It is a kind of international popular high quality long life electric heating equipment. It is used for heating, heat preservation and heating of flowing liquid and gas media. When the heating medium passes through the heating chamber of the electric heater under the action of pressure, the huge heat generated in the operation of the electric heating element is uniformly carried away by using the principle of fluid thermodynamics, so that the temperature of the heated medium can meet the technological requirements of the user.
The Joule effect of electric current is used to transform electric energy into heat energy to heat objects. It is 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 current flowing, the heated object itself (such as electric heating ironing machine) will heat up. The object that can be heated by direct resistance must be a conductor, but it must have a high resistivity. Because the heat is generated by the heated object itself, which belongs to internal heating, the thermal efficiency is very high. Indirect resistance heating needs to be made of special alloy materials or non-metallic materials into heating elements, which generate heat energy and transmit it to the heated object by means of radiation, convection and conduction. As the heated object and heating element are divided into two parts, the types of heated objects are generally unlimited and easy to operate.
The materials used for indirect resistance heating heating elements generally require high resistivity, low resistance temperature coefficient, small deformation at high temperature and not easy to embrittle. Commonly used are ferrous aluminum alloy, nickel chromium alloy and other metal materials, silicon carbide, molybdenum disilicide and other non-metallic materials. The maximum working temperature of metal heating element can reach 1000-1500 ℃ according to the type of material, and that of non-metal heating element can reach 1500-1700 ℃. The latter is easy to install and can be replaced in the furnace, but it needs pressure regulating device when it works, and its service life is shorter than that of alloy heating element. It is generally used in high temperature furnace, places where the temperature exceeds the maximum allowable working temperature of metal heating element and some special occasions.
The thermal effect of induced current (eddy current) generated by conductor in alternating electromagnetic field is used to make conductor itself heated. According to different heating process requirements, the frequency of AC power supply used for induction heating includes power frequency (50-60 Hz), intermediate frequency (60-10000 Hz) and high frequency (higher than 10000 Hz). Power frequency power supply is usually used in industry, the power frequency of most countries in the world is 50 Hz. The voltage of power frequency power supply for induction heating applied to induction device must be adjustable. According to the power of the heating equipment and the capacity of the power supply network, the high-voltage power supply (6-10KV) can be used to supply power through the transformer, or the heating equipment can be directly connected to the 380 V low-voltage power grid.
If power supply used to use if generator set in a long time. It consists of intermediate frequency generator and asynchronous motor. The output power of the unit is generally in the range of 50-1000 kW. With the development of power electronic technology, the thyristor inverter intermediate frequency power supply has been used. This medium frequency power supply uses thyristor to convert power frequency alternating current into direct current, and then convert direct current into alternating current of required frequency. Because of its small volume, light weight, noiseless and reliable operation, the inverter has gradually replaced the intermediate frequency generator set.
High frequency power supply usually uses transformer to raise the three-phase 380V voltage to about 20000 v high voltage, then uses thyratron or high-voltage silicon rectifying element to rectify the power frequency alternating current into direct current, and then uses electronic oscillating tube to transform the direct current into high frequency and high voltage alternating current. The output power of high-frequency power equipment ranges from tens to hundreds of kilowatts.
Induction heated objects must be conductors. When the high frequency AC current passes through the conductor, the skin effect is produced, that is, the surface current density of the conductor is large and the center current density of the conductor is small.
Induction heating can heat the whole object evenly and the surface layer; it can smelt metal; in the high frequency section, it can change the shape of heating coil (also known as inductor), and it can also heat any part.
Use the high temperature generated by the arc to heat the object. Arc is a gas discharge phenomenon between two electrodes. The arc voltage is not high but the current is very large. Its strong current is maintained by a large number of ions evaporated on the electrode, so the arc is easily affected by the surrounding magnetic field. When an arc is formed between electrodes, the temperature of the arc column can reach 3000-6000k, which is suitable for high temperature melting of metals.
There are two kinds of arc heating: direct and indirect. The arc current heated by direct arc directly passes through the heated object, which must be an electrode or medium of the arc. The arc current heated by indirect arc does not pass through the heated object, but mainly depends on the heat radiated by the arc. The characteristics of electric arc heating are: high temperature and energy concentration of the electric arc, the surface power of the molten pool of the steelmaking electric arc furnace can reach 560-1200 kW / m2. But the arc noise is large, and its volt ampere characteristic is negative resistance characteristic. In order to maintain the stability of the arc during the arc heating, the instantaneous value of the circuit voltage is greater than the value of the arc starting voltage when the arc current is instantaneous zero crossing, and in order to limit the short-circuit current, a certain number of resistors must be connected in series in the power circuit.
Electron beam heating
Electrons moving at high speed under the action of electric field are used to bombard the surface of the object and make it heated. The main component of electron beam heating is electron beam generator, also known as electron gun. The electron gun is mainly composed of cathode, bunching pole, anode, electromagnetic lens and deflection coil. The anode is grounded, the cathode is connected to the negative high position, the focus beam is usually at the same potential with the cathode, and an accelerating electric field is formed between the cathode and the anode. Electrons emitted by cathode