Electric type resistance elements consist of a high temperature resistance alloys: nickel-chrome (NiCr); iron chrome aluminum (FeCrAl); or refractory metals such as molybdenum (Mo), tungsten (W) or related alloys. Elements are formed from wire, strip or mesh and can take numerous shapes such as sinuous or zig-zag, loops or helically-wound coils. Depending on the materials or temperature, elements may be free-standing, supported by refractory or alloy hooks, suspended, supported by comb-style refractory insulators, buried or mounted in ceramic insulators.
Alloy selection depends on furnace temperature and atmosphere.
Elements are often designed to deliver rated kilowatts at rated voltage when cold. Elements can be designed around a process as well, if the element’s operating temperature can be provided.
Power delivered by an element will vary as the square of the voltage. A 1-% increase in voltage is about a 2% increase in wattage, and vice-versa, a 1% reduction in voltage is about a 2% reduction in wattage. Most metallic element alloys have a lower resistance at room temperature than when hot. The amount of change with temperature depends on the alloy, for instance 875 FeCrAl exhibits a 4% increase in resistance from 20°C to 1200°C. tungsten exhibits a much larger increase, greater than 400% in that temperature range.
Resistance of open atmosphere metallic elements increases with age due to the reduction of cross section by loss of oxides, and possibly to elongation due to creep. This results in decreased furnace power and eventually failure. Such failure due to age is normal for metallic elements and can be avoided by a maintenance program with regular inspection and replacement.
Certain impurities damage metallic elements. Sensitivity to impurities depends on the element’s alloy and operating temperature. Impurities may be carried by incoming gas, may be given off by the work in the furnace and are often unanticipated. Cutting oils/fluids left on machined parts can be a source of contamination. Components found in clay, glazes or glass formulas can also cause degradation of elements and can be a source of frustration when the formula has changed or become contaminated without notice.
Sulfur, halogens, halide salts and oxides once in contact with an element, even in small quantities, can cause rapid deterioration. Carburizing atmospheres tend to increase the carbon content of the heating element causing it to become brittle and to develop a lower melting point. Lead, tin, or zinc will also attack metallic elements. Contact with heating elements by these materials should be avoided. If these are part of the material to be heated or part of the process, steps should be taken to minimize the impact on the elements if possible.
MAINTENANCE and REPAIR
Inspect heating elements at regular intervals to be sure they are properly hung on their supports. Elements must be supported at all times to function correctly. Only the support hooks, hangers, ceramic, refractory bricks or retainers hold elements in place. Elements will sag and fail without proper supports in place. Replace damaged or broken hangers, hooks/pins, ceramic parts and refractory which have fallen out, or which have broken.
Keep element runs straight so the spacing between the runs are uniform. If the element is brittle (FeCrAl will become brittle due to age hardening), it may not be possible to straighten a run so a stable refractory structure is key to element performance and longevity.
Remove any dirt or debris from the element. This is especially important with floor elements. Some debris is electrically conductive and will cause rapid failure of an element due to short circuits or leakage of current. Also, heating elements may become insulated by debris and become too hot. Over-heated elements can melt and damage their supports.
DO NOT INSTALL NEW ELEMENTS INTO and/or ON REFRACTORY SUPPORTS CONTAMINATED BY FAILED ELEMENTS.
Spare heating elements, refractory, hooks, hangers, ceramic spools, and welding wire/rod should be kept in stock for maintenance personnel’s successful repair of heating elements. If refractory cannot be replaced the surface must be repaired by removing contaminants and possibly coated with an inert repair ceramic material before installing new elements.
Keep terminal stuffing tightly packed, add or replace packing if necessary. Leaky packing may cause work to be discolored in the furnace and cause terminals to over-heat.
Terminal connections should be clean, and tight. Loose connections due to thermal expansion will sizzle, oxidize, and over-heat. Power should be disconnected and all terminal connections should be re-tightened after initial startup.
Resistance of metallic elements increases with age. New elements should not be put in series with old elements. Mixing old and new forces a greater distribution of energy onto the older weaker element, accelerating failure of the older element.