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Personseatedatrest,350Btu/HrSeated,LightOfficeWork,420Btu/HrStandingLightWork,640Btu/HrWalking3mph,1040Btu/HrBowling,960Btu/HrHeavyFactoryWork,1600Btu/HrHeavyAthletics,1800Btu/Hr
Theseunitsexpresstheefficiencyofaheatingorcoolingsource.ThemostimportantunitsofHVACsystemfromtheelectricaldesignerpointofvieware:
1- COP– “CoefficientofPerformance”:ItisthemeasurechillerefficiencymeasuredinBtuoutput(coolingcapacity)dividedbyBtuinput(electricpower).Typicalvaluesare2– 4.
Coolingcapacityisspecifiedintonsofrefrigeration;1tonisequivalentto12000Btuperhour.
1kWhofelectricpowerisequivalentto3412Btuperhour;multiplyingtheCOPby3.412yieldsenergyefficiencyratio.
BTUH– “BritishThermalUnitsperHour”:ItisarateofheatingorcoolingexpressedintermsofBtuperhour.(1kW=3412Btu)
3- Ton:Onetonofcoolingistheheatextractionrateof12000Btuperhour.
Theoreticallyitisenergyrequiredtomeltonetonoficeinonehour.
HVACSystemClassifications:
theHVACsystemincludesthefollowingmajorparts:
1.Heatingsystems.2.Coolingsystems.
Alocalheatingsystemservesasinglethermalzoneandhasitsmajorcomponentslocatedwithinthezoneitself.Servingonlyasinglezone,localheatingsystemswillhaveonlyonepointofcontrol,typicallyathermostatforactivesystems.Alocalheatingsystemwillconsistofoneormoreself-containedequipmentunitscontainingheatsource,distribution,anddeliveryfunctionsinasinglepackage.
Portableelectricheaters,built-inelectricresistanceheaters,electricresistancebaseboardradiators,infraredheaters,fireplaces,andwoodstovesareexamplesoflocalheating-onlysystems.
Note: Ingeneralcentralsystemispreferableforlargebuildingshavingmultiplezonesandlocalheatingisaplus,ifmajorityofareasremainunoccupiedandifthepeoplepreferencesrequiredifferenttemperaturesortheydisagreeaboutthemostcomfortabletemperature.
Equipmentusedtogenerateheatingincentralheatingsystems:
1.Furnace.2.Hotwater&steamboiler.3.HeatPump.
Furnace:
Afurnaceisaheatingsystemcomponentdesignedtoheatairfordistributiontovariousbuildingspaces.Allfourheatsourcecategoriesareusedwithfurnaces.
Forcedarefurnacesareequippedwithfanstocirculatewarmairovergreaterdistancethroughaductworksystem.Coolreturnairfromoccupiedspacespassesfirstthroughafilter,theblower,andtheheatingchamber,arrivingatthesupplyductworkataraisedtemperature.Theexhaustgases(includingcarbon-mono-oxide)areventedtotheexteriorofthebuilding.Theunitmayalsoincludeahumidifierthatevaporatesmoistureintotheairasitpassesthrough.
TypesofFurnaces:
Therearethreetypesoffurnaces:
1- Single-StageFurnace:Singlestageimpliesthefurnacefancontrolissimply“onandoff”.
Sinceeveryspacehasaunique"heatload”whichvariesthroughouttheday,themanufacturerswiselyputoptionsinthefurnacefanspeed.
2- Two-StageFurnace:Two-Stagefurnacesweredevelopedwithcomfortinmind.Hereishowtheyfunction.Whenthethermostatactivatesthefurnace,itcomesonat2/3rdstrength(burninggasat65%ofmaximum).If,after10minutesofoperation,thethermostatisstillcallingforheat- thefurnacewillswitchto100%.Whatthataccomplishesisauniformheatingofentirespace.Two-Stagefurnacesaremoreefficientandmoreeffectivewaythansinglestagefurnaces.
3- Two-StageVariableFurnace:Two-stagevariablesfurnaceincorporatesanadditionalfeatureofvariablespeedblower.Inasingle-stagefurnace,thecontrolisOn-Offwhereasinthevariablefurnace,thefanturnsoverslowlyandvariestheaircirculationpertheload.Thisoffersbetterenergyefficiency.
