Unit 25HeatingEngineeringText Forover10,000years,manhasusedfiretowarmhimself.Inthebeginning,interiorheatingwasjustanopenfire,butcomfortandhealthwasgreatlyimprovedbyfindingacavewithaholeatthetop.Later,fireswerecontainedinhearthsorsunkenbeneaththefloor.Eventually,chimneyswereaddedwhichmadeforbetterheating,comfort,health,andsafetyandalsoallowedindividualstohaveprivaterooms.Next,camestovesusuallymadeofbrick,earthenware,ortile.Inthe1700s,BenjaminFranklinimprovedthestove,thefirststeamheatingsystemwasdeveloped,andafurnaceforwarm-airheatingusedasystemofpipesandfluesandheatedthespacesbygravityflow.Inthe1800s,highspeedcentrifugalfansandaxialflowfanswithsmall,alternatingcurrentelectricmotorsbecameavailableandhigh-pressuresteamheatingsystemswerefirstused[1].The1900sbroughttheScotchmarineboilerandpositive-pressurehydroniccirculatingpumpsthatforcedhotwaterthroughtheheatingsystem.Theheatingterminalswerehotwaterradiators,whichwerelong,low,andnarrow,ascomparedtosteamradiators,andallowedforinconspicuousheating.Centrifugalfanswereaddedtofurnacesinthe1900stomakeforced-airheatingsystems. Theheatingplants(furnacesandboiler)discussedinthiscontextareoftencalledtheprimarysystems.Systemsintendedtodistributeheatproducedbytheprimarysystemsarecalledsecondarysystemsandincludeductsandpipes,fansandpumps,terminaldevices,andauxiliarycomponents.ThetermsprimaryandsecondaryareequivalenttothetermsplantandsystemusedbysomebuildinganalystsandHVACsystemmodelers.Text Thegoalofthiscontextistohavethereaderunderstandtheoperationofvariousheatgenerationortransfersystemsandtheirperformance.FurnacesandBoilers Furnacesareusedtoheatairstreamsthatareinturnusedforheatingtheinteriorofbuildings.Aboilerisadevicemadefromcopper,steel,orcastirontotransferheatfromacombustionchamber(orelectricresistancecoil)towaterineithertheliquidphase,vaporphase,orboth.Boilersarepressurevesselsusedtotransferheat,producedbyburningafuel,toafluid.Boilersareclassifiedbothbythefuelusedandbytheoperatingpressure.Themostcommonheattransferfluidusedforthispurposeinbuildingsiswater,intheformofeitherliquidorvapor. Pressureclassificationsforboilersforbuildingsare: LowPressure:Steamboilerswithoperatingpressuresbelow15psig(100kPa).Hotwaterboilerswithpressuresbelow150psig(1000kPa);temperaturesarelimitedto250°F(120°C). HighPressure:Steamboilerswithoperatingpressuresabove15psig(100kPa).Hotwaterboilerswithpressuresabove150psig(1000kPa);temperaturesareabove250°F(120°C).Text Thekeydistinctionbetweenfurnacesandboilersisthatairisheatedintheformerandwaterisheatedinthelatter.Thefuelsusedforproducingheatinboilersandfurnacesincludenaturalgas(i.e.,methane),propane,fueloil,wood,coal,andotherfuelsincludingrefuse-derivedfuels.Sinceboilersandfurnacesoperateatelevatedtemperatures(andpressuresforboilers),theyarehazardousdevices.Asaresult,abodyofstandardshasbeendevelopedtoassurethesafeoperationofthisequipment.Heatpumps

Aheatpumpextractsheatfromenvironmentalorothermediumtemperaturesources(suchastheground,groundwater,orbuildingheatrecoverysystems),raisesitstemperaturesufficientlytobeofvalueinmeetingspaceheatingorotherloads,anddeliversittotheload[2].Heatpumpsareavailableinsizesrangingfromsmallresidentialunits(10kW)tolargecentralsystems(upto15MW)forcommercialbuildings.Largesystemsproduceheatedwaterattemperaturesupto220°F(105°C).Centralsystemscanusebothenvironmentalandinternalbuildingheatsources.Inmanypracticalcircumstancestheheatgainsinthecorezonesofacommercialbuildingcouldsatisfytheperimeterheatlossesinwinter.TextAheatpumpcouldbeusedtoefficientlyconditionbothtypesofzonessimultaneously.Heatpumpsrequireacompressorandtwoheatexchangers.Intheenergybookkeepingthatonedoesforheatpumps,thepowerinputtothecompressorisaddedtotheheatremovedfromthelowtemperatureheatsourcetofindtheheatdeliveredtothespacetobeheated.Increasedheatingcapacityatlowairsourcetemperaturescanbeachievedbyoversizingthecompressor.Toavoidpartloadpenaltiesinmoderateweather,avariablespeedcompressordrivecanbeused.

Figure25-1 Air-to-airheatpumpdiagram. Areciprocatingcompressorisused.Thisdesignallowsoperationasaheatpumporanairconditionerbyreversingtherefrigerantflow.Text Heatpumpefficiencyisgreateriflowerhighsidetemperaturescanbeused.Inordertoproduceadequatespaceheatinsuchconditions,alargercoilmaybeneededintheairstream.However,ifthecoilissizedforthecoolingload,itwillnearlyalwayshaveadequatecapacityforheating.Insuchacase,adequatespaceheatcanbeprovidedatrelativelylowairtemperatures,95–110°F(35–43°C).Table25-1summarizesadvantagesanddisadvantagesofairandwatersourceheatpumps.Table25-1 Theadvantagesanddisadvantagesofairandwatersourceheatpumps.TypeAdvantagesDisadvantagesAirsourceIndoordistributionpermitsairconditioningandhumiditycontrolOutdoorairsourcereadilyavailableSimpleinstallationLeastexpensiveEstablishedcommercialtechnologyDefrostrequiredLowcapacityatcoldoutdoortemperatureLowerefficiencybecauseoflargeevaporator?T≈30°FIndoorairdistributiontemperaturemustbehighforcomfortreasonsReliabilityatlowtemperatureisonlyfair,duetofrostingeffectsMustkeepevaporatorclearofleaves,dirt,etc.WatersourceMultiplefamilyandcommercialinstallationascentralsystemIncommercialinstallations,goodcouplingtocoolingtowersNorefrigerantreversalneeded;reversewaterflowinsteadNeedswatersourceatusefultemperatureEfficiencypenaltyduetospaceheatexchanger?TTextHeatexchangers Aheatexchangerisapieceofequipmentbuiltforefficientheattransferfromonemediumtoanother.Themediamaybeseparatedbyasolidwall,sothattheynevermix,ortheymaybeindirectcontact.Theyarewidelyusedinspaceheating,refrigeration,airconditioning,powerplants,chemicalplants,petrochemicalplants,petroleumrefineries,naturalgasprocessing,andsewagetreatment.Thetypesofheatexchangersareasfollows:Shellandtubeheatexchanger Shellandtubeheatexchangersconsistofaseriesoftubes.Onesetofthesetubescontainsthefluidthatmustbeeitherheatedorcooled.Thesecondfluidrunsoverthetubesthatarebeingheatedorcooledsothatitcaneitherprovidetheheatorabsorbtheheatrequired.Asetoftubesiscalledthetubebundleandcanbemadeupofseveraltypesoftubes:plain,longitudinallyfinned,etc.Shellandtubeheatexchangersaretypicallyusedforhigh-pressureapplications(withpressuresgreaterthan30barandtemperaturesgreaterthan260°C).Thisisbecausetheshellandtubeheatexchangersarerobustduetotheirshape.TextPlateheatexchanger Anothertypeofheatexchangeristheplateheatexchanger.Oneiscomposedofmultiple,thin,slightlyseparatedplatesthathaveverylargesurfaceareasandfluidflowpassagesforheattransfer[3].Thisstacked-platearrangementcanbemoreeffective,inagivenspace,thantheshellandtubeheatexchanger.Advancesingasketedbrazingtechnologyhavemadetheplate-typeheatexchangerincreasinglypractical.InHVACapplications,largeheatexchangersofthistypearecalledplate-and-frame;whenusedinopenloops,theseheatexchangersarenormallyofthegaskettypetoallowperiodicdisassembly,cleaning,andinspection.Therearemanytypesofpermanentlybondedplateheatexchangers,suchasdip-brazed,vacuum-brazed,andweldedplatevarieties,andtheyareoftenspecifiedforclosed-loopapplicationssuchasrefrigeration.Plateheatexchangersalsodifferinthetypesofplatesthatareused,andintheconfigurationsofthoseplates.Someplatesmaybestampedwith"chevron",dimpled,orotherpatterns,whereothersmayhavemachinedfinsand/orgrooves.TextPlateandshellheatexchanger Athirdtypeofheatexchangerisaplateandshellheatexchanger,whichcombinesplateheatexchangerwithshellandtubeheatexchangertechnologies[4].Theheartoftheheatexchangercontainsafullyweldedcircularplatepackmadebypressingandcuttingroundplatesandweldingthemtogether.Nozzlescarryflowinandoutoftheplatepack(the'Plateside'flowpath).Thefullyweldedplatepackisassembledintoanoutershellthatcreatesasecondflowpath(the'Shellside').Plateandshelltechnologyoffershighheattransfer,highpressure,highoperatingtemperature,compactsize,lowfoulingandcloseapproachtemperature.Inparticular,itdoescompletelywithoutgaskets,whichprovidessecurityagainstleakageathighpressuresandtemperatures.Adiabaticwheelheatexchanger Afourthtypeofheatexchangerusesanintermediatefluidorsolidstoretoholdheat,whichisthenmovedtotheothersideoftheheatexchangertobereleased.Twoexamplesofthisareadiabaticwheels,whichconsistofalargewheelwithfinethreadsrotatingthroughthehotandcoldfluids,andfluidheatexchangers.TextHeatDistribution AnHVACcentrifugalpumpisaelectricalpower-drivenmachinethatisusedtoovercomesystemresistanceandproducerequiredwaterflow.Asthepumpimpellerisrotated,centrifugalforcethrowsthewateroutwardfromtheimpeller.Thecentrifugalforceandotherdesigncharacteristicsreducethepressure(apartialvacuumiscreated)attheinletoftheimpellerandallowmorewatertobeforcedinthroughthepumpsuctionopeningbyatmosphericorexternalpressure.Thismakesthepump’sdischargepressurehigherthanthepump’ssuctionpressure.Afterthewaterentersthepump’ssuctionopening,there’safurtherreductionofpressurebetweenthisopeningandtheinletoftheimpeller.Thelowestpressureinthesystemisatthepumpinlet.Thewaterleavestheimpelleratarelativelyhighvelocity.Then,inthepumpcasing,thevelocity(velocitypressure)isreducedandconvertedintostaticpressure.Thesizeofthepumpimpelleranditsrotationalspeeddeterminesthestaticheadpressuredevelopedbythepump.AtypicalHVACcentrifugalpumpwillhaveavolute(spiral)casingwithoneormoreclosedbackwardcurvedradialflowimpellers.Ifthepumphasoneimpeller,itisasinglestagepump.Ifithastwoormoreimpellersinseriesonacommonshaftitisamultistagepump.Theinlettothepumpmaybeonjustoneside.TextThisisasingleinletpump.Iftheinlettothepumpisonbothsidesofthepumpitisadoublesuctioninletpump.Thesuctionpipemaybethesamesizeorlargerthanthedischargepipe.MostHVACwaterpumpsareconstantvolumemachinesandarecoupleddirectly(directdrive)toaconstantspeedmotor.Somedirectdrivepumpsaredrivenbyavariablefrequency-variablespeedmotor.Varyingthespeedofthemotorchangesthespeedofthepump.Varyingthepumpspeedmakesthepumpvariablevolume.However,someHVACpumpsarebeltdriven.Thesepumpsmaybeeitherconstantspeedandvolumeorvariablespeedandvolume.Hydronicsistheprocessofheatingorcoolingwithwater.Someoftheclassificationsforhydronicpipingsystemsareopen,closed,seriesloop,one-pipe,two-pipe,three-pipe,four-pipe,direct-return,reversereturnandcombination.Hydronicsystemsaregenerallytwo-orfour-pipeheatingandcoolingcircuits.Thesepipingcircuitsprovideheatedorchilledwatertocoilsincentralairhandlingunits,fan-coilunits,ductwork,andterminalboxes.Hydroniccoilsalsoarefoundinunitheaters,valenceunits,andinfin-tuberadiation. Waterflowiscontrolledthroughtheuseofvarioustypesofvalves.Automaticandmanualflowcontrolvalvesandmanualbalancingvalvesareusedtoregulateflowrate.Servicevalvesareusedtoisolatepartorallofthesystem.TextCheckvalvesareusedtolimitthedirectionofflow.Therearethreebasictypesofmanualvalves:flowcontrolorbalancingvalves(Thetypesofflowcontrolorbalancingvalvesinclude:ballvalves,butterflyvalves,globevalves,combinationvalves,plugvalves,andcalibratedbalancingvalves.),servicevalves,andcheckvalves.Specifictypesaredescribedinthefollowingsections. Automatictemperaturecontrolvalves(ATCV)canbeclassifiedaccordingtotypeofconstructionTwo-wayvalves:singleseatedanddoubleseated;andthree-wayvalves:single-seatedmixingvalvesanddouble-seateddivertingvalves[5]. Flowmeterssuchasannular,orificeplate,venturi,andcalibratedbalancingvalvearepermanentlyinstalleddevicesusedforflowmeasurementsofpumps,primaryheatexchangeequipment,distributionpipesandterminals.Forflowmeterstogiveaccurate,reliablereadingstheyshouldbeinstalledfarenoughawayfromanysourceofflowdisturbancetoallowtheturbulencetosubsideandthewaterflowtoregainuniformity.Themanufacturersofflowmetersusuallyspecifythelengthsofstraightpipeupstreamanddownstreamofthemeterneededtogetgoodreadings.Straightpipelengthsvarywiththetypeandsizeofflowmeterbuttypicalspecificationsarebetween5to25pipediametersupstreamand2to5pipediametersdownstreamoftheflowmeter.Text Afterthewatersystemisconstructeditisfilledwithwaterthroughthecitysupplymainorotherappropriatesource.Thepressurereducingvalve(PRV)isadjustedandthesystemistested.Waterexpandswhenheatedandcontractswhencooled.Whentheboilerisstarted(fired)thewaterisheatedandbeginstoexpand.Iftheexpandingwaterhasnowheretogo,theincreasedpressureinthesystemcouldbreakapipeordamageotherequipment.Waterexpansiontanksareusedtokeepthisfromhappening.Thesetanksmaintaintheproperpressureonthesystemandaccommodatethefluctuationsinwaterexpansionandcontractionwhilecontrollingpressurechangesinthesystem.Expansiontanksareusedinopensystems.Compressiontanksareuseinclosedsystems.However,inmostcasescompressiontanksarecalledexpansiontanks.Anexpansiontankissimplyanopentankusedinanopenwatersystemtocompensateforthenormalexpansionandcontractionofthewater.Asthewatertemperatureincreasesthewatervolumeinthesystemincreasesandthewaterintheexpansiontankrises.Corrosionproblemsareassociatedwithopenexpansiontanksasaresultoftheexposuretotheairandevaporationand/orboilingofthewater.Becauseofthis,expansiontanksarelimitedtoinstallationshavingoperatingwatertemperaturesof180°Forless.hearth n.爐膛，爐缸，壁爐地面earthenware n.陶器，瓦器combustion v.燃燒，氧化bookkeeping v.記賬，簿記penalty n.不利后果；劣勢(shì)；苦難petrochemical n.石油化學(xué)產(chǎn)品；石油化學(xué)的configuration v.構(gòu)造；形狀，外貌，輪廓；配置nozzle n.管嘴，噴嘴adiabatic v.絕熱的；不傳熱的impellern.（離心泵、壓縮機(jī)等里的）葉輪，轉(zhuǎn)子casing

