What are the heating modes of electric heaters

Electric heater is an international popular electric heating equipment. It is used to heat, heat and heat the liquid and gaseous media of the flow. When the heating medium is under pressure, the heating chamber is heated through the electric heater, and the huge heat generated by the working of the electric heating element is uniformly carried out by using the fluid thermodynamics principle, so that the temperature of the heated medium reaches the user's technological requirement.

Heating mode:

Resistance heating

The Joule effect of the current is used to convert electrical energy into heat to heat an object. It is usually divided into direct resistance heating and indirect resistance heating. The power voltage of the former is directly added to the heated object. When the current flows through, the heated object itself (such as the electric heating and pressing machine) will be heated. An object that can be heated by a direct resistance must be a conductor, but with a higher resistivity. Because the heat is produced in the heated object itself, it is internal heating, and the heat efficiency is very high. Indirect resistance heating should be made of special alloy or non-metallic materials, and heat generated by heating elements can be transmitted to heated objects by radiation, convection and conduction. The heating object and the heating element are divided into two parts, so the type of the heated object is generally unrestricted, and the operation is simple.

The materials used for heating elements with indirect resistance generally require large resistivity, small resistance temperature coefficient, small deformation at high temperature and not easy to embrittlement. There are metal materials such as iron aluminum alloy, nickel chromium alloy and other non-metallic materials, such as silicon carbide, two molybdenum silicide and so on. The highest working temperature of a metal heating element is up to 1000~1500 C according to the type of material, and the maximum working temperature of a non-metallic heating element is up to 1500~1700 degrees C. The latter is easy to install and can be replaced by hot stove, but it needs voltage regulator when working, and its life is shorter than that of alloy heating element. It is generally used for high temperature furnace and temperature exceeding the allowable maximum working temperature of metal material heating element and some special occasions.

Induction heating

The thermal effect formed by the induction current (eddy current) produced by the conductor in the alternating electromagnetic field causes the conductor to heat up. According to different heating process requirements, the frequency of AC power used for induction heating is of power frequency (50~60 Hz), intermediate frequency (60~10000 Hz) and high frequency (10000 Hz). Power frequency power is usually used as an AC power supply in industry. The power frequency of most countries in the world is 50 Hz. The power frequency power supply for induction heating and the voltage on the induction device must be adjustable. According to the size of heating equipment and the size of power supply network, it can be supplied by a high-voltage power supply (6~10 kV) through a transformer, or directly connect heating equipment to 380 volt low voltage power grid.

If the medium frequency power supply was used for a long time, the medium frequency generator set was used. It is composed of a medium frequency generator and a driving asynchronous motor. The output power of this unit is generally within the range of 50~1000 kW. With the development of electric and electronic technology, the frequency power supply of the thyristor inverter has been used. This medium frequency power converter uses thyristor to convert the power frequency alternating current into DC, and then converts the DC to the required frequency. Because of the small volume, light weight, no noise, reliable operation and so on, it has gradually replaced the medium frequency generator set.

High frequency power supply is usually used to increase the voltage transformer three-phase 380 volts high voltage of about 20 thousand volts or so, and then use the thyristor or the high voltage silicon rectifier rectifier DC to AC, then the electron oscillation tube to DC power into high frequency and high voltage alternating current. The output power of the high frequency power supply equipment is from dozens of kilowatts to hundreds of kilowatts.

An object that is induced to be heated must be a conductor. When the high frequency AC current passes through the conductor, the conductor produces the skin effect, that is, the current density of the conductor surface is large and the current density in the center of the conductor is small.

Induction heating can uniformly heat the surface of the object and heat the surface. It can smelt metals. In the high frequency section, the shape of the heating coil can also be changed, and any local heating can be done.

Arc heating

An object is heated by a high temperature produced by an arc. The arc is the phenomenon of gas discharge between the two electrodes. The voltage of the arc is not high but the current is large. Its powerful 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 the arc is formed between the electrodes, the temperature of the arc column can reach 3000 ~ 6000K, which is suitable for high temperature smelting of metal.

There are two kinds of electric arc heating: direct and indirect arc heating. The electric arc current of the direct arc heating directly passes through the heated object, and 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, which is heated mainly by the heat of the arc radiation. The characteristic of arc heating is that the arc temperature is high and the energy is concentrated. The surface power of the melting tank of the steelmaking arc furnace can reach 560~1200 kW / sq. But the noise of the arc is large, and its volt ampere characteristic is the characteristic of negative resistance (descent characteristic). In order to keep the arc stable during the arc heating and the instantaneous zero crossing of the arc current, the instantaneous value of the circuit voltage is greater than the arcing voltage. At the same time, in order to limit the short-circuit current, a resistor with a certain value must be connected in the power supply loop.

Electron beam heating

The electron bombardment of the surface of the object by high speed motion under the action of an electric field is heated. The main component of the electron beam heating is the electron beam generator, also known as the electronic gun. The electronic gun is mainly composed of the cathode, the bunching pole, the anode, the electromagnetic lens and the deflection coil. The anode is grounded, the cathode is connected to the high position, the focus beam is usually the same as the cathode, and the accelerating electric field is formed between the cathode and the anode. The electrons emitted from the cathode accelerate to a high speed under the action of accelerating electric field, focus by electromagnetic lens, and then control by deflection yoke, so that the electron beam will shoot to the heated object in a certain direction.

The advantages of electron beam heating: electron beam current control value Ie can conveniently and rapidly change the heating power; the electromagnetic lens can freely change the heated part or can freely adjust the electron beam bombardment part of the area; the increase of power density, in order to make the point bombarded substances in flash evaporation off.

Infrared heating

Using infrared radiation objects, the object absorbs infrared rays and turns the radiation energy into heat and is heated.

Infrared is a kind of electromagnetic wave. In the solar spectrum, outside the red end of visible light, it is an invisible radiation energy. In the electromagnetic spectrum, the wavelength range of infrared is between 0.75 and 1000 microns, and the frequency range is between 3 * 10~4 and 10 Hz. In industrial applications, infrared spectra are often divided into several bands: 0.75 ~ 3 microns are near infrared regions, 3 to 6 microns are middle infrared regions, 6 to 15 microns are far infrared regions, and 15 to 1000 micrometers are far infrared regions. The ability of different objects to absorb infrared rays is different, even the same object, and the ability to absorb infrared rays at different wavelengths is different. Therefore, the application of infra-red heating should be based on the type of heated object, and select the appropriate infrared radiation source, so that the radiation energy will be concentrated in the absorption wavelength range of the heated object, so as to get a good heating effect.

Electric infrared heating is actually a special form of resistance heating, the tungsten, iron nickel or nickel chromium alloy materials such as radiator, made of radiation source. After electricity, heat radiation is produced due to the heat of its resistance. The commonly used electric and infrared heating radiation sources include three kinds of lamps (reflected type), tube type (quartz tube type) and plate type (plane type). The lamp is an infrared light bulb, with tungsten radiator, tungsten inert gas in a sealed glass shell filled in as general lighting lamp. Electricity heating radiator (bulb temperature than the general lighting, low) to emit a large number of wavelengths is about 1.2 micron infrared. If the reflective layer is placed on the inner wall of the glass shell, the infrared can be radiated to one direction, so the lamp type infrared radiation source is also called the 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 emission wavelength of lamp and tube type is in the range of 0.7 to 3 microns, and the working temperature is low. It is generally used for heating, baking, drying and infrared physical therapy in medical and light industries. The radiation surface of the plate infrared radiation source is a plane, consisting of a flat resistance plate. The front side of the resistive plate is coated with materials with large reflection coefficient. The reverse side is coated with materials with small reflection coefficient, so most of the heat energy is radiated from the front side. The working temperature of the plate can be above 1000 degrees C, and can be used for the annealing of the weld 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 it is absorbed for objects, it will instantly change to heat energy. Before and after infrared heating, the energy loss is small, the temperature is easy to control and the heating quality is high, so the application of infrared heating is developing rapidly.

Medium heating

The insulation material is heated by high frequency electric field. The main heating object is the dielectric. When dielectric is placed in alternating electric field, it will be repeatedly polarized (the phenomenon of equal or opposite charge on the surface or inside of the dielectric under the action of electric field), so that the electric energy in the electric field can be changed to heat energy.

The electric field used by medium heating is very high. In the medium, short and ultra short waves, the frequency is from several hundred kHz to 300 MHz, which is called high frequency dielectric heating. If it is above 300 MHz and reaches the microwave band, it is called microwave dielectric heating. Usually, the high frequency dielectric heating is carried out in the electric field between the two plates, while the microwave heating is carried out in the waveguide, resonant cavity or under the radiation field of the microwave antenna.

When the dielectric is heated in a high frequency electric field, the electric power in its unit volume is P=0.566fE e RTG delta x 10 (Watts / centimeter).

If expressed in heat, it is: H=1.33fE e RTG delta x 10 (card / sec. Cm)

The F is the frequency of the high frequency electric field, the epsilon R is the relative dielectric constant of the dielectric, the delta is the dielectric loss angle, and the E is the electric field strength. It is known from the formula that the electric power absorbed by the dielectric from the high frequency electric field is proportional to the square of the electric field intensity E, the frequency f of the electric field and the loss angle of the dielectric. E and F are determined by the applied electric field, while the epsilon r depends on the properties of the dielectric itself. So the main object of medium heating is the material with larger dielectric loss.

Dielectric heating is generated inside the dielectric (heated object) because of heat. Therefore, compared with other external heating, the heating speed is fast, the thermal efficiency is high, and the heating is uniform.

Dielectric heating can heat gel in the industry, drying Cereals, paper, wood and other fiber materials, preheating molded plastics, and bonding rubber vulcanization, wood and plastics. A suitable electric field frequency and device can be used to heat the adhesive only when the plywood is heated, without affecting the plywood itself. For homogeneous material, it can be heated as a whole.