Converting electrical energy into thermal energy to heat objects. It is a form of electrical energy utilization. Compared with general fuel heating, electric heating can achieve higher temperatures (such as arc heating, temperature can reach above 3000℃), and it is easy to realize automatic temperature control and remote control. Car electric heating cup
The heated object can be maintained at a certain temperature distribution as needed. Electric heating can generate heat directly inside the heated object, thus having high thermal efficiency, fast heating speed, and can achieve overall uniform heating or local heating (including surface heating) according to the heating process requirements. It is easy to realize vacuum heating and controlled atmosphere heating. During the electric heating process, less waste gas, residue, and smoke are produced, keeping the heated object clean and not polluting the environment. Therefore, electric heating is widely used in production, scientific research, and testing fields. Especially in the manufacturing of single crystals and transistors, mechanical parts and surface quenching, smelting of ferroalloys, and manufacturing of artificial graphite, electric heating methods are adopted.
Classification
According to different methods of electrical energy conversion, electric heating is usually divided into resistance heating, induction heating, arc heating, electron beam heating, infrared heating, and dielectric heating.
Resistance Heating
Using the Joule effect of electric current to convert electrical energy into thermal energy to heat objects. It is usually divided into direct resistance heating and indirect resistance heating. In the former, the power supply voltage is directly applied to the heated object. When current flows, the heated object itself Electric heating ironing machine
generates heat. Objects that can be directly resistance heated must be conductors but have high resistivity. Since heat is generated inside the heated object itself, it is internal heating with high thermal efficiency. Indirect resistance heating requires special alloy materials or non-metallic materials to make heating elements, which generate heat, through radiation, convection, and conduction to the heated object. Since the heated object and the heating element are separated, the type of heated object is generally not limited, and operation is simple.
The materials used for heating elements in indirect resistance heating generally require high resistivity, small temperature coefficient of resistance, small deformation at high temperatures, and not easy to embrittle. Commonly used are metal materials such as iron-chromium-aluminum alloys and nickel-chromium alloys, and non-metallic materials such as silicon carbide and molybdenum disilicide. The maximum working temperature of metal heating elements can reach 1000～1500℃ depending on the material type; the maximum working temperature of non-metallic heating elements can reach 1500～1700℃. The latter is easy to install and can be replaced while the furnace is hot, but it requires a voltage regulating device during operation and has a shorter lifespan than alloy heating elements. It is generally used in high-temperature furnaces, places where the temperature exceeds the maximum working temperature allowed for metal heating elements, and some special occasions.
Induction Heating
Using the thermal effect of induced current (eddy current) generated when a conductor is in an alternating electromagnetic field to heat the conductor itself. According to different heating process requirements, 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 (above 10000 Hz). Power frequency power supply is the AC power supply commonly used in industry, and the power frequency in most countries in the world is 50 Hz. The voltage applied to the induction device for induction heating using power frequency power supply must be adjustable. Depending on the power of the heating equipment and the capacity of the power supply network, high-voltage power supply (6～10 kV) can be used through a transformer, or the heating equipment can be directly connected to a 380 V low-voltage power grid.
Intermediate frequency power supply used to use intermediate frequency generator sets for a long time. It consists of an intermediate frequency generator and a driving asynchronous motor. The output power of this type of set is generally in the range of 50～1000 kW. With the development of power electronics technology, now thyristor inverter intermediate frequency power supplies are used. This type of intermediate frequency power supply uses thyristors to first convert power frequency AC into DC, and then convert DC into AC of the required frequency. Due to the small size, light weight, no noise, and reliable operation of this frequency conversion equipment, it has gradually replaced intermediate frequency generator sets.
High-frequency power supply usually first uses a transformer to raise the three-phase 380 V voltage to about 20,000 V, then uses a thyratron or high-voltage silicon rectifier to rectify the power frequency AC into DC, and then uses an electronic oscillator tube to convert DC into high-frequency, high-voltage AC. The output power of high-frequency power supply equipment ranges from tens of kilowatts to hundreds of kilowatts.