Whenafurnaceisinstalled inabasement itisconsideredan"Upflow"furnace,meaningthecoolerairisdrawnatthebaseofthefurnace,andthewarmairexitsoutthetopofthefurnace.
Heatpumpworksbymovingexistingheatfromoneareatoanother inoneofthefollowingways:
ItUsesairastheoutdoorsourceofheatingorcoolinganddeliversheatingorcoolingtoairindoors.Acondenserabsorbsheatfromtheoutdoorair(eventhecoldestaircontainssomeheat)andtransfersittoanindoorheatexchanger.Indoorairiswarmedintheheatexchangerandcirculatedthroughouttheinteriorspace.Duringthesummer,theprocessisreversedtocoolanddehumidifythehome.
ItUseswaterasthesourceofheating/cooling,deliversheating/coolingtoairindoors.Insteadofextractingheatfromoutsideair,thistypeofpumpabsorbsheatfromgroundwaterorsurfacewater,suchasafarmpond.
InFloororUnderFloorHeating
Underfloorheatingisaformofcentralheatingandcoolingwhichachievesindoorclimatecontrolforthermalcomfortusingconduction,radiationandconvection.modernunderfloorheatingsystemsuseeitherelectricalresistanceelements("electricsystems")orfluidflowinginpipes("hydronicsystems”)toheatthefloor.Eithertypecanbeinstalledastheprimary,whole-buildingheatingsystemoraslocalizedfloorheatingforthermalcomfort.Electricalresistancecanonlybeusedforheatingsowhenspacecoolingisalsorequired,hydronicsystemsareused.
Equipment used to generate heating in Local heating systems:
1.Portable electric heaters. 2.Built-in electric resistance heaters. 3.Electric resistance baseboard radiators. 4.Infrared heaters. 5.Fireplaces. 6.Wood stoves
3- Centralizedsystems(CentralHydronicsystems): Thiscanbedividedtothreemajortypesasfollows:
CentralizedDucted“All– Air”Systems.CentralizedFluidBasedHydronicSystems.Combined(Hybrid)WaterandAirSystems.
- DecentralizedSystems(IndividualRoomSystems
Thesearesystemsinwhichheatingandcoolingisconductedlocally,thesesystemsarerelativelycommoninschools,smallhotels,domesticapplications,residentialhomesandsmalloffices.
DecentralizedSystemsareessentially directexpansion(DX)systems,whichoperateusingdirectexpansionofrefrigerantinthefinnedtubesacrosstheairpath.
Insmallerbuildings,smallerroomairconditionersarelessexpensivetooperatethancentralunits,eventhoughtheirefficiencyisgenerallylowerthanthatofcentralairconditioners.
Themajortypesofdecentralizedsystemsare:
SplitCoolingUnits.WindowUnits.SplitHeatPumps.
A- SplitCoolingUnits
Thesplitsystemsareindividualsystemsinwhichthetwoheatexchangersareseparated(oneoutside,oneinside).Thesplitairconditionercanbeinstalledinroomsandofficeswhereyoudon’twanttodisturbthesetupoftheroomandavoiddemolitionsinyourfavoritespace.
WindowUnitConstruction:Condenser(outdoorcoil).
Condenserfan.
Hermeticcompressor.
Capillarytube.
Evaporator(indoorconditioningcoil).
Evaporatorfan.
Controls:Thecontrolsforwindowunitsaresimpleandinbuilt,witharotatingswitchmarkedwithahot-coldscalewithnotemperaturesettings.Mostunitswillheataswellascool.Freshaircontrol,ifitexists,isnormallyadjustedbyamanuallever.
Acoolingonlysystemcoolstheindoorairbutaheatpumpprovidescoolinginsummerseasonandheatinginwinterseason.DXsystemoperatinginreversevaporcompressioncycleisclassifiedasHeatpump.Throughanadditionofaspecial four-wayreversingvalve,heatflowinmechanicalrefrigerationloopcanbereversedsothatheatisextractedfromoutsideairandrejectedintothebuilding.Suchafacilityisrequiredduringwinterseasontoheatthe indoorusingthesamephysicalcomponents.Duetoaddedheatofcompression,theefficiencyofheatpumpishighercomparedtothecoolingcycle.Asupplementaryelectricresistanceheatermayalsobeusedtoassisttheheatpumpatloweroutdoortemperatures.Incolderclimates,heatspumpsrequiredefrostperiod.Duringdefrosttimestheelectricheateristheonlymeansofheatingtheinteriorofthebuilding.SplitheatpumpsareAir-to-Airtypesandtheyarethemostcommontypeofheatpumps.Itisparticularlysuitableforfactory-builtunitaryheatpumps,andhasbeenwidelyusedforresidentialandcommercialapplication.Airisusedastheheatsourceandheatsink.Extendedsurface,forcedconvectionheattransfercoilsarenormallyemployedtotransfertheheatbetweentheairandtherefrigerant.Whenselecting ordesigninganair-sourceheatpump,twofactorsinparticularmustbetakenintoconsideration:Thevariationintemperatureexperienced inagivenlocality.Theformationoffrost
2- Semi- centralizedsystems(packagedsystems):
2- PackagedAirConditionerswithAirCooledCondensersInthispackagedairconditionersthecondenseroftherefrigerationsystemiscooledbytheatmosphericair.
Thereisanoutdoorunitthatcomprisesoftheimportantcomponentslikethecompressor,condenserandinsomecasestheexpansionvalve.Theoutdoorunitcanbekeptontheterraceoranyotheropenplacewherethefreeflowoftheatmosphericairisavailable.Thefanlocatedinsidethisunitsuckstheoutsideairandblowsitoverthecondensercoilcoolingitintheprocess.Thecondensercoilismadeupofseveralturnsofthecoppertubinganditisfinnedexternally.ThepackagedACswiththeaircooledcondensersareusedmorecommonlythantheoneswithwatercooledcondenserssinceairisfreelyavailable itisdifficultmaintaincontinuousflowofthewater.
Thecoolingunitcomprisingoftheexpansionvalve,evaporator,theairhandlingblowerandthefilterarelocatedonthefloororhangedtotheceiling.Theductscomingfromthecoolingunitareconnectedtothevariousroomsthataretobecooled.
B- Twopiecessystems(DuctedSplitsystem)Inthissystem,theevaporatorisseparatefromthecondenser/compressor.Thesearecommonlyfoundinresidentialandsmallcommercialinstallationswithcapacityrangesvarying1to50TRandsuitableforanareaof100– 10000squarefeet.Thenewductlesssystemswhichcanbeconvenientlymountedontheceilingorwallareinthisfamily.Asplitsystemdescribesanairconditioningorheatpumpsystemthatissplitintotwosections-anoutdoorsectionandanindoorsection.Theoutdoorunitiscomprisedofacompressorandcondenserandislocatedoutsideusuallyonthegroundbutsometimesontheroof.Theindoorsection,usuallylocatedinaninteriorclosetorgarage,consistsofafan,indoorcoolingcoil,furnace,andfilter.
Inan‘All-Airsystem’,therefrigerantorchilledwaterisusedtocoolanddehumidifytheairintheairhandlingunit(AHU).Thecoolairisthencirculatedthroughoutthebuildingthrutheductwork.Heatingcanalsobeaccomplishedeitherbyhotwaterorelectricalstripheaters.
Thecentralizationofthesesystemsallowforbettermanagementandsystemoperation.Ontheotherhand,theyalsorequireeitheramechanicalroomadjacenttothecontrolledspaceforlocatingtheAHUandlargeductworkinbuildingspace.
Freshairisdrawnintothebuildingthroughtheintakelouver,mixedwithreturnair,heatedorcooledtoacontrolledtemperature,circulatedaroundthebuildingandprovidedtotheoccupiedspace.Localtemperaturecontrolisprovidedbyaterminalreheatunitattachedtoatemperaturecontrollerwithintheoccupiedspace.Exhaustairisextractedfromthespaceanddumpedtotheoutside.Ingeneral,themajorityofthereturnairisrecycledviathereturnairduct.
SystemConstruction: