The Joule effect of electric current is used to convert electrical energy into heat 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, and when current flows, the heated object itself (such as an electrically heated iron) heats up. An object that can be heated directly resistively must be a conductor, but with high resistivity. Since the heat is generated from the heated object itself, it belongs to internal heating, and the thermal efficiency is high. Indirect resistance heating needs to be made of special alloy materials or non-metallic materials to make heating elements, which generate heat energy and transmit to the heated object through radiation, convection and conduction. Since the heated object and the heating element are divided into two parts, the type of heated object is generally not limited and easy to operate.
The materials used in the heating element of indirect resistance heating generally require large resistivity, small resistance temperature coefficient, small deformation at high temperature and not easy to embrittle. Commonly used are iron-aluminum alloy, nickel-chromium alloy and other metal materials and silicon carbide, molybdenum disilicide and other non-metallic materials. The maximum working temperature of metal heating elements can reach 1000~1500 °C according to the type of material; The maximum working temperature of non-metallic heating elements can reach 1500~1700 °C. The latter is easy to install and can be replaced by a hot furnace, but it needs a pressure regulating device when working, and its life is shorter than that of alloy heating elements, and is generally used in high-temperature furnaces, places where the temperature exceeds the maximum operating temperature allowed by metal material heating elements and some special occasions. The thermal effect of the conductor itself is heated by the induced current (eddy current) generated by the conductor in an alternating electromagnetic field. According to different heating process requirements, the frequency of AC power supply used in induction heating is power frequency (50~60 kHz), medium frequency (60~10000 Hz) and high frequency (higher than 10000 Hz). Power frequency power supply is usually used in industry AC power supply, most countries in the world power frequency is 50 Hz. The voltage applied to the induction device by the power 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 power supply (6~10 kV) can be used to supply power through the transformer; The heating device can also be connected directly to the 380 volt low-voltage grid.
Medium frequency power supply has used medium frequency generator sets for a long time. It consists of a medium frequency generator and a drive asynchronous motor. The output power of this unit is generally in the range of 50~1000 kilowatts. With the development of power electronics technology, thyristor inverter medium frequency power supplies have been used. This medium-frequency power supply uses thyristors to convert power frequency alternating current into direct current, and then convert direct current into alternating current of the required frequency. Due to the small size, light weight, no noise, reliable operation, etc. of this frequency conversion equipment, it has gradually replaced the medium frequency generator set.
High-frequency power supply usually uses a transformer to raise the three-phase 380 volt voltage to a high voltage of about 20,000 volts, and then uses a thyristor or high-voltage silicon rectifier element to rectify the power frequency alternating current into direct current, and then uses an electronic oscillator to convert direct current 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.
Objects that are inductively heated must be conductors. When high-frequency AC current passes through the conductor, the conductor produces a skin effect, that is, the surface current density of the conductor is large, and the current density of the center of the conductor is small.
Induction heating can heat the object uniformly and surface as a whole; Can melt metals; In high frequency bands, changing the shape of the heating coil (also known as the inductor) can also be used for arbitrary local heating. Heating objects using the high temperatures generated by an electric arc. Arcing is a gas discharge between two electrodes. The voltage of the arc is not high but the current is large, and 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 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 heated by the direct arc passes directly 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 is mainly heated by the heat radiated by the arc. The characteristics of arc heating are: high arc temperature, energy concentration, and the surface power of the steelmaking arc furnace pool can reach 560~1200 kW/square meter. However, the noise of the arc is large, and its volt-ampere characteristic is 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 starting voltage value when the arc current crosses zero, and in order to limit the short-circuit current, a resistor of a certain value must be connected in series in the power supply circuit. Electrons moving at high speed under the action of an electric field are used to bombard the surface of an object and heat it. 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, beam polyelectrode, anode, electromagnetic lens and deflection coil. The anode is grounded, the cathode is connected to the negative high position, the focusing beam is usually 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 the accelerating electric field, focused by the electromagnetic lens, and then controlled by the deflection coil, so that the electron beam shoots towards 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 easily and quickly change the heating power; (2) The electromagnetic lens can be used to freely change the heated part or the area of the electron beam bombardment part can be freely adjusted; (3) The power density can be increased so that the substance at the bombardment point evaporates in an instant. Using infrared radiation objects, the object absorbs infrared rays, converts the radiant energy into heat energy and heats it.
