Electric resistance heating
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Transcript of Electric resistance heating
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LECTURE38
ElectricResistanceHeating
Contents
Principlesofelectricheating
Requirementsforaheatingelement
AvailableMaterials
Lifeoftheelements
KeyWords:electricheating,heatingelements,power,refractorymaterials
Principlesof
electric
heating
WhenacurrentIflowsthroughaconductorofresistanceR,heatisproducedandthereistemperatureriseofconductorsinelectricalcircuits.Temperatureriseisundesirableandmustbelimited
bydesigntoavoidoverheatingoftheconductororitselectricalinsulation.Thisisoftentermedas
permissiblesurfaceloadingofaconductor.Theconductorcouldbeintheformofwire,stripsorany
othershape.
Permissiblesurfaceloadingisoftenaprimeconsiderationinchoosingthebestsizeoftheconductoror
heatingelement.Therelationshipis
q in watt IR UA T (1)Inwhich iscurrent, isresistance, isoverallheattransfercoefficient, issurfaceareaofthe
conductorandTistemperaturedifferenceresultingduetoflowofcurrent.
Considerawireofcircularcrosssection.TheresistanceRis
R K ,
InwhichKisresistivityin.m,lislengthofconductorinm and
(2)
isdiameterin .Thesurfacearea
ofthewire
A l D (3)
Combining1,2and3weget,
D 0 . 7 4
(4)
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Thisrelationshipgivesminimumdiameterin ofaconductorofspecificresistivityK whichcanbeusedtocarrycurrentI withoutincreasingthetemperatureoftheconductortoavaluemoretheTabovethesurrounding.
Requirementsforaheatingelement
Itshouldhavemeltingpointgreaterthanfurnacetemperature.
00 m
tRhal
00
Itshouldnotreactwiththeatmosphereofthefurnace.Commonlyfurnaceatmospheresare
oxidizing,reducingandmaycontainwatervapour.
Itshouldberesistanttothermalandmechanicalshock.
Itshouldhaveenoughresistivity.Resistivityistheabilityofamaterialtoinhibitflowofcurrent
inpresenceofappliedvoltage.
AvailableMaterials
Broadlyspeakingthreedifferenttypesofmaterialsareavailablenamelymetallic,refractorymetalsand
nonmetallic
ones.
Metallic:Amongmetallicmaterials,NickelchromeandironchromeAlarecommonlyused.Nickel
chromecanbeusedupto120 .IronchromeAlcommonlyknownasKanthalcanbeusedattemperatures120 to1400 .Pureplatinumcouldbeuseduptown1400 whereasalloysplatinuandrhodiumcanbeusedupto1700.P loyscanbeusedinreducingatmospheres.
Refractorymetals:
Molybdenumandtungstenareused.Mocanbeusedupto25 and upto3000 .Tungstenheatingelementscanbeusedindryhydrogenatmosphereandundervacuum.
Siliconcarbideheatingelementsareusedinhydrogenandreducingatmospheresupto1200oC.Inair,
Siliconandgraphiteareusedeitherinreducingorinaninertatmosphere.Maximumtemperatureis
Elementtype Advantages Disadvantages
Non metallic
Molybdenumdisilicidealsoknownassuperkanthalcanbeusedupto1750 .Thesafetemperaturedependsontemperature.Inair,molybdenumdisilicideheatingelementscanbeusedupto1700 ,inreducingatmosphereupto1400.
thetemperatureis1500oC.
2200.
Metallic i) Available in strip and wire form so i)Relatively lowmaximumelement
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thatfabricationiseasy
ii)Lessexpensive thanSiCandMoSiO2
elementsforthesamepowerinput
iii)Linevoltagecanbeused
temperature
ii)Needsupportingmechanism
iii) Resistance increases with time
due to reduction in cross section
caused by oxidation and creep,
these results in decreased power
outputandfailure.
SiliconCarbide i)Elements are self supportive. They
can be used in furnaces that are too
wideortoolong
ii) Available in wide range of shapes
andsizes.
iii) Capable to operate at higher watt
loading than metallic elements and
hence higher temperature can be
obtained
iv)Elements can be changed while the
furnaceishot
i) Costly power control equipment
is required than metallic elements
(multitap transformers are
needed)
ii) Brittle in nature and hence
breakseasily
iii) Electrical resistance increases
withtime,maintenanceisrequired
Molybdneum
disilicidei)
Available
in
a
variety
of
shapes
and
sizes
ii) Without material degradation fast
thermalcyclingispossible
iii) Relatively easy to change while the
furnaceishot
iv) Stable resistance, that means new
andoldelementscanbeconnected.
v) Can operate at higher watt loading
and temperature than metallic
elements.
i)
More
costly
power
control
equipmentthanmetallicelements
ii)Elementsareexpensive
iii)Riskoffracture.
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Thelife
of
the
elements
depends
on
1.Fluctuationinsupplyvoltage: Forevery1%increaseinvoltage,therewillbe a2%increaseinpower.
2.Wattdensityoftheheatingelements.
3.Ifmorepowerisneeded,increasethediameteroftheelementwireorreducethelengthofthe
element.
4.Theelementsmustbeoperatedatthedesignvoltage.
5.Forthecyclicnatureapplicationselementsneedadequatespacetomoveontheirhangersor
supports.Elementsshouldnotbelocatedclosetothebottomortoarefractoryshelfbecauseexpansion
maycausethemtorestontherefractorywhereheatwillnotdissipatefromtheelement,allowingahot
spottodevelop.
.
Source Daniel H.Herring [email protected]
