Load Calculations in HVAC.pdf

Effect of Shading Devices, Infiltartion-stack effect, wind pressures, Internal Heat Gains, System Heat gain, cooling and heating load estimates, Energy conservations in airconditioning buildings

1. Unit- 4: Load Calculations
in HVAC
Prepared by
Ankur Sachdeva
Assistant Professor, ME

2. Distribution of Solar Radiation
•Solarradiationformsthegreatestsinglefactorofcoolingloadinbuildings.
•Itis,therefore,necessarytostudythesubjectnotonlyforthepurposeofloadcalculationbutalso
fromthepointofviewofloadreduction.
•Thesunisasphereofintenselyhotgaseousmatter.Itisafusionreactor—themostimportantofits
reactionsisthecombinationofhydrogentoformhelium,thedifferenceinmassbeingconvertedto
energy.
•Thisfusionenergyisproducedintheinteriorofthesolarsphereatatemperatureofmanymillions
ofdegrees.Theenergyistransferredtothesurfaceofthesunbyradiationandconvection.
•Forallpracticalpurposes,itmaybeconsideredtoberadiatingenergyasablackbodyatan
effectivetemperatureof6000K.
•Thespectrumofthewavelengthofradiationstretchesfrom0.29to4.75μm.Asaconsequenceof
hightemperature,themaximumradiationintensityisfoundtobeatawavelengthof0.5μm.

3. Distribution of Solar Radiation
•Themassofthesunisabout332,000timesthatoftheearthanditsdiameterisabout1,392,400
km.Theearthisabout12,710kmindiameter.Itmakesonerotationaboutitsaxisin24hours,and
arevolutionaroundthesuninaperiodofapproximately365.25days
•Theearthrevolvesroundthesuninanellipticalorbit.TheearthisclosesttothesunonJanuary1,
andremotestfromitonJuly1(about3.3percentfartheraway).
•Themeandistanceoftheearthfromthesunis149,500,000km.Theintensityofsolarradiation
outsidetheearth’satmospherevariesinverselywiththesquareofthedistancebetweenthecenter
oftheearthandthecenterofthesun.
•Accordingly,theearthreceives7percentmoreradiationinJanuarythaninJuly.
•Theearth’saxisofrotationis,however,tilted23.5°withrespecttoitsorbitaroundthesun.This
angleoftiltisessentiallyresponsibleforthedistributionofsolarradiationovertheearth’ssurface
and,consequently,thechangeofseasons.

4. Earth’s position with respect to the sun
during summer and winter

5. Solar Radiation intensity
•Thesolarradiationintensity—normaltothesun’s
raysincidentuponaplanesurfacesituatedinthe
outerlimitsoftheearth’satmosphere—varieswith
thetimeoftheyearasthedistanceoftheearth
fromthesunchanges.
•Itsvaluewhentheearthisatitsmeandistance
fromthesuniscalledthesolarconstant.The
presentlyacceptedstandardvalueofthesolar
constantasdeterminedbyThekaekaraand
Drummondin1971is1353W/m
2
•Thevariationofthenormalsolarradiation
intensityoutsidetheearth’satmosphere,denoted
asIn0withthetimeoftheyearcanbeobtainedby
applyingacorrectionfactortothesolarconstant
asgivenintheSmithSomanPhysicalTables.
Solar Constant Correction Factors

6. Direct and Diffuse radiation
DirectRadiation
•Fromoutsidetheearth’satmosphere,thesolarheat
reachesanypartoftheearth’ssurfaceintwoways.
•Apartofthesun’sradiationtravelsthroughthe
atmosphereandreachestheearth’ssurfacedirectly.
Thispartiscalleddirectorbeamradiation.
•Itisspecularinnatureandisincidentonasurface
atananglewhichisdeterminedbythelinejoining
thecentreofthesuntothecentreoftheearth.
•Thusiftheorientationofthesurfaceischanged,
thisradiationcanbeincreasedordecreased.Itis
maximumwhenthesurfaceisnormaltothesun’s
rays,andzerowhenitisparalleltothem.

7. Direct and Diffuse radiation
Diffuseradiation
•Amajorpartofthesun’sradiationisscattered,reflectedbackinto
spaceandabsorbedbytheearth’satmosphere.Apartofthis
radiationisre-radiatedandreachestheearth’ssurfaceuniformly
fromalldirections.Itiscalleddiffuseorskyradiation.Itisdiffuse
innatureanddoesnotnormallychangewiththeorientationofthe
surface.
•Thetotalsolarradiationreachingasurfaceisequaltothesumofthe
directanddiffuseradiations.
•Thedifferencebetweenthesolarradiationoutsidetheearth’s
atmosphereandthetotalradiationreachingtheearth’ssurfaceis
governedbythedistancetravelledbytheradiationthroughthe
atmospheretoreachthesurface,andtheamountofhazeinthe
atmosphere.
•Theskyradiationisusuallyaverysmallpartofthetotalradiationin
aclearsky.Butwithahazyorcloudysky,theskyradiation
increaseswhiledirectradiationisdepletedconsiderably.

8. Depletion of Direct Solar Radiation by
Earth’s Atmosphere
•Afterenteringtheearth’satmosphere,thesolarradiationis
scatteredinalldirectionsbyairandwatervapour
moleculesanddustparticles.
•Apartofthisradiationisalsoabsorbed,particularlyby
ozone,intheupperatmosphereandwatervapourand
carbondioxideintheatmosphereneartheearth.Tosome
extent,itisalsoabsorbedbyoxygenaswell.
•Thus,thedepletionofthedirectsolarradiationisquite
largeevenoncleardayswhenmostoftheair-conditioning
loadrequirementoccurs.
•Unitairmasscorrespondstotheconditionofaclearsky
andthesunatthezenith,i.e.,whenthesunisdirectly
overheadoratanaltitudeangleof90°.
•Thealtitudeangleisdefinedastheangleinaverticalplane
betweenthesun’sraysandtheprojectionofthesun’srays
onahorizontalplane.
•Theairmasscanbedefinedasthepathlengthofsolar
radiationthroughtheatmosphere,assumingthevertical
pathatthemeansealevelasunity.

