The Joule effect of current is used to convert electrical energy into thermal energy to heat an object. Usually divided into direct resistance heating and indirect resistance heating. The former power supply voltage is directly applied to the object to be heated. When an electric current flows, the object itself (such as an electric heating ironing machine) will heat up. An object that can be directly resistance heated must be a conductor, but it must have a high resistivity. Because the heat is generated in the heated object itself, it belongs to internal heating, and the thermal efficiency is very high. Indirect resistance heating requires a special alloy material or non-metallic material to make the heating element. The heating element generates heat energy and transmits it to the heated object through radiation, convection and conduction. Because the heated object and the heating element are divided into two parts, the type of the heated object is generally not limited, and the operation is simple.
The materials used for heating elements for indirect resistance heating generally require large resistivity, small temperature coefficient of resistance, small deformation at high temperature and not easy to be brittle. Commonly used are ferrous aluminum alloys, nickel-chromium alloys and other metal materials and non-metallic materials such as silicon carbide and molybdenum disilicide. The maximum working temperature of metal heating element can reach 1000 ～ 1500 ℃ according to the type of material; the maximum working temperature of nonmetal heating element can reach 1500 ～ 1700 ℃. The latter is easy to install and can be replaced by a hot furnace. However, it requires a pressure regulating device when it works. Its life is shorter than that of alloy heating elements. It is generally used in high-temperature furnaces, where the temperature exceeds the maximum operating temperature of metal materials, and some special occasions.
The conductor itself generates heat by using the thermal effect of the induced current (eddy current) generated by the conductor in an alternating electromagnetic field. According to the requirements of different heating processes, the frequency of the 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). The power frequency power supply is the AC power supply commonly used in industry. The power frequency of most countries in the world is 50 Hz. The voltage applied to the induction device by the industrial frequency power supply for induction heating must be adjustable. According to the power of the heating equipment and the capacity of the power supply network, the high-voltage power supply (6-10 kV) can be used to supply power through the transformer; the heating equipment can also be directly connected to the 380-volt low-voltage power grid.
Intermediate frequency power supply used intermediate frequency generator set for a long time. It consists of an intermediate frequency generator and a driving asynchronous motor. The output power of this unit is generally in the range of 50 to 1000 kilowatts. With the development of power electronics technology, a thyristor inverter intermediate frequency power supply has been used. This intermediate frequency power source uses a thyristor to convert power frequency AC power to DC power first, and then converts DC power to AC power at a desired frequency. Due to the small size, light weight, noiselessness and reliable operation of this frequency conversion equipment, it has gradually replaced the intermediate frequency generator set.
High-frequency power supply usually uses a transformer to raise the three-phase 380 volts to a high voltage of about 20,000 volts, and then uses a thyristor or a high-voltage silicon rectifier to rectify the power frequency AC power to DC power, and then uses an electronic oscillator Direct current is converted into high frequency, high voltage alternating current. The output power of high-frequency power supply equipment ranges from tens of kilowatts to hundreds of kilowatts.
Induction-heated objects must be conductors. When a high-frequency AC current passes through a conductor, the conductor produces a skin effect, that is, the current density on the surface of the conductor is large, and the current density on the center of the conductor is small.
Induction heating can uniformly and surface heat the object; it can smelt metal; at high frequency, change the shape of the heating coil (also known as the inductor), and can also perform arbitrary local heating.
Uses the high temperature of an arc to heat an object. An arc is a phenomenon of gas discharge between two electrodes. The voltage of the arc is not high but the current is large. Its strong current is maintained by a large amount of ions evaporated on the electrode, so the arc is easily affected by the surrounding magnetic field. When an arc is formed between the electrodes, the temperature of the arc column can reach 3000 ~ 6000K, which is suitable for high temperature melting of metals.
There are two types of arc heating: direct and indirect arc heating. The arc current of direct arc heating directly passes through the heated object. The heated object must be an electrode or medium of the arc. The arc current of indirect arc heating does not pass through the heated object and is mainly heated by the heat radiated by the arc. The characteristics of arc heating are: high arc temperature, concentrated energy, and surface power of the melting tank of the steelmaking arc furnace can reach 560 ~ 1200 kW / m2. However, the noise of the arc is large, and its volt-ampere characteristic is a negative resistance characteristic (falling characteristic). In order to maintain the stability of the arc when the arc is heated, the instantaneous value of the circuit voltage is greater than the arcing voltage value when the arc current instantaneously crosses zero, and to limit the short-circuit current, a certain value of resistor must be connected in series in the power circuit.
Electron beam heating
The surface of an object is bombarded with electrons moving at high speed under the action of an electric field, causing it to be heated. The main component for electron beam heating is the electron beam generator, also known as the electron gun. The electron gun is mainly composed of cathode, condenser, anode, electromagnetic lens and deflection coil. The anode is grounded, the cathode is connected to a negative high position, the focused beam is usually at the same potential as the cathode, and an accelerated electric field is formed between the cathode and the anode. The electrons emitted by the cathode are accelerated to a very high speed under the action of an accelerating electric field, focused by an electromagnetic lens, and then controlled by a deflection coil, so that the electron beam is directed toward the heated object in a certain direction.
The advantages of electron beam heating are: ① control the current value Ie of the electron beam, which can change the heating power conveniently and quickly; ② the electromagnetic lens can freely change the heated part or the area of the electron beam bombarded part; ③ can Increase the power density so that the material at the bombardment site evaporates away instantly.
The object is irradiated with infrared rays. After the object absorbs infrared rays, it converts radiant energy into heat energy and is heated.
Infrared is an electromagnetic wave. In the solar spectrum, beyond the red end of visible light, it is an invisible radiant energy. In the electromagnetic spectrum, the wavelength range of infrared rays is between 0.75 and 1000 microns, and the frequency range is between 3 × 10 and 4 × 10 Hz. In industrial applications, the infrared spectrum is often divided into several bands: 0.75 to 3.0 microns is the near-infrared region; 3.0 to 6.0 microns is the mid-infrared region; 6.0 to 15.0 microns is the far-infrared region; 15.0 to 1000 microns is the extreme far infrared Area. Different objects have different infrared absorption capabilities. Even the same object has different infrared absorption capabilities. Therefore, when applying infrared heating, you must select a suitable infrared radiation source according to the type of the object to be heated, so that its radiant energy is concentrated in the absorption wavelength range of the object to be heated to obtain a good heating effect.
Electric infrared heating is actually a special form of resistance heating, which uses tungsten, iron nickel or nickel-chromium alloy as a radiator to make a radiation source. After being energized, heat radiation is generated due to its resistance heating. Commonly used electric infrared heating radiation sources are lamp type (reflective type), tube type (quartz tube type) and plate type (planar type). The lamp type is an infrared light bulb that uses tungsten wire as a radiator. The tungsten wire is sealed in a glass case filled with inert gas, just like a general lighting bulb. The radiator heats up after being energized (the temperature is lower than that of a general lighting bulb), and thus emits a large amount of infrared rays with a wavelength of about 1.2 microns. If a reflective layer is plated on the inner wall of the glass shell, infrared rays can be concentrated in one direction, so the lamp-type infrared radiation source is also called a reflective infrared radiator. The tube of the tube type infrared radiation source is made of quartz glass with a tungsten wire in the middle, so it is also called a quartz tube type infrared radiator. The infrared light emitted by the lamp type and the tube type has a wavelength in the range of 0.7 to 3 microns, and the working temperature is relatively low. It is generally used for heating, baking, drying in the light and textile industries, and infrared physiotherapy in medical treatment. The radiation surface of the plate-type infrared radiation source is a flat surface, which is composed of a flat resistance plate. The front surface of the resistance plate is coated with a material with a large reflection coefficient, and the reverse surface is coated with a material with a small reflection coefficient. Therefore, most of the heat energy is radiated from the front surface. The working temperature of the plate can reach more than 1000 ℃, which can be used for annealing the welds of steel materials and large diameter pipes and vessels.
Infrared has a strong penetrating ability and is easily absorbed by objects. Once absorbed by the object, it is immediately transformed into thermal energy. The energy loss before and after infrared heating is small, the temperature is easy to control, and the heating quality is high. Therefore, the application of infrared heating has developed rapidly.
The high-frequency electric field is used to heat the insulating material. The main heating object is the dielectric. The dielectric is placed in an alternating electric field and will be repeatedly polarized (the phenomenon that an equal amount of opposite polarity charges appear on the surface or inside of the dielectric under the action of the electric field), thereby transforming the electric energy in the electric field into thermal energy.
The frequency of the electric field used for dielectric heating is high. In the medium-, short-wave and ultra-short-wave bands, the frequency is several hundred kilohertz to 300 megahertz, which is called high-frequency dielectric heating. If it is higher than 300 megahertz and reaches the microwave band, it is called microwave dielectric heating. Generally, high-frequency dielectric heating is performed in the electric field between the two plates; while microwave dielectric heating is performed in a waveguide, a resonant cavity, or under the radiation field of a microwave antenna.
When a dielectric is heated in a high-frequency electric field, the electric power absorbed in a unit volume is P = 0.566fEεrtgδ × 10 (Watts / cm)
If expressed in terms of heat:
H = 1.33fEεrtgδ × 10 (cal / sec · cm)
Where f is the frequency of the high-frequency electric field, εr is the relative permittivity of the dielectric, δ is the dielectric loss angle, and E is the electric field strength. It can be known from the formula that the electric power withdrawn by the dielectric from the high-frequency electric field is proportional to the square of the electric field strength E, the frequency f of the electric field, and the loss angle δ of the dielectric. E and f are determined by the applied electric field, and εr depends on the nature of the dielectric itself. Therefore, the object of medium heating is mainly the material with large dielectric loss.
Dielectric heating because the heat is generated inside the dielectric (the object being heated), compared with other external heating, the heating speed is faster, the thermal efficiency is higher, and the heating is uniform.
Medium heating in the industry can heat thermal gels, dry grains, paper, wood, and other fibrous materials; it can also pre-heat plastic before molding, and rubber vulcanization and wood, plastic bonding. Selecting the appropriate electric field frequency and device can only heat the adhesive when heating the plywood without affecting the plywood itself. For homogeneous materials, overall heating can be performed.