The principle of resistance heating of air heater

Air heaters utilize the Joule effect of electric current to convert electrical energy into heat to heat objects. These are generally categorized as direct resistance heating and direct resistance heating. In the former, the power supply voltage is applied directly to the object being heated. When current flows, the object itself generates heat. For direct resistance heating, the object must be a conductor with a high resistivity. Since heat is generated within the object itself, it is considered internal heating and has high thermal efficiency. Direct resistance heating requires heating elements made of specialized alloys or non-metallic materials. Heat energy is generated by the heating element and transferred to the object through radiation, convection, and conduction.

Because the object and the heating element are separate entities, the type of heated object is generally unrestricted, making operation simple. The heating element material for direct resistance heating typically requires high resistivity, a low temperature coefficient of resistance, minimal deformation at high temperatures, and resistance to brittleness. Commonly used materials include metals such as iron-aluminum alloys and nickel-chromium alloys, and non-metallic materials such as silicon carbide and molybdenum disilicide. The maximum operating temperature of metal heating elements can reach 1000-1500℃, depending on the material type; the maximum operating temperature of non-metallic heating elements can reach 1500-1700℃. The latter is easy to install and can be replaced in a hot furnace, but it requires a voltage regulator during operation and has a shorter lifespan than alloy heating elements. It is usually used in high-temperature furnaces, places where the temperature exceeds the maximum operating temperature of metal heating elements, and certain special places.