Lab_report_CMT348_1322748

Page1of13

ShortLaboratoryReport2015

MP2.2VapourCompressionRefrigerationCycle

ChloéMarieTaylor

1322748

LabGroup:Mech18

DateofExperiment:10thNovember2015DateofLabReport:24thDecember2015

Page2of13

Contents

1–Summary------------------------------------------------------------------------------------------------------------22–Introduction--------------------------------------------------------------------------------------------------------33–AimsofExperiment----------------------------------------------------------------------------------------------34–TheoryofVapourCompressionRefrigerationCycles--------------------------------------------------.45–Set-upandProcedure-------------------------------------------------------------------------------------------66–ObservationsandResults---------------------------------------------------------------------------------------67–DiscussionandAnalysisofResults---------------------------------------------------------------------------88–SourcesandDiscussionofError------------------------------------------------------------------------------119–Conclusion----------------------------------------------------------------------------------------------------------1210–References---------------------------------------------------------------------------------------------------------1211–Appendix------------------------------------------------------------------------------------------------------------13

1–Summary

Anairconditioningunitwasusedfortheexperiment.Thepurposeoftheexperimentwastostudytheperformanceofavapourcompressionrefrigerationcyclebycalculatingtheunit’scoefficientof performance (COPR). Air was heated, humidified, cooled and dehumidified and then reheatedduringtheprocess.Temperaturereadingsfortheairandtherefrigerantandworkingparametersoftheunitwererecordedduringtheexperiment.Thestateoftheairateachpointwasplottedonapsychometricchart,whichshowedthattheunitdidnotreducethehumidityoftheairasmuchasexpected.Thiswassuggestedtobeduetotherefrigerantnotremovingasmuchheatfromtheairasexpected.Therefrigerantcyclewasplottedonap-hdiagram,wheretheidealandactualcycleswereshownalongwith irreversibleprocesses. Itwas shown that theactual cyclemovedmore into thesuper-heatedregionmeaningmoreheatwasrejectedintotheenvironmentthantheidealcycle.

The cooling loads from the air and into the refrigerant were calculated, giving values of2.55kWand2.78kWrespectively.Thedifferencebetweenthesevalueswassaidtobeduetotheunitbeingbadlyinsulatedhencetherefrigerantabsorbedheatfromtheenvironmentaswellastheair.COPRvaluesfortherefrigerationcyclewerecalculatedbasedontheairandtherefrigerantcoolingloads,yieldingvaluesof1.19and1.3respectively.TheCOPRbasedontherefrigerantwassaidtobemoreaccurateasitdidnothavetheinaccuraciesofthewetbulbtemperaturereadingsthattheairbasedCOPRhad.Sourcesoferror,howtominimisethemandimprovementsfortheexperimentandtheairconditioningunitarediscussed.

Page3of13

2-IntroductionIntroduction Heat pumps are very important in today’s society, being the basis for refrigerators andfreezersaswellassomeheatingmethods.Heatpumpsaremachineswhichmoveheatfromalowertemperature reservoir (called the source) to a higher temperature reservoir (called the heat sink)drivenbyaworkinput.Dependingontheaim,theycanbeheatersbyrejectingheatintotheheatsink,orrefrigeratorsbyabsorbingheatfromthesource.Theirperformanceismeasuredbytheratioofcoolingobtainedtoworkrequired,calledthecoefficientofperformance(COP)showninequation1below.

!"#$ = '() ,+,. 1

Where:'(=coolingload(heatremovedfromsource)(W))=Workinput(W)

COPvalueswillgenerallybeabove1asmoreheatwillberemovedfromthesourcethanworkisputinduetothenatureofthecycle(discussedin3–TheoryofVapourCompressionRefrigerationCycles).

ThisexperimentaimstofindtheCOPofavapourcompressionrefrigerationcycleasitisusedtocoolanddehumidifyair.Irreversibilitiesoftheprocessandotherpointsofinterestwillbediscussed.

