According to different methods of electrical energy conversion, electric heating is usually divided into resistance heating, induction heating, arc heating, electron beam heating, infrared heating, and dielectric heating.
Fold Resistance Heating Coil
Using the Joule effect of current to convert electrical energy into thermal energy to heat objects. It is usually divided into direct resistance heating and indirect resistance heating. In the former, the power supply voltage is directly applied to the object to be heated. When current flows, the object itself heats up. Objects that can be directly resistance heated must be conductors but have high resistivity. Since heat is generated within the object itself, it is internal heating with high thermal efficiency. Indirect resistance heating requires special alloy or non-metallic materials to make heating elements, which generate heat and transfer it to the object through radiation, convection, and conduction. Since the object and the heating element are separate, the type of object is generally unrestricted and operation is simple.
Materials used for heating elements in indirect resistance heating generally require high resistivity, low temperature coefficient of resistance, minimal deformation at high temperatures, and resistance to embrittlement. Common materials include iron-chromium-aluminum alloys, nickel-chromium alloys, and non-metallic materials such as silicon carbide and molybdenum disilicide. The maximum working temperature of metal heating elements ranges from 1000 to 1500°C depending on the material; non-metallic heating elements can reach up to 1500 to 1700°C. The latter are easy to install and can be replaced while the furnace is hot, but they require a voltage regulation device during operation and have a shorter lifespan than alloy heating elements. They are generally used in high-temperature furnaces, places where temperatures exceed the maximum allowable working temperature of metal heating elements, and certain special applications.
Fold Induction Heating Coil
Using the thermal effect of induced currents (eddy currents) generated when a conductor is placed in an alternating electromagnetic field to heat the conductor itself. Depending on the heating process requirements, the frequency of the AC power used for induction heating includes mains frequency (50-60 Hz), intermediate frequency (60-10000 Hz), and high frequency (above 10000 Hz). Mains frequency power is the common industrial AC power, with most countries using 50 Hz. The voltage applied to the induction device from mains frequency power must be adjustable. Depending on the power of the heating equipment and the capacity of the power supply network, high-voltage power (6-10 kV) can be used via a transformer, or the equipment can be directly connected to a 380 V low-voltage grid.
Intermediate frequency power was historically provided by intermediate frequency generator sets, consisting of an intermediate frequency generator and a driving asynchronous motor. The output power of such sets typically ranges from 50 to 1000 kW. With the development of power electronics, thyristor-based intermediate frequency power supplies are now used. These supplies use thyristors to first convert mains frequency AC to DC, then convert DC to AC of the desired frequency. Due to their small size, light weight, no noise, and reliable operation, they have gradually replaced intermediate frequency generator sets.
High-frequency power supplies typically use a transformer to step up three-phase 380 V voltage to about 20,000 V, then use thyratrons or high-voltage silicon rectifiers to rectify the mains frequency AC into DC, and finally use electronic oscillator tubes to convert DC into high-frequency, high-voltage AC. The output power of high-frequency power supply equipment ranges from tens of kilowatts to hundreds of kilowatts.
Objects for induction heating must be conductors. When high-frequency AC passes through a conductor, the skin effect occurs, meaning the current density is higher on the surface and lower in the center.
Induction heating can be used for uniform heating of the entire metal object or surface heating; it can melt metals, be used for brazing, perform various heat treatments (quenching, tempering, normalizing), and for forging heating; in the high-frequency range, by changing the shape of the heating coil (also called inductor), localized heating can be achieved arbitrarily.
