KANTHALHANDBOOKResistance Heating Alloys andSystems forIndustrial FurnacesCopyright Kanthal AB.Printed in Sweden by PRIMAtryck, 2001Catalogue 1-A-5B-3 09.2001 5000KANTHAL, NIKROTHAL, TUBOTHAL,HOT ROD, CRUSILTIE, FIBROTHAL are registred trademarks of Kanthal AB, Sweden.TM MEANDERTHAL.This information, which may be subject to change, is offered solely for yourconsideration and should not be taken as a warranty or representation forwhich we assume legal responsibility. It is not to be understood as permissionor recommendation to practice a patented invention without a licence and theuser should determine whether relevant patents exist.1Contents PageMetallic Heating Elements from Kanthal 2KANTHAL or NIKROTHAL? 3KANTHAL APM Heating elements 4Physical and Mechanical Properties 5Furnace Wall Loading 6Element Surface Load 7Operating Life and Maximum Permissible Temperature 8Key Data for Kanthal Elements 10Tubothal Heating Elements 12KANTHAL APM Extruded Tubes 15KANTHAL APM standard product range 17Tables KANTHAL A-1 and APM 19 KANTHAL AF 20 KANTHAL D 21 NIKROTHAL 80 22 NIKROTHAL 70 23 Terminals 24KANTHALHANDBOOKResistance Heating Alloys andSystems forIndustrial FurnacesMetallicheatingelementsfrom KanthalThis booklet contains technical data for ourresistance heating alloys KANTHAL and NIKROTHAL.We also include some instructions for thecalculation and design of heating elements forindustrial furnaces as well as examples ofsupport systems and insulation. The APMtubes are described shortly. For more informa-tion, ask for the special Tube Handbook.The latest product- and application informa-tion is found on www.kanthal.com.We can assist you: in choosing suitable element material, ele-ment type, support system and insulation, by supplying complete heating elementsready for installation, to upgrade electric furnaces to latest heating technology for maximum output andeconomy to replace radiant tubes with Kanthal APMtubes in both gas- and electrically heatedfurnaces and to supply complete recuperati-ve systems (SER).Our modern workshops have developed consi-derable experience in manufacturing heating elements and can manufacture to anyspecifications.Our delivery times are short and our serviceobjectives are high.3KANTHAL or NIKROTHAL?There are two main types of electric resistancealloys. Nickel-chromium (e.g. 80 Ni, 20 Cr) wasdeveloped around the turn of the century andwas soon used as heating element material inindustrial furnaces as well as in electric house-hold appliances.In the thirties Kanthal introduced a newresistance heating alloy based on iron-chro-mium-aluminium with a longer life and a highermaximum operating temperature than nickel-chromium.The two main types of alloys have their ownspecific properties, with advantages and dis-advantages, and are supplied in many differentgrades and forms. Kanthal manufactures bothtypes of alloys under the names NIKROTHAL(nickel-chromium) and KANTHAL (iron-chro-mium-aluminium).In general KANTHAL is superior to NIKROTHAL in respect of performance andlife and is therefore nowadays a standardmaterial choice when it comes to metallic heating elements for industrial furnaces. Themost important advantages are: Higher maximum element temperature of1425C 2600F compared to 1250C 2280F Longer life (24 times) Higher surface load Higher resistivity Lower density No spalling oxide, which may contaminatethe goods and the furnace and also causeshort circuit or failure of elements and gasburners.For the furnace user, using KANTHAL resultsin less material at a lower price and also a longer life. Table 1 shows an example ofweight saving and lower cost obtained byusing KANTHAL instead of nickel-chromiumalloys. This lower element weight will alsoresult in considerable cost savings in the support system, because fewer suspensionhooks are necessary.The NIKROTHAL alloy may have specialadvantages if you need a heating elementhaving very good mechanical properties in thehot state. KANTHAL APM has, however, creepstrength at elevated temperatures in the samelevel as NiCr 80/20.NiCr 80/201000 18301068 19553.611.053.445.5 0.2173.09 19.9224.9 73844.4 98Element DataFurnace temperature, C FElement temperature, C FHot resistance, RwTemperature factor, CtCold resistance, R20Wire diameter, mm inSurface load, W/cm2W/in2Wire length, m, ft 3 elementsWire weight, kg, lb 3 elementsKANTHAL AF1000 18301106 20253.611.063.415.5 0.2173.98 25.7174.6 57329.6 6544.429.6= 33%44.4= 33%9865= 33%98= 33%Weight saving:Table 1. A 120 kW furnace equipped with R.O.B. elements. 3 elements of 40 kW each, 380 V.4Kanthal APM heating materialKanthal APM is a resistancematerial which can be used toimprove the performance at hightemperatures, where conventionalmetallic elements can cause pro-blems (e.g. bunching, creeping,oxide spalling) and to open upnew applications where metallicelements are not used today.The great advantagesof Kanthal APM are:Improved hot strength, giving: much better form stability ofthe heating element less need for element support low resistance change (ageing) longer element lifeExcellent oxide, giving: good protection in most atmos-pheres, especially corrosiveatmospheres no scaling and impurities a longer element lifeComparison between Kanthal APM (top) and conventional FeCrAl after 1250 h at max 1225C, 2240F element temperature.Fig. 1 Elongation at 1300C, 2370F element temperature.Fig. 2 Sagging test dia. 9.5 mm, 1300C, 2370F and 1400C,2550F, 300 mm between supports.Time (h) 1000C 1832FMPa psi100 5.6 8121000 3.4 47810000 2.2 320TemperatureTime (h) 1200C 2192FMPa psi100 3.3 4781000 1.6 23210000 0.7 100TemperatureTime (h) 1400C 2552FMPa psi100 1.3 1891000 0.5 7210000 0.2 30TemperatureCreep rupture strength151050530201000 100 200 300 500 500 6000 50 100 150FeCrAlElongation (%)Sagging (mm)APMAPM 1400C 2550F1400C 2550F FeCrAlAPM 1300C 2370F1300C 2370F FeCrAlTime (h)Time (h)5KANTHALAPMKANTHALA-1KANTHALAFKANTHALDNIKROTHAL80NIKROTHAL70NIKROTHAL60NIKROTHAL40Physical and Mechanical PropertiesKANTHAL and NIKROTHAL alloys are generally available in wire or strip form. Physical and mecha-nical properties of the alloys are listed in Table 2.Table 2. KANTHAL and NIKROTHAL, Wire and Strip.Max continuousoperating temp., C 1425 1400 1300 1300 1200 1250 1150 1100operating temp., F 2600 2550 2370 2370 2190 2280 2100 2010Nominal composition, % Cr 22 22 22 22 20 30 15 20Nominal composition, % Al 5.8 5.8 5.3 4.8 Nominal composition, % Fe Rest Rest Rest RestNominal composition, % Ni 80 70 60 35Resistivity at 20C, mm-2m-11.45 1.45 1.39 1.35 1.09 1.18 1.11 1.04Resistivity at 68F, /cmf 872 872 836 812 655 704 668 626Density, g/cm37.10 7.10 7.15 7.25 8.3 8.1 8.2 7.9Ib/in30.256 0.256 0.259 0.262 0.300 0.296 0.296 0.285Coefficient of thermalexpansion, K-120-750C, 681380F 14.10-614.10-614.10-614.10-617.10-616.10-616.10-618.10-620-1000C, 681838F 15.10-615.10-615.10-615.10-618.10-617.10-617.10-619.10-6Thermal conductivity at 20CW m-1K-113 13 13 13 15 13 13 1368F, Btu in ft2 h1 F190 90 90 90 104 90 90 90Specific heat capacityKJ kg-1 K-1, 20C 0.46 0.46 0.46 0.46 0.46 0.46 0.46 0.50Btu Ib1F1, 68F 0.110 0.110 0.110 0.110 0.110 0.110 0.110 0.119Melting point, C 1500 1500 1500 1500 1400 1380 1390 1390Melting point, F 2730 2730 2730 2730 2550 2515 2535 2535Mechanical properties (approx.)