The joule effect of using current transform electric energy into heat energy to heat the object. Usually divided into direct resistance heating and indirect resistance heating. Power supply voltage of the former directly to the heated object, when have a current flows through the heated object itself, such as electric heating ironing machine) and fever. Direct resistance heating object must be a conductor, but must have the high resistivity. Due to the heat generated by heated object itself, belongs to the internal heating, high thermal efficiency. Indirect resistance heating should be made of special alloy materials or non-metallic materials, heating element, heat generated by the heating element, through radiation, convection and conduction to the heated object. Because of the heated object is divided into two parts and heating elements, so heated object types is generally not restricted, easy to operate.
Indirect resistance heating heating element materials, general requirements resistivity, temperature coefficient of resistance small, little deformation at high temperature and not easily embrittlement. Commonly used a iron alloy, nickel chromium alloy and other metal materials and silicon carbide, such as molybdenum disilicide non-metallic materials. The highest working temperature, metal heating element according to the types of materials can reach 1000 ~ 1000 ℃; Non-metallic heating element of the highest working temperature is 1500 ~ 1700 ℃. Which features easy installation, can change hot stove, but it requires pressure regulating device at work, life, shorter than alloy heating elements, commonly used in high temperature furnace, temperature more than metal materials heat element allows the highest working temperature place, and some special occasions.
Use of conductor in alternating electromagnetic field induced current (eddy current) formed by the thermal effect of heating conductor itself. According to the requirements of the different heating process, induction heating USES a power frequency ac power frequency (50 ~ 60 hz), intermediate frequency (60 ~ 10000 hz) and high frequency (above 10000 hz). Power frequency power is usually of the ac power industry, the vast majority of countries in the world of power frequency is 50 hz. Induction heating power supply to the power frequency voltage on the sensor must be adjustable. According to the size and heating equipment power supply network size, can use a high voltage power supply (6 ~ 10 kv) through transformer power supply; Also can directly to the heating equipment on 380 v low voltage power grid.
Medium frequency power supply had in a long time by intermediate frequency generator. It is made up of intermediate frequency generator and asynchronous motor drive. The output power of the unit generally in the range of 50 ~ 1000 kw. With the development of power electronic technology, is the use of a thyristor inverter medium frequency power supply. This kind of medium frequency power supply using thyristor power frequency alternating current into direct current (dc) first, then the direct current into the frequency of the alternating current. Because of the frequency conversion equipment is small in size, light weight, no noise, reliable operation, etc., has gradually replaced the intermediate frequency generator.
Usually high frequency power transformer three-phase 380 v voltage rise to about 20000 v high voltage, and then use thyratron or high voltage silicon rectifying device for direct current, alternating current (ac) power frequency rectifier with electron oscillation tube the direct current into high frequency and high voltage alternating current (ac). The output power of high frequency power supply equipment is from dozens of kw to hundreds of kilowatts.
The object of induction heating must be the conductor. When high frequency alternating current through a conductor, a conductor skin effect, namely the conductor surface current density is big, small center conductor current density.
Induction heating can be an object for the overall uniform heating and surface heating; Metal smelting; At high frequencies, change the shape of a heating coil (also called sensors), also can be in any local heating.
The heat generated by electric arc heating object. Arc is phenomenon of gas discharge between two electrodes. Arc voltage is not high but the current is large, its powerful current supported by evaporation of a large number of ions on the electrode, and therefore vulnerable to the effects of the magnetic field around the arc. When the arc is formed between the electrode and arc column temperature can reach 3000 ~ 6000 k, suitable for high temperature smelting metal.
Arc heating has two kinds of direct and indirect arc heating. Direct electric heating arc current directly through the heated objects, heated object must be an electrode arc or medium. Indirect arc heating arc current is not through the heated objects, mainly by electric arc heat radiation heating. Electric heating features are: high arc temperature, energy concentration, steel-making arc furnace of the welding pool surface power up to 560 ~ 1200 kw/m2. But the noise of the electric arc, the volt ampere characteristics of negative resistance characteristics (down). In order to maintain the stability of arc when arc heating, the arc current of instantaneous zero circuit voltage instantaneous value is greater than the arc voltage, to limit short circuit current at the same time, in the power supply circuit, must through a certain value of the resistor
Electron beam heating
Used in the electric field under the action of high speed movement of the electron bombardment surface, make it is heated. Is a major part of the electron beam heating electron beam generator, also called electron gun. Electron gun is mainly composed of electromagnetic lens, bunching, anode and cathode and deflection coil and other parts. High negative anode grounding, cathode, focused beam usually with cathode potential, accelerating field formed between cathode and anode. By the electron emitting cathode, accelerating field under the action of accelerated to high speed, through the lens of electromagnetic focus, then through deflection coil control, make the electron beam in a certain direction toward the heated object.
