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: