Applied Psychrometrics for Air Conditioning Systems€¦ · 02/05/2020 · Kg per Kg of dry air...
Transcript of Applied Psychrometrics for Air Conditioning Systems€¦ · 02/05/2020 · Kg per Kg of dry air...
By
Eng. Chandana Dalugoda, CEng MIE(SL), FASHRAE, MCIBSE, GCGI (UK), MConsE (SL) Managing Partner, Chandana Dalugoda ConsultantsColombo, Sri Lanka
Applied Psychrometrics for Air Conditioning Systems
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CourseDescription
Firstlyunderstandingthebasiccontentsofthepsychrometricchartandthendiscussthebasicpsychrometricprocesses.AdetailedPsychrometricapproachiscarriedoututilisingthedataavailablefromcoolingloadcalculationstodeterminethecoilconditionsandselectionoftheairconditioner.Allfreshair,highlatentloadandhumiditycontrolapplicationsarediscussed.ImportanceofSensibleheatratioisdiscussedanddistinguishesthedeferencebetweenhigh&lowsensibleheatapplications.ThispresentationissuitableforyoungaswellasexperiencedengineersthatprovidebasicknowledgeofPsychometryanditsapplicationsinpractice.
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LearningObjectives1. ExplainPsychrometricDefinitions2. DescribePsychrometricProperties3. UnderstandthebasicPsychrometricprocesses&theChart4. Discussairmixingprincipleandcalculations5. ExplainhowPsychrometriccalculationsuseddefinecoilconditions6. UnderstandtheimportanceofapparatusdewpointtomaintainroomRH%7. RecognisetheimportanceofBy-passFactor8. Highlatentheatapplication9. HighSensibleheatapplication10.Alloutdoorairapplication
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PsychrometricDefinitions
Dry-bulbTemperature–Temperatureasregisteredbyanordinarythermometer
Wet-bulbTemperature–Temperatureasregisteredbyathermometerwiththebulbcoveredbyawettedwickandexposedtomovingair.(ThermodynamicWetBulb-AdiabaticSaturationtemperature)
DewpointTemperature–Temperatureatwhichcondensationofmoisturebeginswhentheairiscooled
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RelativeHumidity–Ratioofactualwatervapourpressureofairtothesaturatedwatervapourpressureofairatthesametemperature
SpecificHumidity-WeightofwatervapouringramsorKgperKgofdryair(MoistureContent)
Enthalpy -Quantityofheatintheairaboveanarbitrarydatum,in KJ/Kg.ofdryair.
(thedatumfordryairis0oCandmoisturecontent,waterat0oC)
PsychrometricDefinitions
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SensibleHeatFactor-RatioofSensibleHeattoTotalHeat
AlignmentCircle-Locatedat24oCDBand50%RHusedwithsensibleheatfactortoplotvariousairconditioningprocesses
PsychrometricDefinitions
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Usefulconversionfactors
Enthalpy Btu/lbofdryairx2.33gives
KJ/Kgofdryair
Mass 1lb=7000grains=0.454Kg
Flowrate Cubicfeet/minx0.472givesl/sec
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History-PsychrometricPioneers1.Observation&PhilosophicalPeriod
1. AanaximenesofMiletus,(6thCenturyB.C.)GreekPhilosopherin570B.C.toGaius,PlinusSecundus,(23-79A.D.)–RomanNaturalist.
2.TheExperimentalSciencePeriod1. Alberti,LeoneBattista(1414-1472)ItalianPhilosophertoBockmann,CarlWilhelm,Jr(1773-1821),
German.3.TheBeginningofUnifyingTheoryRegardingTheBehaviourofAirAndWaterVapour
1. Dalton,John(1766-1844),EnglishScientisttoSwann,W.F.G.,EnglishScientist4.TransitionFromEmpiricalToRationalPeriod
1. Carrier,WillisH.(1876-1950)toKeenanJosephH.andKeyes,FredG.(1936),professorsofMIT,USA.5.TheMatureSciencePeriod
1. Goodman,William(1903-1993)ConsultingengineerChicago,IllinoistoNelson,H.F.andSauer,H.J.(2001),USACurtesyofDonaldP.Gatley-UnderstandingPsychrometrics
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• Dry-bulbtemperature• Wet-bulbtemperature• Dew-pointtemperature• Relativehumidity• Humidityratio• SpecificVolume• SpecificEnthalpy
PsychrometricPropertiesofAir
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Dry-BulbTemperature
Dry-bulbtemperaturesarereadfromanordinarythermometerthathasadrybulb.
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32°C DB
Dry Bulb Temperature
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Wet-BulbTemperature
Wet-bulbtemperaturesarereadfromathermometerwhosebulbiscoveredbyawetwick.Thedifferencebetweenthewet-bulbtemperatureandthedry-bulbtemperatureiscausedbythecoolingeffectproducedbytheevaporationofmoisturefromthewick.Thisevaporationeffectreducesthetemperatureofthebulband,therefore,thethermometerreading.
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27°C WB
Wet Bulb Temperature
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Thethirdproperty,dew-pointtemperature,isthetemperatureatwhichmoistureleavestheairandcondensesonobjects,justasdewformsongrassandplantleaves.
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Dew-pointTemperature
19°C DPDew Point
Temperature
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Humidity
Watervapourcondensationonwindowduetohighhumidity
Humidityistheamountofwaterintheair.Airisamixtureofgasses&watervapour.
HighHumidityaffectsdiscomforttooccupantsinbuildings.Alsoitaffectsthefurniture,equipment&material.Oftenwallsgetdampandcausefungus&mouldstoform,causinghealthriskstohumans.Highhumiditycanshortcircuitminiatureelectroniccircuits,hencesomeITapplicationswantsclosecontrolofRelativeHumidityandTemperatureinAir-conditionedrooms.
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RelativeHumidity
RelativeHumidity
Amountofmoisturethatagivenamountofairisholding
=Amountofmoisturethatagivenamountofaircanhold
Thefourthproperty,relativehumidity,isacomparisonoftheamountofmoisturethatagivenamountofairisholding,totheamountofmoisturethatthesameamountofaircanhold,atthesamedry-bulbtemperature.
DefinitionTheratiooftheactualwatervapourpressuretothesaturatedwatervapourpressureatthesamedrybulbtemperature
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RelativeHumidity
40%RH
Relative Humidity
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HumidityRatio(MoistureContent)Finally,humidityratiodescribestheactualweightofwaterinanair–watervapourmixture.HumidityratiocanbeexpressedasKgofmoistureperKgofdryair,orasgramsofmoistureperKgofdryair.
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13g/KgDA
Humidity ratio
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WhatisFog?
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Fogintheforeground
Fogbeginstoformwhenwatervapourcondensesintohnyliquidwaterdropletsthataresuspendedintheair.Fogcanbeconsideredatypeoflow-lyingcloudusuallyresemblingstratus,andisheavilyinfluencedbynearbybodiesofwater,topography,andwindcondihons
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FogOccursWhenAirisSaturatedWhenthedry-bulb,wet-bulb,anddew-pointtemperaturesarethesame,theairissaturated.Itcanholdnomoremoisture.Whenairisatasaturatedcondition,moistureenteringtheairdisplacesmoisturewithintheair.Thedisplacedmoistureleavestheairintheformoffinedroplets.Whenthisconditionoccursinnature,itiscalledfog
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FogareaofthePsychrometricchart
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SlingPsychrometer
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PsychrometricChart
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PropertiesofPsychrometricChart
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Temperature&RH
» Comparisonofoutdoorconditionsofdeferentlocations
Unit Colombo Chennai Delhi London
drybulb °C 33 38.5 43.8 28.3
wetbulb °C 28 28.3 29.6 19.8
RH % 68 46 36 52
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EXAMPLESummerDesignConditions
» 32°CDB(drybulb)» 27°CWB(wetbulb)
Forexample,let'sassumethatthesummerdesignconditionsare32°Cdrybulband27°Cwetbulb.Whatistherelativehumidity,humidityratio,anddewpoint?