9. Heat Gain Through Glass
•Glassconstructionformsasignificantpartofmodernbuildingstructures.
•Solarradiation—directanddiffuse—incidentuponaglasssurfaceis,inparts,transmitted,
reflected,andabsorbed.
•Ifτ,γandα,representtherespectivefractionsknownastransmissivity,reflectivityand
absorptivity,then
τ+γ+α=1
•Theheatgainofaspacethroughglassthencomprises
(i)allthetransmittedradiation,
(ii)apartoftheabsorbedradiationthattravelstotheroom,and
(iii)theheattransmittedduetothedifferencebetweentheoutsideandinsidetemperature.
•Thedirectradiationentersthespaceonlyiftheglassisreceivingthedirectraysofthesun.The
diffuseradiationentersthespaceevenwhentheglassisnotfacingthesun.

10. Heat Gain Through Glass
•Theabsorbedradiationraisesthetemperatureofglass,and
theglassthentransmitsthisheatpartlytotheoutsideand
partlytotheinside.
•Ift
grepresentsthetemperatureoftheglass,thentheheat
gainofthespaceisgivenby
•wheref
iistheinsidefilm-coefficientofheattransfer,
subscriptsDandddenotethetermsfordirectanddiffuse
radiationsrespectively,A
sunistheglassareadirectly
exposedtothesunandAisthetotalglassarea.

11. Effect of Shading Device
(Venetian Blinds)
•Theeffectoftheshadingdeviceistofurthercurtailthe
heatgainoftheconditionedspace.
•Theeffectivenessismoreiftheshadingdeviceisoutside
thespacethanwhenitisinside.
•Thisisbecausetheinsideshadingdevicesdissipateallof
theirabsorbedheatintotheconditionedspace.
•Theymustalsoreflectthesolarheatbacktotheglass
whichabsorbssomeofit.
•Ontheotherhand,theoutsideshadingdevicesdissipateall
theirabsorbedaswellasthereflectedradiationintothe
surroundings.

12. Effect of Shading Device (Venetian Blinds)
•ConsideraradiationIincidentonanordinaryglasssurfaceat
anangleofincidenceofq,sayequalto30°forwhich
transmissivityis0.86.
•Apartofthisradiationwillbereflected,apartwillbe
transmittedandasmallpart,oftheorderof5to6percent,will
beabsorbed.
•Theheatabsorbedbyglassincreasesitstemperatureabovethat
oftheoutsideandinsideair.
•Apartofthisheat,therefore,travelstotheoutsideandapartto
theinside.
•Thetotalheatgainofthespacefromthedirectsolarradiationis
thenequaltothesumofthetransmittedradiationandabout40
percentoftheabsorbedradiation.
•Thesolarheat-gainfactorrepresentstheratiooftheradiation
heatgainwiththeshadingdevicetothatofplainglass.

13. Radiation properties of glass
and shading device

14. Tables for Heat Gain Through
Ordinary Glass
•Thefactorsaffectingthesolarheatgainthroughordinaryglassare:
(i)Locationofthepointontheearth’ssurfacegivenbythelatitudeoftheplace
(ii)Timeofday
(iii)Timeofyear
(iv)Facingdirectionofwindow.
•Theheatgainincludesthedirectanddiffusesolarradiationplustheportionoftheheatabsorbedby
glassthatenterstheroom.
•Itistobenotedthatthetransferofheatacrosstheglass—becauseofthetemperaturedifferencebetween
theoutsideandtheroomair—isnotincluded.
•Asthesolarandtransmissionheatgainthroughglassformsamajorcomponentofthecoolingloadin
buildings,itisdesirablefromthepointofviewofenergyconservationtominimizetheglassareas.
•Itisrecommendedthattheglassareasshouldnotexceed25percentofthefloororcarpetareasin
buildings.

15. Tables for Heat Gain Through
Ordinary Glass

16. Fabric Heat Gain
•Afterthesolarandtransmissionheatgainthroughglass,themostimportantheatgainorlosstobeconsideredinthe
airconditioningofbuildingsistheheattransferthroughwalls,roof,ceiling,floor,etc.,i.e.,thebuildingstructure.
•Theloadduetosuchheattransferisoftenreferredtoasthefabricheatgainorloss.Inthisconnection,itistobe
consideredwhetheraparticularwallorroofisexposedtothesunornot.
•Inthecaseofasunlitwallorroof,theheatgainoftheroomwillbemoreincomparisontoashadedone,asthe
outsidesurfacetemperatureofthewallorroofwillincreaseabovetheoutsideairtemperatureduetotheincident
solarradiation.
•Theconductionheattransferthroughthewallorroofwilldependonthethicknessandthethermalconductivityof
thematerialused.Inaddition,therewillbeconvectionandradiationfromboththeoutsideandinsidesurfaces.
•Hence,thesteady-stateheattransferisexpressedintermsofanoverallheat-transfercoefficientUandtheoverall
temperaturedifferencebetweentheoutsideandinsidetemperaturedifference.
•Awallmaybecomposite,consistingofmanysectionsofdifferentconstructionandinsulatingmaterials.Also,the
outsideandinsidewallsurfacesmayexchangeheatbyconvectionandradiationwiththesurroundingatmosphere.
Thus,therewillbemorethanonethermalresistancetoheattransfer.

17. Thermophysical
properties of
selected building
and insulating
material

18. PERIODIC HEAT TRANSFER THROUGH
WALLS AND ROOFS
•Heattransmissionthroughthewallsandroofsofbuildingstructuresisnotsteady
andistherefore,difficulttoevaluate.Thetwoprincipalfactorscausingthisare:
•(i)Thevariationoftheoutsideairtemperatureoveraperiodof24hours.
•(ii)Thevariationofthesolarradiationintensitythatisincidentuponthesurface
overaperiodof24hours.
•Thephenomenonisfurthercomplicatedbythefactthatawallhasathermal
capacityduetowhichacertainamountofheatpassingthroughitisstoredandis
transmittedtotheoutsideand/orinsideatsomelatertime.
•Figureshowsatypicalvariationoftheoutsideairtemperatureandradiationona
hotsummerday.Themaximumtemperaturesusuallyoccurjust2–3hoursafter
solarnoonwhiletheminimumtemperaturesoccurjustbeforesunrise.
•Theoutsideairtemperaturet
0followsnearlyaharmonicvariation.
•Themeanlineisshownattemperaturet
0m

19. Sol-Air Temperature
•Forcalculationsofheattransferthroughstructures,ithasbeenfound
convenienttocombinetheeffectoftheoutsideairtemperatureand
incidentsolarradiationintensityintoasinglequantity.
•Forthispurpose,anexpressionfortherateofheattransferfromthe
environmenttotheoutsidesurfaceofthewallmaybewrittenas
•Introducinganequivalenttemperaturetewemaywritefortheheat-
transferrate
•Thistemperaturet
eiscalledthesol-airtemperatureandcanbeconsidered
asanequivalentoutsideairtemperaturesuchthatthetotalheattransferred
isthesameasduetothecombinedeffectoftheincidentsolarradiation
andoutsideairandthewalltemperaturedifference.

