2Air Conditioning System.docx

2Air Conditioning System.docx

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2AirConditioningSystem

LessontwoAirConditioningSystem

Airconditioningforpeopleisthecontroloftemperature,humidity,airmovementandaircleanliness,heatradiationsometimes[e.g.bychilledceiling],normallywithmechanicalmeans,toachievehumanthermalcomfort. 

Airconditioningsystemscanbecategorizedaccordingtothemeansbywhichthecontrollablecoolingisaccomplishedintheconditionedspace.Theyarefurthersegregatedtoaccomplishspecificpurposesbyspecialequipmentarrangement.

Inselectingasuitableairconditioningsystemforaparticularapplication,considerationshouldalsogotothefollowing:

-

-Systemconstraints:

Coolingload,Zoningrequirements,Heatingandventilation

-ArchitecturalConstraints:

Sizeandappearanceofterminaldevices,acceptablenoiselevel,Spaceavailabletohouseequipmentanditslocationrelativetotheconditionedspace,acceptabilityofcomponentsobtrudingintotheconditionedspace

-FinancialConstraints:

Capitalcost,Operatingcost,Maintenancecost

Therearefourbasicsystemcategories:

1  Centralchilledwaterairconditioningsystems-AllAirSystems

1.1Singlezone

1.2Reheat

1.3VariableAirVolume

1.4DualDuct

1.5Multizone

2Centralchilledwaterairconditioningsystems-Air-andWaterSystems

2.1Induction

2.2FanCoil

2.3Two-pipe

2.4Three-pipe

3Centralchilledwaterairconditioningsystems-AllWaterSystems,includingcoolingtowerswhichcanalsobeappliedtosystems1,2above

3.1Fan-coilunits

3.2Centralchilledwaterairconditioningsystemwithfancoilsandotherdevices

3.3Watercoolingtower

4DirectexpansionSystems[i.e.directexpansionofrefrigerant,withoutthechilledwatercoolingmedium]

4.1Windowairconditioners

4.2UnitaryandRooftopAirConditioners

4.3Splittypeandpackageairconditioningsystems

4.4Heatpumps

1.Centralchilledwaterairconditioningsystems-AllAirSystems

Anall-airsystemprovidescompletesensibleandlatentcoolingcapacityinthecoldairsuppliedbythesystem.Heatingcanbeaccomplishedbythesameairstream,eitherinthecentralsystemorataparticularzone.All-airsystemscanbeclassifiedinto2categories:

-

 -Singleductsystems

-Dualductsystems

SystemAdvantages

◆Thecentralplantislocatedinunoccupiedareas,hencefacilitatingoperatingandmaintenance,noisecontrolandchoiceofsuitableequipment.

◆Nopiping,electricalwiringandfiltersarelocatedinsidetheconditionedspace.

◆Allowstheuseofthegreatestnumbersofpotentialcoolingseasonshousewithoutsideairinplaceofmechanicalrefrigeration.

◆Seasonalchangeoverissimpleandreadilyadaptabletoclimaticcontrol.

◆Givesawidechoiceofzonability,flexibility,andhumiditycontrolunderalloperatingconditions.

◆Heatrecoverysystemmaybereadilyincorporated.

◆Allowsgooddesignflexibilityforoptimumairdistribution,draftcontrol,andlocalrequirements.

◆Infringesleastonperimeterfloorspace.

◆Adaptstowinterhumidification.

SystemDisadvantages

◆Requiresadditionalductclearancewhichcanreducetheusablefloorspace.

◆Air-balancingisdifficultandrequiresgreatcare.

◆Accessibilitytoterminalsdemandsclosecooperationbetweenarchitectural,mechanicalandstructuralengineers.

Distributionsystemshaveanumberofimportantcomponents:

1.TheAirHandlingUnitisacabinetthatincludesorhousesthecentralfurnace,airconditioner,orheatpumpandtheplenumandblowerassemblythatforcesairthroughtheductwork.

2.TheSupplyDuctworkcarriesairfromtheairhandlertotheroomsinahouse.Typicallyeachroomhasatleastonesupplyductandlargerroomsmayhaveseveral.

3.TheReturnDuctworkcarriesairfromtheconditionedspacebacktotheairhandler.Mosthouseshaveonlyoneortwomainreturnductslocatedinacentralarea.

4.SupplyandReturnPlenumsareboxesmadeofductboard,metal,drywallorwoodthatdistributeairtoindividualductsorregisters.

5.TheDuctworkisabranchingnetworkofroundorrectangulartubesgenerallyconstructedofsheetmetal,fiberglassboard,oraflexibleplasticandwirecompositemateriallocatedwithinthewalls,floors,andceilings.Thethreemostcommontypesofductmaterialusedinhomeconstructionaremetal,fiberglassductboard,andflex-duct.

6.Flex-ductisinstalledbetweentheregisterandplenumbox,orplenumboxandairhandler,usuallyinasingle,continuouspiece.Whileflex-ducthasfewerseams,theinnerliningandouterinsulatedcoveringcantearorbepinchedclosed.Alsolongerflex-ductrunscanrestricttheflowofair;properdesignandinstallationisveryimportant.

7.Bothmetalandfiberglassductboardarerigidandinstalledinpieces.Fiberglassductboard,likeflex-duct,ismadeofaninsulationmaterial.Ductsarebuiltofsectionsoftheductboard.Theseamsintheductboardshouldbecarefullysealedwithmasticorhighqualityducttape.