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27°C WB
68%
25.4°C DP
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Given32◦CDBand27◦CWB.Whatisthedew-pointtemperature,RH%&MoistureContent
20.5g/KgDA
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30°C DB
20.4°C WB
40%
Given30oCDBand40%RHWhatisthewetbulbtemperature
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32°C DB
27°C WB0.0208 Kg/Kg
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Given15oCDBand12oCWB,whatisthemoisturecontentandenthalpy
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PsychrometricsProcesses
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PsychrometricProcesses
1. OA-SensibleCooling2. OB-Cooling&De-Humidification3. OC-De-Humidification4. OD-Heating&De-humidification5. OE-SensibleHeating6. OF-Heating&Humidification7. OG-Humidification8. OH-Cooling&humidification
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SensibleCooling
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Nochangeinmoisturecontent
h2
h1
Ø1
Ø2
w1=w2
SensiblecoolingtDB2>tDB1tWB2>tWB1
RH2<RH1
w2=w1
h2>h1Vs2>Vs1
t2t1
Sensible Cooling
ActiveChilledBeam
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SensibleHeating
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Nochangeinmoisturecontent
h2
h1
Ø1
Ø2
w1=w2
SensibleHeatingtDB1<tDB2tWB1<tWB2
RH1>RH2
w1=w2
h1<h2Vs1<Vs2
t2t1
Sensible Heating
HotWaterCoil
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Humidification/De-humidification
Humidification
0
Enth
alpy
decr
ease
1
2
De-humidification
Enth
alpy
Incr
ease
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w0
w2
w1
Ø1
Ø2
Ø0
HumidificationtDB0=tDB1twb1>twb0RH0<RH1
w1>w0
h1>h0Vs1>Vs0
t2t1=
De-HumidificationtDB0=tDB2Twb0>twb2RH2<RH0
w0>w2
h0>h2Vs0>Vs2
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1
Cooling & de-humidification
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Actualcooling&de-humidificationprocess
h2
h1
Ø1
Ø2
w1
Cooling&De-humidification
w2
t2 t1
Cooling&De-HumidificationtDB1>tDB2tWB1>twb2RH1<RH2
w1>w2
h1>h2Vs1>Vs2
Cooling&De-humidifyingCoil
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1Heating & humidification
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SpaceconditioningPsychrometricprocess
h2
h1
Ø1
Ø2
w1
Heating&Humidification
w2
Heating&HumidificationtDB1<tDB2tWB1<twb2RH1>RH2
w1<w2
h1<h2Vs1<Vs2
t2t1
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2c
1Cooling & humidification
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PsychrometricprocessforDesertCoolerstw&hconstant
Cooling&humidification Cooling&HumidificationtDB1>tDB2atWB1<tWB2a
RH1<RH2a
w1<w2a
Evaporativecooling
tWB1=twb2h1=h2b
Cooling & humidification
Evaporative cooling2b
2a
t1
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EvaporativeCooling
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Chemicalde-humidificationDesiccantdrying
Heating&de-humidification C&HtDB1<tDB2tWB1>twb2RH1>RH2
w1>w2
h1>h2Vs1<Vs2
Heating & de-humidification
t1
1
2
Ø1
Ø2
w1
w2
t2
Chemical de-humidifiers
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AirMixingCalculation
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1
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Mixingconditionh2
h1
w1
t2 t1
w2
w33
t3
t3 = (t1 x m1) + (t2 x m2) (m1 + m2)
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AppliedPsychrometricsforAirConditioningSystems
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PsychrometricAnalysisofAirConditioningSystem
Outdoorconditions :Chennai38.5CDB,28.3CWBIndoorconditions :24CDB,50%(17CWB) Fromcoolingloadcalculations,totalcoolingcapacity:26.5kW,Sensiblecapacity :16.5kWOccupancy :50peopleDeterminetheenteringandleavingconditionsofthecoil&supplyairvolume
Airconditioner
38.5°CDB28.3°CWB
Roomconditions24°CDB17°CWB
24°CDB17°CWB
24°CDB17°CWB
1
534
22
Lectureroom50people115m2
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Step-1Locatepoints1&2onthechart
1
250 RH%
Indoor&outdoorConditions
38.5CDB/23.2CWB
24CDB/50%RH
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Step-2DeterminetheSHRanddrawtheRSHFlineThroughthepoint-2
RSHF=SH/TH=16.5/23.5=0.7
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ConstructingSHRline
38.5CDB/23.2CWB
24CDB/50%RH
0.7SHRline
0.7SHR
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Step-3Findtheleavingconditionofthecoil.Allwaysairleavesthecoolingcoilalmostsaturated,thusleavingaircanbeassumedAs90%RH.RSHFlineintersectswith90%saturationline,givesthecoilleavingcondition.ThisisOFFCoiltemperature,10CDB,9.4CWB
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90RH%
CoilLeavingConditions
38.5CDB/23.2CWB
10.7CDB/9.8CWB
0.7SHRlineCoilleavingat10C
5
SpecificVolume0.814m3/kg
24CDB/50%RH
1
SpecificVolume0.899m3/kg
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Step-4Tofindtheenteringconditionsofthecoil,airmixingequationisused. t3=[(t1xm1)+(t2xm2)]/(m1+m2)However,atthisstagewedonothavem2,thatissupplyairflowrate.
Henceweshallfirstdeterminethem2supplyairflowrate.
Tofindthesupplyairvolumeinwecanusethefollowingformula q=mCpΔT
q=ρvCpΔT(massflowrateisreplacedwithdensity&volumeflowrate
Then; v=q/ρvCpΔT
Volumeflowratev=16.5/(1.2x1.02x(24-10)) =0.9628m3/sSpecificvolumeatsupplyairconditionreadfromthechartas0.814m3/kg
Thenmassflowrateofsupplyair;m=v/vs=0.9628/0.855=1.182kg/s
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Step-4continued….
TofindthemassflowrateoffreshairVolumeflowrateV=50x8.0L/s.p(approximation.NeedstofindVbz) =0.4m3/sSpecificvolumeatoutdoorconditionreadfromthechartas0.90m3/kgThenmassflowrateoffreshair=v/vs=0.4/0.90=0.44kg/s
Step-5
Tofindtheenteringconditionsofthecoil,airmixingequationisused. t3=[(t1xm1)+(t2xm2)]/(m1+m2)m2=1.182,m1=0.44 t3=[(38.5x0.44)+(24x1.187)]/(0.44+1.187) t3=27.9=28.0C
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2
5
Step-6
Drawaverticallinefrom28Cuntilitintersectstheline1-2andthisisairmixingatcoilenteringpoint.ThisisONCoiltemperature.ThisisONCoiltemperature28CDB,19CWB
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4
ADP-apparatusdewpoint
Coilleaving
Coilentering
CoilEnteringCondition
28CDB/19CWB
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2
5
PsychromtericProcessesofTheAir-ConditioningSystem
Ifweneglectthefanheatandductheatgains,theairconditioningsystemLookslikebelow.3Psychrometricprocessesthatareinvolved;1. Airmixing2. Cooling&de-humidification3. Heating&humidification 3
4
Heating&
humidification
AirmixingCooling&de
-
humidification
AirConditioningCycle
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2
5
PsychromtericProcessesofTheAirConditioningSystem
Withfanheatandductheatgains,theairconditioningsystemisgivenbelow.4Psychrometricprocessesthatareinvolved;
1. Airmixing2. Cooling&de-humidification3. Heating&humidification4. Sensibleheating
3
4Heatin
g&
humidification
AirmixingCooling&de
-
humidification
Fan&ductheat
ActualAirConditioningCycle
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ΔhRoom
RoomloadcanbedeterminebyQ=Δh room x m
RoomTotalLoad
1
2
5
3
4
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ΔhCoil
CoilcapacityCoilcapacitycanbedeterminebyQ=Δhcoil x m
CoolingCoilCapacity
1
2
5
3
4
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PsychrometricAnalysisofActualAirConditioningSystem
1.Coilprocess:mtocc2.Coilleaving:cc3.Heatgainsupplyplenum:cctosf4.Supplyfan:sf5.Heatgainfromsupplyfan&duct:sftos6.Roomprocess:stoR7.Heatgainreturnplenum:Rtorp8.Heatgainreturnduct&fan:rptor
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Tomaintainindoordesignconditionsof24CDB&do%RH,CoilDPshouldbebelow13C,RequiresCHWreturnnear13C.IfCHWreturnincreaseabove13Cwouldincreasetheroomconditions.CHWoutlettemperaturerestrictedminimum4.5C,toavoidfreezing.Hence,chilleroperatingconditionsusedasCHWout6.6C(44F)&CHWreturn12.7C(54F).
