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ME492MATERIALSINENGINEERINGDESIGN
April5,2012
MaterialsandProcess
SelectionforaCryogenic
HeatExchanger
GROUP#4DawsonJamesJeffreyPowellJamesStevensonDerekVisvanathan
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ExecutiveSummary
Thereisanincreasingdemandfornaturalgasasitisthecleanestburningofallfossilfuels.
Whenpipelinescannotbeusedtotransportthenaturalgasitisliquefied(liquefaction
temperature163)asithasagreatlyreducedvolume,makingtruckandnavaltransportation
morefeasible.Inordertoachievethelowtemperaturesrequiredtoliquefythenaturalgasa
cryogenicheatexchangermustbeused.Thisreportoutlinestheselectionprocedureforthe
type,materialandprocessesrequiredtofabricateacryogenicheatexchanger.
Aspiralinshellheatexchangerutilizesthintubesfilledwithcoolantwoundontheinsideofa
cylindricalshell.Thisdesignmaximizestheamountofsurfaceareabetweenthetubingandthe
naturalgasallowingforahighheatflowfromthegastothecoolant.Thetubesrequireless
advancedprocessestofabricatethanfinsandthespiralshellheatexchangerrequires
significantlylessmaintenancethanaplateandfinexchanger.Heatexchangersusedforthis
purposearequitelargewithlengthsupto500manddiametersof5m.
Wroughtaluminum2026wasfoundtobethebestmaterialforthetubinginsidetheheat
exchanger.Aluminumalloyshaveahighthermalconductivityresultinginthedesiredheatflow
inthecoolant.Aluminumisalsolessexpensivethannickelalloysandwillnothaveadverse
reactionswiththenaturalgaswhichwasaproblemwithcopperandbrass.Alongwiththe
thermalconductivityandcost,aluminumwillbeabletowithstandthepressuresinsidetheheat
exchangerpassingallobjectives.
Tomaximizeheattransferthinwalledtubingwillbeused.Therearemanyprocessesthatare
abletocreatethethinwalledtubingbutwiredrawingwasfoundtobethebestoption.Wire
drawingcancreatesmallcrosssectionalareasrequiredforthethinwalltubingandis
compatiblewiththeselectedaluminumalloys.Wiredrawingcanalsoyieldtighttolerances.
Anotherprocessthatissuitableisrollforming.Withsimilarspecificationsaswiredrawingitwill
alsoperformtherequiredfunction.Wiredrawinghaslowertoolingandcapitalcoststhanroll
formingandisthereforethemostidealprocess.
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TableofContentsExecutiveSummary........................................................................................................................................i
ReasonsandProcessofLiquefyingNaturalGas...........................................................................................1
IntroductionandDesignStatement..............................................................................................................1
DesignConstraintsandSelectionCriteria.....................................................................................................2
Function:...............................................................................................................................................2
Constraints:..........................................................................................................................................2
Objectives:............................................................................................................................................2
FreeVariables:......................................................................................................................................2
MaterialIndices:...................................................................................................................................2
Conceptualdesigns.......................................................................................................................................4
MaterialSelectionforHeatExchangerTubes...............................................................................................5
Limits:.......................................................................................................................................................5
MaterialSelectionSummary....................................................................................................................6
ProcessSelectionforHeatExchangerTubes................................................................................................8
JustificationforMaterialsandProcesses.....................................................................................................9
CostEstimation...........................................................................................................................................11
FinalDesignandMaterials..........................................................................................................................11
SummaryandConclusion...........................................................................................................................12
Bibliography................................................................................................................................................13
Appendix.....................................................................................................................................................14
MaximizeHeatFlowPerUnitArea:........................................................................................................14
MaximizeHeatFlowPerUnitMass:.......................................................................................................15
MaximizeHeatFlowPerUnitCost:........................................................................................................16
Costestimationresults...........................................................................................................................18
1
ReasonsandProcessofLiquefyingNaturalGas
Naturalgasconsumptionisincreasingduetothedemandforcleanerenergyproduction.Inmanycases
pipelinesarenotavailabletosupplythenaturalgasdirectlyfromtheextractionplant.Initsnatural
gaseousstate,naturalgastakesupalargeamountofvolumewhichmakestransportationvery
expensive.Inordertodecreasetheshippingvolume,naturalgascanbeliquefiedwhichreducesits
volumetoapproximately1/600thofitssize.Themaindifficultyinliquefyingnaturalgasisits
classificationasacryogenicfluid,meaningitcondensesatbelow150C.
Anotherreasonforliquefyingnaturalgasisthattheprocessremovesimpuritieswhichmeanonceit
reachesitsdestinationitonlyneedstoberegasifiedpriortodistribution.Liquidnaturalgasisalsonon
toxicandnoncorrosivehowever;itisexplosivewhenputincontactwithwater.Oncenaturalgasis
extracted,itfirstgoesthroughseveralcleaningprocesses.AnycondensatesareremovedalongwithCO2
mercury&H2S.Thenaturalgasalsogoesthroughadehydrationstagetoremoveanytraceamountsof
water.Thegasthengoesthroughseveralcoolingstageswithairfinheatexchangersandcompressors
untilitreachesthefinalcryogenicheatexchanger.Thecryogenicheatexchangerusesanotherliquefied
gas,typicallyliquidnitrogenoroxygenintheliquefactionprocess.Finally,theliquidnaturalgasisput
intocryogenicseacarriersorcryogenicroadtankersandshippedtothefinaldestination.
IntroductionandDesignStatement
Thematerialsandprocessselectionwillbedeterminedforacryogenicheatexchangerusedinthe
liquefactionofnaturalgas.Theprocessofliquefyingnaturalgasrequiresatemperaturebelow163
degreesCelsiusandinthisprocesstheothertraceelementspresentinthenaturalgasareseparatedout
leavingpureliquidmethane.Thisprocessoccursatambientpressureonthenaturalgassidebutthe
refrigerantthattravelsthroughtheheatexchangermaynotoperateatthispressuredependingonthe
typeofprocessutilized.Thereforethestressduetopressuredifferencemustbeconsideredalongwith
thethermalconductivityofthetubingmaterial.Therearetwotypesofheatexchangersusedinthis
application,oneisacoiledtubeheatexchangerandtheotherisfinandplateheatexchangertheyare
commonlymanufacturedusingaluminumandtitaniumalloys,thisreportwillinvestigatetheoptimum
materialforthistypeofheatexchanger.Theoptimumprocessformanufacturingwillalsobe
determined.
2
DesignConstraintsandSelectionCriteria
Function: Cryogenic heat exchanger used to cool natural gas to its saturation temperature (-163C) at
which point it will liquefy and become liquefied natural gas (LNG).
Constraints:
Withstandthepressuredifferencebetweenworkingfluids, Operateattemperaturebelowto163 Moderateductilitysotubingcanbebent Doesnotcorrodeduetoworkingfluidsorbyproducts(suchasH2S) Haveexcellentlowtemperature(cryogenic)durability
Objectives:
Maximizetheheattransfer/flowperarea, Minimizecost,
FreeVariables:
Wallthicknessoftubing, Materialchoice
MaterialIndices:
Themethodofheattransferthroughthetubingwillbeconductionwhichisgivenby:
Where: istheheatflux( ),
isthethermalconductivity( ),
isthetemperaturedifferencebetweentheworkingfluids(,and
isthetubewallthickness().
Heatflow,,isgivenby:
Where: istheheatflow(),and
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isthesurfaceareaofthetubing() 2.
Substitutingtheheatfluxfromconductionyieldsthefollowing:
Wallthicknessisaremainingfreevariablewhichrelatesthepressuredifferencesbetweentheworking
fluids.
Where: isthematerialsyieldstrength(),
istheradiusofthetube(),
isthepressuredifferencebetweentheworkingfluids(),and
isthewallthickness.
Finallybysubstitutingthicknessamaterialindexbasedonbothyieldstrengthandthermalconductivity
canbeattained.
Inordertheminimizecosttheheatflowperunitmassisused(/).