20. Empirical Methods To Evaluate Heat
Transfer Through Walls And Roofs
•Therearetwoapproachestoempiricalcalculationsofheattransferthroughwalls
androofs.Theyare:
•(i)Thedecrementfactorandtimelagmethod.
•(ii)Theequivalenttemperaturedifferentialmethod.
•Bothmethodsuseanalytical-experimentalresultsfortheirformulations.
•Theequivalenttemperaturedifferentialmethodismorecommonlyusedbyair-
conditioningengineersasitisalsoapplicabletosunlitwallsandroofs.

21. Decrement Factor and Time-lag Method
•If the thermal capacity of the wall is ignored, then the instantaneous rate of heat transfer through
the wall at any time t is given by
•and on an average basis, the mean heat flow is given by
•But most building materials have a finite thermal capacity which is expressed as
mC= ρCV= ρC(A Δx)
•where m = Mass of wall
•ρ, C = Density and specific heat of wall material
•A = Cross-sectional area of wall
•Δx= Wall thickness.

22. Decrement Factor and Time-lag Method
•Ithasbeenseenthatthereisatwo-foldeffectofthermalcapacityonheattransfer:
•(i)Thereisatimelagbetweentheheattransferattheoutsidesurfaceq
0andtheheattransferatthe
insidesurfaceq
i
•(ii)Thereisadecrementintheheattransferduetotheabsorptionofheatbythewalland
subsequenttransferofapartofthisheatbacktotheoutsideairwhenitstemperatureislower.
•Thermalcapacityofmostmaterials,therefore,essentiallydependsontheirdensityandthickness.
TheIHVEGuidegivesthevalueofthetimelaganddecrementfactorasafunctionofthewall
thicknessanddensityofconstructionmaterials.
•Consideringtheeffectofthermalcapacity,theactualheattransferatanytimeτis
•wherete
τ-ɸisthesol-airtemperatureattimeτ-ɸ,i.e.,ɸ hoursbeforetheheattransferistobe
calculated.

23. Decrement Factor and Time-lag Method

24. Decrement Factor and Time-lag Method
•Ifthewallisthick,thedecrementfactorwillbesmallasisalsoseenfromTables1
and2.
•Forexample,fromTable2,thedecrementfactorfora15cmconcreteroofis0.48
whereasfora5cmconcreteroof,itis0.83.

25. Decrement Factor and Time-lag Method
•Accordingly,forthickwalls,theheatgaindoesnot
varymuch,whereas,forthinwalls,itvaries
considerablyover24hours.
•Alightwallwithalowthermalcapacityhavingatime
lagofabout3hourshasamaximumheatgainat3
p.m.andgreatvariationinheattransferover24hours.
•Aheavierwallwithhighthermalcapacityhasa
reducedandmoreuniformheatgain,andthepeak
occursmuchlater,sayat12midnight,witha
correspondingtimelagof12hours.
•Astillheavierconstructionmayresultinaverysmall
anduniformheat-transferrate.
•Inalocality,wherethedailyrangeoftemperatureis
large,itisdesirabletohavethickwallstocutthe
coolingloadinsummerandtheheatingloadinwinter.

26. Natural Ventilation through Infiltration
•Infiltrationisthenamegiventotheleakageofoutsideairthroughdooropenings,
andthroughcracksandintersticesaroundwindowsanddoorsintoconditioned
space.
•Eventhoughtheairinsideisslightlypressurized,theleakagedoestakeplace
whichisprincipallyduetothefollowingfactors:
(i)Stackeffect,particularlyintallbuildings
(ii)Windpressure
(iii)Entryandexitofoccupantseffectingchangeofairduetodooropenings.
•Correspondingtoeveryinfiltrationthereisanequivalentamountofexfiltration.
•Ineffect,infiltrationinvolvesanexchangebetweentheoutsideandinsideair.
•Infiltration,asaresultofstackeffect,windeffect,andthroughdoorsandwindows
andotheropeningscanbetreatedascontributingtonaturalventilation.

27. Stack Effect
•Differencesbetweentemperaturesandhumiditiesproduce
differencesinthedensitiesofairbetweentheoutsideandinside
ofbuildings.Asaresult,pressuredifferencesoccurcausingflow
ofairknownasthechimneyorstackeffect.
•Whentheinsidetemperatureislowerthantheoutside,thestack
effectproducespositiveinsidepressureatlowerlevelsand
negativeinsidepressureathighlevels.
•Consequently,theoutwardflowofairtakesplaceatlowerlevels
andtheinwardflowathigherlevels,withtheneutralzoneinthe
middle.
•Thereverseistruewhentheinsideisatahighertemperaturethan
theoutside.Thus,wehave
•InSummerInfiltrationatthetopandexfiltrationatthebottom.
•InWinterInfiltrationatthebottomandexfiltrationatthetop.
•Theinfiltrationfromthestackeffectisgenerallysmallbutis
greatlyinfluencedbytheheightofthebuildingandthepresence
ofstaircasesandelevators.

28. Wind Action
•Theflowofairduetowindoverabuildingcreatesregionsin
whichthestaticpressureishigherorlowerthanthestatic
pressureintheundisturbedairstream.
•Thepressureispositiveonthewindwardsideresultinginthe
infiltrationofair,andnegativeontheleewardsideresultingin
exfiltration.
•Inatallbuilding,thewindvelocityisveryhightowardsthetop
ofthebuildingandhencetheleakagerateisalsohigher.
•Therearetwomethodsofestimatingtheinfiltrationofairinto
conditionedspaceduetowindaction.Theyare:
(i)Crackmethod,and
(ii)Air-changemethod.

29. Infiltration Due to Door Openings
•Infiltrationthroughdoorsdependsonthetypeofdoor,aswellasitsusage.
•Oftenthetablesdevelopedfordoorsgivetheinfiltrationratesthroughdoorswhich
includetheleakageratesthroughcracksduetodooropenings.
•Tablesbelowgivetheinfiltrationratesthroughdoorsonthewindwardsidefor
variousdoorconstructionsandusage,andforawindvelocityof12kmph.

