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Published by the EPRl Centeroraterialsabrication Vol. 3, No. 7 Revised 1994. .

Clean, Controlled HeatingElectric resistance furnaces offer asafe, efficient, reliable and cleanmethod for heat treating, melting,heating prior to forming, and brazingmetals. Electric furnaces are also easy

to control, and operate over a widetemperature range. In addition tohe?ting metals, they are used formelting glass, sintering ceramics, andcuring coatings. And the number ofapplications continues to grow astechnological developments broadenthe operating temperature range ofelectric furnaces, and the demand for?utomatic process control increases.

Resistance heating is based on theprinciple that, when a current ispassed through an electrical resistor,electrical energy is converted tothermal energy. The thermal energythen is transferred to the partbyconvection, radiatbn and/or conduc-tion. This issue of TechCommentarydescribes indirect resistance heatingand discusses the echnical andeconomic factors to consider whendeciding if the process could benefit aparticular application or product.Direct resistance heating and encasedresistance heaters are discussed inTechCommentary, Vol. 3, No. 8.Advantages

Often, an electric resistancefurnace and a gas-fired furnace areequally appropriate for a particularapplication, and the choice is basedon economics. However, severalcharacteristics of electric resistancefurnaces may make them the betterchoice for your application. The7dvantages of electric resistance..urnaces include:H Flexibility. Both operating tempera-ture and furnace atmosphere canbe varied. Resistance heatingelements are available for alltemperatures of .importance in

Lc

Schematic diagramof an indirectresistance heating furnaceshowing the arrangementof theheating elements on the walls.Below about 1250 F, heat transferto the workpiece s primarily by'convection. sometimes with the aidof fans, as in "forced" convectionfurnaces. Radiation s the majormode of heat transfer at tempera-tures above about1250 F and invacuum furnaces where theres noatmosphere to support convection.

industrial processing, whereas themaximum temperature ofgas-flredfurnaces is limited to approximately2400 F. Heating in a controlledatmosphere or a vacuum is easilyachieved with an electric furnace.Such control is more difficult in agas furnace unless radiant tubes rprotective muffles are used.a Automatic temperature control.Microprocessors and solid-stateswitches can control furnacetemperature automatically. Becausethe location and configuration ofelectric elements can be easdytailored to a given application,temperature is more uniform thanwith other types of furnaces.

Improved working conditions.Electric furnaces are quiet because

there is no combustion or blowernoise (except in orced alr furnaces,where there IS some fan noise). Theabsence of smoke and hot fluegases makes the plant both cleanerand cooler, thus mmimizing con-cerns for worker safety and environ-mental pollution.a Cost savings. For some applica-tions resistance furnaces are moreenergy efficient and save space.Reslstance furnace efficiency isrelatlvely Independent of tempera-ture, whereas the efficiency of gas-fired furnaces drops sharply withincreasrng temperature. Waste heat

Is mlnlmized since there are no hotflue gases In an electric furnace,Space is saved because there is noneed to store or plpe in flammablefuel or remove exhaust gases.H Safety. There s ittle explosionhazard connected with the heating

system with an electric resistancefurnace.H Serviceability. Every ndustrlalcompany is likely to have anelectrician with the skilis to repair anelectric heating system. Burnerexperts are less common, but arenecessary to maintain the efficiencyof a gas-fired system.Applications

Indirect resistance heating is usedprimarily in the metals, ceramics,electronics and glass industries. Themore frequently used processes thatincorporate this heating techniqueinclude:H Heat treatment of metalsH MetalmeltingH Heating prior to forminga BrazingH Sinteringceramicse Curing coatingsH Glass empering.

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Heat tre'atment of metals-Indirectresistance heating,is used for anneal-ing, austenitizing, normalizing, harden-ing, tempering, nitriding, carburizing,and sintering a wide range of ferrousmaterials. I t is also used for annealing,solution treating, and aging nonferrousmetals. These processes are carriedout at temperatures ranging from300 to 2400 F, as shown in Table 1 .