The object to be induction heated must be a conductor. When high-frequency AC current passes through a conductor, the conductor exhibits the skin effect, that is, the current density on the surface of the conductor is high, and the current density in the center of the conductor is low.
Induction heating can achieve overall uniform heating and surface heating of objects; it can melt metals; in the high-frequency range, by changing the shape of the heating coil (also called inductor), arbitrary local heating can be performed.
Arc Heating
Using the high temperature generated by an electric arc to heat objects. An electric arc is a gas discharge phenomenon between two electrodes. The voltage of the arc is not high but the current is large. The strong current is maintained by a large number of ions evaporated from the electrodes, so the arc is easily affected by the surrounding magnetic field. When an arc is formed between electrodes, the temperature of the arc column can reach 3000～6000 K, suitable for high-temperature melting of metals.
Arc heating has two types: direct and indirect arc heating. In direct arc heating, the arc current directly passes through the heated object, and the heated object must be one electrode of the arc or the medium. In indirect arc heating, the arc current does not pass through the heated object, and heating mainly relies on the heat radiated by the arc. The characteristics of arc heating are: high arc temperature, concentrated energy, the surface power of the molten pool of an electric arc steelmaking furnace can reach 560～1200 kW/m². However, the arc has high noise, and its volt-ampere characteristic is negative resistance characteristic (drooping characteristic). To maintain the stability of the arc during arc heating, ensure that the instantaneous value of the circuit voltage is greater than the arc striking voltage value when the arc current crosses zero instantaneously, and to limit the short-circuit current, a certain value of resistor must be connected in series in the power supply circuit.
Electron Beam Heating
Using high-speed electrons moving under the action of an electric field to bombard the surface of an object, thereby heating it. The main component for electron beam heating is the electron beam generator, also called electron gun. The electron gun mainly consists of a cathode, a focusing electrode, an anode, an electromagnetic lens, and a deflection coil. The anode is grounded, the cathode is connected to a negative high potential, the focusing electrode is usually at the same potential as the cathode, and an accelerating electric field is formed between the cathode and the anode. Electrons emitted from the cathode are accelerated to very high speeds 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 is directed to the heated object in a certain direction.
The advantages of electron beam heating are: ① By controlling the current value of the electron beam Ie, the heating power can be easily and quickly changed; ② Using the electromagnetic lens, the heated part can be freely changed or the area bombarded by the electron beam can be freely adjusted; ③ The power density can be increased so that the material at the bombarded point evaporates instantly.
Infrared Heating
Using infrared radiation to irradiate objects. After absorbing infrared rays, objects convert radiant energy into thermal energy and are heated.
Infrared rays are electromagnetic waves. In the solar spectrum, they lie beyond the red end of visible light and are invisible radiant energy. In the electromagnetic spectrum, the wavelength range of infrared rays is between 0.75 and 1000 micrometers, and the frequency range is between 3×10 and 4×10 Hz. In industrial applications, the infrared spectrum is often divided into several bands: 0.75～3.0 micrometers is the near-infrared region; 3.0～6.0 micrometers is the mid-infrared region; 6.0～15.0 micrometers is the far-infrared region; 15.0～1000 micrometers is the extreme far-infrared region. Different objects have different abilities to absorb infrared rays, and even the same object has different absorption abilities for different wavelengths of infrared rays. Therefore, when using infrared heating, it is necessary to select an appropriate infrared radiation source according to the type of heated object, so that its radiation energy is concentrated within the absorption wavelength range of the heated object to achieve good heating results.
Electric infrared heating is actually a special form of resistance heating, using materials such as tungsten, iron-nickel, or nickel-chromium alloys as radiators to make radiation sources. When energized, they generate thermal radiation due to their resistance heating. Common electric infrared heating radiation sources include lamp type (reflective), tube type (quartz tube type), and plate type (flat type). The lamp type is an infrared bulb, with a tungsten filament as the radiator, sealed in a glass shell filled with inert gas, similar to an ordinary lighting bulb. When energized, the radiator heats up (temperature lower than ordinary lighting bulbs), emitting a large amount of infrared rays with a wavelength of about 1.2 micrometers. If a reflective layer is plated on the inner wall of the glass shell, the infrared rays can be concentrated to radiate in one direction, so the lamp-type infrared radiation source is also called a reflective infrared radiator. The tube-type infrared radiation source uses a quartz glass tube with a tungsten filament inside, so it is also called a quartz tube infrared radiator. The wavelength of infrared rays emitted by lamp-type and tube-type is in the range of 0.7～3 micrometers, with low working temperature, generally used for heating, baking, drying in light and textile industries, and infrared physiotherapy in medical treatment. The plate-type infrared radiation source has a flat radiation surface, composed of flat resistance plates. The front side of the resistance plate is coated with material with high reflection coefficient, while the back side is coated with material with low reflection coefficient, so most of the heat energy is radiated from the front side. The working temperature of the plate type can reach above 1000℃, and it can be used for annealing of steel materials and welds of large-diameter pipes and containers.
Because infrared rays have strong penetration ability, are easily absorbed by objects, and once absorbed, immediately convert into thermal energy; infrared heating has little energy loss before and after, easy temperature control, and high heating quality. Therefore, the application of infrared heating has developed rapidly.
Dielectric Heating
Using high-frequency electric fields to heat insulating materials. The main heating object is dielectrics. When a dielectric is placed in an alternating electric field, it will be repeatedly polarized (the phenomenon where equal and opposite charges appear on the surface or inside of the dielectric under the action of an electric field), thereby converting the electrical energy in the electric field into thermal energy.
The electric field frequency used for dielectric heating is very high. In the medium-wave, short-wave, and ultra-short-wave bands, frequencies from several hundred kHz to 300 MHz are called high-frequency dielectric heating. If it exceeds 300 MHz and reaches the microwave band, it is called microwave dielectric heating. Usually, high-frequency dielectric heating is carried out in the electric field between two plates; while microwave dielectric heating is carried out under the radiation field of a waveguide, resonant cavity, or microwave antenna.
When a dielectric is heated in a high-frequency electric field, the electric power absorbed per unit volume is P=0.566fEεrtgδ×10 (W/cm)
If expressed in terms of 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, E is the electric field strength. From the formula, it can be seen that the electric power absorbed 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 external electric field, while εr depends on the nature of the dielectric itself. Therefore, the main objects of dielectric heating are substances with large dielectric loss.
Since heat is generated inside the dielectric (heated object), compared with other external heating methods, dielectric heating has fast heating speed, high thermal efficiency, and uniform heating.
In industry, dielectric heating can be used to heat thermal gel, dry grains, paper, wood, and other fibrous materials; it can also be used for preheating plastics before molding, rubber vulcanization, and bonding of wood and plastics. By selecting appropriate electric field frequency and device, when heating plywood, only the adhesive can be heated without affecting the plywood itself. For homogeneous materials, overall heating can be achieved.
Working Principle
High-frequency large current flows into a heating coil wound into a ring or other shape (usually made of copper tube). This generates a strong magnetic flux with instantaneous polarity changes inside the coil. When a metal or other heated object is placed inside the coil, the magnetic flux penetrates the entire heated object, generating a corresponding large eddy current in the opposite direction to the heating current inside the heated object. Due to the resistance inside the heated object, a lot of Joule heat is generated, causing the object's temperature to rise rapidly. Achieving the purpose of heating all metal materials.
Applications of Electric Heating Machines
1. Heat Treatment: Local or overall quenching, annealing, tempering, through heating of various metals;
2. Hot Forming: Whole forging, local forging, hot upsetting, hot rolling;
3. Welding: Brazing of various metal products, welding of various tool blades, saw blade teeth, welding of steel pipes, copper pipes, welding of similar and dissimilar metals;
4. Metal Melting: (Vacuum) melting, casting, and evaporation coating of metals such as gold, silver, copper, iron, aluminum;
5. Other Applications of High-Frequency Heating Machines: Semiconductor single crystal growth, thermal fitting, bottle mouth heat sealing, toothpaste tube heat sealing, powder coating, metal insertion into plastic
