Industrial heat treatment systems tend to use electrical resistance, i.e. using an electrical coil, as a heat generation method due to its high efficiency.
Gas furnaces, which use combustion to generate heat, can also be used. Gas chambers must depend on convection and directed insulation to distribute heat over materials in the oven, and it can be difficult to ensure uniform treatment. For these reasons gas furnaces tend to be less efficient than electrical heating and problems can be encountered for sensitive processing.
Factors to Consider When Choosing a Heating Element
The heating elements in electrical furnaces, consisting of a heating element coil wrapped around an inert ceramic core, are used to convert electrical energy into heat. The metallic alloy used determines if the heating element can operate in an oxygen-containing atmosphere or if it requires a protective environment.
Heating elements produced using a ceramic such as silicon carbide will not undergo oxidation so do not require a protective atmosphere to operate at high temperatures. Conversely, metallic/alloy heating elements containing graphite, molybdenum, tungsten or tantalum can be oxidized at higher temperatures so a protective atmosphere is required.
The other two important factors to consider when choosing a heating element is the required operating temperature, and if a special atmosphere is needed.
The operating temperature of a furnace is defined by the temperature of the element itself, and not the temperature of the chamber. The elemental temperature depends on the watt loading of the element, its radiating capacity and the temperature of the chamber surrounding it.
For example, when using a heating element of silicon carbide (ceramic) a watt loading of 10 – 14 Wcm-2 to give a furnace temperature of 800 °C (1,472 °F), the requirement on the loading value is reduced as the temperature is increased to 1,700 °C (3,092 °F).
Furnaces that use heating coils made of an alloy of iron, chrome and aluminum have a steeper relationship with the maximum loading point occurring at 1400 °C (2552 °F). It is not only watt-loading efficiency that should be considered, there are also other factors that must be taken into account when choosing the heating coil material.
For example, when compared to ceramic heating elements, heating coils produced from iron-chrome-aluminum alloys are cheaper to install but are less versatile and have a shorter lifetime. Ceramic parts facilitate processing with better chemical resistance and a lower density at temperatures up to 1300 °C (2372 °F).
For high temperature applications, where the temperatures go above 1,600 °C (2912 °F) silicon carbide elements are preferred. The material itself is rigid and can be provided as either multi-leg or spiral-cut elements, or as a rod. Whilst silicon carbide has better thermal abilities than metallic elements it can be more easily damaged by thermal shock.
Thermcraft’s Heating Elements
Thermcraft is a leading supplier of thermal systems to commercial, industrial and academic institutions. Their heating elements (both ceramic and metallic) can be installed into custom furnaces for a multitude of applications including curing, sintering and annealing.
This information has been sourced, reviewed and adapted from materials provided by Thermcraft Inc.
For more information on this source, please visit Thermcraft Inc.