Both gas-fired and indirect resis-tance furnaces can be used at lowtemperatures and where there are nospeccal atmosphere demands. How-ever, aerospace alloys and tool steelsoften have to be treated in a controlledatmosphere. For example, titaniumalloys, which are prone to contamina-tion, particularly hydrogen pickup,must be processed in a vacuum or aninert atmosphere. Similarly, tool steelsoften must be heat treated in avacuum or special atrnosphere toprevent vaporization of importantalloying elements at the surface. Thesespecial conditions are easily arrangedin an indirect resistance furnace butdifficult io achieve in a gas furnace.Ind LAo n is occasionally used frapid heating is required. However, thecost of coils and power suppliesmakes it economical only in highvolume applications.Metal melting-Indirect resistancefurnaces for melting and holdingmetals, especially nonferrous alloys,have become popular in the castingindustry in the last ten years. Thereason for this populari?y is the

increased availability of light-weightrefractory materials for buildingindirect-resistance-heatedcruciblefurnaces, The low thermal mass ofthese materials makes the furnacesvery energy efficient.is often an electric resistance cruciblefurnace at each casting machine tohold the molten aluminum at theoptimal casting temperature ( I 100- .1200 F depending on the alloy). Gasfurnaces can be used, butt is moredifficult to maintain their contents at aconstant temperature. In addition toallowing excellent temperature control,electric heating minimizes gas pickupby the molten metal, or oxidation of thesurface. Together, these factors canreduce metal loss by half. Electricheating also limits hot spots andthermal stresses in the furnace; and,because there are no combustionproducts, the crucible is not eroded byhot gases.

The capital cost of an electriccrucible furnace is somewhat higherthan that of a gas-fired furnace($18,000versus $14,000 for500 Ibcapacity ), but the extra cost can bequickly paid back by decreasedmaterial loss.or refractory metals like titanium andmolybdenum, must be melted in avacuurn. While this can be done n aresistance heated furnace, vacuummelting is usually done with induction,arc or electron beam heating since

In modern aluminum foundries there

Some materials, especially reactive

these methods result In faster meitrngand a more homogeneous productHeating prior to forming-In t h eforging Industry, electric-basedheating is gaining in popularity forbillet preheating The reason IS thatelectric furnaces are considerablymore efficient than gas-fired furnaces,even those with recuperators, atpreheating temperatures (around2100 F, the exact temperaturedepending on the specific material).

Direct resistance and inductronheating are two other methods usedfor preheating materials. Both heatfaster than indirect resistance becausethe heat is generated within theworkpiece. However, they only workefficiently for simply-shaped blanks.such as rods and bars, madeof fairlyhlgh-resistivity metals. There areessentially no restrictions on thematerials or workpiece shapes thatcan be heated n an indirect resistancefurnace. And a single furnace can beused to heat a wide variety of parts.Brazing-Metal components areoften brazed in indirect resistancefurnaces because of the need for acontrolled atmosphere (o r vacuum) ora carefully controlled thermal cycle.Induction brazing is also done, but iseconomical only for high-volumeapplications because the complexcoils required are so expensive.Sintering ceramics-Ceramicmaterials are widely used in theproduction of various electroniccomponents including capacitors,resistors and piezoelectric elementsThey are also being developed fo r usein internal combustion engines.Theseceramic materials must often be fired,or slntered, at temperatures as high as3000 F, and both temperature andfurnace atmosphere have to beprecisely controlled. The electroncsindustry also uses indirect resistancefurnaces for growing, purifying andprocessing the silicon and germaniumcrystals and wafers used in manysemiconductor devices.Cur ing coatings-Resistancefurnaces are used in the finishingindustry for baking vitreous enamelcoatings onto metal substrates and fordrying and curing organic coatings,such as paints and varnishes, on avariety of materials. The main competi-.tors to indirect resistance furnaces aremedium and short wave infraredovens.

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Glass ternpering-Glass is tem-pered by heating to a carefully con-trolled temperature (usually around1100 to 1200 F) followed by rapid butuniform coolmg. Tempering results inresidual compressive stresses on thesurface that provlde greater resistanceto fracture and damage. Typicalproducts include automobile andarchitectural glass. Tempering isalmost always carried out in resistancefurnaces.Technical Considerations

Indirect resistance furnaces areusually designed for a particularapplication, although modifying anexisting furnace to operate at adifferent temperature range is quitefeasible.

Important factors to consider inselecting an indirect resistancefurnace are:rn Heating element characteristicsrn Form and arrangement of heating

Material handlingPower requirements.

Heating element characteristics-The choice of furnace heatingelements is influenced by both thetemperature required and the furnaceatmosphere. Commercially availableresistance heating elements arebroadly characterized as metallic ornonmetallic. Common metallic ele-ments, made of alloys containlngvarious percentages of iron, nickel,chromium and aluminum, work up toapproximately 2400 F. For tempera-tures above about 2200 F, however,noble or refractory metals, such asplatinum, molybdenum, tantalum, ortungsten, or ceramic elements,including silicon carbide, molybdenumdisilicide and graphite, are used.

The specific heating process andthe workpiece material determinewhether the furnace should have anair, inert, oxidizing, or reducingatmosphere, or whether a vacuum isappropriate. Many metallic heatingelements can be used in air. However,molybdenum, tantalum, and tungstenelements cannot be exposed ooxygen. Molybdenum and tungstenaork well in hydrogen or inert gas andin a vacuum. Tantalum absorbshydrogen as it cools below 1200F andalso absorbs residual oxygen in anatmosphere. Therefore, tantalumelements are best used only in a

elements

~ fvlaxlmurn OperatingperatingElementaterialemperature F Environment

Metal-sheathed elements 1300 Alr* Fe-Ni-Cr alloys 1850 Alr* Ni-Cr alloys 2000 Air* Fe,Cr-AI alloys 2400 Air

Platinum 2900 AirPlatinum-rhodium alloys 3250 AirMolybdenum 3400ydrogen

Tantalum 4000 Heliurn/argonTungsten 5425 Vacuum

4080 Vacuum4710 Vacuum

Silicon carbide 3100 AirMolybdenum disilicide 3275 AirCarbonigraphite450 Helium/argon

* These elements can also be used In a protective atmosphere. The maximum Operattngtemperature will dependon the composition of the atmosphere. Consult heating elementmanufacturers for further details. -~Table 2.Operating Characteristics of Various Heating Element Mate.r.ig!s

vacuum. Because they oxidize rapidly,graphite elements must also be usedonly in an inert or vacuum envircn-ment. However, silicon carbide andmolybdenum disilicide can be used athigh temperatures and in air. Maxi-mum operating emperatures andenvironments for various heatingelement materials are shown inTable 2.by both operating emperature andfurnace environment. However,average values are:

Heating element life is influenced

Operating ElementTemperature F Life Years

Below 1200 More than 41400-1 600 2-4

2000 1-2Form and arrangement f heatingelements-Metallic heating elementscome in a variety of forms, mostcommonly wires, rods, strips, ribbons,sheets, plates, and bars (Figure 1 ) .Nonmetallic elements are usually rod-

I I

Figure 1. Common shapes and formsof metallic elements.

shaped. Element form, as well asplacement of elements within thefurnace, is generally determined bythe furnace manufacturer.A variety offactors inf luence the final choice. Theyinclude volume and shape of thefurnace enclosure and the areaavailable for the heating elements, andease of installation and replacement ofelements. Also considered are thetypes of treatment to be performed,the mode of heat transfer (radiation orconvection), furnace insulation, themethod of transporting materialthrough the furnace, and the mechani-cal strength of the heating elements.

Element strength determineswhether heating elements are ar-ranged horizontally or vertically in thefurnace. Horlzontally mounted metalltcelements tend to sag at high tempera-tures. Suspending them verticallyrelieves this problem. Element strengthalso determines to a certain extentwhether radiant heat is transferreddirectly or indirectly from the elementsto the charge. A "muffle" or tubefurnace is used for indirect radiantheating. Heat passes by radiatton fromthe heating element to the walls of thetube and then to the workpiece withinthe tube; such a design protects heheating elements and evens outtemperature gradients and hot spots.Material handling-Indirectresistance furnaces are available withboth batch and continuousmaterialhandling. Batch furnaces can be usedfor a wider variety of products and

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parts, and have more flexible cycletimes. In contlnuous furnaces, thecharge IS moved through at a constantrate, either on a high-temperaturemesh conveyor belt or by a mechani-cal pusher.Power requirements-Because of themany material and p rocessing param-eters involved, precise estimates ofpower requirements for indirectresistance furnaces are best left toexperienced furnace manufacturers orutility experts.Economic Considerations

The following factors should betaken into consideration in determiningthe economic feasibility of indirectresistance furnaces:

EquipmentcostEnergycostMaintenance costs.

