Electric heater is a device that converts electric energy into heat energy (the conversion efficiency is not necessarily 100%). Compared with the original traditional fuel heating, it has the advantages of sanitation, no pollution, high thermal efficiency, convenient temperature control and adjustment, and is widely used in daily life, such as electric cookers, electric mattresses, electric soldering irons, electric water heaters, and so on.
When there is current in the conductor, it will generate heat and convert electric energy into internal energy. This phenomenon is calledThermal effect of current。Main utilizationCurrent heating effectThe device to work is called electric heater.The main heating component of the traditional electric heater is the heating resistance element, but with the technological innovation, the electric heating film as a new heating element of the electric heater is more and more widely used.Electric heating is a basic property of conductor, which is related to the size, materialThicknessofMost electric heaters are heated by thermistor elements.Since the electric energy consumed is calculated according to the average electric power and service time, and it is convenient to measure and calculate, the thermal calculation of the electric heater is still based on the average electric power.In fact, sinusoidal alternating current is used in production and daily life.Therefore, the instantaneous heating rate of the heating resistance element, that is, the heat flow generated by the instantaneous electric power, also changes with time and is not a constant.[1]
working principle
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The working principle of electric heater isThermal effect of current(It is a phenomenon that the conductor temperature rises when the current passes through various conductors), which is based on the Joule Lenz law.The formula of Joule's law is:
。When the current passes through the conductor, if all electric energy is converted into internal energy, but not into other forms of energy, then electric heating is just equal to electric work.In this case, Joule's law can also use the electric work formula andOhm's lawThe formula can be transformed into:
。
The resistance type electric heater is heated by the heat generated when the current passes through the conductor with resistance.As the current through the conductor needs to do work to overcome the resistance of the conductor, and the energy conversion is measured by doing work, so how much work the current does, how much electric energy is converted into heat energy.resistivityLarge resistance wire with a certain wire diameter will continuously generate heat, thus producingheat effectThis is the electric heater.
Classification of electric heaters
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There are certain electric heatersresistanceIts resistance is selected according to the set power. The higher the power, the smaller the total resistance.The resistivity of the heating wire has an appropriate range. If the resistivity is too large or too small, a very short or long, thin or thick heating wire is required, which makes it difficult to manufacture and use large and small power heaters.According to the different purposes of electrification, electric heaters can be divided into pure resistance type and non pure resistance type. Among them, pure resistance type electric heaters are only used for heating, while non pure resistance type electric heaters are converted intomechanical energymeanwhilecoilHeat dissipating appliances, such as electric fans, and non pure resistance can be divided into ordinary non pure resistance hot spots and special non pure resistance hot spots.
Pure resistance circuit
AC circuitIf there is only resistance in this circuit, it is calledPure resistance circuit, such as electric lamps, electric soldering irons, irons, etc., only convert electric energy into heat energy, in which the light energy of electric lamps is converted from the heat generated, so they are all pure resistance circuits.
The power calculation formula of pure resistance circuit is:
, where
Is the voltage,
Is the current.Power is mainly andresistivityThere are close connections.Generally speakingElectric heating wireThe larger the diameter, the smaller the resistance and the greater the power.After being wound into a coil, the size and shape of the coil have little effect on the power.
Pure resistance electric heater
Ordinary non pure resistance circuit
ordinaryImpure resistance circuitOur equipment includes generators, motors, electric fans, electrolytic cells, etc. They do work outside in addition to heating, so these are non pure resistance circuits.The energy calculation of ordinary non pure resistance circuit equipment generally uses the following three formulas:
Among them, the voltage symbol U, current symbol I, resistance symbol R, R are called object'sResistance coefficientorresistivityIt is related to the material of the object and is numerically equal to the resistance value of the object with unit length and unit area at 20 ℃.
The heat calculation of special non pure resistance circuit generally needs to consider the following points:
(1) First, determine the temperature rise time (H) and temperature difference (℃).If the time requirement is very short, the heating power required may be very large, wasting energy;If the time is too long, the equipment preparation time will be long, and a balance point is needed.
(2) Volume of air in main equipment and connecting pipes (m3).
(3) Air specific gravity 1.16 (Kg/m3), specific heat 0.24kcal/kg ℃
（4）Heating efficiency, generally 0.5~0.6.
The heating power can be calculated according to the following formula: heating power (KW)=(volume X specific gravity X specific heat X temperature difference)/(860 X heating time X heating efficiency).
Resistivity of common conductors
The resistivity (Ω · m) of common conductors at 20 ℃ is as follows:
Electric heating wireAn important parameter ofresistivity。If the resistivity is too high, the surface power will be large after a certain current is applied.Therefore, small diameter and small current are selected for small power appliances, but the current depends on the resistance. The length and diameter of the heating wire are calculated.
Use of large industrial electric furnaceElectric ribbon heaterThat is to increase the surface area to ensure that the surface power will not exceed the standard.At the same time, in order to ensure high power, there must be enough current, so that the length, cross-sectional area and shape of each ribbon heater are comprehensively considered.Therefore, when designing an electric heater, the electric wire (belt) used in the electric heater needs various factors of power.For example, the power required by the electric heater, the technical requirements of the electric heater, the materials used, the allowable surface power of this material, the cross-sectional area of the heating wire (tape), the length of the heating wire, the shape of the heating wire (straight, spiral, broken line, winding, etc.), etc.
When the resistivity is high, when the resistance wire with a certain diameter is used to wind the heater with a certain resistance value, the length of the resistance wire required is short, which reduces the volume of the heater.To ensure that the heater can still work normally under high temperature, the melting point of the resistance wire winding the heater must be high.For example, electric furnace wire is usually usedNickel chromium alloyMaterial, temperature resistance can exceed 1000 ℃.
Development trend of heating elements
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Metal heating element
The main materials of traditional electric heating elements are generally divided into two categories: (1)Metallic materials: Precious metals and their alloys, such as platinum and iridium;Heavy metals and their alloys, such as tungsten and keys;Nickel base alloy, such as nickel chromium, nickel chromium iron;Iron base alloy, such as iron aluminum, iron chromium aluminum;Copper base alloy, such as constantan and new constantan.(2) Non metallic material: silicon key rod, the main raw materials are
，
；Silicon carbide, the main raw materials are
；PTC (Positive Temperature Coefficient of Resistance), the main raw materials are
。[2]
The metal electric heating tube is a tubular electric heating element, which uses a metal tube as the shell, puts alloy resistance wire into the tube as the heating element, and fills the tube with crystalline magnesium oxide with good insulation and thermal conductivityFar infrared radiation coatingThe development of its application technology can also add far-infrared radiation coating to the heating part of the pipe wall to become far-infrared radiation electric heating elements.Electric heating tube features: easy to use, fast temperature rise, small thermal ductility, high thermal efficiency, low power consumption, long service life, it can heat static or flowing air, can be immersed in water or other liquids for heating, and can smelt light metal and metal mold for heating.Therefore, it has been widely used by various sectors of national economy and production, such as the heating of saltpeter tanks, water tanks, oil tanks, acid and alkali tanks in all walks of life, as well as various air heaters, drying ovens, water boilers, etc.The metal electric heating tube is made of high-quality stainless steel tube material, high temperature resistant insulating filler material and strict process technology. The nozzle is sealed by special process and can work normally under 85% relative humidity.Its maximum operating temperature is 800 ℃.
The heating electric heating tube in the air is installed in the air blowing pipe of the air heating system for blowing hot air, and can also be used as the heating element of various ovens, electric furnaces, display cabinets, freezers, refrigerators, dryers and defrosters.[3]
Quartz tube
quartz glassElectric heating tube, quartz electric heating tube is made of opal quartz glass tube, and the spiral heating wire with bracket is installed in the tube as the heating element. It is the only uncoated selective far-infrared radiation heating element. Compared with other heating elements, it has good spectral radiation matching absorption characteristics, long-term use of radiation performance without degeneration, high electric heating conversion efficiency, fast temperature rise, small thermal inertia, high temperature resistanceCorrosion resistance, good thermal and chemical stability, long service life, high insulation strength, no pollution, and energy saving of about 30% compared with other electric heating elements.
It can be widely used for air heating in industrial, agricultural and civil fields.Double hole gold-plated heater with half side gold-plated: it can obtain broad spectrum infrared radiation and has better heating effect. It is mainly used in paint drying, tobacco industry, textile industry, plastic industry, printing industry, agriculture, grain industry, pharmaceutical industry, rubber industry, bone glue industry, paper making and carton industry, wood and wood manufacturing industryCeramics, enamel and other industries, as well as the environment requiring no dye, and corrosive heating occasions including acid and alkali.[3]
Halogen electric heating tube
Halogen heating tube is a sealed luminous heating tube, which is filled withHalogen group element, which can effectively ensure the service life of the heating wire.In the use of the oven, it is required to comply with GB/T241993 Basic Environmental Test Procedures for Electrical and Electronic Products, and GB4706.1-19921 General Safety Requirements for Household and Similar Electrical Appliances.
It has the following basic performance:
(1) The electrical performance is stable, the electric heating power is stable, the temperature rises quickly, and the electric heating conversion rate is up to 70%.
(2) High thermal efficiency, non oxidizing heating, service life of more than 3000 hours, safe and reliable.
(3) The light emitted during operation is mainly visible light and near-infrared light, and more than 84% is concentrated in the 0.75 μ m-3.5 μ m area.
It can be widely used for air heating in industrial, agricultural and civil fields.The shape can be processed into straight pipe or bent pipe.[3]
Ceramic electric heating tube
Ceramic far-infrared electric heating element is a direct heated far-infrared electric heating element with positive temperature coefficient, which is made of semiconductor thick film on ceramic substrate and sintered at high temperature.The ceramic electric heating element film is firmly combined with the substrate, and the far-infrared emissivity is more than 90%. It has the characteristics of wide heating temperature zone, large power density, high electric heating conversion efficiency, energy saving, safety (no open flame), long life, and can work under AC and DC voltage to meet the requirements of non inductive heating. Because its energy is mainly concentrated in the 4-10um frequency band,It is especially suitable for baking organic materials and has therapeutic and health care effects on human body.It has good substitutability and transformability for traditional heating methods in industries, agriculture, national defense, scientific research, civil appliances and other fields.
Ceramic conductive glaze direct heating typeFar-infrared radiationElectric heating tube (ceramic electric heating tube for short) is a new generation of high-tech energy-saving electric heating product, which has successfully solved many defects of traditional heating elements, such as: silicon carbide plate, metal tube, quartz tube, electric heating film and other traditional electric heating materials with low heating conversion efficiency, short life, poor environmental adaptability, poor safety performance and many other defects.
It is widely used in industry, agriculture, light industry, chemical industry, automobile, medical treatment, textile, household appliances and other industries.For example, the micro electric ceramic tube/sheet is made by sintering a layer of resistance material on the high-strength ceramic tube/sheet.It has the characteristics of small size, light weight, high power density, long service life and good weather resistance.The power density of Φ 6 porcelain tube can reach 500W/m.The minimum diameter can reach Φ 3mm, and the sheet thickness can reach 1mm.Products can be customized as required.The specifications of commonly used porcelain pipes are generally Φ 3, Φ 6, Φ 12, Φ 16, Φ 20, Φ 25, etc.It can be used as air heating or liquid flowing in heating pipe.[3]
Carbon fiber heater
Carbon fiber is a new type of high performanceFiber reinforced materialsIt has high strength, high modulus, high temperature resistance, wear resistance, corrosion resistance, fatigue resistance, creep resistance, conductivity, thermal conductivityFar-infrared radiationAnd many other excellent performances.It can reduce the weight of components in the form of composite materials, thus improving the technical performance of components.It has been widely used in aerospace, new textile machinery, petrochemical industry, medical equipment, automobile, machinery manufacturing, construction industry, stationery, telecommunications, electric heating, etcHigh tech field。Its wide application will greatly change our way of life and improve our quality of life.
Carbon fiber far-infrared hot wireNon metallic materialsThe carbon fiber is the heating element, with a thermal efficiency of more than 99%, non oxidation, stable resistance, no magnetic field, and can be folded and washed.In addition, its far infrared function is the best material for health care products, known as the "soft gold" in fibers.Carbon fiber heater is the most competitive high-tech material in this century. Its appearance in the field of electric heating has set off a new revolution. It will become an inevitable trend for carbon fiber heater to replace metal heater.The heater has the advantages of large power margin, high temperature resistance, high heat capacity, long service life, and power can be adjusted at will.Carbon fiber quartz electric heating tubeThe product has the following basic performance characteristics:
(1) Stable electrical performance;
(2) High thermal efficiency, more than 30% energy saving than ordinary metal heating body, and extremely fast heating speed;
(3) Long service life;
(4) In addition, unlike the metal heater, the carbon fiber has its own characteristics, which completely avoids the generation of electromagnetic fields.[3]
Electrothermal membrane
Electric heating film belongs toNon metallic materials。It is a thin layer resistance, commonly known as electric heating film, also known as printing resistance or spraying resistance, formed by directly brushing or spraying a conductive thin sheet or fine powder substance on glass, ceramics or insulated metal carrier after being evenly mixed with adhesive, and then drying at room temperature and sintering at constant temperature.The base material of electric heating film is a very stable closely arranged hexagonal crystal, which will not be oxidized and has a high melting point.Even if the electric heating film is scratched, broken or damaged in other forms, it can also be easily repaired by brushing or spraying. It is a promising technologynew materials 。
As the electric heating film is directly brushed or sprayed on the carrier, it does not need to add grooved accessories like the resistance wire, which simplifies the structure and process, reduces the weight, shortens the production cycle, reduces the product cost, and saves the reactive power consumption caused by the temperature rise of the accessories.The electric heating film is close to the carrier, forming a great direct heat conduction surface, and the heat is directly absorbed by the carrier, so the thermal efficiency is high.[2]
Special electric heater
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Electrothermal principle
Some special electric heaters, such asElectric blanketIt is required that the heat dissipation area is large and the unit power is low. If the usual nickel chromium and iron chromium aluminum heating wires are used, the heating wires with extremely thin cross section area are required, and they cannot be made at all.So the electric blanket is made of manganese copper wireresistivityIt is much smaller than ferrochrome aluminum, so it has enough length.In theory, the greater the power per unit area of the heater,Calorific valueThe greater the temperature, the shorter the time to reach the maximum temperature under the same conditions.However, there is a limit to the power within a certain area, which cannot be too high, otherwise, the insulation sheet or nickel chromium wire will be burnt out once the power is turned on.
