Understanding psychrometrics

310 Pages·2013·8.55 MB·English

Understanding
Psychrometrics
Third Edition
Donald P. Gatley
ISBN978-1-936504-31-2
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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
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QC915.G372012
551.57'10287--dc23
2012042195
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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