What are the heating methods of electric heaters?

The electric heater is an international popular electric heating equipment. Used for warming, holding and heating a flowing liquid or gaseous medium. When the heating medium through the electric heating chamber under pressure, the fluid thermodynamics principle even take huge heat generated by the heating element in the work of the heating medium temperature reaches the requirement of customers.

Heating method:

Ohmic heating

The Joule effect of electric current transforms the electric energy into heat energy to heat the object. They are usually divided into direct resistance heating and indirect resistance heating. The supply voltage of the former is applied directly to the heated object, and when the current passes through it, the heated object itself, such as an electric ironing machine, is heated. An object that can be heated by direct resistance must be a conductor, but with a higher resistivity. Because the heat is generated by the heated object itself, it belongs to the internal heating, and the heat efficiency is very high. Indirect resistance heating needs to be made of special alloy material or nonmetal material. The heating element is produced by heating element, and it is transmitted to the heated object by radiation, convection and conduction. Because the heated object and the heating element are divided into two parts, the types of the objects to be heated are generally unrestricted, and the operation is simple.

The materials used for heating elements with indirect resistance heating generally require high resistivity and low resistance temperature coefficient. They are small in deformation and difficult to be brittle at high temperature. Commonly used are iron aluminum alloy, nickel chromium alloy and other metal materials and silicon carbide, two molybdenum silicide and other non-metallic materials. The maximum working temperature of the metal heating element can be up to 1000~1500 degrees according to the material type, and the maximum working temperature of the non-metal heating element can be up to 1500~1700 DEG C. The latter has the advantages of convenient installation, heat furnace replacement, but it need to work pressure regulating device lifetime than alloy heating element is short, generally used in high temperature furnace, the temperature exceeds the allowable maximum working temperature of the metal material of the heating element and some special occasions.

induction heating

The conductor itself is heated in an alternating electromagnetic field to produce an induced current (eddy current), causing the conductor to heat itself. According to different heating process requirements, induction heating adopts AC power frequency with frequency (50~60 Hz), intermediate frequency (60~10000 Hz) and high frequency (higher than 10000 Hz). Frequency power supply is usually used in industrial AC power supply, most countries in the world power frequency of 50 hz. The mains frequency power supply for induction heating shall be adjustable to the voltage on the induction device. According to the power size of heating equipment and the capacity of the power supply network, the power supply can be supplied by the high-voltage power supply (6~10 kV), and the heating equipment can be directly connected to the 380 volt low voltage power grid.

Intermediate frequency power supply has used intermediate frequency generator set for a long time. The utility model is composed of an intermediate frequency generator and a driving asynchronous motor. The power output of this unit is usually within 50~1000 kilowatts. With the development of power electronic technology, thyristor frequency converter and intermediate frequency power supply have been used. The intermediate frequency power supply uses the thyristor to convert the power frequency alternating current into the direct current, and then converts the direct current into the alternating current at the required frequency. Because of its small size, light weight, no noise and reliable operation, the frequency conversion equipment has gradually replaced the intermediate 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. High frequency power supplies have power output ranging from tens of kilowatts to hundreds of kilowatts.

An object heated by induction must be a conductor. When the high frequency AC current passes through the conductor, the conductor has a skin effect, i.e., the conductor surface current density is large, and the conductor center current density is small.

Induction heating can make the whole body evenly heated and heated on the surface. It can smelt metal; in the high frequency section, it can change the shape of the heating coil (also called inductor), and also can carry out any local heating.

Arc heating

The use of electric arc to produce high temperature heating objects. Arc is the phenomenon of gas discharge between two electrodes. The arc voltage is not high, but the current is very large, and the strong current is maintained by a large number of ions evaporated on the electrode. Therefore, the arc is susceptible to the influence of 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 the high temperature smelting of metal.

There are two kinds of arc heating, direct and indirect arc heating. The arc current of a direct arc heating is passed directly through the heated object, and the heated object must be an electrode or medium of the arc. The arc current of indirect arc heating is not heated by the heated object, mainly by the heat of the arc radiation. Arc heating is characterized by high arc temperature and concentrated energy, and the surface power of the molten steel arc furnace can reach 560~1200 kW / m2. But the arc noise is big, and its volt ampere characteristic is negative resistance characteristic (descent characteristic). In order to keep the instantaneous arc in arc heating stability, zero voltage in arc current instantaneous value is greater than the voltage value, at the same time in order to limit the short-circuit current in the power supply circuit, must be connected a numerical resistor.