2
/
2
/
2
4
/ 2
/
2
Oncethisindexisderivedmultiplyingmassbycostwillresultintheindexforcost.
Conceptualdesigns
Thereareseveraltypesofheat
exchangersthatareeffectivefor
theliquefactionofnaturalgas.
Eachexchangerwillusethe
coolantliquidmethaneinorder
tobringthenaturalgastoits
saturationtemperaturethrough
theuseofatypicalvapour
compressionrefrigerationcycle.
Oneofthesimplestheat
exchangerisashellandtubeheatexchangerwheremanysmalltubesareranthroughareservoirof
coolant.Thesmalltubesallowformaximumsurfaceareatobeincontactwiththecoolantallowingfor
thehighestheatflowfromnaturalgastothecoolant.Themajorityofheatexchangersarecounterflow
inwhichtheworkingfluidsarepumpedinoppositedirections.Sotheinletsideofthenaturalgaswillbe
theoutletsideofthecoolantandviceversa.Multipletubepassescanbeusedtoincreasetheamountof
surfaceareaandheattransferthatoccurs.Amodificationoftheshellandtubeheatexchangeristhe
Figure1 Shellandtubeheatexchanger
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spiralwoundheatexchanger.Inthiscasethenaturalgasisintheshellarea(AAshellstream)while
coolantrunsthroughthetubestreams.Thisconfigurationhasaveryhightubesurfaceareabutisfar
morecomplexthanatypicalshellandtubeheatexchanger.Plateandfinheatexchangerscanhave
highersurfaceareaforheattransfer.Theyarealsoabletowithstandhighpressures.Platesare
sandwichedtogetherwithsmallfinsinbetweentofurtherincreasethesurfacearea.Thistypeofheat
exchangerismoredifficulttomanufacturethanthosethatutilizetubing.Inadditiontomanufacturing
complicationstheincreasedsurfaceareamakesthefluidpathwaysverysmall.Thiscanleadtoan
increaseincloggingdependingontheworkingfluidsbeingused.
MaterialSelectionforHeatExchangerTubes
WhiledeterminingthematerialsneededforthetubesintheLNGcryogenticheatexchangerthreeobjectiveswereconsidered:
MaximizeHeatFlowperUnitArea MaximizeHeatFlowperUnitMass MaximizeHeatFlowperUnitCost
Limits:Elongation:>20%Strain
MaximumServiceTemperature:
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ThermalConductororInsulator:GoodConductor
TolerancetoCryogenicTemperatures:Excellent
MaterialSelectionSummaryUsinglevel2CESEduPackwiththelimitsstatedabove,thefollowingaretheninematerialsthat
passthelimitstage:
AgehardeningwroughtAlalloys Brass Bronze Commerciallypurelead Copper Leadalloys Nickel Nickelbasedsuperalloys NonagehardeningwroughtAlalloys
Thetopfivematerialsusinglevel2foreachobjectiveare:
MaximizeHeatFlowperUnitArea:
AgehardeningwroughtAlalloys Brass Copper Nickel NonagehardeningwroughtAlalloys
MaximizeHeatFlowperUnitMass:
AgehardeningwroughtAlalloys Brass Copper Nickel NonagehardeningwroughtAlalloys
MaximizeHeatFlowperUnitCost:
AgehardeningwroughtAlalloys NonagehardeningwroughtAlalloys Brass Bronze
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Copper Breakingdowneachalloyusinglevel3CESEduPackallowedforamoreaccuratematerial
selectionforeachofthestatedobjectives.Copper,brass,andbronzewererejectedfromthe
materialselectionprocessastheyhaveanadversereactionwithH2Swhichisabyproductof
theliquefactionprocess.Thefollowingarethetoptenmaterialsinorderofperformanceindex
forAluminumandNickelalloys:
MaximizeHeatFlowperUnitArea(seeFigure2):
1) Aluminum,2026,wrought,T3511 2) Aluminum,2024,wrought,T4 3) Aluminum,6082,wrought,T4 4) Nickel,DuranickelAlloy301,annealed&aged 5) NickelCoCralloy,UDIMET700,bar 6) Nickel,PermanickelAlloy300,annealed7) NickelFeCralloy,UDIMET630,bar 8) NickelCoCralloy,AEREX350,coldworked,aged9) 45Ni3MoFesoftmagneticalloy10) Nickel,commercialpurity,grade200,soft(annealed)
MaximizeHeatFlowperUnitMass(seeFigure4):
1) Aluminum,2026,wrought,T3511 2) Aluminum,2024,wrought,T4 3) NickelFeCralloy,UDIMET630,bar 4) NickelCoCralloy,AEREX350,coldworked,aged 5) Nickel,DuranickelAlloy301,annealed&aged 6) NickelCoCralloy,UDIMET700,bar 7) NickelCoCralloy,EP741NP 8) Aluminum,6082,wrought,T4 9) NickelFeCralloy,D979,bar 10) 45Ni3MoFesoftmagneticalloy
MaximizeHeatFlowperUnitCost(seeFigure6):
1) Aluminum,2026,wrought,T3511 2) Aluminum,2024,wrought,T4 3) Aluminum,6082,wrought,T4 4) Aluminum,S520.0:LM10TB,cast 5) Aluminum,5154,wrought,O
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6) Aluminum,6060,wrought,T4 7) Aluminum,2024,wrought,T0 8) Aluminum,5251,wrought,O 9) Aluminum,commercialpurity,1080,wrought,O 10) 45Ni3MoFesoftmagneticalloy
Materialsthatmeeteachofthethreedesignobjectivesare:
1) Aluminum,2026,wrought,T3511 2) Aluminum,2024,wrought,T43) Aluminum,6082,wrought,T44) 45Ni3MoFesoftmagneticalloy
ProcessSelectionforHeatExchangerTubes
Thefieldofavailableprocesseswasfirstnarrowedusingthetreeselectiontouseonly
processescompatiblewithaluminum.Usinglimitselectionthelistwasnarrowedtoinclude
onlyprimaryshapingprocesses,circularprismaticobjectsandcontinuousmethods.Finallydue
tothethinwallthicknessrequired,processeswhichcouldntprovideathicknessoflessthan1
mmwereremoved.Thefinalfiveprocesseswerethenrankedaccordingtotherangeof
thicknessavailablebelow1mm.
Thetopfiveprocessesforshapingthetubesare:
1. Wiredrawing
2. Rollforming
3. Shapedrawing
4. Coldshaperolling
5. Powderextrusion
Wiredrawingconsistsofpullingasolidcylindricalblankthroughahardeneddieinorderto
reduceitscrosssectionalarea.Tubedrawingisasubsetofwiredrawingwherethecylindrical
blankhasahollowcenterandamandrelisplacedinsidetokeeptheinteriordiameteratthe
valuethatisrequired.Thisprocessallowsforawallthicknessdownto0.1mmandatolerance
of0.010.04mm.Rollformingconsistsoffeedingacontinuoussheetofmetalthroughrollers
inordertocreatetheshape.Fortheheatexchangertubes,anadditionalweldingprocess
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wouldhavetobedoneinordertosealtheseambetweenthesections.Shapedrawingissimilar
towireortubedrawingwhereacontinuousblankispulledthroughadietocreatethefinal
geometry.Thedifferenceisthatamandrelisnotusedtocreatethedesiredwallthickness
whichlimitstheminimumwallthickness.Coldshaperollingissimilartorollformingexceptthe
processisdoneatroomtemperatureandproducesbettersurfacefinish.Thedownsidetocold
shaperollingisthehigherstressesinvolvedwhichdontallowforasectionthicknesslessthan1
mm.finally,powderextrusionusesheatedloosemetalpowdereitherformedintoabilletor
placeddirectlyintoachamber.Thematerialisthenpressedthroughadiewhichformsitinto
therequiredshape.Powderextrusionhasthesameproblemascoldshaperollingwherethe
sectionthicknessmustbegreaterthan1mm.
JustificationforMaterialsandProcesses
Inordertodetermineappropriatematerialsforthecryogenicheatexchangertubes,objectives
andconstraintshadtobedefined.Themainconstraintsassociatedwiththeheatexchanger
tubematerialare:
MinimumElongationof20%Strain MaximumServiceTemperatureLessThan163C GoodThermalConductor ExcellentTolerancetoCryogenicTemperatures
Additionally,objectivesweredeterminedtoincorporateperformanceindiceswithourmaterial
selections.Sincetheheatexchangertubesmainpurposeistotransferheatfromonefluidto
another,thermalconductivityisamajoraspectwiththismaterialselection.Therefore
consideringmass,area,andcostwiththermalconductivity;threeobjectiveswereconsidered:
MaximizeHeatFlowperUnitArea MaximizeHeatFlowperUnitMass MaximizeHeatFlowperUnitCost
Usinglevel2CESEduPack2011withthelimitsandobjectivesstatedabove,thetopfive
materialsdeterminedwerefoundtobe:
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AgehardeningwroughtAlalloys Brass Copper Nickel NonagehardeningwroughtAlalloys
Duetohighcorrosionrateswithhydrogensulphide,whichisabyproductofnaturalgas,
copperandbrasswereeliminatedfromourselection.Withcopperandcopperalloysremoved
fromourselectionlist,alevel3CESEduPackanalysisofthetopmaterialswasconducted
incorporatingthethreeperformanceindicesstatedearlier.Thisproducedthetopthree
materialswhichallperformedwellineachobjective.Therefore,thetopthreeoverallmaterials
fortheheatexchangertubes,inorderofperformanceindex,are:
5) Aluminum,2026,wrought,T3511
6) Aluminum,2024,wrought,T4
7) Aluminum,6082,wrought,T4
Wiredrawingwaschosenforitswiderrangeofavailablecrosssectionalareas.Thisprocessisalso
simplerthanrollforming,whichmakesitlessexpensive.Wiredrawingalsoconsistsofoneprocess
doneinstepsunlikerollformingwhichwouldrequireanextraweldingstep.Wiredrawingallowsa
rangeofcrosssectionalareasof.01to10mmwithanexcellenttolerancerangeof0.01to0.04mm.
Theprocessisalsocontinuouswhichallowsfortheproductionofverylongtubes,thisisidealdue
toatypicalheatexchangerbeing500minlength.
Figure3Wiredrawingprocess
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CostEstimation
Thetotalamountoftubingwasbasedonanaverageweightof91000kg.Thecomparisonof
potentialmaterialscanbeseeninTable1oftheappendix.Fromthetableitcanbeseenthat
Aluminum6082isthecheapest,withacostperunitweightof$1.258/kgwhichbringsthetotal
costofthematerialto$243,780.Thiscostdoesnotincludethecontainmentvesselandthe
weldedmountingstructure.
Forthechosenmaterialmorefactorswereconsideredsuchasthermalconductivityand
aluminum,2026,wrought,T3511waschosen.Outofthetwoavailableprocesses,onlythe
capitalcostandthetoolingcostcouldbeconsidered.Thecheapestprocessturnedouttobe
wiredrawingasseeninTable2withacombinedcostof$104,720.Thetotalcostoftheheat
exchangertubingincludingmaterialandprocesscameto$360,430.
FinalDesignandMaterials
Astherearesignificantbyproductscreatedintheliquefactionofnaturalgastheplateandfin
heatexchangerwasrejectedbecausethesmallpathwaysmaybecomecloggedduring
operation.Itwasdecidedthataspiralwoundheatexchangerwouldbettersuitedforthis
application.Thisheatexchangerhasmuchhighersurfaceareaoftubingresultinginhigherheat
flowfromthenaturalgastothecoolantwhencomparedwithamoreconventionaltubeinshell
exchanger.Also,thermalexpansionandcontractioncanoccurinaspiraltubeheatexchanger
withoutbreaking.Thetubingforthisheatexchangerwillbemadeoutofthefollowing
aluminumalloys(inorderfrommosttoleastdesirable):
1) Aluminum,2026,wrought,T3511
2) Aluminum,2024,wrought,T4
3) Aluminum,6082,wrought,T4
Aluminumhasahighthermalconductivityallowingformaximumheattransferthroughoutthe
heatexchanger.Aluminumalloyswillnotreactwithanyofthecoolants,naturalgasorby
productsfromtheliquefactionsothereisnoriskofcorrosion.Aluminumisalsoreadily
availableandinexpensivematerialthatiswidelyusedinheatexchangersandother
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mechanism.Thereisanaddedadvantagebecausealuminumcanbebenttocreatecomplex
pipingnetworkswithoutbreakingandiscompatiblewiththeprocessselected,wiredrawing.
Aluminumhasloweryieldstrengththannickelalloysbutthe2000seriesaluminumalloyswill
besufficientforthisfunction.Typicallytheheatexchangersusedtoliquefynaturalgasare5m
indiameterandrangebetween200mand500mlong.Usingaluminumwillkeepweightand
costdownwhilethespiralheatexchangerwillreducethemaintenancerequiredwhich,
consideringitssize,isvital.
SummaryandConclusion
Themostdesirablematerialswereallagehardenedaluminumalloyswhicharelowincost,
havehighyieldstrengthandhighthermalconductivity.Thesematerialsweresimilartothose
currentlyusedinthesetypesofnaturalgasheatexchangerswhichtypicallyusealuminum.
Othermaterials,suchascopper,wouldbeattractivehadtheynothavehadadversereactions
withbyproductsofthereaction.Aluminumisalsoductilesoitcanbebentintothespiral
shapesonceithasbeenmanufacturedintothinwalledtubes.Perhapsthemostimportant
constraintthataluminummetwasitslowminimumoperatingtemperature(below200C).
Afterpassingallconstraintsaluminumscoredhighinallthreeofthematerialindices:heatflow
perunitarea,heatflowperunitmassandheatflowperunitcost.
Wiredrawingistheprocessthatbestfitstherequirementsformakingsmallthinwalledtubes
whilestillhavinglowcostswhencomparedtoothermethods.Asanaddedbenefittherewillbe
noadditionalprocessesrequiredotherthanthefinalassembly.Afterresearchingcurrent
materialsandprocesswefoundthatthemethodsandmaterialsselectedforthecryogenicheat
exchangeraresimilartowhatiscurrentlyused.
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Bibliography
AirProducts.(2008,February).RetrievedMarch23,2012,from
http://www.airproducts.com/~/media/Files/PDF/industries/energylngbrochure0408.ashx
Ashby,M.F.(2011).MaterialsSelectioninMechanicalDesign4thEd.Burlington:Butterworth
Heinemann.
TheLindeGroup.(2008,December09).GryogenicHeatExchangersforLNGPlants.Retrieved
March23,2012,from
http://www.hts.org.uk/downloads/Linde_LNG_HEX_09Dec2008_Extract.pdf
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Appendix
MaximizeHeatFlowPerUnitArea:MaterialIndices: Slope=1
Figure4Level2CESEduPack2011Materials
Figure5Level3CESEduPack2011Al,NiAlloys
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MaximizeHeatFlowPerUnitMass:
MaterialIndices:
Slope=1
Figure6Level2CESEduPackMaximizeHeatFlowperUnitMass
Figure7Level3CESEduPack2011Al,NiAlloys
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MaximizeHeatFlowPerUnitCost:
MaterialIndices:
Slope=1
Figure8Level2CESEduPackMaximizeHeatFlowperUnitCost
Figure9Level3CESEduPack2011Al,NiAlloys
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Figure10Level3CESEduPackProcessrankofsectionthicknes
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Costestimationresults
Table1Materialcost
Material UnitCost($/kg) Avg.TubingWeight
(kg)
TotalCost
($)
Aluminum,2026,wrought,
T3511
2.81 91,000 255,710
Aluminum,2024,wrought,T4 2.85 91,000 259,350
Aluminum,6082,wrought,T4 2.58 91,000 234,780
Table2Processcost
Process CapitalCost
($)
ToolingCost
($)
TotalCost($)
WireDrawing 95,200 9,520 104,720
RollForming 667,000 19,000 686,000