Equ ipment cost-The cost of anindirect resistance furnace s deter-

mined mostly by size and operatingtemperature.A 2 x 3 1.5 t. furnaceoperating up to about 2000 F wouldcost around $60,000. Increasing thesize to 5 x 6 x 5 f t . would increase thecost to $2.00,000.urnaces operatingto higher temperatures are moreexpensive because of increasedmaterial costs.Ene rgy cost-Energy cost is deter-mined by fuel cost and the operatingefficiency of the furnace. On a per-BTU basis, natural gas usually costsabout one-third as much as electricity.However, at 1800-2200 F a gas-firedfurnace without a recuperatoris onlyabout 20% efficient. Adding arecuperator increases efficiencyo35-40%. But electric furnaces aretypically 60-70% efficient. Thus,increased efficiency can often com-pensate for higher energycosts. Theactual utilization break point at whichelectric resistance furnaces are moreeconomical than gas-fired furnacesdepends on the relative costs ofelectricity and gas.

Maintenance costs-lndlrect resis-tance furnaces require no burneradjustment or other special mainte-nance. The major tasks are replaclngburned out elements, whlch is usuallyeasy, and cleanmg out the furnacewhen necessary.In Summary

Indirect resistance heating isappropriate for a variety of heattreating, preheating, sintering, andbrazing processes. The ease oftemperature control, atmospherecontrol, and maintenance, combmedwith high energy efficiency, make t anattractive technique in the metals,ceramics, electronics, and glassindustries, among others.

The information discussed in thisissue of Techcommentary s anoverview and intended onlyto familiar-ize you with the basic aspectsofIndirect resistance heating. If you areinterested in more detailed informationplease contact CMF or an equipmentmanufacturer.

TheEPRI Center for Materials Fabrication (CMF) is an R&DApplications Center sponsored by the Electric Power Research lnsfitute (EPRI) andoperated by Battelle. EPRl s a nonprofit organization whose mission is to discover, develop, and deliver high value technological advancesthrough networking and partnership with the electricity industry.CMF's mission s to assist EPRl mem-bers and their industrial customers nimplementing cost-efficient electrotech-nologies that inlprove efficiency,productivity, and address environmentalissues. TechCommentary is one commu-nication vehicle that the Center usesotransfer technology to ndustry throughan electric utility network. The Centeralso conducts applications developmentprojects that demonstrate innovativeuses of electrotechnologies.This issue of TechCommentary wasmade possible through he cooperationof Battelle staff members Lee Semiatin,Senior Research Scientist, JohnHallowell, Industry Applications Engi-neer, Laura Cahill, Manager, Marketingand Communications, andDenise

Sheppard, Publications Coordinator; andAnne Moffat and Dorothy TonjesofProwrite. Technical review was providedby Tom Groeneveld, Battelle; RobertWatson, The Kanthal Corporation; andSteve Maus, Lindberg Parts and Service.The source used in this issueofTechCommentarywas:Orfeuil, M. Nectric Process Heating:Technologies/Equipment/Applications,Battelle Press, 1987.Photograph courtesy of The KanthalCorporation, Bethel, CT.Applicable SIC Codes34-21, 23, 25, 29,41-43, 49 , 51, 52,

62, 63, 82-84, 89, 93, 96, 9735-1 1 , 19, 23, 24,31-33,41, 42, 44-46,

49, 51, 62, 63, 66, 68, 72-74, 76,79, 92, 99

36-1 2, 21, 43-46, 9137-11, 13-15, 21, 24, 28, 31, 32, 43,

51, 61, 64,95, 9938-1 1, 41-43, 6139-1 1 , 14 , 15, 44, 61, 64For ordering information, callEPRImAMP at 1-800-4320-AMP.Members are welcome o quote verbatlmfrom this publication provided CMFscredited. CMF approvals required foreditorial incorporation,as pertaining toU.S. copyright laws.

For technical information, contact

CENTER FOR MATERIALS FABRICATIONAn EPRl RBD Appl lcat lons Center505 Ktng Avenue Columbus,Ohto 43201-2693(614) 424-7742 FAX (614) 424-4320Copyrtght 0 1994 All Rlghts ReservedEleclrlc Power Research Institute. Palo Alto, CANATS # TC-103922-V3P7