Infrared is an electromagnetic wave. In the solar spectrum, beyond the red end of visible light, is an invisible form of radiant energy. In the electromagnetic spectrum, the wavelength range of infrared is between 0.75~1000 microns, and the frequency range is between 3×10~4×10 kHz. In industrial applications, the infrared spectrum is often divided into several bands: 0.75~3.0 microns for the near-infrared region; 3.0~6.0 microns for the mid-infrared region; 6.0~15.0 microns for the far infrared region; 15.0~1000 microns for the extremely far infrared region. Different objects have different ability to absorb infrared rays, even if the same object has different ability to absorb infrared rays of different wavelengths. Therefore, the application of infrared heating, according to the type of heated object, select the appropriate infrared radiation source, so that the radiation energy is concentrated in the absorption wavelength range of the heated object to obtain a good heating effect.
Electric infrared heating is actually a special form of resistance heating, that is, materials such as tungsten, iron-nickel or nickel-chromium alloy are used as radiators to make radiation sources. When energized, thermal radiation is generated due to the heat generated by its resistance. Commonly used electrical infrared heating radiation sources are lamp type (reflective), tube type (quartz tube type) and plate type (flat type). The lamp type is an infrared bulb, with tungsten filament as the radiator, and the tungsten filament is sealed in a glass shell filled with inert gas, just like ordinary lighting bulbs. When the radiator is energized, it heats up (the temperature is lower than that of a normal lighting bulb), thus emitting a large amount of infrared rays with a wavelength of about 1.2 microns. If the inner wall of the glass shell is plated with a reflective layer, infrared rays can be concentrated in one direction, so lamp-type infrared radiation sources are also called reflective infrared emitters. The tube of the tubular infrared radiation source is made of quartz glass, and a tungsten wire is in the middle, so it is also called a quartz tubular infrared emitter. The wavelength of infrared emitted by lamp type and tube type is in the range of 0.7~3 microns, and the working temperature is low, which is generally used for heating, baking, drying and infrared physiotherapy in the light and textile industry. The radiation surface of the plate-type infrared radiation source is a plane, composed of a flat resistance plate, the front side of the resistance plate is coated with a material with a large reflection coefficient, and the reverse side is coated with a material with a small reflection coefficient, so most of the heat energy is radiated out by the front. The working temperature of the plate type can reach more than 1000 °C, which can be used for annealing of welds of steel materials and large diameter pipes and containers.
Because infrared has strong penetration ability, it is easy to be absorbed by objects, and once absorbed by objects, it is immediately converted into heat 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 is developing rapidly. High-frequency electric fields are used to heat insulating materials. The main heating object is the dielectric. When the dielectric is placed in an alternating electric field, it will be repeatedly polarized (the phenomenon that the dielectric has an equal amount of charge opposite polarity on its surface or inside under the action of the electric field), thereby converting the electrical energy in the electric field into heat energy.
The electric field frequency used for medium heating is high. In the medium, short wave and ultra-short wave bands, the frequency is several hundred kilohertz to 300 MHz, which is called high-frequency medium heating, and if it is higher than 300 MHz and reaches the microwave band, it is called microwave medium heating. Usually high-frequency medium heating is carried out in the electric field between the two plates; Microwave medium heating is carried out under the radiation field of waveguides, resonators or microwave antennas.
When the dielectric is heated in a high-frequency electric field, the electrical power drawn in the unit volume is P=0.566fEεrtgδ×10 (W/cm)
If expressed in heat, it is:
H=1.33fEεrtgδ×10 (cal/s·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 seen from the formula that the electric power drawn 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, while εr depends on the properties of the dielectric itself. Therefore, the object of medium heating is mainly the substance with large dielectric loss.
Since the heat is generated inside the dielectric (the object being heated), the heating speed is fast, the thermal efficiency is high, and the heating is uniform compared to other external heating.
Media heating can be used industrially to heat thermogels to dry grain, paper, wood, and other fibrous materials; It is also possible to preheat plastics before molding, as well as rubber vulcanization and bonding of wood, plastics, etc. Choosing the appropriate electric field frequency and device can only heat the adhesive when heating the plywood, without affecting the plywood itself. For homogeneous materials, integral heating is possible.
