Since the discovery of the thermal effect of electric current, the electrothermal method was first used in household appliances, and then used in small electric furnaces. With the invention of nickel chromium alloy, resistance furnace (box furnace) has been widely used in industry by 1920s. The resistance furnace used in industry is generally composed of electric heating elements, masonry, metal shell, furnace door, furnace machinery and electrical control system. The heating power ranges from less than one kilowatt to several kilowatts. If the working temperature is below 650 ℃, it is a low temperature furnace; if the temperature is 650-1000 ℃, it is a medium temperature furnace; if the temperature is above 1000 ℃, it is a high temperature furnace. It is mainly heated by radiation in high and medium temperature furnaces. In the low-temperature furnace, it is heated by convection heat transfer. The electric heating element is installed in the air duct, and the air in the furnace is forced to circulate by the fan to strengthen the convection heat transfer. Resistance furnace has room type, well type, trolley type, push rod type, step type, muffle type and tunnel type. Controlled atmosphere furnace, vacuum furnace, flowing particle furnace are also resistance furnaces.
The electric heating element has high heat resistance, high temperature strength, low resistance temperature coefficient and good chemical stability. The commonly used materials are metal and non-metal. The materials of metal electric heating elements include nickel chromium alloy, chromium aluminum alloy, tungsten, molybdenum, tantalum, etc., which are generally made into helix, wave line, wave belt and wave plate. The materials of non-metallic electric heating elements include silicon carbide, molybdenum disilicide, graphite and carbon, which are generally made into the shapes of rods, tubes, plates and belts. The distribution and wiring of electric heating elements are determined by the power and temperature requirements of the furnace.
Compared with the flame furnace, the resistance furnace (experimental furnace) has the advantages of simple structure, uniform furnace temperature, easy to control, good heating quality, no dust, no noise, etc., but the use cost is higher.