Pipeline Heater

Electric Heating Coil

The Joule effect of electric current is used 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 generates heat. Electric heating ironing machine
Objects that can be directly resistance heated must be conductors with high resistivity. Since heat is generated inside 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 operating temperature of metal heating elements can reach 1000–1500°C depending on the material; non-metallic heating elements can reach 1500–1700°C. The latter are easy to install and can be replaced while hot, but require a voltage regulation device and have a shorter lifespan than alloys.

Induction Heating Coil

The heat generated by induced currents (eddy currents) in a conductor placed in an alternating electromagnetic field causes the conductor itself to heat up. Depending on the heating process requirements, the frequency of the AC power used for induction heating can be power frequency (50–60 Hz), intermediate frequency (60–10000 Hz), or high frequency (above 10000 Hz). Power frequency is the standard industrial AC supply, with most countries using 50 Hz. The voltage applied to the induction device from the power frequency supply must be adjustable. Depending on the power of the heating equipment and the capacity of the power grid, a high-voltage supply (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 an asynchronous motor drive. The output power of such sets typically ranges from 50 to 1000 kW. With the development of power electronics, thyristor inverter intermediate frequency power supplies are now used. These convert power frequency AC to DC and then to the required frequency AC. 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 the three-phase 380 V voltage to about 20,000 V, then use thyratrons or high-voltage silicon rectifiers to convert the power frequency AC to DC, and finally use electronic oscillator tubes to convert DC to high-frequency, high-voltage AC. The output power of high-frequency power supplies ranges from tens of kilowatts to hundreds of kilowatts.
Objects for induction heating must be conductors. When high-frequency AC flows through a conductor, the skin effect occurs, meaning the current density is higher on the surface and lower in the center.
Induction heating can achieve uniform heating of the entire object or surface heating; it can melt metals; in the high-frequency range, by changing the shape of the heating coil (also called inductor), localized heating can be performed arbitrarily.

Electric Heating Coil

Heating objects using the high temperature generated by an electric arc. An arc is a gas discharge phenomenon between two electrodes. The arc voltage is low but the current is high, sustained by a large number of ions evaporated from the electrodes, making the arc susceptible to surrounding magnetic fields. When an arc forms between electrodes, the temperature of the arc column can reach 3000–6000 K, suitable for high-temperature metal melting.
Arc heating can be direct or indirect. In direct arc heating, the arc current passes directly through the object, which must be one of the electrodes or the medium. In indirect arc heating, the arc current does not pass through the object; heating relies mainly on radiation from the arc. Characteristics of arc heating include high arc temperature, concentrated energy, and surface power of the molten pool in an electric arc furnace for steelmaking can reach 560–1200 kW/m². However, arcs are noisy and have a negative resistance characteristic (falling characteristic). To maintain arc stability during heating, ensure that the instantaneous voltage of the circuit is greater than the arc ignition voltage when the arc current crosses zero, and to limit short-circuit current, a resistor of a certain value must be connected in series in the power circuit.