n.框；殼，罩；套，套管volute

n.渦形,渦形花樣,渦螺，渦形的,螺旋形的butterfly

n.蝴蝶；蝶形物；節(jié)氣門annular

a.環(huán)的，環(huán)形的；圓環(huán)orifice

n.（管子等的）口；孔；噴嘴venturi

n.文氏管，文丘里管（一種流體流量測(cè)定裝置）contraction

v.收縮；縮短heatpump

n.熱泵shellandtubeheatexchanger

n.殼管式換熱器NewWordsandPhrasesplateheatexchanger n.板式換熱器plateandshellheatexchanger n.板殼式換熱器adiabaticwheelheatexchanger n.絕熱輪換熱器centrifugalpump n.離心泵flowcontrolvalve n.流量調(diào)節(jié)閥NewWordsandPhrasesNotes（1）Inthe1800s,highspeedcentrifugalfansandaxialflowfanswithsmall,alternatingcurrentelectricmotorsbecameavailableandhigh-pressuresteamheatingsystemswerefirstused. 十九世紀(jì)，配有小型交流電動(dòng)機(jī)的高速離心式風(fēng)機(jī)和軸流式風(fēng)機(jī)開始出現(xiàn)，高壓蒸汽供熱系統(tǒng)開始使用。（2）Aheatpumpextractsheatfromenvironmentalorothermediumtemperaturesources(suchastheground,groundwater,orbuildingheatrecoverysystems),raisesitstemperaturesufficientlytobeofvalueinmeetingspaceheatingorotherloads,anddeliversittotheload. 熱泵從周圍環(huán)境或其他介質(zhì)溫度源（如地面、地下水或建筑熱回收系統(tǒng)）獲取熱量，自身溫度升至空間加熱或其他負(fù)荷所需要的溫度值，然后將熱量傳遞給負(fù)荷。（3）Anothertypeofheatexchangeristheplateheatexchanger.Oneiscomposedofmultiple,thin,slightlyseparatedplatesthathaveverylargesurfaceareasandfluidflowpassagesforheattransfer. 另一種類型的換熱器是板式換熱器。它由多層適當(dāng)分開的薄板組成，其傳熱的表面積和流體通道非常大。Notes（4）Athirdtypeofheatexchangerisaplateandshellheatexchanger,whichcombinesplateheatexchangerwithshellandtubeheatexchangertechnologies. 第三種類型的換熱器是板殼式換熱器，其融合了板式換熱器與殼管式換熱器技術(shù)。（5）Automatictemperaturecontrolvalves(ATCV)canbeclassifiedaccordingtotypeofconstructionTwo-wayvalves:singleseatedanddoubleseated;andthree-wayvalves:single-seatedmixingvalvesanddouble-seateddivertingvalves. 自動(dòng)溫度控制閥(ATCV)可根據(jù)其構(gòu)造類型分為兩類：兩通閥（單座閥和雙座閥）以及三通閥（單座混合閥和雙座分流閥）。1.以對(duì)流換熱為主要方式的供熱，稱為對(duì)流供暖。系統(tǒng)中的散熱設(shè)備是散熱器，因此這種系統(tǒng)也稱為散熱器供暖系統(tǒng)，它是以對(duì)流方式向室內(nèi)供暖。輻射供暖是以輻射傳熱為主的一種供暖方式。輻射供暖系統(tǒng)的散熱設(shè)備主要是金屬輻射板或以建筑物部分頂棚、地板或墻壁作為輻射散熱面。2.按系統(tǒng)循環(huán)動(dòng)力的不同，可分為重力循環(huán)系統(tǒng)和機(jī)械循環(huán)系統(tǒng)?？克拿芏炔钸M(jìn)行循環(huán)的系統(tǒng)，稱為重力循環(huán)系統(tǒng)；靠機(jī)械力進(jìn)行循環(huán)的系統(tǒng)，稱為機(jī)械循環(huán)系統(tǒng)。重力循熱水供暖系統(tǒng)的循環(huán)作用壓力的大小，取決于水溫（水的密度）在循環(huán)環(huán)路的變化狀況。3.Thehumanbodycontinuouslyproducesheatthroughaprocesscall“metabolism”.Thisheatmustbeemittedfromthebodytomaintainafairlyconstantcoretemperature,andideally,acomfortableskintemperature.Weproduceheatataminimumratewhenasleep.Asactivityincreases,fromsittingtowalkingtorunning,sothemetabolicheatproducedincreases.4.Steamresultsfromboilingwater.Asthewaterboils,ittakesuplatentheatofvaporizationandexpandstoabout1,600timesitsoriginalvolumeatatmosphericpressure.Steamisagas,andinaves