3-AimsofExperiment

Anairconditioningunitoperatingavapourcompressionrefrigerationcyclewillbeusedtoheatair to a warm and humid state, then dehumidify and cool air. Temperatures of the air and therefrigerantworkingfluidwillberecordedfromtheairconditioningunit.Otherworkingparametersincludingdifferentialpressures,massflowratesandvoltage/currentvalueswillberecorded.Thesesetofresultswillallow:

• Thepathoftheairconditioningprocesstobeplottedonapsychometricchart• Thestateoftheairateachpointtobefoundandcommentstobemadeonthefinalquality

oftheair• Thecoolingloadfromtheairduringtheairconditioningcycletobecalculated• Theidealandrealcyclefortherefrigerantonap-hdiagramtobeplotted• Thecoolingloadfortherefrigerantduringthecycletobecalculated• Thecoefficientofperformanceoftherefrigerator(COP)tobecalculated

Page4of13

4-TheoryofVapourCompressionRefrigerationCyclesCarnotCycle Thisexperimentusesavapourcompressionrefrigerationcyclewhichcoolsairdown.Thoughthe air is heated and cooled in the air conditioning unit it is the refrigeration cycle (using R12refrigerantastheworkingfluid)whichisthemainfocusoftheexperiment.Therefrigerationcycleremoves heat from the air and rejects it to the environment, acting as a reverse heat engine –commonlycalledaheatpump.HeatpumpsarebasedaroundreverseCarnotcycles,whichareexactlythesameasCarnotcyclebuttheprocessdirectionsandhenceworkandheatinputsandoutputsarereversed.Figure1and2belowarethepressure-volume(p-v)andtemperature-entropy(T-s)diagramsforareverseCarnotcycle.

Figure1:PressureVolumediagramshowingFigure2:Temperature-Entropygraphwith

Carnotcycle.sssssssssssssssssssssssddfffffffffffhsslkcsdasaturationcurveshowingCarnotcycle.

Theprocessesrelatingtothepointsinfigure1and2aredetailedbelow:1-2:Isentropiccompression–workisdoneontheworkingfluidataconstantentropy,resultinginanincreaseintemperature

2-3:Isothermalheatrejection–heatisrejectedintothehightemperaturereservoir3-4:Isentropicexpansion–workisdonebytheworkingfluidataconstantentropy,resultinginadecreaseintemperature

4-1:Isothermalheatabsorption–heatisabsorbed('()fromalowtemperaturereservoir

Notethatfigure2showsthesaturationcurveforthecycle.Enclosedinthesaturationcurvethe refrigerant is a mixed state of vapour and liquid. To the left of the saturated liquid line therefrigerant is a sub-cooled liquid, to the right of the saturated vapour line the refrigerant issuperheatedvapour.Theimportanceofthisisdiscussedbelow. TobasearefrigerationcyclearoundthereverseCarnotcycle,aworkingfluidmustbefoundthatiscapableofisothermalheatrejectionandabsorptionatthetemperaturesofthehighandlowtemperaturereservoirs(THandTL).Thiscanbeobtainedbyusingfluidsthatcondenseandevaporatearoundthesetemperaturesasphasechangesareprocessesinwhichheatisinputoroutputwithnotemperature change. This can be understood more by looking at the Carnot cycle encased in asaturationcurve(figure2).Bychoosingarefrigerantwhichhassaturationtemperatures(THandTL)attherequiredpressures,thephasechangesoftherefrigerantcanbeusedtofacilitatetheisothermalprocesses2-3and4-1.Therefrigerantwillhavealowboilingpointallowingittochangephase(andabsorbheat)fromanalreadycooltemperature.

P

V

11

3

4

2

T

s

1

3

4

2TH

TL

Page5of13

Practicalvapourcompressionrefrigerationcycle InpracticethereverseCarnotcycleisadjustedtothevapourcompressionrefrigerationcycleshownbelowinfigure3.Theworkingfluidisarefrigerantthatsuitsthehighandlowtemperaturereservoirstemperatures.

Figure3:TemperatureEnthalpydiagramshowing

vapourcompressionrefrigerationcycle.

Theprocessesinfigure3areasfollows:1-2:Isentropiccompression–therefrigerantiscompressedtoasuper-heatedvapour2-3:Heatrejection–thesuperheatedvapouriscooledtothesaturationtemperature(TH)thenrejectsheatatconstanttemperatureuntilitreachesthesaturatedliquidpoint3-4:Expansion – the refrigerant is cooledquicklybyuseofa throttlevalve toTL, forminga liquidvapourmixture4-1: Heat absorption – the refrigerant absorbs heat ('() from low temperature reservoir until itreachesthesaturatedvapourpoint

Thedifferences in figure3aremainlydueto thepracticalitiesof runninga reverseCarnotcycle.Stage1-2isthemostnotablydifferentasitisshiftedcompletelyoutofthesaturationcurve.Compressorsdonothavehighefficiencyandrequiremoremaintenancewhentheyoperatewithamixedstatemedium,hencethecompressionstageisshiftedintothesuperheatedvapourregion.Thismeansthatprocess2-3cannolongerbeisothermalasthesuper-heatedvapourmustfirstbecooledtoitssaturationtemperature(TH)beforeitcanundergothephasetransitiontothesaturatedliquid.

Process3-4isanirreversibleexpansionprocessusingathrottlingvalve.Throttlingvalvesareusedratherthanisentropicexpansionengines(whichexpandsaturatedliquids)asthereisonlyasmallamountofworkoutputtobegained–thecostsforanenginewouldnotbejustifiable.Thethrottlevalve reduces the pressure of the saturated liquid abruptly, causing flash evaporation (partialevaporationofliquidduetosuddendropinpressure)oftheliquidandreducingitstemperature.Itisideallyaconstantenthalpyprocess.

Figure3showsanidealrefrigerationcycle.Inrealitythecompressionprocess(1-2)islikelytonotbeisentropichenceanincreaseinentropywillbeseen.Thiswillalsomeanthatthesuper-heatedvapourwillalsoreachahighertemperaturemeaningtheheatrejectionprocesswillhavetoreducethetemperaturemorebeforereachingthesaturationtemperature(TH).Therewillbeothergenerallyfrictionallossesduringtheprocesswhichwillcancausepressuredropswithintheprocesses.

T

s

1

TH

TL

4

2

3

Page6of13

5-Set-upandProcedureEquipment

Theapparatususedisanairconditioningunit.Theunitisdividedintostationswherevariousreadingscanbetaken.AtstationAairistakeninfromtheroomandenterstheunit.StationBisformixingre-circulatingairthathasalreadypassedthroughtheunitinwiththefreshairfromtheroom.Airwillnotbere-circulatedduringtheexperimentsoanyreadingsfromstationAandBshouldbeidentical.BetweenstationBandCelectricpre-heatersandsteaminjectionoccurstobringtheairtothe conditions of a warm humid climate. Between station C and D the air passes though anevaporator where it is cooled and excess moisture is condensed out. The evaporator usesdichlorodifluoromethane(R12)refrigerant–thecycleoftherefrigerantisofkeyimportancetotheexperimentandwillbediscussedinmoredetailinSection7–DiscussionandAnalysisofResults.Between stationD and E there is re-heating of the air to increase its temperature and reduce itshumidity.Eachstationhastwothermocoupledthermometers–awetandadrybulb.Bytakingthesetworeadingswecandeterminethestateoftheair(itsrelativehumidity).Otherreadingsabouttheunit such asmass flow rates, current in compressors, relative pressures and temperatures in therefrigerantcyclewillberecorded.

Set-upandProcedure Hotandhumidairmustbeproducedfortheairconditioningcycle.Thisairwillthenbecooledanddehumidifiedandthenreheatedtoarequiredtemperatureof20°C.

Theunitisturnedonandairflowissettoaminimumof0.07kgs-1(anylowerandthereliabilityof thewetbulb thermocoupleswillbepoor).Theboilersandrefrigeratorunitsare turnedonandallowedtostabiliseforabout5minutes.Oncesteamisbeingproducedbytheboilers,twoofthethreeboilersareturnedoffandthere-heaterbetweenstationDandEisturnedon,adjustingitsothattheairbeforethecoolingisheatedto25°Candarelativehumidityof90%(awarmhumidclimate).Theunit is then left again to stabilise for 10minutes, after which the re-heater is adjusted to give atemperatureofabout20°Candrelativehumidityof50-60%afterthecoolingasitexitstheunit.Afterallowingtheunittostabilisefor10minutesoncemore,thermocouplereadingsandotherparametersateachstationcanbetaken.Theresultscanbeseeninthesectionbelow.

6-ObservationsandResults

Thetemperaturereadingsfromthethermocouples(wetanddrybulbs)wererecordedandshownintable1.Thecorrectedvaluesforthewetbulbtemperaturearealsoshown–thecorrectionvaluewasfoundusingfigure6whichshowstherelationshipbetween‘Screen’and‘Sling’wetbulbtemperatureswhen the dry bulb temperature is known. Figure 6 and a brief explanation of howcorrectionvaluesareobtainedisshownintheAppendix.

Station DryBulbTemperature(°C)

WetBulbTemperature(observed)(°C)

WetBulbTemperature(corrected)(°C)

A–Intake 20.0 16.0 16.4B–AfterMixing 21.0 17.0 17.4C–AfterPre-heatingandSteamInjection

27.5 27.5 27.5

D–AfterCoolingandDehumidification

18.3 18.3 18.3

E–AfterRe-heating 24.0 22.0 22.2Table1:Dry,Wetbulbandcorrectedwetbulbtemperaturereadingsfromeachstation.

Page7of13

NotethatwhenthereadingsforstationCandDweretakenthewetbulbtemperatureswerehigherthanthedrybulbtemperatures.Thewetbulbismeanttobelessthanorequaltothedrybulbso the wet bulbs were given the same temperature as the dry bulb. No correction values wereobtainedforthesetemperatures.Thisisfurtherdiscussedinthesection8-SourcesofError.

TheairatAandBisexpectedtobeidenticalasthereisnochangeintheairbetweenthesetwopoints.HoweveratstationBthereisanincreaseintemperatureof1°Cforboththewetandthedry bulb. This could be due to the air conditioning unit acting as an insulation to the outsideenvironmentsoairfurtherintheunit(stationB)willbewarmerwhereasstationAisnotwellenclosed.Itcouldalsobeduetoatooshortstabilisationperiod.

Thetemperaturereadingsfromtherefrigerantcycleareshownintable2.Thesewillbeused

inSection7–DiscussionandAnalysisofResultstoplotamorerealisticcycleonap-hdiagram.

R12Temperature: Temperature(°C)BeforeExpansionvalve 33AfterExpansionvalve 0AfterEvaporator 21.5AfterCompressor 92Table2:TemperatureofR12refrigerantatvariousstages

throughcycle.

Thevoltagesupply,andcurrentsthroughtherelevantequipmentarerecorded,shownintable3.

ReadingVoltage(V) 233Pre-heatercurrent(A) 2Boilercurrent(A) 8Compressorandcoolingfancurrent(A) 9.2Re-heatercurrent(A) 2Circulatingfancurrent(A) 0.8Table3:Voltageandcurrentreadingsfromtheunit.

Using the value of voltage and compressor and cooling fan current we can calculate theamountofpowerthecompressorandcoolingfanrequire.Thisiscalculatedusingequation2belowandweobtainapowerof2.144kW.

#/0+1 = 23 = 9.2×233 = 2143.6),+,. 2Thisvalueofpoweristheworkinputtotherefrigerantcycle.Itwillbeusedlatertofinda

valuefortheCOPRoftherefrigerantcycle.

Othervariablesoftheunitwhichwererecordedareshownintable4.TheywillbeusedinlatercalculationsfindingtheCOPandmassflowratesoftheairandrefrigerant.

Reading UnitsOrificeDifferential(intake) mmH20 0.7OrificeDifferential(outlet) mmH20 0.7EvaporatorPressure(gauge) kPa 200CondenserPressure(gauge) kPa 750Refrigerantmassflowrate kgs-1 0.024

Table4:Orificedifferentialintakeandoutlet,evaporatorandcondenserpressureandrefrigerantmass

flowrateasrecordedfromtheunit.

