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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