applied psychrometrics for air conditioning systems€¦ · 02/05/2020 · kg per kg of dry air...

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

1 2-APPLIED PSYCHROMETRICS FOR AC SYSTEMS - May 2, 2020

©ChandanaDalugoda

03/03/2012

CourseDescription

Firstlyunderstandingthebasiccontentsofthepsychrometricchartandthendiscussthebasicpsychrometricprocesses.AdetailedPsychrometricapproachiscarriedoututilisingthedataavailablefromcoolingloadcalculationstodeterminethecoilconditionsandselectionoftheairconditioner.Allfreshair,highlatentloadandhumiditycontrolapplicationsarediscussed.ImportanceofSensibleheatratioisdiscussedanddistinguishesthedeferencebetweenhigh&lowsensibleheatapplications.ThispresentationissuitableforyoungaswellasexperiencedengineersthatprovidebasicknowledgeofPsychometryanditsapplicationsinpractice.

©ChandanaDalugoda

03/03/2012

LearningObjectives1. ExplainPsychrometricDefinitions2. DescribePsychrometricProperties3. UnderstandthebasicPsychrometricprocesses&theChart4. Discussairmixingprincipleandcalculations5. ExplainhowPsychrometriccalculationsuseddefinecoilconditions6. UnderstandtheimportanceofapparatusdewpointtomaintainroomRH%7. RecognisetheimportanceofBy-passFactor8. Highlatentheatapplication9. HighSensibleheatapplication10.Alloutdoorairapplication

©ChandanaDalugoda 4

PsychrometricDefinitions

Dry-bulbTemperature–Temperatureasregisteredbyanordinarythermometer

Wet-bulbTemperature–Temperatureasregisteredbyathermometerwiththebulbcoveredbyawettedwickandexposedtomovingair.(ThermodynamicWetBulb-AdiabaticSaturationtemperature)

DewpointTemperature–Temperatureatwhichcondensationofmoisturebeginswhentheairiscooled

RelativeHumidity–Ratioofactualwatervapourpressureofairtothesaturatedwatervapourpressureofairatthesametemperature

SpecificHumidity-WeightofwatervapouringramsorKgperKgofdryair(MoistureContent)

Enthalpy -Quantityofheatintheairaboveanarbitrarydatum,in KJ/Kg.ofdryair.

(thedatumfordryairis0oCandmoisturecontent,waterat0oC)

SensibleHeatFactor-RatioofSensibleHeattoTotalHeat

AlignmentCircle-Locatedat24oCDBand50%RHusedwithsensibleheatfactortoplotvariousairconditioningprocesses

Usefulconversionfactors

Enthalpy Btu/lbofdryairx2.33gives

KJ/Kgofdryair

Mass 1lb=7000grains=0.454Kg

Flowrate Cubicfeet/minx0.472givesl/sec

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

• Dry-bulbtemperature• Wet-bulbtemperature• Dew-pointtemperature• Relativehumidity• Humidityratio• SpecificVolume• SpecificEnthalpy

PsychrometricPropertiesofAir

©ChandanaDalugoda

Dry-BulbTemperature

Dry-bulbtemperaturesarereadfromanordinarythermometerthathasadrybulb.

32°C DB

Dry Bulb Temperature

©ChandanaDalugoda

Wet-BulbTemperature

Wet-bulbtemperaturesarereadfromathermometerwhosebulbiscoveredbyawetwick.Thedifferencebetweenthewet-bulbtemperatureandthedry-bulbtemperatureiscausedbythecoolingeffectproducedbytheevaporationofmoisturefromthewick.Thisevaporationeffectreducesthetemperatureofthebulband,therefore,thethermometerreading.

27°C WB

Wet Bulb Temperature

©ChandanaDalugoda

Thethirdproperty,dew-pointtemperature,isthetemperatureatwhichmoistureleavestheairandcondensesonobjects,justasdewformsongrassandplantleaves.

Dew-pointTemperature

19°C DPDew Point

Temperature

Humidity

Watervapourcondensationonwindowduetohighhumidity

Humidityistheamountofwaterintheair.Airisamixtureofgasses&watervapour.

HighHumidityaffectsdiscomforttooccupantsinbuildings.Alsoitaffectsthefurniture,equipment&material.Oftenwallsgetdampandcausefungus&mouldstoform,causinghealthriskstohumans.Highhumiditycanshortcircuitminiatureelectroniccircuits,hencesomeITapplicationswantsclosecontrolofRelativeHumidityandTemperatureinAir-conditionedrooms.

©ChandanaDalugoda

RelativeHumidity

Amountofmoisturethatagivenamountofairisholding

=Amountofmoisturethatagivenamountofaircanhold

Thefourthproperty,relativehumidity,isacomparisonoftheamountofmoisturethatagivenamountofairisholding,totheamountofmoisturethatthesameamountofaircanhold,atthesamedry-bulbtemperature.

DefinitionTheratiooftheactualwatervapourpressuretothesaturatedwatervapourpressureatthesamedrybulbtemperature

RelativeHumidity

Relative Humidity

©ChandanaDalugoda

HumidityRatio(MoistureContent)Finally,humidityratiodescribestheactualweightofwaterinanair–watervapourmixture.HumidityratiocanbeexpressedasKgofmoistureperKgofdryair,orasgramsofmoistureperKgofdryair.

13g/KgDA

Humidity ratio

©ChandanaDalugoda

WhatisFog?

Fogintheforeground

Fogbeginstoformwhenwatervapourcondensesintohnyliquidwaterdropletsthataresuspendedintheair.Fogcanbeconsideredatypeoflow-lyingcloudusuallyresemblingstratus,andisheavilyinfluencedbynearbybodiesofwater,topography,andwindcondihons

©ChandanaDalugoda

FogOccursWhenAirisSaturatedWhenthedry-bulb,wet-bulb,anddew-pointtemperaturesarethesame,theairissaturated.Itcanholdnomoremoisture.Whenairisatasaturatedcondition,moistureenteringtheairdisplacesmoisturewithintheair.Thedisplacedmoistureleavestheairintheformoffinedroplets.Whenthisconditionoccursinnature,itiscalledfog

FogareaofthePsychrometricchart

©ChandanaDalugoda

SlingPsychrometer

©ChandanaDalugoda

PsychrometricChart

PropertiesofPsychrometricChart

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

©ChandanaDalugoda

EXAMPLESummerDesignConditions

» 32°CDB(drybulb)» 27°CWB(wetbulb)

Forexample,let'sassumethatthesummerdesignconditionsare32°Cdrybulband27°Cwetbulb.Whatistherelativehumidity,humidityratio,anddewpoint?

©ChandanaDalugoda32°C DB

27°C WB

25.4°C DP

Given32◦CDBand27◦CWB.Whatisthedew-pointtemperature,RH%&MoistureContent

20.5g/KgDA

©ChandanaDalugoda

30°C DB

20.4°C WB

Given30oCDBand40%RHWhatisthewetbulbtemperature

©ChandanaDalugoda

32°C DB

27°C WB0.0208 Kg/Kg

34 KJ/Kg

Given15oCDBand12oCWB,whatisthemoisturecontentandenthalpy

©ChandanaDalugoda

PsychrometricsProcesses

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

©ChandanaDalugoda

SensibleCooling

Nochangeinmoisturecontent

SensiblecoolingtDB2>tDB1tWB2>tWB1

RH2<RH1

h2>h1Vs2>Vs1

Sensible Cooling

ActiveChilledBeam

©ChandanaDalugoda

SensibleHeating

Nochangeinmoisturecontent

SensibleHeatingtDB1<tDB2tWB1<tWB2

RH1>RH2

h1<h2Vs1<Vs2

Sensible Heating

HotWaterCoil

©ChandanaDalugoda

Humidification/De-humidification

Humidification

De-humidification

HumidificationtDB0=tDB1twb1>twb0RH0<RH1

h1>h0Vs1>Vs0

De-HumidificationtDB0=tDB2Twb0>twb2RH2<RH0

h0>h2Vs0>Vs2

©ChandanaDalugoda

Cooling & de-humidification

Actualcooling&de-humidificationprocess

Cooling&De-humidification

Cooling&De-HumidificationtDB1>tDB2tWB1>twb2RH1<RH2

h1>h2Vs1>Vs2

Cooling&De-humidifyingCoil

©ChandanaDalugoda

1Heating & humidification

SpaceconditioningPsychrometricprocess

Heating&Humidification

Heating&HumidificationtDB1<tDB2tWB1<twb2RH1>RH2

h1<h2Vs1<Vs2

©ChandanaDalugoda

1Cooling & humidification

PsychrometricprocessforDesertCoolerstw&hconstant

Cooling&humidification Cooling&HumidificationtDB1>tDB2atWB1<tWB2a

RH1<RH2a

w1<w2a

Evaporativecooling

tWB1=twb2h1=h2b

Cooling & humidification

Evaporative cooling2b

EvaporativeCooling

Chemicalde-humidificationDesiccantdrying

Heating&de-humidification C&HtDB1<tDB2tWB1>twb2RH1>RH2

h1>h2Vs1<Vs2

Heating & de-humidification

Chemical de-humidifiers

AirMixingCalculation

Mixingconditionh2

t3 = (t1 x m1) + (t2 x m2) (m1 + m2)

©ChandanaDalugoda

AppliedPsychrometricsforAirConditioningSystems

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

Lectureroom50people115m2

Step-1Locatepoints1&2onthechart

250 RH%

Indoor&outdoorConditions

38.5CDB/23.2CWB

24CDB/50%RH

Step-2DeterminetheSHRanddrawtheRSHFlineThroughthepoint-2

RSHF=SH/TH=16.5/23.5=0.7

ConstructingSHRline

38.5CDB/23.2CWB

24CDB/50%RH

0.7SHRline

0.7SHR

Step-3Findtheleavingconditionofthecoil.Allwaysairleavesthecoolingcoilalmostsaturated,thusleavingaircanbeassumedAs90%RH.RSHFlineintersectswith90%saturationline,givesthecoilleavingcondition.ThisisOFFCoiltemperature,10CDB,9.4CWB

CoilLeavingConditions

38.5CDB/23.2CWB

10.7CDB/9.8CWB

0.7SHRlineCoilleavingat10C

SpecificVolume0.814m3/kg

24CDB/50%RH

SpecificVolume0.899m3/kg

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

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

Step-6

Drawaverticallinefrom28Cuntilitintersectstheline1-2andthisisairmixingatcoilenteringpoint.ThisisONCoiltemperature.ThisisONCoiltemperature28CDB,19CWB

ADP-apparatusdewpoint

Coilleaving

Coilentering

CoilEnteringCondition

28CDB/19CWB

PsychromtericProcessesofTheAir-ConditioningSystem

Ifweneglectthefanheatandductheatgains,theairconditioningsystemLookslikebelow.3Psychrometricprocessesthatareinvolved;1. Airmixing2. Cooling&de-humidification3. Heating&humidification 3

Heating&

humidification

AirmixingCooling&de

humidification

AirConditioningCycle

PsychromtericProcessesofTheAirConditioningSystem

Withfanheatandductheatgains,theairconditioningsystemisgivenbelow.4Psychrometricprocessesthatareinvolved;

1. Airmixing2. Cooling&de-humidification3. Heating&humidification4. Sensibleheating

4Heatin

humidification

AirmixingCooling&de

humidification

Fan&ductheat

ActualAirConditioningCycle

ΔhRoom

RoomloadcanbedeterminebyQ=Δh room x m

RoomTotalLoad

ΔhCoil

CoilcapacityCoilcapacitycanbedeterminebyQ=Δhcoil x m

CoolingCoilCapacity

PsychrometricAnalysisofActualAirConditioningSystem

1.Coilprocess:mtocc2.Coilleaving:cc3.Heatgainsupplyplenum:cctosf4.Supplyfan:sf5.Heatgainfromsupplyfan&duct:sftos6.Roomprocess:stoR7.Heatgainreturnplenum:Rtorp8.Heatgainreturnduct&fan:rptor

Tomaintainindoordesignconditionsof24CDB&do%RH,CoilDPshouldbebelow13C,RequiresCHWreturnnear13C.IfCHWreturnincreaseabove13Cwouldincreasetheroomconditions.CHWoutlettemperaturerestrictedminimum4.5C,toavoidfreezing.Hence,chilleroperatingconditionsusedasCHWout6.6C(44F)&CHWreturn12.7C(54F).

ImportanceofApparatusDewPoint

MinimumCoilConditions4.5C

RoomConditions24CDB/50%RH

BypassFactor(BF)

Isafunctionofthephysicalandoperatingcharacteristicsoftheconditioningapparatusand,assuch,representsthatportionofairwhichisconsideredtopassthroughtheconditioningapparatuscompletelyunaltered.

BypassFactorisaffectedby 1.Availableapparatusheattransfersurface

Finspacing,numberofrowsetc. 2.VelocityofairthroughthecoilBypassFactor=A/B

BF=(tldb–tadp)/(tedb–tadp)

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)

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

SpecificationsoftheAirconditioneris;1.TotalcoolingcapacitykW2.SensiblecoolingcapacitykW3.SensibleHeatRatioSHR4.CoilconditionsONcoil/OFFcoiltemperature5.SupplyairvolumeL/s6.OutdoorfreshairvolumeL/s

SpecificationsofAirConditioner

WhenhighoccupancyprevailssuchasAuditoriumorBanquetHall,psychrometricProcessbecomeHighLatentloadapplication.Whenthelatentloadincreases,RoomSHRlinedonotintersectwithcoilratioline,whichisthecoilleavingconditions.Fromtheleavingconditions,ahorizontallinehastoDraw,untilRoomSHRlineintersects.Thishorizontallinerepresentthereheatrequiredafterthecoiloutlet.Thisre-heatingwoodbeanenergywaste,henceheatexchangerssuchasHeatpipeorface-bypassisusedWithoutelectricalenergybeingused.

Reheat

Supplyairtoconditionedspace

HighLatentLoad

Re-Heating

Electricre-heatingcoilsareplaceddownstreamofthecoolingcoilforde-humidificationofsupplyairinhighlatentloadapplications.Usuallytheseare3-stagedelectricheatersworkingon3-phaseelectricity.WithincreasingelectriccostsusageofelectricheatingisdiscouragedbyASHRAEStandards

ElectricHeatersuseforRe-Heating

Wrap-around,de-humidification,noncontrolledheatpipe(DHP)useforre-heatingtheairleavingevaporatorforde-humidificationwithzeroenergyinput.

Heatfromreturnairpickedupbyupstreamheatpipecoilandtransferittothedownstreamheatpipecoilplacedafterthecoolingcoil,whichre-heatstheairtoachievefurtherde-humidificationrequired.

Theseweresuccessfullyusedinworld’ssecondPassiveHouseRatedbuildinginSriLanka,tomaintainrelativehumiditydownto52~54%withoutcontrol.

HeatPipeuseforRe-HeatingwithZeroEnergyInput

Face&BypassDampersforRe-HeatingwithZeroEnergyInputTherearesetofphaseandbypassdampersareinstalledupstreamofthecoolingcoil.Normaloperationairpassesthroughfacedampersandenterthecoolingcoil,wherebypassdampersareclosed.Whenyouneedextrade-humidificationforhighlatentlands,bypassdampersopensandsomeofthereturnairbypassesthecoilandenterssupplyairstreamthusaddingmoreheatingtoair.

ServerroomCoolingLoadscomprisesofSensibleheatingonlyandroomSHR=0.9maybe1.00.Nolatentloadsoroomratioandcoilratiolinesarehorizontal.Moisturecontentofairisconstant.Coilloadisfrom4to1andtheroomloadis4to3.

SuchapplicationisServerRoomairconditioning.Deferencebetweenserverroom&comfortcoolingisheatdensity;5000W/m2vs250W/m2.Highdensityheatandclosecontroloftemperature/relativehumidityaretheDesigntargetsofITcooling.

HighSensibleHeatApplications

Δh Coil

RoomAir

Supplyfanoutlet

HeatingCoiloutletCoolingCoiloutlet

Δh Room

ITCooling&ServerRoomApplications

ServerroomorITcoolingrequireclosecontrolofTemperature&Humidity.Indoorconditionsasper1999ASHRAEApplications;22℃±1℃and50±5%RH.Typicalsupplyairtemperature14℃andreturnairtemperatureis24℃&45%RH.

Coilneedstohandlelargelatentload,coildepthupto8to10rowsmayrequire.Evaporatorcapacitycontrolrequiredwithfreezingprotection.Usuallycoilloadsare2.5to3.0timeslargerthancomfortACsystems.ApplicationsareOperatingRooms,ICU,CCRetc.

OutdoorairDirectlypassthroughthecoolingcoilandCooled&De-humidifiedtocoilleavingconditionsandenterstheroom.Noairwillbecirculated,allairisexhaustedwhilekeepingspacepositivelypressurised.

ALLOutdoorAirApplications

ΔhCoil

OutdoorAirIntake

RoomCondition

AllFreshAirApplicationsforOperatingTheatres

100%freshairfromoutdoors,filteredandcooled&de-humidifiedbeforeenteringtheOT.Allroomairexhausted,creatingpositivepressureintheOT.SupplyairUni-directionalflow(laminarflow)iscreatedandreturnair(exhaustair)pickedupbywallextractgrillsatfloorlevel.

10/4/2016

applied psychrometrics for air conditioning systems€¦ · 02/05/2020 · kg per kg of dry air...

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