*Tensile strength, N mm-2680 680 680 680 750 875 750 750Tensile strength, psi 99000 99000 99000 99000 109000 127000 109000 109000Yield point, N mm-2445 445 445 445 450 450 450 450Yield point, psi 65000 65000 65000 65000 65000 65000 65000 65000Hardness, Hv 230 230 230 230 180 185 180 180Elongation at rupture, % 19 19 19 19 30 30 30 30Tensile strength at 900C, Nmm-240 34 37 34 100 120 100 120Tensile strength at 1650F, psi 5800 5000 5400 5000 14500 17400 14500 17400Creep strength at 800C, N mm-214 6 8 6 15 15 15 20Creep strength at 1000C, N mm-21.8 1 1.5 1 4 4 4 4Creep strength at 1470F, psi 2000 870 1160 870 2160 2160 2160 2160Creep strength at 1830F, psi 260 145 215 145 580 580 580 580Magnetic properties Magnetic (Curie point 600C 1100F)Non Non Slightly NonEmissivity, fully oxidized condition 0.70 0.70 0.70 0.70 0.88 0.88 0.88 0.88*) The values given apply for sizes of 4 mm, 0.16 in diameter for the KANTHAL alloys and of 1 mm, 0.04 in for the NIKROTHAL alloys.6Furnace wall loadingFigure 3 shows the maximum recommendedwall loading for four different element types.Please note that the furnace wall loadingdepends on both element type and elementsurface load. The lower the surface load, thelonger the element life will be. (For descriptionof the element types, see page 7).When elements are placed on the base of afurnace, special attention must be paid toavoid overheating of the elements. For exam-ple, with a hearth plate having a thermal con-ductivity of ()=1.0 Wm1K16.39 Btu in ft2h1F1and thickness of 15 mm, 0,63 in, at apower concentration on the bottom surface of15 kW/m2, 1.4 kW/ft2, a temperature drop of225C, 435F is obtained through the plate.The total temperature difference between thebase elements and the furnace temperaturewould thus be about 375C, 700F. This impo-ses a furnace operating temperature of 1000C,1830F even when using the high-temperatureKANTHAL A-1 alloy, since the element tempe-rature will be about 1375C, 2500F.The example illustrates the significance ofchoosing a hearth plate of a material havinggood thermal conductivity, for example siliconcarbide or heat-resistant steel. Beside measu-ring the temperature in the furnace chamber, itmay also be advisable to measure the tempe-rature of the base elements by means of aseparate thermocouple.Fig. 3 Maximum recommended wall loading as afunction of furnace temperature for different elementtypes.a) Wire and strip elements in groovesb) Wire elements on ceramic tubesc) Vertically hanging, corrugated wire elementsd) Vertically hanging, corrugated strip elementsFurnacetemperature700 800 900 1000 1100 1200 1300C1290 1470 1650 1830 2010 2190 2370FkW/m2100 _____90 _____80 _____70 _____60 _____50 _____40 _____30 _____20 _____10 _____kW/ft2______ 9.3______ 8.36______ 7.4______ 6.5______ 5.6______ 4.6______ 3.7______ 2.8______ 1.9______ 0.93KANTHALA-1/AF/APM7Element surface loadSince KANTHAL alloys can be operated athigher temperatures than NIKROTHAL alloys,a higher surface loading can be achieved wit-hout jeopardizing the life. Element design isalso of great importance. The more freely radi-ating the element form, the higher the maxi-mum surface load. Therefore the R.O.B. (RodOver Bend) type element (corrugated heavywire, mounted on the surface), can be loadedthe highest, followed by the corrugated stripelement.Spiral elements on ceramic tubes can beloaded higher than spiral elements in grooves.The values in Fig. 4 are given for the follo-wing design conditions:Element aWire and strip element in grooves. Wire diame-ter min. 3 mm, 0.12 in, strip thickness min. 2mm, 0.08 in.Element bWire element on ceramic tubeWire diameter min. 3 mm 0.12 in.Element types c (strip) andd (heavy wire)Strip thickness min. 2.5 mm, 0.1 in. Wire diame-ter min. 5 mm, 0.2 in. Pitch min. 50 mm, 2.0 inat maximum loop length and maximum surfaceload.Maximum recommended loop length: