Electric heater is an internationally popularElectric heating equipment。It is used for temperature rising, heat preservation and heating of flowing liquid and gaseous media.When the heating medium passes through the heating chamber of the electric heater under pressure, the principle of fluid thermodynamics is used to evenly remove the huge heat generated during the operation of the electric heating element, so that the temperature of the heated medium can meet the user's process requirements.
Electric heater
Electric heating is the process of converting electric energy into heat energy.Since it is found that the power supply can occur through the wireheat effectSince then, many inventors in the world have been engaged in the research and manufacture of various electric heating appliances.The development and popularization of electric heating, like other industries, follows such a rule: from advanced countries to countries all over the world;From city to countryside;From collective use to family and then to individuals;The products have developed from low-grade to high-grade.In the 19th century, most of the electric heating appliances in the embryonic stage were poor. The first electric heating appliances were used for life. In 1893, the prototype of the electric comfort bowl first appeared and was used in the United States, and then in 1909Electric cookerThe use of electric heater is to place electric heater in the stove, that is to say, heating is transferred from firewood to electricity, that is, from electric energy to heat energy.But really electricElectric appliance industryThe rapid development ofNickel chromium alloyAfter the invention of.In 1910, the United States successfully developed nickel chromium alloyElectric heating wireThe structure of the electric iron is fundamentally improved, and the use of iron is rapidly popularized.By 1925, the products with electric heating elements installed in Japanese cookers had become the prototype of modern rice cookers.In this stage, laboratory use also appears in industryelectric furnace, glue melting furnace, heater and other electric products.The period from 1910 to 1925 was a big development stage in the history of electric heating appliances. In the family and industry, the emergence and popularization of various types of electric heating appliances have been rapidly developed, especially in the family.Therefore, the invention of nickel chromium alloy is the foundation for the development of electric heating appliance industry.
After the 1920s, there were not as many new applications and developments as in the previous period. However, during this period, all kinds of electric heating appliances were reset and continuously improved, becoming an improvement stage in the history of electric heating appliances.In terms of household electric heating appliances, various appliances are designed to be more beautiful, durable and sturdy, and most of them have automatic temperature and time control.
According to the process, draw the heatedProcess Flow Diagram(Material form and specification are not involved).
Calculate the heat required for the process.
Calculate the heat and time required for system startup.
Redraw the heating process flow chart, and determine the total power of the heater by considering the appropriate safety factor.
Determine the sheath material and power density of heating elements.
Determine the form, size and quantity of heaters.
Determine the power supply and control system of the heater.
purpose
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According to the Analysis Report on Prospects and Transformation and Upgrading of China's Electric Heater Industry from 2013 to 2017, the electric heater is mainly used in the following five aspects:
1、 Heat treatment: local or overall quenching, annealing, tempering and diathermy of various metals;
2、 Hot forming: whole forging, partial forging, hot heading, hot rolling;
3、 Welding: various metal productsBrazingWelding of various knife blades and saw blades, welding of steel pipes and copper pipes, and the same and different kindsMetal welding；
4、 Metal smelting: (vacuum) smelting, casting molding and evaporation coating of gold, silver, copper, iron, aluminum and other metals;
5、 Other applications of high-frequency heater: semiconductor single crystal growth, thermal matching, bottle mouth heat sealing, toothpaste skin heat sealing, powder coating, metal implant plastic.
6、 The heater is the most popular electric heating equipment in today's society. It not only has high quality, but also has a long life.It is for flowing liquid.[1]
Explosion proof electric heater
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working principle
Fluid explosion-proof electric heaterIt is used to convert the consumed electric energy into heat energy to heat the materials to be heated.During operation, the low temperature fluid medium enters its input port under pressure through the pipeline, along the specific heat exchange channel inside the electric heating vessel, and uses the path designed by the principle of fluid thermodynamics to take away the high temperature heat energy generated during the operation of the electric heating element, so that the temperature of the heated medium rises, and the outlet of the electric heater gets the high temperature medium required by the process.The internal control system of the electric heater is based on theTemperature sensorThe signal automatically adjusts the output power of the electric heater to make the medium temperature at the output port uniform;When heatingelementIn case of overtemperature, the independent overheat protection device of the heating element will immediately cut off the heating power supply to avoid coking, deterioration and carbonization caused by overheating of the heating materials, which will lead to burning of the heating element in serious cases, effectively extending the service life of the electric heater.
Scope of application
Typical applications of fluid explosion-proof electric heater include:
1. Chemical materials in the chemical industry are heated up, some powders are dried under certain pressure, chemical process and spray drying.
2. Hydrocarbon heating, including petroleum crude oil, heavy oil, fuel oil, heat transfer oil, lubricating oil, paraffin, etc
3. Process water, superheated steam, molten salt, nitrogen (air) gas, water gas, etc.
4. Due to the advanced explosion-proof structure, the equipment can be widely used in chemical, military, oil, natural gas, offshore platforms, ships, mining areas and other places requiring explosion-proof.
Functional features
Explosion proof electric heater
1. Small size and large power: the heater mainly adopts the bundle tubular electric heating element
2. Fast thermal response, high temperature control precision and high comprehensive thermal efficiency.
3. High heating temperature: the designed maximum working temperature of the heater can reach 850 ℃.
4. The medium outlet temperature is uniform and the temperature control precision is high.
5. Wide application range and strong adaptability: the heater can be used in explosion-proof or ordinary occasions, with explosion-proof level up to d Ⅱ B and C, and pressure resistance up to 20MPa.
6. Long service life and high reliability: the heater is made of special electric heating materials, with low design surface power load and multiple protections, which greatly increases the safety and service life of the electric heater.
7. Fully automatic control: According to the requirements, the heater circuit can be designed to facilitate the automatic control of outlet temperature, flow, pressure and other parameters, and can be networked with a computer.
8. The energy-saving effect is remarkable, and the heat generated by electric energy is almost 100% transferred to the heating medium.
Change electric energy into heat energy to heat objects.It is a form of electric energy utilization.Compared with general fuel heating, electric heating can obtain a higher temperature (such as arc heating, the temperature can reach more than 3000 ℃), which is easy to realize automatic control and remote control of temperature. (such as vehicle mounted electric heating cup) can make the heated object maintain a certain temperature distribution as required.Electric heating can generate heat directly inside the heated object, so it has high thermal efficiency and fast heating speed. It can realize overall uniform heating or local heating (including surface heating) according to the heating process requirements, which is easy to achievevacuumHeating and controlled atmosphere heating.In the process of electric heating, less waste gas, residues and smoke are generated, which can keep the heated objects clean and not pollute the environment.Therefore, electric heating is widely used in production, scientific research, test and other fields.Especially in the manufacture of single crystals and transistors, mechanical parts and surface quenchingferroalloyElectric heating is used for smelting of graphite and manufacturing of artificial graphite.
The Joule effect of current is used to transform electric energy into heat energy to heat objects.It is generally divided into direct resistance heating and indirect resistance heating.The power supply voltage of the former is directly applied to the heated object. When there is current flowing, the heated object itself (such as electric heating ironing machine) will generate heat.Objects that can be directly heated by resistance must be conductors with high resistivity.As the heat is generated from the heated object itself, it belongs to internal heating and has high thermal efficiency.Indirect resistance heating requires special alloy materials orNon metallic materialsIt is made into heating elements, which generate heat energy and transmit it to the heated object through radiation, convection and conduction.Since the heated object and heating element are divided into two parts, the type of heated object is generally unlimited and easy to operate.
Materials used for heating elements heated by indirect resistance generally require high resistivityResistance temperature coefficientSmall, small deformation at high temperature and not easy to embrittlement.Commonly used areIron aluminum alloy, nickel chromium alloy and other metal materials and silicon carbideMolybdenum disilicideAnd other non-metallic materials.The maximum working temperature of metal heating element can reach 1000~1500 ℃ according to the material type;The maximum operating temperature of non-metallic heating elements can reach 1500~1700 ℃.The latter is easy to install and can be replaced by a hot furnace, but it needs a pressure regulating device when it works, and its service life is shorter than that of alloy heating elements. It is generally used in high-temperature furnaces, and its temperature exceedsMetallic materialsThe place where the heating element allows the highest working temperature and some special occasions.
induction heating
Use conductor to generate induced current in alternating electromagnetic field（vortex）The resulting thermal effect heats the conductor itself.According to different heating process requirements, the frequency of AC power supply used for induction heating includes power frequency (50-60 Hz), medium frequency (60-10000 Hz) and high frequency (higher than 10000 Hz).Power frequency power supply is the AC power supply commonly used in industry. The power frequency of most countries in the world 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 heating equipment and the capacity of power supply networkhigh-voltage power supply(6-10kV) power supply through transformer;The heating equipment can also be directly connected to the 380 V low-voltage power grid.
Intermediate frequency power supply has been used for a long timeIntermediate frequency generator set。It is driven by intermediate frequency generator andasynchronous motorform.The output power of such units is generally in the range of 50~1000 kilowatts.along withpower electronic technology What has been used is the thyristor inverter medium frequency power supply.This medium frequency power supply uses the thyristor to turn the power frequencyalternating currentIt is converted into direct current, and then the direct current is converted into alternating current of the required frequency.Because of its small size, light weight, noiseless and reliable operation, the frequency conversion equipment has gradually replaced the intermediate frequency generator set.
High frequency power supplyUsually, a transformer is used to raise the three-phase 380 V voltage to a high voltage of about 20000 V, then a thyristor or a high-voltage silicon rectifier is used to rectify the power frequency AC power into DC power, and then an electronic oscillator is used to convert the DC power into high-frequency, high-voltage AC power.The output power of high-frequency power supply equipment ranges from tens of kilowatts to hundreds of kilowatts.
Inductively heated objects must be conductors.When high frequencyAC currentWhen passing through the conductor, the conductor producesSkin effectThat 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 conduct overall uniform heating and surface heating for objects;Smeltable metal;In the high frequency band, arbitrary local heating can be carried out by changing the shape of the heating coil (also called the inductor).
Arc heating
Use the high temperature generated by the arc to heat the object.The arc is between two electrodesgas discharge Phenomenon.The voltage of the arc is not high but the current is large. Its strong current is maintained by a large number of ions evaporated on the electrode, so the arc is vulnerable to the influence of the surrounding magnetic field.When an arc is formed between electrodes, the temperature of the arc column can reach 3000~6000K, which is suitable for high temperature melting of metals.
There are direct and indirect arc heating.The arc current heated by direct arc directly passes 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 and is mainly heated by the heat radiated by the arc.The characteristics of arc heating are: high arc temperature, concentrated energy, steelmakingarc furnaceThe surface power of the tank can reach 560~1200 kW/m2.However, the arc is noisy, and its volt ampere characteristic is negative resistance (falling).In order to maintain the stability of the arc during arc heating, the instantaneous value of the circuit voltage is greater than the arcing voltage when the arc current transients to zero, and to limit the short-circuit currentPower circuitA resistor of a certain value must be connected in series.
Electron beam heating
An electron moving at high speed under the action of an electric field is used to bombard the surface of an object and make it heated.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, bunching pole, anode, electromagnetic lens and deflection coil.The anode is grounded, and the cathode is connected to the negative high position. The focusing beam is usually at the same potential with the cathode, and an accelerating electric field is formed between the cathode and the anode.The electrons emitted from 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 at the heated object in a certain direction.
The advantages of electron beam heating are: ① control the current value of the electron beamIe. The heating power can be easily and rapidly changed; ②The area of the heated part or the bombarded part of the electron beam can be freely adjusted by using the electromagnetic lens; ③The power density can be increased, so that the matter at the bombarded point can evaporate in an instant.
Infrared heating
Use infrared ray to radiate an object. After absorbing infrared ray, the object will convert radiant energy into heat energy and be heated.
Infrared is an electromagnetic wave.In the solar spectrum, beyond the red end of visible light, is an invisible radiant energy.In the electromagnetic spectrum, the wavelength range of infrared ray is 0.75~1000 μ m, and the frequency range is 3 × 10~4 × 10 Hz.In industrial applications, the infrared spectrum is often divided into several bands: 0.75~3.0 μ m is the near-infrared region;3.0~6.0 μ m is the middle infrared region;6.0~15.0 μ m is the far infrared region;15.0~1000 μ m is the extremely far infrared region.Different objects have different infrared absorption capabilities. Even the same object has different infrared absorption capabilities for different wavelengths.Therefore, if infrared heating is applied, appropriate infrared heating must be selected according to the type of object to be heatedradiation sourceThe 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, which uses tungsten, iron nickel or nickel chromium alloy and other materials as radiators to make radiation sources.After power on, heat radiation is generated due to the heating of its resistance.Commonly used electric infrared heating radiation sources include lamp type (reflective type), tube type (quartz tube type) and plate type (plane type).The lamp type is an infrared bulb, which uses tungsten wire as the radiator. The tungsten wire is sealed and filled withinert gasIn the glass shell ofGeneral lighting bulb。The radiator generates heat after being powered on (the temperature is lower than that of general lighting bulbs), thus emitting a large number of infrared rays with a wavelength of about 1.2 microns.If a reflective layer is plated 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 reflective infrared radiator.The tube of tubular infrared radiation source isquartz glassIt is made with a tungsten wire in the middle, so it is also called quartz tube infrared radiator.The wavelength of infrared ray emitted by lamp type and tube type is within 0.7~3 μ m, and the working temperature is low. It is generally used for heating, baking, drying in light and textile industries and infrared physiotherapy in medical treatment.The radiation surface of the plate type infrared radiation source is a plane, which is composed of flat resistance plates. The front side of the resistance plate is coated with materials with large reflection coefficient, and the back 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 plate type can reach more than 1000 ℃, which can be used for annealing of steel materials and welds of large-diameter pipes and vessels.
As infrared ray has strong penetration ability, it is easy to be absorbed by objects, and once absorbed by objects, it will be immediately transformed 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 develops rapidly.
Medium heating
Using high-frequency electric field pairInsulating materialApply heat.The main heating object is dielectric.When a dielectric is placed in an alternating electric field, it will be repeatedly polarized (under the action of the electric field, an equal amount of charges with opposite polarity will appear on its surface or inside), thus converting the electric energy in the electric field into heat energy.
The electric field frequency used for medium heating is very high.In the medium, short wave and ultra short wave bands, the frequency ranges from several hundred kHz to 300 MHz, which is called high-frequency dielectric heating. If the frequency is higher than 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 the two plates;Microwave dielectric heating is carried out in waveguide, resonator or under the radiation field of microwave antenna.
When a dielectric is heated in a high-frequency electric field, the electric power absorbed per unit volume isP=0.566fEεrtgδ× 10 (W/cm)
If expressed in heat, it is:
H=1.33fEεrtgδ× 10 (cal/s · cm)
WherefIs the frequency of the high-frequency electric field,εR is dielectricRelative permittivity，δIs dielectric loss angle,Ebyelectric field intensity。According to the formula, the electric power and electric field strength absorbed by the dielectric from the high-frequency electric fieldESquare of, frequency of electric fieldfAnd dielectric loss angleδIs proportional.EandfDetermined by the applied electric field, andεR depends on the nature of the dielectric itself.Therefore, the main objects of medium heating areDielectric lossLarger material.
As the heat is generated inside the dielectric (heated object), the heating speed is fast, the thermal efficiency is high, and the heating is uniform compared with other external heating.
Medium heating in industry can heat thermal gel, dry grain, paper, wood, and other fibrous materials;It can also preheat the plastic before molding, and glue rubber vulcanization and wood, plastic, etc.Selecting appropriate electric field frequency and device can heat only the adhesive when heating the plywood, without affecting the plywood itself.For homogeneous materials, overall heating can be carried out.[3]
