Chapter11Gas-VaporMixturesandAirConditioning

11-1DryandAtmosphericAir11-1-1AtmosphericairTheairintheatmospherenormallycontainssomewatervaporandisreferredtoatmosphericair.Airthatcontainsnowateriscalleddryair.Althoughtheamountofwatervaporintheairissmall,itplaysamajorroleinhumancomfort.ThereforeitisanimportantconsiderationinairconditioningapplicationsThedryairandvaporofatmosphericairinairconditioningapplicationrange(temperaturechangesfrom-10℃to50℃)canbetreatedasidealgas

11-1-2ThepressureofatmosphericairThetotalpressureofatmosphericairThepartialpressureofdryairThepartialpressureofvapor,increaseswiththeamountofvaporinair

sp2Tp1T1pmax

11-1-3Dew-pointTemperatureTp1T11Astovaporstate1,decreasethetemperature,thenpressureofvaporwon’tchangeDew-point

Thedew-pointTdpisdefinedasthetemperatureatwhichcondensationbeginsiftheairiscooledatconstantpressure

11-2Thepropertiesofair11-2-1SpecifichumidityofairTheratioofthemassofwatervaporpresentinaunitmassofdryair,denotedbyωSinceVv=VaandTv=TaRa=287.1J/kg.KRv=461.4J/kg.K

11-2-2RelativehumidityofairThecomfortleveldependsmoreontheamountofmoisturetheairholds(mv)relativetothemaximumamountofmoisturetheaircanholdatthesametemperature(mg).TheratioofthesetwoquantitiesiscalledtherelativehumidityThesaturatedpressureunderthetemperatureofatmosphericair

11-2-3EnthalpyofairSpecificenthalpy:

Inthefieldofair-conditioning,thereferencetemperatureis0℃.Then:ha=1.005tkJ/kg.℃hv=2501+1.863tkJ/kg.℃h=1.005t+ω(2501+1.863t)kJ/kg.℃2501kJ/kgmeanstheenthalpyat0℃1.863kJ/kg.℃istheaveragespecificheatofvapor1.863kJ/kg.℃istheaveragespecificheatofdryair

11-3AdiabaticSaturationandWet-BulbTemperature11-3-1Adiabaticsaturationt1,ω1,f1t2,ω2,f2=100%UnsaturatedairSaturatedairh