PsychrometricChart

• Thepsychrometricchartdisplaystherelationshipbetweendry-bulb,wet-bulb,anddewpointtemperaturesandspecificandrelativehumidity.Givenanytwoproperties,theotherscanbecalculated.• Tousethechart,takethepointofintersectionofthelinesofanytwoknownfactors(interpolateifnecessary),and,fromthatintersectionpoint,followthelinesoftheunknownfactorstotheirnumberedscalestoobtainthecorrespondingvalues.

Thermo-physicalpropertiesfoundonmostpsychrometricchartsDry-BulbTemperature• Dry-bulbtemperatureisthetemperatureofanairsample,asdeterminedbyanordinarythermometer,thethermometerbulbbeingdry.Itistypicallytheabscissaorhorizontalaxisofthegraph.TheSIunitfortemperatureisCelsius,theotherunitisFahrenheit.

Wet-BulbTemperature• Wet-bulbtemperatureisthetemperatureofanairsampleafterithaspassedthroughaconstant-pressure,ideal,adiabaticsaturationprocess,thatis,aftertheairhaspassedoveralargesurfaceofliquidwaterinaninsulatedchannel.• Inpractice,thisisthereadingofathermometerwhosesensingbulbiscoveredwithawetsockevaporatingintoarapidstreamoftheairsample.• Thewet-bulbtemperatureisthesameasthedry-bulbtemperaturewhentheairsampleissaturatedwithwater.

Thermo-physicalpropertiesfoundonmostpsychrometricchartsDewPointTemperature• Dewpointtemperatureisthattemperatureatwhichamoistairsampleatthesamepressurewouldreachwatervaporsaturation.Atthissaturationpoint,watervaporwouldbegintocondenseintoliquidwaterfog.

RelativeHumidity• Relativehumidityistheratioofthemolefractionofwatervaportothemolefractionofsaturatedmoistairatthesametemperatureandpressure.Relativehumidityisdimensionless,andisusuallyexpressedasapercentage.

Thermo-physicalpropertiesfoundonmostpsychrometricchartsHumidityRatio• Humidityratio,alsoknownasmoisturecontent,mixingratio,orspecifichumidity,istheproportionofmassofwatervaporperunitmassofdryairatthegivenconditions.• Foragivendry-bulbtemperature,therewillbeaparticularhumidityratioforwhichtheairsampleisat100%relativehumidity.• Humidityratioisdimensionless,butissometimesexpressedasgramsofwaterperkilogramofdryair.

Thermo-physicalpropertiesfoundonmostpsychrometricchartsSpecificEnthalpy• Specificenthalpy,alsocalledheatcontentperunitmass,isthesumoftheinternal(heat)energyofthemoistairinquestion,includingtheheatoftheairandwatervaporwithin.Intheapproximationofidealgases,linesofconstantenthalpyareparalleltolinesofconstantwet-bulbtemperature.

SpecificVolume• Specificvolume,alsocalledinversedensity,isthevolumeperunitmassoftheairsample.TheSIunitiscubicmetersperkilogramofair;theotherunitiscubicfeetperpoundofdryair.

Problem

• Humidairat28°Chasadewpointof8°C.Usingthepsychrometricchartprovided,determinethefollowing:relativehumidity,absolutehumidity,wet-bulbtemperature,dry-bulbtemperature,humidvolume,specificenthalpy,andmassofairthatcontains2kgofwater,andvolumeoccupiedbyairthatcontains2kgofwater.

Solution

Usepsychrometric chart.Humidairat28°Chasadewpointof8°C.Usingthepsychrometric chartprovided,determinethefollowing:• Relativehumidity=30%• Absolutehumidity=0.007kgwater/kgdryair• Wet-bulbtemperature=16.5°CFollowtheconstantenthalpylinefromtheintersectionofthedry-bulbanddewpointtemperatures.• Dry-bulbtemperature=28°C• Humidvolume=0.86m3/kg• Specificenthalpy=46kJ/kg−0.3kJ/kg=45.7kJ/kg

Solution

• Massofairthatcontains2kgofwater:

Problem

• Thedry-bulbtemperatureismeasuredas20°Candthewet-bulbtemperatureas15°C.Ifthetotalpressureis1atm,whataretherelativehumidityandtheabsolutehumidity?Usethehumiditychart.

Solution

• Usethepsychrometric chart.

• Relativehumidity=59%

• Absolutehumidityormoisturecontent=0.0087 kgwater/kgdryair

Problem

• Airflowingatavolumetricflowrateof0.80m3/hunderatemperatureof25°Candapressureof3barisfedtoaprocess.Theairhasarelativehumidityof80%.Themolarcompositionofdryairis21%oxygenand79%nitrogen.Determinethemolarflowratesofdryairandoxygenenteringtheprocess.

Solution

• First,wecalculatethepartialpressureofwaterbyusingthedefinitionofrelativehumidity:

Solution

• Themolefractionofwateris0.008mol water/molehumidair,andthemolefractionofdryairis0.992.Themolarflowrateofaircanbeobtainedusingidealgasequationofstateusingthevolumetricflowrategivenintheproblemstatement.Aftersubstitutingtheknowninformationintothisequation,wehave

Problem

• Considertaking1000.0kgofoutsideair,whichisthenheatedtomakeyourapartmentcomfortableinthewinter.Theoutsideairhasadry-bulbtemperatureof10.0°Candawet-bulbtemperatureof5°C.Youwanttheairinyourapartmenttobeatadry-bulbtemperatureof25°Candarelativehumidityof60.0%.• Howmuchwatermustbeaddedtothe1000.0kgofoutsideairtoreachthedesiredhumiditylevel?

Solution

• Usepsychrometric chart.• Initialcondition:10.0°Cdrybulbtemperatureandawet-bulbtemperatureof5°Ccorrolates toapproximately40%humidityand0.003kg/kgdryair• Desiredcondition:adry-bulbtemperatureof25°Candarelativehumidityof60.0%whichmeansweareaimingat0.012kg/kgdryairmoisturecontent.• So,0.003kgmoisture/kgdryairX1000=3kgmoisture• Final,0.012kgmoisture/kgdryairX1000=12kgmoisture• 12-3=9kgmoistureshouldbeadded.