What Is Resistance Heating?

Resistance heating refers to the method of electrically heating materials by utilizing the thermal effect of current passing through the resistor. Resistance heating is used in everything from heating molten metal to heating food

Resistance heating is the simplest (and oldest) electric-based heating method. It can heat metal, molten metal, or non-metals. The efficiency can reach almost 100%, and the operating temperature can reach 2000 ° C. Therefore, it can be applied to high temperature heating and low temperature heating. Due to its controllability and rapid temperature rise, resistance heating is used in everything from heating molten metal to heating food.

Resistive heating is the electrical heating of an object using thermal energy generated by the Joule effect of the current flowing through a conductor. Resistance heating can be divided into two categories: indirect resistance heating and direct resistance heating. Indirect resistance heating is to pass current through an electric heating element or a conductive medium, such as a resistance wire,

Resistance heating generally has the following characteristics:

The heating temperature of the material can be selected in a wide range from room temperature to about 3000 ° C according to the needs of the heating process, and can be accurately controlled;

The material can be selected to be heated and processed in the environment required by the process, such as vacuum, controlled atmosphere, liquid medium, etc. (such as vacuum resistance furnace, controlled atmosphere resistance furnace, electric bath furnace, etc.), vacuum degree or some atmosphere The gas composition can be controlled automatically;

Heat evenly;

High thermal efficiency;

Less environmental pollution. ^[2]

Resistance heating is the main form of obtaining electric heating, and it is divided into direct electric heating and indirect electric heating.

1. Direct electrothermal method

The direct electrothermal method refers to passing a current through the object to be heated, and using the resistance of the object to be heated to achieve heating. It is mainly used to heat materials with regular shapes, such as water heaters that use water's own resistance to heat water in household appliances.

When using the direct electric heating method, the two ends of the object to be heated are directly connected to the circuit, and a transformer or a rheostat with a tap is used to adjust the working voltage or working current. In the water heater, the potential difference provided in the external circuit remains unchanged, and the resistance of the water is adjusted by changing the electrode position and the size of the electrode area or the level of the water level. It should be noted that any object that is heated by the direct electrothermal method must have a certain resistance value. The resistance value is too small or too large to be suitable for the direct electrothermal method.

2. Indirect electrothermal method

The indirect electrothermal method is the opposite of the direct electrothermal method. It is a circuit through which electric current passes. It is not an object to be heated, but an electric heating element made of another special material. The electric current generates heat to the electric heating element, and then uses different heat transfer methods (radiation, convection and conduction) to transfer the heat to the heated object. This indirect electrothermal resistance heating form is a widely used heating form today, mainly used to heat and dry objects. As small as a hair dryer and as large as a resistance furnace, this indirect electric heating method is widely used. ^[2]

The resistance heating device has the advantages of simple structure, large temperature adjustment range, and convenient installation and maintenance.

At present, some manufacturers in China directly use electric heating wires as heating elements. That is, the resistance wire is wound into a helical spring shape, and then the electric heating wire is put in a special ceramic tube or a ceramic element with a hole and placed on a heating plate or a mold In the heating hole, pay attention to insulation from metal parts such as the heating plate. Although this method is low in cost, the resistance wire is susceptible to oxidation loss due to direct contact with air and has a short service life. In addition, it is not safe to expose the resistance wire to the outside.

Another form is to heat the surface of the mold with an electric heating jacket or a heating coil. After winding the resistance wire on the mica sheet, it is clamped into a special metal case, and the mica sheet is used between the resistance wire and the metal case. insulation. The heating jacket should match the shape of the mold. The most common ones are rectangular and circular. In use, it can be selected according to the shape of the heating part of the mold. The rectangular electric heating jacket is composed of 4 electric heating sheets, which are connected together by wires and screws, and also clamped on the mold by screws. This heating form is often used in small and medium-sized thermoplastic injection molds. Since one side of the electric heating jacket is in contact with the atmosphere, the heat loss is greater than that of the electric heating rod, that is, heating the same mold requires more power than the electric heating rod.

The most common method is to drill holes in the relevant parts of the mold to cool, and insert electric heating rods. Select the type and number of heating rods according to the heating power required by the mold, and determine the connection method (parallel or series). Because the resistance wire is isolated from the outside air, it is not easy to oxidize and has a long service life. It can be easily performed when the heating rod is burned Replace without having to disassemble the entire mold and heating plate. However, the opening of the heating hole is limited by the structure of the core, the molding insert and the ejection part. ^[3]