30. Load due to Infiltration
•Infiltration involves the heat gain or loss to the conditioned space due to the replacement of the
conditioned inside air by the undesirable outside air.
•This load includes both sensible and latent and is evaluated in the same manner as the ventilation
load from the infiltration rate (cmm).
•If ventilation air is greater than infiltration/ exfiltration air then infiltration may not be considered
separately.

31. Design Considerations in
Load Calculations
•Itisontheprecisionandcareexercisedbythedesignerinthecalculationsofthecoolingloadforsummerandtheheating
loadforwinterthatatrouble-freesuccessfuloperationofanair-conditioningplant,afterinstallation,woulddepend.
•Animportantconsiderationinthisexerciseisthedateandtimeforwhichthesecalculationsaremade.Thedatewould
dependonthelocalclimaticconditions.
•AlthoughthelongestdayinsummerisJune21,thehottestandmosthumiddaymayoccurinJuly.Similarly,thecoldest
daymayoccurinJanuaryoreveninFebruaryinsteadofDecember21.
•Again,thoughthemaximumtemperaturemayoccuroutsideat1or2p.m.,themaximumheatgainoftheroommayoccur
at3or4p.m.duetothedirectsolarradiationthroughglassonthewestside,orevenlaterduetothetimelagfortheheat
transferthroughthestructure.
•Further,theapplicationforwhichthebuildingisintendedtobeusedwouldalsogovernthechoiceoftime.
•Forexample,foranofficebuildinginwinterthatisnotusedatnight,thetimeforloadcalculationsmaybetakenduringthe
earlyhoursofthemorning,althoughthemaximumheatingloadmayoccuratnight.
•Similarly,anofficebuildinginsummermayhavethemaximumcoolingloadat7p.m.duetothetimelag,butsinceno
occupantswouldbepresentatthattime,thetimeforloadcalculationsmaybetakenas4or5p.m.

32. Components of Load
•Themajorcomponentsofloadinbuildingsareduetothedirectsolarradiation
throughthewestglass,transmissionthroughthebuildingfabricorstructure,and
freshairforventilation.
•Inthecaseofapplicationssuchastheatresandauditoriums,theoccupancyloadis
predominant.
•Thus,componentsthatmaycausecoolingloadsincludethefollowing:
•External:Walls,roof,windows,partitions,ceiling,andfloor
•Internal:Lights,people(occupancy),appliances,andequipment
•Infiltration:Airleakageandmoisturemigration
•System:Outsideair(ventilationair),ductgain,reheat,fan,andpumpenergy.

33. Internal Heat Gains
ComponentsofInternalHeatGainsare:
1.SensibleandLatentheatgainsduetooccupants.
2.SensibleandLatentheatgainsduetolights.
3.SensibleandLatentheatgainsduetoappliances.
4.SensibleandLatentheatgainsduetomachines.
5.SensibleandLatentheatgainsduetopipingetc.
6.SensibleandLatentheatgainsduetoproducts.

34. Occupancy Load
•Theoccupantsinaconditionedspacegiveout
heatatametabolicratethatmoreorless
dependsontheirrateofworking.
•Therelativeproportionofthesensibleand
latentheatgivenout,however,dependsonthe
ambientdrybulbtemperature.
•Thelowerthedrybulbtemperature,the
greatertheheatgivenoutassensibleheat.
•Thevaluesforrestaurantsincludetheheat
givenoutbyfoodaswell.
•Theusualproblemincalculatingthe
occupancyloadliesintheestimationofthe
exactnumberofpeoplepresent.
Heat Liberated Due to Occupancy

35. Lightning Load
•Electriclightsgeneratesensibleheatequaltotheamountoftheelectricpowerconsumed.
•Mostoftheenergyisliberatedasheat,andtherestaslightwhichalsoeventuallybecomesheatafter
multiplereflections.
•Lightingmanufacturersgivesomeguidanceastotherequirementofpowerfordifferentfittingsto
producevaryingstandardsofillumination.
•Inconnectionwithfluorescenttubes,itmaybestatedthattheelectricpowerabsorbedatthefittingis
about25percentmorethannecessarytoproducetherequiredlighting.
•Thus,a60Wtubewillneed75Watthefitting.Theexcessof15Wisliberatedatthecontrolgearofthe
fitting.
•Asaroughcalculation,onemayusethelightingloadequalto33.5W/m
2
toproducealightingstandard
of540lumens/m
2
inanofficespace;20W/m
2
istheminimum.
•Afterthewattageisknown,thecalculationoftheheatgainisdoneasfollows:
•Fluorescent:Q=Totalwattsx1.25
•Incandescent:Q=Totalwatts

36. Appliances Load
•Mostappliancescontributebothsensibleandlatentheat.
•Thelatentheatproduceddependsonthefunctiontheappliancesperform,such
asdrying,cooking,etc.
•Gasappliancesproduceadditionalmoistureasaproductofcombustion.
•Suchloadscanbeconsiderablyreducedbyprovidingproperlydesignedhoods
withapositiveexhaustsystemorsuctionovertheappliances.
•Theappliancesintheconditionedspaceareacommonfeatureincafeterias.
•Electricmotorscontributesensibleheattotheconditionedspace.
•Apartofthepowerinputisdirectlyconvertedintoheatduetotheinefficiency
ofthemotorandisdissipatedthroughtheframeofthemotor.Thispoweris
(Input)(1–Motorefficiency)
•Therestofthepowerinputisutilizedbythedrivenmechanismfordoingwork
whichmayormaynotresultinheatgaintothespace.
•Thisdependsonwhethertheenergyinputgoestotheconditionedspaceor
outsideit.

37. Piping, Tanks, Evaporation of Water from
a Free Surface and Steam
•Heatisaddedtotheconditionedspacefromrunningpipescarryinghotfluidsduetoheattransfer.
Ontheotherhand,coldpipestakeawayheatfromthespace.
•Opentankscontainingwarmwatercontributebothsensibleheatandlatentheattothespacedueto
evaporation.Thiscanbecalculatedbyknowingtherateofevaporationandenergybalance.
•Inindustrialairconditioning,productshaveoftentobedried.Thisinvolvesboththelatentheat
gainandthesensibleheatgaintothespacefromthehotsurfacesofthedryerdependinguponthe
dryingrate.
•Whensteamisenteringtheconditionedspace,thesensibleheatgainisverylittle.Itisequalto
onlythedifferenceintheenthalpyofsteamatthesteamtemperatureandtheenthalpyofwater
vapourattheroomdry-bulbtemperature.
•Themainloadisintheformofthelatentheatgain.

38. Product Load
•Inthecaseofcoldstorage,theenclosuresare
insulatedwithatleast10-15cmofthermocoleand
arealmostcompletelysealed.
•Thus,manyoftheloadspresentinbuildingsfor
comfortairconditioningareeitherabsentorlessened
inthecaseofcoldstorage.
•However,inadditiontotheheatwhichisremoved
fromproductsatthetimeofinitialloading,thereis
alsotheheatproducedbythecommoditiesduring
storage.
•Thisheatofrespirationformsasizableproductload
evenatastoragetemperatureof0°C.Athigher
temperatures,itismore.
•Theapproximaterateofevolutionofheatbyvarious
productsatdifferenttemperaturesisgiveninTable.
Heat of respiration of products in J/kg per 24 hours

39. Process Load
•Theprocedureofcalculatingthecoolingandheatingloadforvariousindustrialair-conditioning
processesisspecificforeachprocess.
•Therequirementsfortheprocessmayinvolvethecontrolofoneormoreofthefollowingfactors:
(i)Regainofmoisturecontentbyhygroscopicmaterials,suchascottonsilk,tobacco,etc.,andthe
accompanyingheatliberated.
(ii)Dryingload.
(iii)Rateofchemicalandbiochemicalreactions.
(iv)Rateofcrystallization,freezing,freeze-drying,etc.
(v)Sensiblecoolingload.

40. System Heat Gains
•Thesystemheatgainistheheatgain(orloss)ofanair-conditioningsystem
comprisingitscomponents,viz.,ducts,piping,air-conditioningfan,pumps,etc.
•Thisheatgainistobeinitiallyestimatedandincludedinthetotalheatloadforthe
air-conditioningplant.
•Thesameshouldbecheckedafterthewholeplanthasbeendesigned.

41. Supply Air Duct Heat Gain and Leakage Loss
•Thesupplyair,normally,hasatemperatureof10to15°C.
•Theductmaypassthroughanunconditionedspacehavinganambienttemperatureof40°C.
•Thisresultsinasignificantheatgaintilltheairreachestheconditionedspaceeventhoughtheductmaybe
insulated.
•Theheatgaincanbecalculatedusingthefollowingexpression,Q=UA(t
a–t
s)
whereUistheoverallheat-transfercoefficient,Aisthesurfaceareaoftheductsystemexposedtothe
ambienttemperaturet
aandt
sisthesupplyairtemperature.
•Asaroughestimate,avalueoftheorderof5percentoftheroom’ssensibleheatmaybeaddedtothetotalsensible
heatifthewholesupplyductisoutsidetheconditionedspace,andproportionatelylessifsomeofitiswithinthe
conditionedspace.
•Ithasbeenfoundthatductleakagesareoftheorderof5to30percentdependingontheworkmanship.Airleakages
fromsupplyductsresultinaseriouslossofthecoolingcapacityunlesstheleakagestakeplacewithinthe
conditionedspace.
•Ifallductsareoutsidetheconditionedspacewhich,normally,isstrictlyavoided,a10percentleakageistobe
assumedwhichshouldbeconsideredasacompleteloss.Whenonlyapartofthesupplyductisoutsidethe
conditionedspace,thenonlytheleakagelossofthisportionistobeincluded.Thefractionof10percent,tobeadded
insuchacase,isequaltotheratioofthelengthoutsidetheconditionedspacetothetotallengthofthesupplyduct

42. Heat Gain from Air-Conditioning Fan
•Theheatequivalentofanair-conditioningfanhorsepowerisaddedasthesensibleheattothesystem.Ifthe
fanmotorisoutsidetheairstream,theenergylostduetotheinefficiencyofthemotorisnotaddedtotheair.
Therearetwotypesofairsupplysystems.
Draw-throughSystem
•Inthedraw-throughsystem,thefanisdrawingairthroughthecoolingcoilandsupplyingittotheconditioned
space.Thisisthemostcommonsystem.Inthissystem,thefanheatisinadditiontothesupplyairheatgain.
Theheatshould,therefore,beaddedtotheroomsensibleheat.
Blow-throughSystem
•Intheblow-throughsystem,fanblowsairthroughthecoolingcoilbeforebeingsuppliedtotheconditioned
space.Inthissystem,thefanheatisaddedaftertheroomtothereturnair.Thusthefanheatisaloadonthe
coolingcoil.Theheatshould,therefore,beaddedtothegrandtotalheat.
Thefanefficienciesareoftheorderof70percentforcentralair-conditioningplantfansandabout50percentfor
packageair-conditionerfans.
Thefanhorsepowerdependsonthequantityofairsuppliedandthepressurerise,viz.,thetotalpressure
developedbythefan.Thesupplyairquantityinturndependsonthedehumidifiedrise,whichisoftheorderof8
to14°C.Thefantotalpressuredependsonthesystempressurelosswhichcomprisesthepressuredropthrough
theductwork,grilles,filters,coolingcoil,etc.

43. Blow-Through Vs Draw-Through
Fan System

44. Heat Gain from Air-Conditioning Fan
•Oncethesupplyair-rateandpressuredevelopedareknown,thefanpowercanbecalculated.
•Butthesecannotbeknownuntiltheloadcalculationshavebeencompleted.
•Hencetheprocedureistoinitiallyassumefanheatbetween2.5and7.5percentoftheroom
sensibleheatandcheckthevalueafterthedesignhasbeencompleted.
•Designersusuallytake5%ofRSHasfanheat.
Fan pressure for different duct systems

45. Return Air Duct Heat and Leakage Gain
•Thecalculationoftheheatgainforreturnairductsisdoneinexactlythesame
wayasforsupplyairducts.
•Buttheleakageinthiscaseisthatofthehotandhumidoutsideairintotheduct
becauseofsuctionwithintheduct.
•Iftheductsareoutsidetheconditionedspace,aninleakageofupto3percentmay
beassumeddependingonthelengthoftheduct.
•Ifthereisonlyashortconnectionbetweentheconditioningequipmentandthe
space,thisleakagemaybeneglected.

46. Heat Gain from Dehumidifier Pump and Piping
•Thehorsepowerrequiredtopumpwaterthroughthedehumidifieraddsheattothesystemandisto
beconsideredlikethatofotherelectricmotors.
•Forthispurpose,pumpefficienciesmaybeassumedas50percentforsmallpumpsand70percent
forlargepumps.
•Theheatgainofdehumidifierpipingmaybecalculatedasapercentageofthegrandtotalheatas
follows:
(i)Verylittleexternalpiping:1%ofGTH.
(ii)Averageexternalpiping:2%ofGTH.
(iii)Extensiveexternalpiping:4%ofGTH.
•Itistobenotedthatallheatgainsaftertheroomarenottobeaddedtoroomheatgains,buttothe
grandtotalheatloadthatdirectlyfallsontheconditioningequipment.
•Theseincludethereturnairductheatandleakagegain,dehumidifierpumppower,dehumidifier,
andpipinglosses,asoutlinedabove,andthefansensibleheatinthecaseoftheblow-through
system.

47. Safety Factor
•Safetyfactorisstrictlyafactorofprobableerrorintheestimationof
theload.
•Forthispurpose,anadditional5percentheatshouldbeaddedtothe
room’ssensibleandlatentheat.

48. Break-up Of Ventilation Load and
Effective Sensible Heat Factor
•Theventilation-airrequirements,dependingonindividual
applicationsaregiveninTable.
•Theminimumrequirementistakenas0.2cmmperperson.
•Thisisbasedonapopulationdensityof5to7.5m
2
perperson
andaceilingheightof2.4m.
•Whenpeoplearesmoking,theminimumventilationrequirement
is0.4to0.7cmmperperson.
•Thebypassfactorofthecoolingequipmentaffectsthepositionof
thegrandsensibleheatfactorline.Asamatteroffact,theeffect
ofthebypassfactorissuchastoadd(X)(m
ao)amountofthe
outsideairdirectlytotheroom,andtoallowonly(1–X)(m
ao)to
passthroughtheapparatus
.
•Althoughtheroomairisalsobypassed,this
doesnotaffectthebreak-upoftheloadasthe
roomairgoingtotheroomdoesnotchange
theloadsituation.

49. Break-up Of Ventilation Load and
Effective Sensible Heat Factor
•Thus,wecansaythatapartoftheventilationloadformsacomponentoftheroomload.
•Thisby-passedoutsideairloadisproportionaltothebypassfactorX.
•Ithasbothsensibleandlatentheatcomponents.
•Theotherpart-whichisproportionalto1–X,bothsensibleandlatent,whichisbypassedaroundtheapparatus—isadded
totheequipmentload.
•Thusthebypassedoutsideairloadsontheroomare:
•SH=(OASH)(BPF)
•LH=(OALH)(BPF)
•Theseloadsareimposedontheroominexactlythesamemannerastheinfiltra-tionload.Accordingly,theeffectiveroom
loadsaremodifiedasfollows:
•Effectiveroomsensibleheat,ERSH=RSH+(OASH)(BPF)
•Effectiveroomlatentheat,ERSH=RLH+(OALH)(BPF)
•Theeffectivesensibleheatfactor(ESHF)istheratiooftheeffectiveroomsensibleheattotheeffectiveroomtotalheat.

50. Cooling Load Estimate
RoomLoad
A.RoomSensibleHeat(RSH)
(i)Solarandtransmissionheatgainthroughwalls,roof,etc.
(ii)Solarandtransmissionheatgainthroughglass.
(iii)Transmissiongainthroughpartitionwalls,ceiling,floor,etc.
(iv)Infiltration.
(v)Internalheatgainfrompeople,power,lights,appliances,etc.
(vi)Additionalheatgainnotaccountedabove,safetyfactor,etc.
(vii)Supplyductheatgain,supplyductleakageloss,andfanhorsepower.
•Thesumofalltheabovegivestheroomsensibleheat(RSH)load.
•Forthepurposeofpsychrometricanalysis,thefollowingcomponentisalsoincludedintheroom
sensibleheat.
(viii)Bypassedoutsideairload.
•Thesumofitems(i)to(viii)givestheeffectiveroomsensibleheat(ERSH).

51. Cooling Load Estimate
RoomLoad
A.RoomLatentHeat(RLH)
(i)Infiltration.
(ii)Internalheatgainfrompeople,steam,appliances,etc.
(iii)Vapourtransmission.
(iv)Additionalheatgainnotaccountedabove,safetyfactor,etc.
(v)Supplyductleakageloss.
•Thesumofthesegivestheroomlatentheat(RLH).
•Theotherlatentheatgainconsideredforpsychrometricanalysisis:
(vi)Bypassoutsideairload.
•Thesumofitems(i)to(vi)abovegivestheeffectiveroomlatentheat(ERLH).
•ThesumofitemsAandBabovegivestheeffectiveroomtotalheat(ERTH).
•Butthesumofitems(i)to(vii)inA,and(i)to(v)inBgivestheroomtotalheat(RTH).

52. Grand Total Load On
Air-conditioning Apparatus
A.SensibleHeat
(i)Effectiveroomsensibleheat(ERSH).
(ii)Sensibleheatoftheoutsideairthatisnotbypassed.
(iii)Returnductheatgain,returnductleakagegain,dehumidifierpumphorse-powerand
dehumidifierandpipinglosses.
•Thesumofitems(i)to(iii)givesthetotalsensibleheat(TSH).
B.LatentHeat
(i)Effectiveroomlatentheat(ERLH).
(ii)Latentheatofoutsideairwhichisnotbypassed.
(iii)Returnductleakagegain.
•Thesumofitems(i)to(iii)abovegivesthetotallatentheat(TLH).
•Finally,thesumofAandBabovegivesthegrandtotalheat(GTH).

53. Heating Load Estimate
•Anestimateoftheheatingloadismadeonthebasisofthemaximumprobableheatlossofthe
roomorspacetobeheated.
•Thustheplantfortheheatingsystemistobesodesignedthatithasacapacityjustsufficientto
meettheheatingloadrequirementwhichdevelopswhenmostsevereweatherconditionsoccur.
•Inthisrespectverybriefperiodsofsevereweatherneednotbetakenintoaccount.
•Accordingly,thefollowingpointsinheating-loadcalculationsarenoteworthy.
(i)TransmissionHeatLoss:Thetransmissionheatlossfromwalls,roof,etc.,iscalculatedonthe
basisofjustthedesignoutsideandinsideairtemperaturedifference.
(ii)SolarRadiation:Thereisgenerallynosolarradiationpresentandhence,thereisnosolarheat
gainatthetimeofthepeakloadwhichnormallyoccursintheearlyhoursofthemorning.
(iii)InternalHeatGains:
•Internalheatgainsfromoccupants,lights,motorsandmachinery,etc.,diminishtheheatingrequirement.These
negativeloadsshouldbeaccountedforinapplications,suchastheatres,assemblyhalls,stores,officebuildings,etc.,
wheretheseloadsareconstantlypresent.
•Butallowancefortheseloadsmustbemadeonlyaftercarefulconsideration.
•Animportantaspecttokeepinmindistheuseofthespaceatnight,weekendsorotherunoccupiedperiods.
•Also,theheatingplantshouldhavesufficientcapacitytobringuptheinsidetemperaturetothedesignvaluebefore
theoccupantscomein.

54. Psychrometric Calculations For Cooling
•Figureshowstheconditionofthemixtureofthe
recirculatedroomairandventilationairenteringthe
apparatusat1,andleavingtheapparatusat2whichis
thesameasthesupplyairstates,theeffectivesurface
oftheapparatusbeingatS.
•Theconditionline1–2representsthepsychrometric
processintheair-conditioningapparatus,andhence
theGSHFline.Further,theleavingairstate2is
governedbytheBPFoftheapparatus,although,atthe
sametime,itmustlieontheRSHFlinei–2
•Accordingly,thedehumidifiedairquantitycanbe
calculatedeitherfromroomsensibleheatbalance,
viz.,processs–iintheroom
•From the total sensible heat balance,
viz., process 1–2 in the apparatus.

55. Evaporative Cooling
•Evaporativecoolingisobtainedduringtheprocessofadiabaticsaturation.
•Itisaprocessoftheremovalofsensibleheatfromairandanequivalent
additionoflatentheattoitintheformofaddedwatervapour.
•Evaporativecoolingisaprocessinwhichheatisneitheraddedtonoritis
removedfromthewateroutsidetheairwasher.Waterissimplyrecirculated
byapump.
•Evaporativecoolingiscommonlyusedwhentheoutdoorconditionsarevery
dry.Thismeansthatthewet-bulbdepressionofairisverylarge.
•Inadryclimate,evaporativecoolingcangivesomereliefbyremovingthe
sensibleheatfromtheroom.Butthehumiditycannotbecontrolled.
•Anotherdefectoftheevaporativecoolingsystemisthelargequantityofair
thatmustbesuppliedtomeettheroom’ssensibleheatloadasthe
temperaturedifferencebetweentheroomandsupplyairisgenerallysmall.
•Thus,whereasinairconditioning,thesupplyairquantitymaybeoftheorder
of8–10airchangesperhour,inthecaseofevaporativecooling,thesame
maybeoftheorderof20airchanges.
•Thisquantityincreasesrapidlyasthehumidifyingefficiencyoftheair
washerdecreases

56. Limitations of Evaporative Cooling
(i)ThelowestpossibleDBTofairoffthecoolerisat100%efficiency.Itisthen
equaltoWBTofambientair.Itisconcededthatevaporativecoolingissatisfactory
onlyinareaswheretheDBTexceeds32°C,andtheWBTisbelow21°C.
(ii)Evaporativecoolingremovesonlysensibleheat.Eventhoughasatisfactory
DBTmaybeobtainedintheroom,therelativehumidityoffthecoolerandinthe
roomisveryhigh.
(iii)Supplyairquantityisverylargeleadingtoconditionsofdraft.
(iv)Thecooleristobekeptingoodrepairtoobtainhighefficiency,otherwisethe
DBTofairoffthecoolerwillnotbesufficientlylowtocoolthespace.

57. Building Requirements And Energy
Conservation In Air-conditioned Buildings
•Thetotalamountofenergyconsumptioninairconditioningisquitesubstantial.
•Itisknownthatonetonofrefrigerationincentralair-conditioningplantsrequires1.25kWof
powerapproximately.
•Thisisonthebasisoftheroofnotexposedtosun,andnottoomuchglassareasinthewalls.
•ThisoneTRissufficientforofficespaceof18–22.5m
2
,or12–14seatsinatheatre.
•Thus,foranofficeofapproximately1850m
2
oratheatreof1250seatingcapacity,theA/Cloadis
100TR,requiringapowerconsumptionofabout125kW.
•Thisshowsthatthepowerconsumptionissizableandthereisaneedtominimizeit.
•Further,thecostofairconditioningconstitutesabout60%ofthecostofbuilding.Hence,thea
needtocutcoolingloadstominimizethesizeoftheplant.

58. Building Requirements And Energy
Conservation In Air-conditioned Buildings
•Energy conservation in the air conditioning of buildings can be achieved by
adopting the following measures:
(i)Minimization of solar gain.
(ii)Other building design features and thermal properties of construction materials.
(iii)Minimizing infiltration and ventilation load.
(iv) Use of natural ventilation for cooling.
(v) Use of thermal storage.
(vi) Plant selection.
(vii) Plant maintenance.
(viii) Permitting drift in room design conditions.

59. Minimization of solar gain
•Solarradiationaccountsfor40–70%ofthecoolingloadinmanybuildings.
Factorsaffectingare;theorientationofthebuilding,fenestration,preventive
measurestointerceptsolarheat,etc.
•Itisconcededthatthemaximumareaofexposedwallsandwindowsshouldbein
theNorth-SouthdirectionasagainstEast-Westdirection.Thismeansthelonger
sideofthebuildingtofaceN-Sdirections.
•Further,thefollowingdesignfeatureswillconsiderablydecreasethecoolingload
duetofenestration:
(a)Reductioninglassareasonthewesternside:
TheW-glassaddsabout510W/m
2
duringthehotafternoon.Thus,6.9m
2
of
unshadedW-glasscontributes1TR.ThesameglassareaontheN-sidecontributes
only1/15TR.

