Amorphous metallic alloy ribbons heating elemen1.docx
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Amorphous metallic alloy ribbons heating elemen1.docx
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Amorphousmetallicalloyribbonsheatingelemen1
AmorphousMetallicAlloyRibbons
HeatingElement
M.AGeller,E.T.Brook-Levinson
AdvanceHeatingTechnologyLtd.,Israel
1.Introduction
Varioustypesofheatersarewidelyusedindifferenttechnologicalprocessesanddomesticapplications.Metalsandtheiralloysaremainlyusedasmaterialsforheatingelementconstruction.Themostusableformofsuchelementsiswire.Highservicepropertiesandrelativelylowpricemakesthemanattractivematerialforheatingapplications.Atthesametime,scientistsandengineersarelookingfornewmaterials,whichcanserveasheatingelements.Forexample,recentlyheatersbecameknownusingelectricalconductiveceramicsandevenconductiveplastics.
Theamorphousmetallicribbonsknownformanyyearsandwidelyusedassoftmagneticmaterials,wereneverusedinthepastintheheatingapplicationsalthoughtheamorphousribbonsrevealhighmechanicalandelectricalproperties.Thestudyoftheamorphousribbonasheatingelementshasbeencarryingout.Theresultsofthesestudiesarepresentedinthepresentarticle.
2.Themethodforamorphousribbonproductionandtheirphysicalproperties
Theamorphousmetallicalloyribbonsareusuallyproducedbythemethodofrapidquenchingbypouringthemeltedalloyonarotatingmassivedrum.Linearvelocityatthedrumedgeisabout30m/s.Normallythedrumismadefromcopperorbronzetoprovidehighcoolingratethroughthehighthermalconductivityofthedrummaterial.
Thecoolingrateprovidingamorphousstateofthesolidifiedmetalalloyisabout105-1060C/sdependingonthealloycomposition.Theribbonswiththe20-30mthicknessand5-100mmwidthcanbefabricatedusingrapidquenchingprocedure.
2.1Thetypicalpropertiesofamorphousribbons
1.Hardness2-20GPa
2.Tensilestresses500-1200Mpa
3.Electricalresistivity1-510-6Ohmm
4.Thetemperatureexpansioncoefficientcloseto0
5.Thecrystallizationtemperature250-5000C
2.2Comparisonbetweencrystallineandamorphousmetallicmaterialsuseasheatingelements
2.3Crystallinematerials
Metalsandtheiralloysarethemostlyusedmaterialsforheatingelementfabrication.Theyexposethefollowingproperties:
1.Highelectricalresistivity
2.Highoperationtemperature
3.Highanticorrosionresistance
Canthalandnickel-chromiumalloysaremostlyusedforhightemperatureheaters.Theyhavehighelectricalresistivityof1.4-1.6*10-6Ohmmandhighoperationtemperatureranging8000Cto15000C.Themanganin,constantanandreotanareusedformiddletemperatureheaterswithoperationtemperatureofupto3000C-5000C.Theheatersareproducedmainlyintheformofwirespirals.Thewiresusuallyworkunderhightemperatureduetotheirrelativelylowheattransferarea.Allelectricalpoweristransferredtotheambientinsteadystateheateroperationmode.Thetransferredelectricpowerinsteadystateisdefinedbythewell-knownequation[1]:
P=αST
(1)
HerePistheelectricalpoweroftheheater,αisthecoefficientofheattransferbetweentheheatingelementandambient,Sistheheattransferareaoftheheatingelement,Tisthetemperaturedifferencebetweentheheatingelementandambient.
OnecanreadilycanseefromEq.
(1)thatthesamepowertransfercanbeprovidedwithhightemperaturedifferenceandsmallareaoftheheatingelementor,viceversa,withlowtemperatureandlargeheattransferarea.Thewiresasheatingelementshaveminimumareaperunitvolume.Thisresultsinhightemperatureoftheheatingelement.Forexample,ifwecomparetwoheaters,oneofthemanwireelement0.5mmbydiameterandanotheranamorphousribbon10mmbywidth,provingthesameelectricalpower,wecanconcludethattheiroperatingtemperaturesareessentiallydifferent.Thetemperatureoftheribbonelementisby12timeslowerthenthetemperatureofthewireelement.
Thisexplainswhytheamorphousribbonsareofgreatinterestasheatingelement.Nevertheless,eventhecrystallineribbonsarepracticallynotusedintheheatingelementapplications.Wecanassume,thatthereasonisinconventionalapproachofthedesignersusingthehightemperaturewiresasheatingelementsforalltemperaturerangesoftheheatingapplicationsalthoughthewirepossessesminimumsurfaceareaperunitvolume,whichresultsinhighworkingtemperatureoftheheatingelement.Becausethetemperatureoftheribbonelementsisessentiallylowersuchlowtemperatureheatershaveadvantagesincomparisonwithwireswiththesameelectricalpower.Amongthem:
1.Lowoperationtemperaturereducesrequirementstotheheatandelectricalinsulation
2.Betterenvironmentalfriendliness(thedustisburntonthesurfaceofthehightemperatureheatingelements)
3.Amorphousmetallicalloyribbonasheatingelement
Whyamorphousmetallicalloyribbonisgoodheatingelement?
Thefollowingsetofparametersgivestheanswertothisquestion.
Ribbonsthickness20to30m;
Widthrange2to100mm;
Resistivity>10-6ohmm;
Operationtemperature4000C
Highcorrosionresistance(cobaltandchromiumbasedalloys);
Highribbonductilityandflexibility;
Highheattransferareaduetothelargeribbonwidth;
Shortwarm-uptimetosteadystateduetothelowribbonmass.
Savingofelectricalenergyinunsteadyoftheheatersoperationmodes.
Theheatdesignincludesthefollowingsteps:
1.Calculationoftheheaterpower
2.Calculationoftheheatertemperature
3.Checkingoftheinsulation
4.Designoftheheatingelement
5.Testingoftheheatingelementinaccordancewithstandardsforheatingapplications
Astheamorphousribbonshaveneverbeenusedasheatingelementtheymustbetestedasheatingelementsatallstagesoftheheatelementfabrication.Twoseriousrestrictivefactorsexistwhenusingtheamorphousmetallicalloyribbon:
1)Theoperationtemperatureoftheribboncannotexceedthecrystallizationtemperatureoftheamorphousmaterial.Atthetemperaturehigherthanthecrystallizationtemperaturethematerialbecomescrystallineandtheelectricalresistancesharplydropsdown.
2)Theoperationtemperatureoftheribboncannotexceedtheembrittlementtemperatureofthematerial.Belowthistemperature,themechanicalpropertiesdeteriorateandthematerialcanbeevendestroyed.
Bothfactorsshouldbetakenintoaccountduringthedesignoftheheatingelementbasedonamorphousribbons.
3.1Investigationoftheheattransferbetweentheamorphousribbonandtheambient
Tocalculatethetemperatureoftheribbonelementtheheattransfercoefficientmustbeknown.ThemeasurementschemeispresentedinFig.1.
Theamorphousribbon1mbylengths;25mmbywidthand27mbythicknesshasbeenpluggedtoanelectricalsource.Theelectricalcurrentandvoltageattheribbonhasbeenmeasured.Thechromel-alumelthermocouplewasusedtomeasurethesteadystatetemperatureontheribbonsurface.
Fig1.Measurementschemeforheattransfercoefficient
1-amorphousribbon;2–voltmeter;3–ampermeter;4-temperaturesensor;5–electricsource
TheresultsofthemeasurementsarepresentedinTable1
Table1
Current,voltage,powerandtemperaturedifference
betweentheribbonssurfaceandambientair
No
I,A
V,V
P,W
T,0C
W/m*K
1
0
0
0
0
0
2
1.30
2.96
3.84
8
9.60
3
1.75
4.00
7.00
14
10.00
4
3.35
7.55
25.29
48
10.50
5
4.52
13.27
60.00
92
11.00
6
7.11
16.00
113.76
177
12.80
Theelectricalpowerhasbeencalculatedbythefollowingequation
P=V*I
(2)
Heattransfercoefficienthasbeencalculatedfrom:
P=2αST(3)
Heattransfercoefficientαhasbeencalculatedfrom(3)as
α=P/2ST(4)
TheaveragevalueoftheheattransfercoefficienthasbeencalculatedfromtheexperimentaldatausingtheEXCELprogram.Theaveragevalueoftheheattransfercoefficientwasfoundtobe
α=13W/m20C(5)
4.Designoftheelectricheaterwithamorphousmetallicalloyribbon
4.1Calculationoftheheater
A500Wpowerheaterwasdesignedandtestedatthevoltageof24V.Allstepsofthecalculationsarepresentedbelow.
Step1.Calculationoftheelectricresistance.
R=V2/P=242/500=1.15Ohm
Step2.Thecaseoftheheaterhadthedimensionsof33cmx23cmx40cm.Forsuchdimensionstheoptimumdesignistheradiatorcomprise12platesconnectedinseries.Eachplateconsistof7ribbonsconnectedinparallel.Theelectricresistanceofaplatewas0.095Ohm.
Theamorphousribbonhadthefollowingdimesions:
Ribbonsthickness2.510-5m
Ribbonswidth2.510-2m
Thespecificelectricresistanceis1.410-6Ohmm
Thelengthofoneribbonwas30cmthatallowstheplateresistanceof0.095Ohm.Theelectricresistanceoftheoneribbon30cmlengthis0.672Ohm.
Theelectricresistanceoftheplateconsistingofsevenribbonsconnectedinparallelis0.692/7=0.095Ohm.
Step3.Ribbontemperaturecalculation.Theconsumedelectricpowerequals
P1=P/12=500/12=41.66W
ThetemperaturedifferencecanbecalculatedfromtheNewtonequation (3)
T=(P1/2αS)=41.66/(2*13*2.1*2.510-2)=300C
Attheambienttemperatureof200Ctheribbontemperaturereached500C.ItmustbeunderlinedthattheembrittlementtemperaturefortheFeBCrribboncompositionisinthetemperaturerangeof3500Cto4500C.Asonecansee,theribbonoperationtemperatureisveryfarfromribbonembrittlementtemperature.
Step4.Heatingelementdesign.TheviewoftheoneheatingplateispresentedintheFig.2.
Fig2.Theviewoftheheatingplate
1–amorphousribbon;2–ribbonswidth;3–electricconnectedcopperstrip;4 –thedistancebetweenribbons
Oneplateconsistsof7ribbonsof25mmbywidthand30cmbylength.Thedistance
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