The advantage of electron beam heating is: (1) to control the electron beam current value, Ie can be easily and rapidly changing the heating power; (2) using the electromagnetic lens can freely change the heated part or can freely adjust the area of electron beam bombardment part; (3) can increase the power density, in order to make the point being bombarded substance evaporates in a flash.
After using infrared radiation object, infrared absorption, the radiant energy into heat energy by heating.
Infrared light is an electromagnetic wave. In the solar spectrum, at the red end of visible light, the radiant energy is a kind of invisible. In the electromagnetic spectrum, infrared wavelength range between 0.75 to 1000 micron, frequency range between 3 x 10 ~ 4 x 10 hz. In industrial applications, often used the infrared spectrum is divided into several band: 0.75 ~ 3.0 microns for near infrared area; In 3.0 ~ 6.0 microns for infrared area; 6.0 ~ 15.0 microns for far infrared area; 15.0 to 1000 micron to a far infrared region. Different objects in infrared absorption ability, even if the same object, for different wavelengths of infrared ray absorbing ability. Application of infrared heating, therefore, must according to the types of heated object, select the appropriate infrared radiation source, the radiation energy is focused on the heated object absorption wavelength range, in order to get good heating effect.
Electric infrared heating is actually a kind of special form of resistance heating, i.e., according to the material such as tungsten, iron nickel or nickel chromium alloy as radiator, radiation source. After electrify, due to its electrical resistance heating and heat generation. A commonly used electric infrared heating source type lamp (reflection), pipe type (quartz tube type) and plate type (flat) three. Is a kind of infrared light bulb lamp type, is radiator with tungsten filament, tungsten wire seal within the glass shell filled with inert gases, as a general lighting bulbs. Radiator after electrify fever (temperature is lower than general lighting bulbs), which emit a lot of infrared wavelength of 1.2 microns. If on the inner wall of the glass shell plating reflector, but focuses infrared radiation, in the direction of a type so light reflecting type of infrared radiation sources, also known as infrared radiator. Tube type infrared radiant tubes are made of quartz glass in the middle is a tungsten filament, which is also called quartz tube infrared radiator. The wavelength of the ir lamp type and tube type in 0.7 ~ 3 micron range, low working temperature, generally used for light, heating, baking, drying and spinning industry of medical infrared physical therapy, etc. Plate type infrared radiation source radiation surface is a plane, is composed of the resistance of the flat plate, plate coated with positive reflection coefficient of the resistance of the material, the reverse side is coated with reflection coefficient of the material, so the most heat by radiation out positive. Plate type of working temperature can reach 1000 ℃ above, can be used in iron and steel materials and large diameter pipeline and vessel weld annealing.
Due to the infrared ray has strong penetrating power, easily absorbed by the object, and once for objects absorb, immediately turned into heat energy; Infrared heating and energy loss is small, easy temperature control, heating quality, therefore, infrared heating application development soon.
Using high frequency electric field to heat insulation material. The main object of heating is dielectric. Dielectric put in alternating electric field, will be repeated polarization (under the action of the dielectric in electric field, its surface or internal appear the phenomenon of equal and opposite polarity charge), to the electric field of electric energy into heat energy.
Dielectric heating using high frequency electric field. In the medium and short wave and ultrashort wave band, the frequency of a few hundred KHZ to 300 MHZ, known as the high frequency dielectric heating, if higher than 300 MHZ, achieve microwave band, it is called a microwave dielectric heating. High frequency heating medium is usually done in the electric field between the two plates; And microwave dielectric heating is in the waveguide, cavity, or under the microwave irradiation antenna radiation field.
Dielectric heating due to the heat generated inside the dielectric objects (heated), so compared with other external heating, heating speed, high thermal efficiency, heat up evenly.
Medium heat to heat thermal gel in the industry, drying grain, paper, wood, and other fibrous materials; Can also to preheat before molded plastic and rubber vulcanization and wood adhesive, plastic, etc. Frequency and device, select the appropriate field during heating plywood heated glue only, without affecting the plywood itself. For homogeneous material, can make whole heating.