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63WasteHeatRecoveryJerryT.Zolkowski,PECEMDemandSideEnergyEngineer/ManagerShawIndustriesABSTRACTInvestmentsinwasteheatrecoveryprovideaveryattractivepayback.ThisarticlecoverscasesimplementedoverthepastfouryearsamongShaw’s50manufacturingplants.Heatsourcestappedareaircompressors,wastewater,boilerstackgasses,andboilerblowdown.Thedestinationfortheheatisprocesswaterheatingandboilermake-upwaterheating.Heatexchangersonlyhavebeenused,noheatpumps.ABOUTSHAWANDPROCESSESShawIndustriesisthecountry’slargestcarpetproducerandsecondlargestflooringsupplier,with$5billioninannualsales.Itisverticallyintegratedfromfiberextrusionthroughdistribution.Thecompanyhasover50manufacturingplants,includinghardwoodproduction,woodlaminate,nylonproductionfromcaprolactam,fiberextrusion,fiberpro-cessing,carpetproduction,andonenylonrecyclingplant.Thefacilitiesusingwasteheatrecoveryareextrusionplants,yarnprocessingplants,andcarpetmillswithdyeing.Allofthesefacilitieshavesteamplantsoperatingat120psisaturatedsteam.Typicallythereislittleornocondensatereturnduetotherequirementforlivesteaminmostpro-cesses,yarnheatsettingandcarpetsteamersinparticular.Highvolumes(e.g.250gpm)ofwarmrinsewaterarerequiredfordyeing.Coldboilermake-upwaterandhighblowdownrateswerethenormjustafewyearsago.Inaddition,rinsewaterwasheatedonlythroughtheuseofsteamormorerecentlydirect-firegas.Companywidein2004,workingwasteheatrecoverydevicesconsistedoffourblowdownheatexchangers,twoeconomizers,andoneundersizedwastewaterheatex-changer.64EnergyEngineeringVol.106,No.52009SYSTEMREQUIREMENTSFORWASTEHEATRECOVERYAheatsourceisrequired.TheheatsourcesforShawareboilerexhausts,boilerblowdown,wastewater,andaircompressors.Processexhaustfromdryershasbeeninvestigatedbutonlyusedinonecasesofar.Therecoveredheatmustbeusedtodisplaceheatprovidedfromanenergysourcesuchassteamorfuel.AtShawtherecoveredheatistransferredtoboilermake-upwaterandrinsewater.Usingtheheatforboilerordryermake-upairisalsoagoodapplication,butthathasnotbeenwidelyimplementedyet.Thefinalrequirementisthatthewasteheatmustbeatahighertemperaturethantheincomingwatertobeheated,ifheatexchangersareused.Itispossibletouseheatpumps,butthatislesseconomical.SPECIFICAPPLICATIONSHeatingBoilerMake-upWaterfromWasteWaterDyehousewastewatersareinthe115-145°Frange,dependingontheplant.Sevendyeplantshaveinstalledwastewaterheatexchang-ers,primarilytoheatprocesswater.Fiveofthosedyeplantshavealsotappedintothisheatsourceforboilermake-upwater.A25°Ftempera-tureriseistypical.Sincethisdirectlydisplacessteamusedtoheatthewateratthedeaerator,andcondensatereturnisnegligible,theresultingboilerfuelsavingsisabout2.5%.Thesesystemsareusedprimarilyforheatingrinsewater,sotheadditionalinvestmenttosupplythesteamsystemissmall.Withannualboilerfuelcostsbetween$2Mand$4Mperyear,thesesavingsamountto$50k-$100kperyear,withminimaladdi-tionalinvestment.Thesteamsystemsatyarnplantsaremuchsmaller,withannualfuelcostsof$300k-$1M.Oneoftheyarnheatsettingprocesses(“Su-perba,”whichisacontinuousautoclave)produceswastewatertemper-aturesof140-170°F.Twomanufacturingplantspumpthewastewatertotheboilerroomandpassitthroughasmallheatexchangertowarmboilermake-upwater.Bothoftheseplantsthenheatthemake-upwaterfurther,usingboilerblowdown.Thefuelsavingsforthetwostepheat-ingprocessis7%.Eachheatexchangercostsabout$2500.65HeatingBoilerMake-upWaterwithAirCompressorsAircompressorsdiscardabout80%oftheinputenergyviawasteheat.Intherightapplication,thiscanbeanexcellentheatsource.Manyindustrialplantsdirecttheheatfromair-cooledcompressorsindoorsforbuildingheatinthewinter;someShawplantsdothis.OtherShawplantsusethewastecompressorheatforboilermake-upwaterandtoreducefueluseyear-round.Extrusionplantstendtohavelargecompressedairrequirements(10,000hp)comparedtothesteamplantsize(600boilerhp).Centrifugalaircompressorsareused.Twoplantsaddaftercoolers,whicharecom-pressedair-to-waterheatexchangers.Softenedboilermake-upwaterpassesthroughthewatersideoftheaftercooler,and250°Fcompressedairpassesthroughtheairside.Theairthengoesontoadryerandtheplant;thewatergoesontothedeaerator.Oneplantisgetting170°Fwaterfromthis,andtheotherisgetting126°Fwater.Heatcanalsoberecoveredfromscrewcompressors.Onemethodistouseawatercooledscrewcompressoranduseboilermake-upwaterforcoolingwaterinsteadofprovidingwaterfromacoolingtower.OneShawplantdoesthiswitha100hpcompressor.Figure1.WastewaterandboilerblowdownheatexchangersatShawPlant10inDecatur,TN.66EnergyEngineeringVol.106,No.52009However,air-cooledcompressorsarefarmorecommon.Screwcom-pressorsreject75%oftheirheatviatheoiland25%viathecompressedair.Theoilisconstantlycirculated,andaproportioningvalvesendsavaryingamountofoiltotheheatexchangerasneeded.Byplacingaplate-and-frameheatexchangerontheoillinebeforetheproportioningvalve,heatcanberecoveredfromtheoiltoheattheboilermake-upwater.Intheeventthereisinsufficientcoolingwater,thecompressorwillstillcontinuetofunctionnormally,astheproportioningvalvesendsoiltotheoriginalheatexchanger.Eventhoughthisislessthanidealatcapturingalltheavailableheat,manyplantsliketheideaofbeingabletousethecomp