Resistance Heating Coil
Using the Joule effect of electric current to convert electrical energy into thermal energy to heat objects. It is usually divided into direct resistance heating and indirect resistance heating. In the former, the power supply voltage is directly applied to the object to be heated. When current flows, the object itself generates heat. Objects that can be directly resistance heated must be conductors with high resistivity. Since heat is generated within the object itself, it is internal heating with high thermal efficiency. Indirect resistance heating requires special alloy or non-metallic materials to make heating elements, which generate heat and transfer it to the object through radiation, convection, and conduction. Since the object and the heating element are separate, the type of object is generally unrestricted, and operation is simple.
The materials used for heating elements in indirect resistance heating generally require high resistivity, low temperature coefficient of resistance, minimal deformation at high temperatures, and resistance to embrittlement. Common materials include iron-chromium-aluminum alloys, nickel-chromium alloys, and non-metallic materials such as silicon carbide and molybdenum disilicide. The maximum working temperature of metal heating elements can reach 1000–1500°C depending on the material; non-metallic heating elements can reach 1500–1700°C. The latter are easy to install and can be replaced while hot, but require a voltage regulator during operation and have a shorter lifespan than alloy heating elements. They are generally used in high-temperature furnaces, places where the temperature exceeds the maximum allowable working temperature of metal heating elements, and certain special occasions.
