Unit 9 Air Conditioning System.docx

Unit 9 Air Conditioning System.docx

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Unit9AirConditioningSystem

UnitNineAirConditioningSystem

I.Text

1.Themeaningofairconditioning

Anairconditioningsystemisanassemblyofvariouscomponentswhichoperateinacontrolledmannertoproduceaspecifiedconditionoftheairwithinaspaceorbuilding.Toprovidecompleteairconditioningasystemmustaccomplishallthefollowing:

heat,humidify,cool,dehumidify,ventilate,filter,andcirculate.

1.1Comfortconditioning

Thedesignspecificationforacomfortconditioningsystemisintendedtobetheframeworkforprovidingacomfortableenvironmentforhumanbeingsthroughouttheyear,inthepresenceofsensibleheatgainsinsummerandsensibleheatlossesinwinter.Dehumidificationwouldbeachievedinsummerbuttherelativehumidityintheconditionedspacewouldbeallowedtodiminishaswinterapproached.Therearetworeasonswhythisisacceptable:

first,humanbeingsarecomfortablewithinafairlylargerangeofhumidity,fromabout65percenttoabout20percentand,secondly,ifsingleglazingisuseditwillcausetheinnersurfacesofwindowstostreamwithcondensedmoistureifitisattemptedtomaintaintoohighahumidityinwinter.1

Thus,asystemmightbespecifiedascapableofmaintaininganinternalconditionof22℃dry-bulb,with50percentsaturation,inthepresenceofanexternalsummerstateof28℃dry-bulb,with20℃wet-bulb,decliningtoaninsideconditionof20℃dry-bulb,withanunspecifiedrelativehumidity,inthepresenceofanexternalstateof-2℃saturatedinwinter.

1.2Industrialconditioning

Herethepictureisquitedifferent.Anindustrialorscientificprocessmay,perhaps,beperformedproperlyonlyifitiscarriedoutinanenvironmentthathasvaluesoftemperatureandhumiditylyingwithinwelldefinedlimits.Adeparturefromtheselimitsmayspoiltheworkbeingdone.Itfollowsthatachoiceoftheinsidedesignconditionisnotbasedonastatisticalsurveyofthefeelingsofhumanbeingsbutonaclearlydefinedstatementofwhatiswanted.

Thus,asystemmightbespecifiedtohold21℃±0.5℃,with50percentsaturation±2.5percent,providedthattheoutsidestatelaybetween29.5℃dry-bulb,with21℃wet-bulband–4℃saturated.

2.SystemCategories

Airconditioningsystemsmaybeclassifiedascentral-stationorunitarysystems.Acentral-stationsystemisoneinwhichthecomponentsofthesystemareallgroupedtogetherinonecentralmechanicalroomandtheconditionedairissentfromtheretothespacestobeconditionedthroughextensiveductwork.

Unitarysystemsmakeuseoffactory-assembledandtestedairconditioningunits.Theseareusuallyinstalledinorimmediateadjacenttoazoneorspacetobeconditioned.Thesourceoftheheatandtherefrigerationmaybeineachairconditioningunitorina“central”mechanicalroom.

Anotherpossibleclassificationdividesairconditioningsystemsintocoolingsystemsandheatingsystems.Coolingsystemsmayinturntobeclassifiedintermsofthecoolingmediumandcoolingsurfaceused.Heatingsystemsareclassifiedaswarm-airsystemsorradiant-heatsystem.

Thebasicsystemsandtheirseveralvariantswillbediscussedinthefollowingsection.

2.1Central-stationSystem

Acentral-stationyear-roundairconditioningsystemisonewhichusuallyservesasinglelargebuildingorspace.Theterm“central-station”isordinarilynotappliedtosystemsoflessthanabout25tonsofcoolingcapacity,circulatingabout10,000cfmofconditioningair.Buildingswhosedemandislessthanthisareusuallymoreeconomicallyairconditionedbyunitaryequipments,allfield-assembled.

Ageneralarrangementofthecomponentpartsofacentralsystemsuitableforyear-roundairconditioningwithclosecontroloftemperatureandhumidityisillustratedinFigure9.1.Itusuallyconsistedoffollowingcomponents:

（1）Coilsforcoolinganddehumidifying,eitherforchilledwaterorfordirectexpansionoftherefrigerant

（2）Coilsforheating,suppliedwithsteamorhotwater

（3）Bloweranddrivingmotor

（4）Spraysforcoolinganddehumidifyingorforwashingtheair

（5）Air-cleaningequipmentsuchasfilters,electrostaticprecipitators,odor-removingequipment,andgermicidallamps

（6）Awidevarietyofcontroldevices.

Fig.9.1EquipmentArrangementforCentralSystems

Thecentral-stationairconditioningplantmustbesuppliedwitheitherchilledwaterorrefrigerantforthecoolingcoilsandwithhotwaterorsteamfortheheatingcoils.Refrigerationcondensingunitsandsteamorhot-waterboilersforthesepurposesmaybeinthesamemechanicalroomwiththeairconditionerormaybeinanadjoiningroomorinthebasement.

2.2Capacityconsiderations.

Itwillberecalledthatheattransferbetweenairandmetalsurfacesrequiresratherlargesurfacesareas.Heatingandcoolingcoilshandlingairatfacevelocitiesofaround500fpmmustp