ImportanceofApparatusDewPoint
1
2
3
4
DP13C
MinimumCoilConditions4.5C
RoomConditions24CDB/50%RH
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BypassFactor(BF)
AB
Tadp
TeDB
TlDB
Isafunctionofthephysicalandoperatingcharacteristicsoftheconditioningapparatusand,assuch,representsthatportionofairwhichisconsideredtopassthroughtheconditioningapparatuscompletelyunaltered.
BypassFactorisaffectedby 1.Availableapparatusheattransfersurface
Finspacing,numberofrowsetc. 2.VelocityofairthroughthecoilBypassFactor=A/B
BF=(tldb–tadp)/(tedb–tadp)
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TypicalBypassFactors
Depthofcoils(rows)
Withoutsprays Withsprays8fins/in 14fins/in 8fins/in 14fins/in
Velocity(fpm)300-700 300-700 300-700 300-700
2 0.42–0.55 0.22–0.383 0.27–0.40 0.10–0.234 0.19–0.30 0.05–0.14 0.12–0.22 0.03–0.105 0.12-0.23 0.02-0.09 0.08–0.14 0.01–0.086 0.08–0.18 0.01–0.06 0.06–0.11 0.01–0.058 0.03–0.08 0.02–0.05
(ForFinnedCoils)
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ApplicationsCoil
bypassfactorTypeofapplication Example
0.30to0.50Asmalltotalloadoraloadthatissomewhatlargerwithalowsensibleheatfactor(highlatentload)
Residence
0.20to0.30Typicalcomfortapplicationwitharelativelysmalltotalloadoralowsensibleheatfactorwithasomewhatlargerload
Residence,smallretailshop,Factory
0.10to0.20Typicalcomfortapplication Departmentstore,Bank,
Factory
0.05to0.10 Applicationswithhighinternalsensibleloadsorrequiringalargeamountofoutdoorairforventilation
Departmentstore,restaurant,Factory
0to0.10 Alloutdoorairapplication Hospitaloperatingtheatre,Factory
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SpecificationsoftheAirconditioneris;1.TotalcoolingcapacitykW2.SensiblecoolingcapacitykW3.SensibleHeatRatioSHR4.CoilconditionsONcoil/OFFcoiltemperature5.SupplyairvolumeL/s6.OutdoorfreshairvolumeL/s
SpecificationsofAirConditioner
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1
2
5
WhenhighoccupancyprevailssuchasAuditoriumorBanquetHall,psychrometricProcessbecomeHighLatentloadapplication.Whenthelatentloadincreases,RoomSHRlinedonotintersectwithcoilratioline,whichisthecoilleavingconditions.Fromtheleavingconditions,ahorizontallinehastoDraw,untilRoomSHRlineintersects.Thishorizontallinerepresentthereheatrequiredafterthecoiloutlet.Thisre-heatingwoodbeanenergywaste,henceheatexchangerssuchasHeatpipeorface-bypassisusedWithoutelectricalenergybeingused.
3
4
Reheat
Supplyairtoconditionedspace
HighLatentLoad
Re-Heating
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Electricre-heatingcoilsareplaceddownstreamofthecoolingcoilforde-humidificationofsupplyairinhighlatentloadapplications.Usuallytheseare3-stagedelectricheatersworkingon3-phaseelectricity.WithincreasingelectriccostsusageofelectricheatingisdiscouragedbyASHRAEStandards
ElectricHeatersuseforRe-Heating
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Wrap-around,de-humidification,noncontrolledheatpipe(DHP)useforre-heatingtheairleavingevaporatorforde-humidificationwithzeroenergyinput.
Heatfromreturnairpickedupbyupstreamheatpipecoilandtransferittothedownstreamheatpipecoilplacedafterthecoolingcoil,whichre-heatstheairtoachievefurtherde-humidificationrequired.
Theseweresuccessfullyusedinworld’ssecondPassiveHouseRatedbuildinginSriLanka,tomaintainrelativehumiditydownto52~54%withoutcontrol.
HeatPipeuseforRe-HeatingwithZeroEnergyInput
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Face&BypassDampersforRe-HeatingwithZeroEnergyInputTherearesetofphaseandbypassdampersareinstalledupstreamofthecoolingcoil.Normaloperationairpassesthroughfacedampersandenterthecoolingcoil,wherebypassdampersareclosed.Whenyouneedextrade-humidificationforhighlatentlands,bypassdampersopensandsomeofthereturnairbypassesthecoilandenterssupplyairstreamthusaddingmoreheatingtoair.
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2
ServerroomCoolingLoadscomprisesofSensibleheatingonlyandroomSHR=0.9maybe1.00.Nolatentloadsoroomratioandcoilratiolinesarehorizontal.Moisturecontentofairisconstant.Coilloadisfrom4to1andtheroomloadis4to3.
SuchapplicationisServerRoomairconditioning.Deferencebetweenserverroom&comfortcoolingisheatdensity;5000W/m2vs250W/m2.Highdensityheatandclosecontroloftemperature/relativehumidityaretheDesigntargetsofITcooling.
3
Supplyairtoconditionedspace
HighSensibleHeatApplications
Δh Coil
2
3
14
RoomAir
Supplyfanoutlet
HeatingCoiloutletCoolingCoiloutlet
Δh Room
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ITCooling&ServerRoomApplications
ServerroomorITcoolingrequireclosecontrolofTemperature&Humidity.Indoorconditionsasper1999ASHRAEApplications;22℃±1℃and50±5%RH.Typicalsupplyairtemperature14℃andreturnairtemperatureis24℃&45%RH.
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2
Coilneedstohandlelargelatentload,coildepthupto8to10rowsmayrequire.Evaporatorcapacitycontrolrequiredwithfreezingprotection.Usuallycoilloadsare2.5to3.0timeslargerthancomfortACsystems.ApplicationsareOperatingRooms,ICU,CCRetc.
OutdoorairDirectlypassthroughthecoolingcoilandCooled&De-humidifiedtocoilleavingconditionsandenterstheroom.Noairwillbecirculated,allairisexhaustedwhilekeepingspacepositivelypressurised.
3
Supplyairtoconditionedspace
ALLOutdoorAirApplications
ΔhCoil
2
3
1
OutdoorAirIntake
RoomCondition
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AllFreshAirApplicationsforOperatingTheatres
100%freshairfromoutdoors,filteredandcooled&de-humidifiedbeforeenteringtheOT.Allroomairexhausted,creatingpositivepressureintheOT.SupplyairUni-directionalflow(laminarflow)iscreatedandreturnair(exhaustair)pickedupbywallextractgrillsatfloorlevel.
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Q&A
10/4/2016
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65 2-APPLIED PSYCHROMETRICS FOR AC SYSTEMS - May 2, 2020