The common heating modes are listed for you. (ii)
1. Resistance heating: use the joule effect of current to convert electrical energy into heat energy to heat an object. Usually divided into direct resistance heating and indirect resistance heating. The power supply voltage of the former is directly applied to the heated object. When there is a current flowing through the heated object itself, the ironing machine will generate heat electrically. An object that can be heated directly by resistance must be a conductor with a high resistivity. Since the heat is generated from the heated object itself, it is internal heating, and the thermal efficiency is very high. Indirect resistance heating requires heating elements made of special alloy or non-metallic materials, which generate heat energy that is transmitted to the heated object by means of radiation, convection and conduction. As the heated object and heating element are divided into two parts, the type of heated object is generally unlimited and easy to operate.
The material used for heating element of indirect resistance heating is generally required to have large resistivity, small resistance temperature coefficient, small deformation and not easy embrittlement at high temperature. There are commonly used iron aluminum alloy, nickel chromium alloy and other metal materials and silicon carbide, molybdenum disilicide and other non-metallic materials. The zui high operating temperature of metal heating element can reach 1000 ~ 1500℃ according to the material type. The high operating temperature of zui can reach 1500 ~ 1700℃. The latter is easy to install and can be replaced by hot furnace, but it needs pressure regulating device when working, and its service life is shorter than that of alloy heating element. It is generally used in high temperature furnace, where the temperature exceeds the high working temperature allowed by metal heating element zui and some special occasions.
2. Arc heating: use the high temperature generated by arc to heat objects. Arc is the phenomenon of gas discharge between two electrodes. The arc's voltage is low but its current is high. Its strong current is maintained by a large number of ions vaporized on the electrode, so the arc is susceptible to the influence of the surrounding magnetic field. When an arc is formed between electrodes, the temperature of arc column can reach 3000 ~ 6000K, which is suitable for high temperature melting of metal.
Arc heating has direct and indirect arc heating. The arc current from direct arc heating passes directly through the heated object, which must be an electrode or medium of the arc. The arc current of indirect arc heating does not pass through the heated object. The characteristics of arc heating are: high arc temperature, energy concentration, steel arc furnace melting pool surface power can reach 560 ~ 1200 kw/m2. However, the noise of arc is large, and its volt-ampere characteristic is negative resistance characteristic (descending characteristic). In order to maintain the stability of the arc when the arc is heated, the instantaneous value of the circuit voltage when the arc current instantaneous passes zero is greater than the value of the arc starting voltage, and in order to limit the short-circuit current, a certain number of resistors must be connected in series in the power circuit.