8.Rectangularmetalduct,especiallythekindusedforplenumsandlargertrunkruns,isofteninsulatedontheinsidewithfiberglassductliner.Ifitisnotinsulatedontheinside,metalductsshouldbeinsulatedontheoutsideusingafiberglassbattwithanattachedmetalfoilvaporretarder.Theinsulationshouldbeatleasttwoinchesthick,andthevaporbarrierinstalledontheoutsideoftheinsulationfacingawayfromtheduct.

Theseamsintheinsulationareusuallystapledtogetheraroundtheductandthentaped.Alloftheseamsshouldbesealedbeforeinsulationisinstalled.Allreturnandsupplyductslocatedoutsidetheconditionedspace,inattics,crawlspaces,orbasements,forexample,shouldbesealedandinsulated.

9.DuctworkJointsjoinpiecesofductwork.

10.Elbowsaremanufacturedpiecesofductusedforturns.

11.Bootsconnectductworktoregisters.

12.RegistersandGrillesarethecoveringsforductopeningsintotheconditioned

space.

Thesecomponentswillbringabout:

-

-Heatbalance:

Theamountofheatextractedoutoftheairconditionedroom（bythecoolingsystem,exhaustairsystems,buildingleakage,mustbeequaltotheamountofheatgeneratedinsidetheroom（byhumanbeing,electricalappliances,etc.）andtransferredintotheroom（byconductionthroughthebuildingenvelope,radiationviatheglass,hotairleakageintotheroomthroughgapsinwindows,doors,freshairintroducedintotheroom,etc.）i.e.TotalkWgoingintoroom=TotalkWgoingoutoftheroom.

-Airbalance:

Themassflowrateoftheairgoingintotheroom=Themassflowrateofairgoingoutoftheroom.Freshaircomingintotheroom:

2.5l/sperperson,non-smoking,5l/sperpersonforsmokingaccommodation,goodindoorairquality（IAQ）isimportant.

1.1SingleZoneSystem

Theall-airsingle-zoneairconditioningsystemisthebasiccentralsystemwhichcansupplyaconstantairvolumeoravariableairvolumeatlow,mediumorhighpressure.Normally,theequipmentislocatedoutsidetheconditionedspacebutcanalsobeinstalledwithintheconditionedareaifconditionspermit.Typicalapplicationsinclude:

-

-Spacewithuniformloads

-Smallspacesrequiringprecisioncontrol

-Multiplesystemsforlargeareas

1.2ReheatSystem

Thereheatsystemisamodificationofthesingle-zonesystem.Itprovides:

-

-Zoneorspacecontrolforareasofunequalloading.

-Heatingorcoolingofperimeterareaswithdifferentexposures.

-Closecontrolforprocessorcomfortapplications.

Inthereheatsystem,heatisaddedasasecondaryprocesstoeitherpreconditionedprimaryairorrecirculatedroomair.Theheatingmediumcanbehotwater,steamorelectricity.

Advantages:

Closelycontrolsspaceconditions

Disadvantages:

Expensivetooperate

1.3VariableAirVolumeSystem

 Thevariableairvolumesystemcompensatesforvaryingcoolingloadsbyregulatingthevolumeofcoolingairsuppliedthroughasingleduct.

（a）SimpleVariableAirVolume（VAV）

SimpleVAVsystemstypicallycoolsonlyandhavenorequirementforsimultaneousheatingandcoolinginvariouszones.

（b）VariableAirVolume–Reheat

Itintegratesheatingatorneartheterminalunits.Itisappliedtosystemsrequiringfullheatingandcoolingflexibilityininteriorandexteriorzones.Heatingisturnedonwhentheairflowreachesapredeterminedminimum.

 Advantages

a）Whencombinedwithaperimeterheatingsystem,itoffersinexpensivetemperaturecontrolformultiplezoningandahighdegreeofsimultaneousheating-coolingflexibility.

b）Capitalcostislowersincediversitiesofloadsfromlights,occupancy,solarandequipmentofasmuchas30%arepermitted.

c）Virtuallyself-balancing.

d）Itiseasyandinexpensivetosubdivideintonewzonesandtohandleincreasedloadswithnewtenancyorusageifloaddoesnotexceedtheoriginaldesignsimultaneouspeak.

e）Nozoningisrequiredincentralequipment.

f）Loweroperatingcostbecause

Fansrunlonghoursatreducedvolume

Refrigeration,heatingandpumpingmatchesdiversityofloads

Unoccupiedareasmaybefullycut-off

g）Reducednoiselevelwhenthesystemisrunningatoff-peakloads.

h）Allowssimultaneousheatingandcoolingwithoutseasonalchangeover.

1.4DualDuctSystem

Thedual-ductsystememploystwoairductstosupplycoldairandwarmairtoamixingterminalunitwhichproportionsthecoldandwarmairinresponsetoathermostatlocatedintheconditionedspace.Thesystemiswellsuitedtoprovidetemperaturecontrolforindividualspacesorzones.

Advantages（inadditiontothosecommontoallairsystems）

●Systemswithterminalvolumeregulationareself-balancing.

●Zoningofcentralequipmentisnotrequired.

●Instanttemperatureresponseisach