Electron beam heating

The electrons are bombarded by electrons moving at high speeds under the electric field. The main component of electron beam heating is an electron beam generator, also called an electron gun. The electron gun mainly consists of cathode, beam pole, anode, electromagnetic lens and deflection coil. The anode is grounded and the cathode is negative high. The focusing beam is normally the same as the cathode, and between the cathode and anode

Infrared heating

An object that uses infrared radiation to heat up radiant energy when it absorbs infrared radiation.

Infrared ray is an electromagnetic wave. In the solar spectrum, an invisible radiant energy is visible beyond the red end of visible light. In electromagnetic spectrum, the wavelength range of infrared light is between 0.75 and 1000 microns, and the frequency range is between 3 * 10~4 * 10 hz. In industrial applications, often divided into several IR bands: 0.75 ~ 3 micron for near infrared region; 3 to 6 microns in the infrared region; 6 ~ 15 micron for far infrared region; 15 ~ 1000 micron for far infrared region. The ability of different objects to absorb infrared light is different, even if the same object absorbs infrared light at different wavelengths. The application of infrared heating, according to the variety of the heating object, select the suitable source of infrared radiation, the radiation energy on the heated object absorption wavelength range, to obtain 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 heating, the thermal radiation is generated due to the heating of the resistor. The common electric infrared heating radiation sources are lamp type (reflection type), tube type (quartz tube type) and plate type (flat type) three kinds. 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 reflecting layer is arranged on the inner wall of the glass shell, the infrared radiation can be concentrated in one direction, so the lamp type infrared radiation source is also called a reflecting infrared radiator. The tube of the infrared radiation source is made of quartz glass, and the middle is a tungsten filament. Therefore, it is also called quartz tube type infrared radiator. Light emitting tubes and tube type infrared wavelengths range from 0.7 to 3 microns and have lower operating temperatures. They are generally used for heating, baking, drying and infrared medical treatments in light and spinning industries. The radiation surface type infrared radiation source is a plane, consisting of a flat plate of the resistor and resistor plate is coated with a reflection coefficient of the material, the opposite with the reflection coefficient of small material, so the heat radiated by most positive. The working temperature of the plate type can reach above 1000 degrees, and can be used for annealing the welds of steel materials and large diameter pipes and containers.

Because the infrared has a strong penetrating ability, easy to be absorbed, and once absorbed into heat, immediately before and after heating; infrared energy loss is small, easy to control temperature, high heating quality, therefore, the infrared heating developed quickly.

Dielectric heating

The insulating material is heated by high frequency electric field. The main heating object is dielectric. When a dielectric is placed in an alternating electric field, it is repeatedly polarized (a phenomenon in which an electric field occurs at the same surface or inside the surface of an electric charge), thereby converting the electric energy in the electric field into heat energy.

Dielectric heating uses very high frequency electric fields. In the medium, short wave and ultrashort wave sections, the frequency is several hundred kHz to 300 MHz, which is called high frequency medium heating. If the microwave band is higher than 300 MHz, it is called microwave dielectric heating. In general, high frequency dielectric heating takes place in an electric field between two plates, while microwave dielectric heating takes place in the waveguide, resonator, or radiation field of the microwave antenna.

When the dielectric is heated in a high frequency electric field, the electric power absorbed in its unit volume is P=0.566fE epsilon RTG, delta x 10 (w / cm)

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

In the formula, f is the frequency of the high frequency electric field, epsilon R is the relative dielectric constant of the dielectric, delta is the dielectric loss angle, and E is the electric field intensity. By the formula, 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 of the electric field F and the loss angle of the dielectric. E and F are determined by the applied electric field, while epsilon r depends on the nature of the dielectric itself. Therefore, the main object of dielectric heating is the medium with relatively large dielectric loss.

The medium heats up because the heat is generated inside the dielectric (heated object), so it is faster in heating, more efficient in heat and more uniform in heating compared with other external heating.

Heating medium in industrial heating can gel, drying grain, paper, wood, and other fibrous materials; can also be preheated before molding plastic, adhesive and rubber and wood and plastic etc.. By choosing the proper electric field, frequency and device, the adhesive can be heated only when heating the plywood without affecting the plywood itself. For homogeneous materials, bulk heating can be performed.