Page8of13

7-DiscussionandanalysisofResults

AirCalculationsPsychometricchart

Thetemperaturereadings(dryandwetcorrected)fromtable1areplottedonapsychometriccharttoenablethestateoftheairateachstationtobefound.Thisisshowninfigure4below[1].Eachpointhasbeenlabelledandfromthegraphwecanfindthespecificenthalpy,%saturationandspecificvolumeoftheairateachstation.Thedataobtainedfromthegraphisshownintable5below.

Figure4:Psychometric chart forairat101.325kPawithpointsofairateach station

plottedon.[1]

StationA StationB StationC StationD StationESpecificEnthalpy

(kJ/kg)45 49 87 50 66

%Saturation 72 70 100 100 88SpecificVolume

(m3/kg)0.843 0.848 0.884 0.842 0.863

Table5:Specificenthalpy,%saturationandspecificvolumeforairateachstationreadfrom

figure4.

WecanseethatthestatesofAandBareverysimilar–thisisexpectedandwasdiscussedearlierinsection5-Set-upandProcedure.StationCwasaimedtohavea%saturationof90andatemperatureofaround25°C,table5andtable1showthatactualvalueswere100%saturationand27.5°C.StationEwasaimedtobeat50-60%saturationandaround20°C,table5and1showthatactualvalueswere88%and22.2°C.Theactual%saturationatEisconsiderablyhigherthanexpected,showing there-heatingprocessbetweenDandEdidnot reducetherelativehumidityasmuchasexpected.Thefinaltemperatureoftheairleavingtheconditioningunit(stationE)was2.2°Cabovetheaimedvalue,suggestingthattherefrigerationprocessmaynothavereducedthetemperatureasmuchaswasrequired.Hadtherefrigerationcyclecooledtheairmore,moreheatcouldhavebeen

Page9of13

addedbackduringthere-heatingprocesswhichmayhavedecreasedthe%saturationmore,givingacloservalueofboth%saturationandtemperaturetotheaim.

Otherdifferencesbetweentheaimedandactualvaluesarecommentedoninthesection8-SourcesofError. CoolingLoad Thecoolingloadontheairistheamountofheatremovedfromitasitpassesthroughtheheatexchangerwherethecooledrefrigerantflows.Itcanbecalculatedusingequation3below.

'( = :(ℎ= − ℎ?),+,. 3Where::=massflowrate(kgs-1)(air)ℎ==specificenthalpyofairbeforecooling(stationC)(kJ/kg)ℎ?=specificenthalpyofairaftercooling(stationD)(kJ/kg)

Thevaluesofℎ=andℎ?fromtable5are87kJ/kgand50kJ/kgrespectively.Themassflowrateofaircanbecalculatedusingequation4shownbelow.

: = 0.0757 ∆EFG,+,. 4

Where:∆E=intakeorificedifferentialpressure(mmH20)FG=specificvolumeatintake(stationA)(m3/kg)

Thevaluefor∆Ewasrecordedtobe0.7mmH20(table4)andthevalueofFGwasfoundtobe0.843m3/kg(graph1andtable5).Thesetwovaluescanbesubstitutedintoequation4andgiveavalue of 0.0690kgs-1 for the airmass flow rate. Substituting themass flow rate and both specificenthalpiesforstationDandC(table5)intoequation3acoolingloadontheairof2.55kWisobtained.RefrigerantCalculationsRefrigerationCycleonpressure-enthalpyDiagram Thereadingsthatweretakenduringtheexperimentcanbeusedtoplotap-hdiagramfortherefrigerantcycle.Apressureenthalpy(p-h)diagramshowninfigure5[2]canthenbeusedtofindtheenthalpyoftherefrigerantateachstageandhencethecoolingloadcanbecalculated.ThiscanthenbeusedtocalculatetheCOPRoftherefrigerationcycle.

A pressure enthalpy diagram for dichlorodifluoromethane (R12)was used, by plotting thepointsthechangesinphaseoftherefrigerantcanclearlybeseen.Notethatonthisgraphtherearemultiple axes. In order to plot the points the absolute pressures at which the condenser andevaporatorarerunningonmustbecalculated.Thegaugerunningpressuresoftheevaporatorandcondenserwererecordedtobe200kPaand750kParespectively(table4).Toconvertthevaluestoabsolutepressuretheatmosphericpressure(takenas100kPa)isaddedtoeachvalue.Fromthistheevaporatorpressureisfoundtobe300kPaandthecondenserpressureisfoundtobe850kPa. Eachpointisplottedonthediagram(figure5),notethatthesamenomenclatureforthepointsis used as in the theory section (4) above. Point one represents the refrigerant as a superheatedvapourasitexitstheevaporator.Itisplottedat0.3bar(evaporatorpressure)andonthesaturatedvapour curve. Process 1-2 is a constant entropy compression of the refrigerant to the condenserpressure(0.85bar),sopoint2isplottedat0.85bar(condenserpressure)andatentropyaspoint1(0.7kJ/kg/K).Process2-3isacondensingprocesswheretherefrigerantiscooledandphasechangesintoaliquid.Point3canbeplottedat0.85bar(condenserpressure)andonthesaturatedliquidcurve.Process 3-4 is a constant enthalpy expansion process where the refrigerant is bought to theevaporator pressure (0.3 bar) so point 4 is plotted at the same enthalpy as point 3 and on theevaporatorpressureline.

Page10of13

Point 1 has been plotted assuming that the evaporator only heats the refrigerant to thesaturatedvapourpoint.Theactualpoint1(1a)canbeplottedonthegraphusingthetemperatureoftherefrigerantasitexitstheevaporatorwhichfromtable2isfoundtobe21.5°C.Notethatthisismuchhigherthantheidealtemperaturevalueof0°C.It isseenthatpoint1a is inthesuperheatedregioninthep-hdiagram,meaningtheevaporatorhasheatedtherefrigerantmorethanisexpected.Theimportanceofthiswillbediscussedwhencalculatingthecoolingloadontherefrigerant(laterinsection).

Point2hasbeenplottedassumingtheprocess1-2isisentropic(constantentropy).Inrealityitwillnothavebeenanisentropicprocess,therewillhavebeenasmallamountofheattransferintotherefrigerantresultinginahigherfinaltemperature.Amorerealisticpoint2canbeplottedbyusingthetemperatureoftherefrigerantattheendofthecompressionprocess.Fromtable2thevalueisfoundtobe92°Candcanbeplotted(aspoint2a)onthecondenserpressureline(0.85bar)usingaconstanttemperature line. It isseenthattheactualpoint(2a) isfurthertotherightthanthe idealisentropicpoint(2),astheidealpointisatatemperatureof40°C.Thismeansthatmorecoolingisneeded to cool the superheated refrigerant to a saturated vapour, and also represents anirreversibilityinthecycle.

Point1aand2ahavebeenconnectedanddrawnonfigure5,toallowcomparisonbetweentheactualandidealcycle.

Figure5:Pressure-enthalpydiagramshowingtherefrigerantcycle.Notethatpoint1

and 1a and the isentropic (2) and actual (2a)points of the compression are shown.

Dashedlinesrepresentirreversibleprocesses.[2]

Irreversibilitiesinthevapourcompressionrefrigerationcyclehavebeenshownonfigure5bydashedlines.Process1ato2aisirreversibleasitdoesisnotanisentropicprocess(duetotherebeingsmallamountsofheattransfer)henceitcannotbereversed.Process3to4isirreversibleduetothenatureoftheairconditioningunit,thiswasdiscussedinSection4–TheoryofVapourCompressionRefrigerationCycles.

Speci�c Enthalpy (kJ\kg)

Ab

solu

te P

ress

ure

(M

Pa

)

Constant Speci�c

Entropy (kJ\kg\K)

4

3 2 2

11 a

a

Page11of13

CoolingLoad Thecoolingloadontheaircanbecalculatedusingequation3fromabove,notethatℎ=andℎ?correspondtothespecificenthalpiesoftherefrigerantatpoints1and4onfigure5–186kJ/kgand70 kJ\kg respectively. These values and the mass flow rate of the refrigerant from table 4 of0.024kgs-1canbesubstitutedintoequation3,givingacoolingloadof2.78kW.Notethatthisislargerthanthecoolingloadcalculatedfortheair(2.55kW),showingthattherefrigerantgainedmoreenergythantheairintheunitlost.Thisdifferenceisfurtherdiscussedinthesection8-SourcesofError.CoefficientofPerformance TheCOPRcanbecalculatedfortherefrigerationunitusingequation1shownbelow.

!"#$ = '() ,+,. 1

Where:'(=coolingload(W))=Workinput(powerofthecompressor)(W) The work input into this cycle is the compressor, its power was calculated in section 6 -ObservationsandResultsandwas found tobe2.144kW.Thisand thevalue for cooling load fromabove(2.78kW)canbesubstitutedintoequation1yieldingaCOPRof1.30.ThisCOPRvaluemeansthatforevery1kWofworkinput,1.3kWofheatwillberemovedfromtheair. TheCOPR canalsobe calculatedusing the cooling loadbasedon theair’s calculations. Thecompressorpowerandthecoolingloadbasedontheair(2.55kW)canbesubstitutedintoequation4togiveaCOPRof1.19.ThisissmallerthantheCOPRbasedaroundtherefrigerant,andisduetothecoolingloadoftheairbeinglowerthanthecoolingloadfortherefrigerant.ThisCOPRvaluemeansthatforevery1kWofworkinput,1.19kWofheatwillberemovedfromtheair.Thesignificanceofthisisdiscussedinsection8–SourcesandDiscussionofError.

8-SourcesandDiscussionofError As this experiment was mainly descriptive, it is difficult to quantify any errors. No fits orexpectedtrendscanbecomparedtotheresults,insteadthesystematicandexperimentalerrorsandimprovementsfortheexperimentandtheairconditioningunitarediscussed. Theairconditioningunitwasusedinalargeroomwithotherlargepiecesofequipmentrunningnearby.Intermittentconstructionworkwasgoingonintheroomadjacent,meaningthatthequalityoftheairmayhavefluctuatedduringtheexperiment.Itisalsopossiblethattheairhadahigherdustandparticulatecontentthannormal.Theexperimentwasconductedwithina2hoursession,thistimeconstraintlimitedthetimethatcouldbeusedforstabilisationoftheunit.Iftheexperimentweretobedoneagain,anisolated,airconditionedandfilteredroomshouldbeusedandlongertimesshouldbeallowedforthestabilisationoftheunit. Allreadingsweretakenfromneedlescaleswhichwerefluctuating,increasingtheerrorintheresults.Theunitwasnotcalibratedbeforeusageandhencetheresultsobtainedmayhavesystematicerror. A digital scalemay have increased the precision of the results obtained though thiswouldrequireadjustmentontheunit. Insection6theresultsobtainedshowedthatfortwooftherecordedtemperaturesthewetbulbwashigherthanthedrybulbtemperature-thismayhavehappenedduetotheunitnothavingenoughtimetofullystabilise. Insection7itwasshownthattheaimedtemperatureandsaturationsatstationsCandEweredifferenttothevaluesaimedfor.Thiswassuggestedtobeduetotherefrigerationcyclenotremovingasmuchheatfromtheairasexpected.Thisissupportedbythefactthatthecoolingloadontheairwasfoundtobelessthanthecoolingloadgainedbytherefrigerant(2.55kWto2.78kW,section7),showingtherefrigerantwasremovingmoreheatthanwaslostfromtheair.Theextraheatgainedbytherefrigerantcouldbeduetoitalsoabsorbingheatfromthesurroundingenvironmentastheunit

Page12of13

maynotbewellinsulatedonthesectionwheretheheatexchangeoccurs.Theextraheatabsorptionisalsosupportedbytheactualcycleplottedonthep-hdiagramwheretheevaporatorincreasesthetemperatureoftherefrigerantaswellaschangesitsphase(pushingitintothesuperheatedregion). TwoCOPRvalueswerecalculated,oneusingthecoolingloadtotherefrigerant(1.3)andonefromthecoolingloadfromtheair(1.19).TheCOPRbasedontheairismoreconservativeasitusesthe cooling load that was directlymeasured from the air and is the desired outcome of the air-conditioningprocess.HowevertherefrigerantbasedCOPRismoreaccurateasitdoesnothavetheinaccuraciesinmeasurementofthewetbulbreadingsastheairCOPRdoes. TheCOPRvaluesobtainedfortheunitcouldbeimprovedinanumberofways.Fromequation1itisclearthattheCOPvaluecanbeincreasedbymakingthecoolingloadlarger,ortheworkinputsmaller.Thecoolingloadcouldbemadelargerbyinsulatingtheheatexchangesectiontoensurethatallheatabsorbedbytherefrigerantisfromtheair.Itcouldalsobeincreasedbyusingacontra-currentflowoftherefrigerantandairwhentheexchangetakesplaceasthismaintainsahighertemperaturegradient increasing the amount of heat exchange.Work input could be reduced by using amoreefficientcompressor.

9–Conclusion Theresultsobtainedallowedustoplotthestateoftheaironapsychometricchart.Itwasshownthatthe%saturationsoftheairweredifferenttothevaluesthatwereaimedfor,thiswasattributedtotherefrigerantremovinglessheatfromtheairthanexpectedhenceremovinglesshumidity.Thetemperaturesoftheairwererelativelyclosetotheaimedvalues,thoughthefinaltemperatureoftheairwas2.2°Cabovetheaimedvalue,againsuggestingthattherefrigerationcycledidnotreducetheair’stemperatureenough. Therefrigerantcyclewasplottedonap-hdiagram,showingtheidealandactualcyclealongwithirreversibilities.Theactualcyclemovedintothesuper-heatedvapourregionofthegraphmuchmorethantheactualcycle,meaningthatmoreheatwasrejectedtotheenvironmentthanexpected.The cooling loads on the air and to the refrigerant were calculated, giving 2.55kW and 2.78kWrespectively.Thishigherrefrigerantcoolingloadwassaidtobeduetotheairconditioningunitnotbeing well insulated so heat was removed from the surrounding environment as well as the air. COPRvalueswerecalculatedbasedonthecoolingloadsfortheairandtherefrigerant,yieldingvaluesof1.3and1.19respectively.TheCOPRbasedon theair’scooling loadwassaid tobemoreconservativeasitusedasmallercoolingloadandthedesiredoutcomeoftherefrigeration,howeverit is more inaccurate as it includes the measurements of the wet bulb temperatures which areinherently inaccurate. Improvements for theairconditioningunitwerediscussed includingusingamoreefficientcompressorandacounter-currentflowintheheatexchanger.

10–References[1]-HeikalMorganR.,MillerA.J.(2011).AIRCONDITIONING.Available:http://www.thermopedia.com/content/550.Lastaccessed24thDec2015.Edited.[2]-UniversityofBirmingham(2015).LaboratoryExperimentMP2.2.Appendix5–EnthalpydiagramforR12.p10.Edited. [3]-UniversityofBirmingham(2015).LaboratoryExperimentMP2.2.Appendix6:Relationshipbetween“Screen”and“Sling”wetbulbtemperatures.p11.Edited.

Page13of13

11–AppendixRelationshipbetween“Screen”and“Sling”wetbulbtemperatures

Figure6:Relationshipbetween‘Screen’and‘Sling’wetbulbtemperatures.[3]

ForthereadingsfromstationAvaluesof20°Cand16°Cwereobtainedforthedryandthe

wetbulbrespectively.Toobtainthecorrectionvalueforthewetbulbtemperaturealinewasdrawnupfrom16°Conthexaxisuntilthedrybulbtemperatureof20°Cwasreached.Alinewasthendrawnhorizontallyandweobtainacorrectionvalueof0.4°C.ThesameprocesswasusedforeachsetofthermocoupledreadingsandthecorrectedvaluesareshowninTable1.