edition Donald P. Gatley ISBN978-1-936504-31-2 ©2002,2005,2013ASHRAE.Allrightsreserved. 1791TullieCircle,NE·Atlanta,GA30329·www.ashrae.org ASHRAEisaregisteredtrademarkoftheAmericanSocietyofHeating, RefrigeratingandAir-ConditioningEngineers,Inc. PrintedintheUnitedStatesofAmerica CoverdesignbyTracyBecker ASHRAEhascompiledthispublicationwithcare,butASHRAEhasnotinvestigated,and ASHRAEexpresslydisclaimsanydutytoinvestigate,anyproduct,service,process,proce- dure,design,orthelikethatmaybedescribedherein.Theappearanceofanytechnicaldata oreditorialmaterialinthispublicationdoesnotconstituteendorsement,warranty,orguar- antybyASHRAEofanyproduct,service,process,procedure,design,orthelike.ASHRAE doesnotwarrantthattheinformationinthepublicationisfreeoferrors,andASHRAEdoes notnecessarilyagreewithanystatementoropinioninthispublication.Theentireriskofthe useofanyinformationinthispublicationisassumedbytheuser. No part of this publication may be reproduced without permission in writing from ASHRAE,exceptbyareviewerwhomayquotebriefpassagesorreproduceillustrationsin areviewwithappropriatecredit,normayanypartofthispublicationbereproduced,stored inaretrievalsystem,ortransmittedinanywayorbyanymeans—electronic,photocopying, recording,orother—withoutpermissioninwritingfromASHRAE.Requestsforpermission shouldbesubmittedatwww.ashrae.org/permissions. LibraryofCongressCataloging-in-PublicationData Gatley,D.P. Understandingpsychrometrics/DonaldP.Gatley.—Thirdedition. pagescm Includesbibliographicalreferencesandindex. Summary:“Updatesthesecondeditiontoprovidereadersareferencethatagreeswiththelatest internationalstandards.Thethirdeditionalsoincludesarevisedequationfortheadiabaticsaturation process,ansummaryofthe2009RP-1485ASHRAEresearch,aswellasminoreditstothetext”— Providedbypublisher. ISBN978-1-936504-31-2(hardcover) 1.Hygrometry.2.Humidity.I.AmericanSocietyofHeating,RefrigeratingandAir-Conditioning Engineers.II.Title. QC915.G372012 551.57'10287--dc23 2012042195 ASHRAESTAFF SPECIALPUBLICATIONS MarkOwen,Editor/GroupManagerofHandbookandSpecialPublications CindySheffieldMichaels,ManagingEditor MattWalker,AssociateEditor RobertaHirschbuehler,AssistantEditor SarahBoyle,EditorialAssistant MichshellPhillips,EditorialCoordinator PUBLISHINGSERVICES DavidSoltis,GroupManagerofPublishingServicesandElectronicCommunications TracyBecker,GraphicsSpecialist JayneJackson,PublicationTrafficAdministrator PUBLISHER W.StephenComstock Psychrometrics—thephysicsofmoistair: anappliedsciencedealingwiththepropertiesandprocessesofmoistair. Preface Psychrometricsisasubscienceofphysicsdealingwiththeproperties andprocessesofmoistair.Moistairisdefinedasamixtureoftwogases: dryairandwatervapour(thegasphaseofH O).Dryairwithinthetro- 2 posphereistreatedasanon-varyingmixtureofnitrogen(78.1%),oxygen (20.9%),argon(0.9%),andothertracegases,includingcarbondioxide (<0.04%).Somebroadenthedefinitionofpsychrometricstocovermix- turesofthegasofonesubstance(anydrygascomponent)andthecon- densablevapourofasecondsubstance. Theclockinthemarginofthisparagraphandelsewhereinthisbook (cid:27) indicatestextthatshouldbereadbyastudentornewuserwhowantsto acquiremostofthebasicsofpsychrometricsinfourtosixhoursofreading. (cid:27) Psychrometricsisabasicsciencethatunderliesagriculturalandaero- nauticalengineering;airconditioning;dryingofcrops,grains,andphar- maceuticals; dehydration; dehumidification; humidification; moisture control;meteorology;weatherreporting;foodscienceengineering;and pilotingofaircraft.Itispossibletoworkinthesefieldswithoutagood understandingoffundamental-levelpsychrometricsbytheuseofshort- cutformulae,tables,andcharts.Whileitistruethatdesignerscansurvive inthesefieldswithminimalandincompletepsychrometricskillswithout creating many problems for themselves, their employers, and clients, thereisnojustificationforthislackofknowledgewhenpsychrometrics canbequicklylearned. Manyreadersarefrustratedintheirpursuitoffundamentalpsychro- metricknowledgebecauseexistingtextsareincomplete,overlycompli- cated,notwellorganizedintolearningelements,orcontainobsoleteterms andcalculationsthathavelittlerelevanceintoday'sworldoffastpersonal computers, psychrometric software, and computer-generated psychro- metricchartstailoredtoasite-specificbarometricpressureoraltitude. xi Understanding Psychrometrics, Third Edition (cid:27) Psychrometricscanbesimplyexplainedandissolidlybasedon:(1) theidealgasequation,(2)Dalton’smodelofpartialpressures,(3)con- servationofenergy,and(4)conservationofmass.Intheauthor’sopinion, thereiseveryreasontopursueasolidfoundationinpsychrometricswhen ittakeslessthanfourhourstoacquirethisbackgroundforalifetimeof use.Suchknowledgemaynotberequiredintypicaldesigns,butitcanpre- vent costly mistakes when altitude or barometric pressure differ from standardsealevelvalues,whensubfreezingtemperaturesexist,orwhen thespecificvolumeofthemoistairdifferssubstantiallyfromthatofStan- dardTemperatureandPressure(STP)air(arbitrarilyfixedat15°Cand 101,325Pa). Whatisdifferentaboutthispsychrometrictext?Itisdevotedsolely topsychrometrics.Psychrometricsisusuallycoveredinoneortwochap- tersofathermodynamicstextbook.Thistextiswrittenforeaseofunder- standingandnotwithbrevityinmind,butatthesametimetheinformation ispresentedinsuchawaythatthereadercanchoosenotonlythechapters toexplorebutalsothelevelofdetailwithineachchapter.Italsoincludes anextensivelistingofthepioneersofpsychrometrics. (cid:27) Theunderlyingreasonsforthisnewpsychrometricpublicationare: (1)toaddressthechangesbroughtonbytheshiftfrommanualcalcula- tionsandplottingonprintedpsychrometricchartstosoftware-basedpsy- chrometriccalculationsandgraphics,(2)topresentinonetextathorough coverageofpsychrometricfundamentals,and(3)toassistdesignersand practitionersinthetransitiontotheSystèmeInternationalsystemofunits andcalculations.Therefore,theobjectivesofthispublicationareto: (cid:27) • Provide a resource equivalent to a reference book as well as a basic refresher course for those who use psychrometrics on a recurringbasis. • Provide students and air-conditioning designers, meteorologists, processengineers,andotheruserswithafour-hourcompletepsy- chrometricslearningmodule. • Puttoresttheimpressionthatpsychrometriccalculationsbasedon ideal gas formulations (at normal air-conditioning temperatures andpressures)areimprecise,inaccurate,orbasedlargelyonempir- icalformulae.Thetextincludescomparativedataforidealgascal- culationsandtheultimate-in-accuracyrealmoistairformulations by Herrmann-Kretzschmar-Gatley (ASHRAE RP-1485). In con- trast to statements in some texts, ideal-gas-based psychrometric calculationsarefarmoreaccuratethanheatgain,ductpressureloss, and other calculations used by air-conditioning engineers and meteorologists. xii Preface • Providepractitionersandthosenewtothefieldwithmultipledef- initionsofbasictermstohelpintheirunderstandingandcommu- nication. • Providethosefluentininch-poundormetricpsychrometriccalcu- lations with an easy transition to Systéme International psychro- metrics. • Provide allofthe algorithms required for psychrometric calcula- tions; e.g., few texts provide the correlation between barometric pressure and altitude, nor do they provide formulas for wet-bulb and dew-point temperatures below freezing. Algorithms can be inputintohand-heldprogrammablecalculatorsandpersonalcom- puterstoaidengineersandmeteorologistsintheirwork. • Motivatemanufacturersofcoolingcoilsanddesiccantdehumidi- fierstoutilizeprovenalgorithmsintheirselectionandpsychromet- ricsoftwaretoeliminateinconsistency. Endnote Eachreaderwilldeterminethedegreetowhichthisbookmeetsthe author’s objectives. Once the basic principles are understood, the psy- chrometric practitioner will undoubtedly find progressively easier and moreefficientwaystoapplytheminthesolutionofmeteorological,air- conditioning,drying,dehumidification,humidification,andotherprob- lems.Fortheair-conditioninganddryingindustries,thismayleadtothe developmentofcompoundorhybridcyclesthataremoreefficientandless costly. You,thereader,willbetheultimatetestofmyfour-hourlearningcon- (cid:27) viction.Hopefully,thatconvictionwillprovetrueforthemajority.Forall, itishopedthatthisbookwillbetheresourcethat(1)takesthemysteryout ofpsychrometriccalculations,(2)makesaconvincingcasefortheaccu- racyofcalculationsbasedontheidealgasequation,(3)givesafarbetter grasp of dew-point, moisture, and psychrometric processes, and (4) allowspractitionerstoservetheirclientsandthepublicbetter. Pleasee-mailsuggestedcorrections,comments,andcriticismtothe [email protected]. DonaldP.Gatley,P.E. January2002(1stedition) xiii Preface to the Third Edition Inthelate1990s,whenthefirsteditionofthisbookwaswritten,psy- chrometrics was a mature science, and I thought that little if anything would change over the following century. Since then, the universal gas constant has been revised by CODATA, the molar mass of dry air has increasedby0.0001kg/kmoleveryfourtofiveyearsduetotheincrease ofCO inEarth’satmosphere,IAPWShasissuednewmodelsforthecal- 2 culation of H O properties, and in 2009 ASHRAE replaced the 1983 2 Hyland-Wexlerreal-moist-airnumericalmodelwiththeresearchproject RP-1485 LibHuAirProp model.These