Resistance Heating Elements
Electric type resistance elements consist of a high temperature resistance alloy either nickel-chrome alloy or iron chrome aluminum, usually formed in sinuous, loops or coils. The elements may be supported from the furnace sidewalls on refractory hooks or alloy hooks, suspended from the roof with alloy hanger; hooks, or may be laid on the floor in comb type refractory insulators.
The type of alloy used depends on the furnace temperature rating and the type of atmosphere used. The most common types are Chromel "A" 80% nickel -20% chromium, Chromel "D" 35% ,nickel -19% chromium -46% iron, Hoskins "875" 22-1/2%-Chromium - 5-1/2%-Aluminum balance iron.
Elements are designed to deliver rated kilowatts at rated voltage only when hot. If actual voltage differs from rated voltage, the power delivered will vary as the square of the voltage. Remember 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. The resistance of the heating elements will be lower at room temperature than when hot. The resistance of the elements will increase with age, due to the reduction in cross section by oxidation, and also, due to elongation of the loops. This will result in decreased power to the furnace and ultimate failure. Such failure represents the normal life of the elements.
Certain impurities in the atmosphere will attack the alloy in the elements. These impurities may be in the incoming gas, or may be given off by the work entering the furnace. Cutting oils/fluids are major sources of impurities, typically carbon and sulfur.
Sulfur even in small quantities; will cause rapid deterioration of heating elements. 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, and halides will attack the element. These materials should not be put in a furnace.
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, retainers hold elements in place. They will sag and fail without these parts in place. Replace damaged or broken hangers, hooks/pins, ceramic parts and refractory which have fallen out, or which have broken.
Keep the loops straight so the spacing between ribbons is uniform. If ribbon is brittle, it may not be possible to straighten the loop.
Remove any dirt or debris from the element. This is especially important with floor elements. Heating elements that are buried in debris will over heat due to heat entrapment under the debris. Controllers calling for more heat that cannot escape from under debris will cause excessive element operating temperature. Heating elements can melt quickly and damage their supports.
DO NOT INSTALL NEW ELEMENTS INTO and/or ON REFRACTORY SUPPORTS CONTAMINATED FROM PREVIOUS MOLTEN 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.
Keep terminal stuffing boxes packed tightly, adding or replacing packing washers if necessary. Leaky stuffing boxes may cause work to be discolored in the furnace and the terminals to over-heat.
Terminal connections should be clean, and tight. Loose connections due to thermal expansion will sizzle, oxidize, and over-heat. Connections should be check for tightness after initial startup. See general maintenance instructions item 7 on next page.
Since the resistance of the elements may increase with age, a large percent of new element (more than 25 percent) should not be put in series with a section of old element, as this will change the distribution of heat and can cause rapid failure of the element section.
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