Resistance furnaces can be divided into indirect heating resistance furnaces and direct heating resistance furnaces according to different heat generation methods. The so-called indirect heating resistance furnace refers to the heating element or conductive liquid made of special resistance materials inside the furnace. When the power supply is connected to the heating element or the conductive liquid, the heat generated by the heating element or conductive liquid is indirectly transmitted to the charge in the furnace and heated by conduction, convection and radiation. In a direct-heating resistance furnace, the power supply is directly connected to the material being treated to heat it.

　　The vast majority of resistance furnaces used in industry are indirectly heated resistance furnaces.

　　Indirect heating resistance furnace, according to the operation mode of the furnace, can be divided into intermittent operation type resistance furnace and continuous operation type resistance furnace. The so-called intermittent operation type resistance furnace refers to the resistance furnace that does not enter and exit the charge during the heating process of the electric furnace. Intermittent operation type resistance furnace can be divided into low temperature, moderate and high temperature three categories. The working temperature of the low-temperature resistance furnace is generally below 700 ° C, which includes a variety of drying furnaces (drying boxes), steel tempering furnaces, heat treatment furnaces for light alloy parts and heating furnaces, and the ovens commonly used in the factory belong to this.

　　The operating temperature of this electric furnace is generally not more than 300 ° C, which can be used for heating and drying below 300 ° C, and its heat transfer mode is mainly convection. In order to improve the heat transfer effect, a fan can be installed in the furnace for forced ventilation. The working temperature of the medium temperature resistance furnace is 700-1200℃, and its heat transfer mode is mainly radiation, which is mainly used for the tempering, annealing and quenching of general steel products. The most widely used medium temperature resistance furnaces are box furnaces and well furnaces, which are suitable for heat treatment of shaft and tube workpieces. The working temperature of high temperature resistance furnace is above 1200℃, which can be used for heat treatment of advanced alloy steel parts and sintering and melting of metals.

　　Continuous operation resistance furnaces are different from intermittent operation resistance furnaces: continuous operation resistance furnaces generally have two furnaces, one is the feed port and one is the discharge port. When working, the heated charge enters the furnace from the inlet, moves gradually along the length of the furnace and is heated, and after the heating process is over, the charge comes out of the discharge port, so the whole operation is continuously carried out.

　　The continuous operation type resistance furnace structure is more complex than the intermittent operation type, so it needs a mechanism to move the workpiece. Continuous operation resistance furnace can be divided into low temperature, medium and high temperature three categories according to the working temperature. The heating elements and wall structure of various resistance furnaces are basically the same as those of intermittent resistance furnaces with the same temperature. Low temperature furnaces are generally equipped with fans.

　　The power of the heat treatment resistance furnace is related to the production task of the furnace, the heating temperature of the workpiece, the heat treatment process time, the structure of the furnace and the operating system. There are two methods for calculating power: empirical method and theoretical method. The empirical method is simple to calculate, but the limitation is large, only suitable for a certain type of furnace, for the designer with practical experience, the choice of strong empirical calculation method to determine the power quickly and accurately. The theoretical method uses heat balance calculation to comprehensively consider various factors affecting the power, which has universal significance, and the results are more accurate and reliable, but the calculation program is complicated, and many parameters are involved in the calculation, if the improper selection, it will cause large errors. The heat balance calculation method can help the designer to analyze and discuss the various factors affecting the furnace power. For some special furnaces, or designers lack of practical experience, the heat balance calculation method is more secure.