changes have little effect on air- conditioningandmeteorologicalpsychrometriccalculations,butstudents andpractitionersshouldhaveareferencethatagreeswiththelatestinter- nationalstandards. Thethirdeditionincludestheabovechanges,astraightforwardand moreelegantequationfortheadiabaticsaturationprocessinthewet-bulb temperature chapter, an appendix summarizing the 2009 RP-1485 ASHRAEresearch,aswellasminoreditstothetext. xv Understanding Psychrometrics, Third Edition Supporting Files Thispublicationisaccompaniedbyalimiteddemonstrationversionof theASHRAELibHuAirPropadd-in.Alsoincludedarethehw.exeprogram from the second edition and PDF files of 13 ultra-high-pressure and 12 existingASHRAEpsychrometriccharts,plus3new0°Cto400°C,0–1.0 humidityratiochartsfor5.53,101.325,and2000kPa. TheLibHuAirPropadd-inallowsforduplicationofportionsofthereal moist-airpsychrometrictablesinASHRAEHandbook—Fundamentalsfor both standard sea-level atmosphere pressure and pressures from 5 to 10,000kPa.Thehw.exeprogramisincludedtoenableuserstocomparethe 2009ASHRAEnumericalmodelrealmoist-airpsychrometricproperties withthe1983ASHRAE-Hyland-Wexlerproperties. Thesefilescanbedownloadedatwww.ashrae.org/UP3.Ifthefiles orinformationatthelinkarenotaccessible,pleasecontactthepublisher. xvi Contents Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ix Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xi Preface to the Third Edition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv Introduction 1—How to Use this Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2—Etymology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3—Moist Air—The Psychrometric Substance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 4—Water Vapour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19 5—Basics of the Psychrometric Chart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 6—Underlying Theory and Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 7—Ideal Gas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 8—Psychrometric Pioneers and Charts from the First 100 Years . . . . . . . . . . . . . . . . . 49 Part I: Psychrometric Pioneers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Part II: A Selection of Psychrometric Charts from the First 100 Years . . . . . . . . . 79 Psychrometric Properties 9—Psychrometric Properties and Conventions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .105 10—Dry-Bulb Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .109 11—Wet-Bulb Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113 12—Dew-Point Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .121 13—Relative Humidity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127 vii Understanding Psychrometrics, Third Edition 14—Barometric Pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .133 15—Specific Enthalpy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .137 16—Specific Volume. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .143 17—Humidity Ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .147 18—Water Vapour Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .153 Basic Psychrometric Processes 19—Psychrometric Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .159 20—Process Calculations and Definitions of Sensible and Latent Enthalpy Change . . . .183 21—Why Do Air-Conditioning Engineers Need Psychrometrics? . . . . . . . . . . . . . . . . . .197 Processes in Meteorology 22—Adiabatic Expansion and Adiabatic Compression. . . . . . . . . . . . . . . . . . . . . . . . 207 Additional Information 23—Fan Temperature Rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .213 24—Frequently Asked Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .221 25—Altitude Effects on Psychrometrics and HVAC . . . . . . . . . . . . . . . . . . . . . . . . . . 235 26—Psychrometric Program Listing and Comparison Table. . . . . . . . . . . . . . . . . . . . 245 Appendices I—ICAO Equation Relating Barometric Pressure and Altitude . . . . . . . . . . . . . . . . . . 255 II—Water—Facts and Trivialities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .261 III—Other Psychrometric Substances. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 IV—Charts, Tables, and Forms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .271 V—Real Gas Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 Bibliography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365 Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .371 Conversion Equalities: I-P to SI Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .381 viii Introduction