60. Minimization of solar gain
(b)DirectsunlightonW,N-WandS-WshouldbeavoidedThiscanbedonebysuitablesunshades
whichwillpermitjustenoughlightbutlimitdirectsolarradiation.Theamountofoverhangcanbe
calculatedfrom
Overhang=FactorxShadowheight
wherethefactorrecommendedfortheperiodApril-Septemberforvariouslatitudes
(c)Usingcurtains/Venetianblindsonwindowsinsidethespace.However,externalshadingismore
effectivethaninternalshading.
(d)Indiumoxide(In
2O
3)coatedontheexternalsidereflectingglasssurfacescanbeused.
(e)Heatabsorbing/tintedglasscanbeused.

61. Other Building Design Features And Thermal
Properties Of Construction Materials
•Themeasuresincludethefollowing:
(a)Airisagoodinsulator.Hollowtileswithairtrappedinthemaremostideallysuitedforwalls.
Similarly,roofsandfloorscanbecastwithhollowspacefullofair.Adouble-wallconstructionwith
hollowbricksandair-gapiscommonlyusedinhotdesertareas.
(b)Roofexposedtosunmustbeinsulatedwithaminimumof5cmthickexpandedpolystyreneor
equivalentinsulation.Thiswillreducetheroofloadfrom3TRto0.1TRinthecaseof100m
2
of
concreteslab.
(c)Roofcanbepaintedwithaluminiumpainttoreflectsolarradiation.
(d)Sprayingtheroofswithwaterduringsunperiods.
(e)Useofsmallsurface-to-volume(A/V)ratio.DivisionofabuildinglikeblocksAandBseparated
asshowninFig.increasesthesurfaceareaandhencetransmissiongain.Inthesameway,itcouldbe
seenthatbuildingswithlargeraspectratioswillhavemoretransmissionheatgain.

62. Minimizing Infiltration and Ventilation Load
•Infiltrationloadisgenerallyveryheavy.
•If,however,infiltrationcannotbereducedthennoventilationair
needstobetakenseparatelywiththerecirculatedroomairbeforethe
A/Capparatus.
•Itmustbenotedthatevery5cmmoffreshairrequiresabout1TR.
•Itisrecommendedtokeeptheoutsideairdampersclosedatalltimes
unlessthebuildingiscompletelyoccupied.
•Infiltrationisexpectedtotakecareofthefreshairrequirementwhen
thedampersareclosed.
•Aninterestingmethodtoconserveenergyistocooltheventilationair
regenerativelywiththehelpofexhaustroomairusingaheat-pipeheat
exchanger.
•Figureshowstheconstructionofheatpipes(a)withoutawickand(b)
withawick.
•Anumberofsuchheatpipesareemployedtoformaregenerativeheat
exchangerasshowninFig.

63. Use of Natural Ventilation For Cooling
•WhentheoutsideWBTislowerthanroomWBTof18-20°Ccorrespondingto
roomdesignconditionsof25°CDBTand50±5%RH,theairconditioningplant
canbeshutdown,andonlynaturalventilationemployedinstead.
•Forthispurpose,largerfreshairintakeshavetobeprovidedattheplantinlet.
•SuchaconditionoccursformanydaysduringMarch-AprilandSeptember-
October,andalsoatnight.
•Ifitisresortedto,itwillresultinconsiderablepowersavingandlongerlifeofthe
plant.

64. Use of Thermal Storage
•Themethodinvolvestheuseofproperlysizedandheavilyinsulatedchilledwater
tankswhicharechargedatnightstoringsurplusrefrigerationwhencoolingload
demandislow,andoutsideWBTisalsolow.
•Thischilledwatercanbeusedduringthedayatthepeakloadperiod.
•ThemethodmainlyenablestheplanttooperateathigherCOPthusdecreasing
powerconsumption.Althoughitalsodecreasesthesizeoftheplant,thecapital
costisnotreducedasthecostofthetanksandinsulationisadded.
•Thissensibleheatstorageisnotreallythebestasitrequiresverylarge-sized
chilledwatertanks.
•Latentheatstorageemployingachangeofphaseofmaterialsisabettermethod.
Onesuchmaterialissodium-sulphatedecahydrate(Na
2SO
4:10H
2O)whichfreezes
at7–10°C.Thiswillrequireonly10%volumeascomparedtochilledwater.

65. Plant Selection
•Selecting high efficiency compressors
•Capacity control of reciprocating compressors
•Multistage compression in centrifugal compressors
•Multi-evaporators with individual or multistage compressors
•Optimal design of refrigeration and air conditioning equipment
•Most designers oversize the equipment
•Thermostat construction and adjustment

66. Plant Maintenance
•Cooling tower maintenance
•Removing fouling from condenser and chiller tubes
•Cleaning of air filters
•Overhauling of compressors

67. Permitting drift in room design conditions
•Mostoftheairconditioningequipmentisgovernedbybypasscontrolwhichmaintainsaconstant
DBTintheroom.
•ThesystemanditspsychrometryareillustratedinFigs23.16and23.17respectively.
•Inthis,theenteringrecirculatedroomairandoutdoorairmixtureispurposelydivertedaroundthe
coolingcoil.
•Asagainsttheinherentcoilbypass,thisisreferredtoassystembypass.
•Inaddition,therefore,onecanprovidecontrolsallowingforthespacetemperatureswingto26°C
insummerand16°Cinwinteratthetimeofpeakcoolingandheatingloads,thusreducingthesize
andenergyconsumptionoftheA/Csystemforthebuilding.
•Ithasnowbeenfoundthatsmalldriftsintemperatureandhumidityfromtherecommendedsteady-
stateconditionsdonotresultinadeclineincomfortlevel.
•Anacceptablelevelofchangeintemperatureis£0.6°Cperhourfroma25°Cbase,andin
humidityis£16torrwithnormalsummerclothingandsedentaryactivity.Thecontrolscanbe
devisedaccordingly.

68. Other Energy Conservation Measures
(i)Winterheatingbyheatpump.Itwillprovide3.4kWofheatingfor1kWof
powerconsumption.
(ii)Providingairlocksinallmajorentrancestominimizeinfiltration.
(iii)Shadingofthebuildingbytreesbloominginsummer.Thetreesprovideshade
andalsoactasevaporativecoolers.
(iv)Replacingallresistance-typeregulators,dimmers,etc.,withelectronicones.
(v)Installingtimers,controls,etc.,toautomaticallyswitchofflights,andair
handlingunits(AHUs)whenthespaceisnotinuse.
(vi)PreferringcentralA/Cplantsthatemploywater-cooledcondensersinplaceofa
numberofwindow-typeA/Csthatemployair-cooledcondensers.

69. Thank You!!!