Current situation, challenge and progress of electric heating tube technology

On the basis of summarizing the current situation of electric heating tube technology, the technical challenge and development trend of electric heating tube are analyzed and summarized, and the technical progress direction of electric heating tube in material selection, structure, production process, production equipment, product performance and engineering application is given. And through some practical cases, the direction and realization of electric heating tube technology progress are deeply interpreted, in order to give their own ideas and insights for the development and production of electric heating tube with high quality, high performance, new uses and special environment, and the progress and improvement of electric heating tube technology in our country.



1. Electric heating tube is also called electric heating rod

As a general electric heating element, it can not only be directly used for heating various media and materials, but also through different technical means to form a heating device in the shape of plate, cylinder, cluster, etc., and even can directly become a functional part of other equipment; Not only used in a variety of daily appliances, equipment, but also widely used in petroleum, chemical, metallurgy, building materials, light industry, food, HVAC and other industries related equipment and facilities, and can even play an important role in photovoltaic, nuclear power, laboratories, aviation, aerospace, ships and other high-tech fields. The production and manufacturing of electric heating tubes has become a special industrial category with complete categories, relatively mature technology, relatively important technical status, relatively large economic scale and significant impact on the efficiency of social energy utilization.



It is precisely because the electric heating tube is widely used and the process position is more critical, the controllability, reliability, safety and economy of the electric heating tube have been widely concerned, and the technical requirements are becoming more and more stringent. In addition, the progress of other related technologies will continue to put forward new technical requirements for electric heating tubes, especially the requirements of high-tech fields are often very challenging. On the other hand, the accumulation of relevant technologies, such as new electric heating materials, insulation materials, heat-resistant materials, thermal insulation materials, anti-corrosion materials, new heat transfer, molding, sealing, insulation and other theoretical research results, new electric heating tube applications, technical conditions and technical requirements, have made great progress in recent years.



In view of this, it is necessary to review and summarize the current status of electric heating tube technology, summarize and analyze the technical challenges faced by electric heating tube, and speculate on the progress direction of electric heating tube technology.



It is worth pointing out that new electric heating elements such as porous glass carbon heating element, polymer graphite heating element, quartz tube infrared heating element, ceramic embedded heating element [1], which have appeared in recent years, although similar to electric heating tube in appearance, but the principle, structure, materials and application conditions are very different, so they are not within the scope of discussion in this paper.





2. Brief history of the development of electric heating tubes

In 1859, the British Simpson proposed the invention of an electric heating tube with spiral metal wire and ceramic insulating material armored in a metal tube, but there are still big problems in the material and performance of the electric heating wire, the stability, reliability, heat transfer effect and service life of the electric heating tube. In 1917, in response to the problems existing in Simpson electric heating tube, American Wiegand proposed the invention of electric heating tube with metal shell, mineral powder filler, alloy heating wire and reduced diameter compaction [2].



On this basis, in order to adapt to different working conditions, improve technical performance and extend service life, various new theories, new materials and new processes on electric heating tubes continue to emerge, and category refinement has been carried out. At present, there may be thousands of product varieties of electric heating tubes.



3. General classification of electric heating tubes

According to the application field, electric heating tube can be divided into daily electric heating tube, HVAC electric heating tube, electric vehicle heating tube, industrial electric heating tube and other types. Because the application conditions and technical requirements of electric heating tubes in various fields are very different, they can be more finely divided. For example, the daily electric heating tube can be subdivided into electric kettle, rice cooker, electric baking pan, electric soy milk machine, electric water heater, coffee machine, electric steam box, electric oven, electric heater, air conditioner and other special electric heating tube.



According to the type of contact material or atmosphere, electric heating tubes can be divided into gas electric heaters, liquid electric heaters, solid electric heaters and other types. Among them, gas can be subdivided into oxidizing gas, reducing gas, corrosive gas, flammable and explosive gas, liquid can be subdivided into acidic liquid, alkaline liquid, solvent, paste, oil, molten salt, molten metal, etc., solid can be subdivided into metals, minerals, organic matter, food and so on.



According to the range of heating temperature of the material, the electric heating tube can consider the classification method shown in Table 1. According to the material of the electric heating wire, the electric heating tube can have Constantan, nickel iron, iron chromium aluminum, nickel chromium, PTC (Positive Temperature Coefficient positive temperature coefficient) alloy and other electric heating wire material types. According to the material of the protective sleeve, the electric heating tube can be copper, brass, white copper, aluminum, carbon steel, stainless steel, heat-resistant steel, corundum, silicon carbide and other protective sleeve material types.



According to the material of the conductive rod, the electric heating tube can be copper, low carbon steel, stainless steel, stainless iron and other conductive rod material types.



According to the material of the insulating filler, the electric heating tube can have quartz sand, alumina, magnesium oxide, aluminum nitride and other insulating filler material types.



According to the shape of the product, the electric heating pipe can be divided into double straight pipe, single straight pipe, L-shaped bend, U-shaped bend, double U-shaped bend, snake bend, double snake bend, round bend, spiral bend, disc bend and other appearance types.



According to the electrical characteristics, the electric heating tube can have four circuit wiring methods: double-ended single-phase (220V), double-ended two-phase (380V, the outside of the tube is combined into Δ connection), three-ended three-phase (380V, there is no zero line Y connection in the tube), four-ended three-phase (380V, there is zero line Y connection in the tube), but the latter three are mainly used for high-power and high-temperature heating occasions, which is relatively rare. In addition, the electric heating tube can also have a certain auxiliary temperature control effect by selecting an electric heating wire with PTC effect.



According to the type of heat exchange surface, the electric heating tube can be divided into optical tube, spiral fin tube, brazed fin tube and other types, if according to the fin material can also be subdivided into aluminum fins, copper fins, carbon steel fins, stainless steel fins and so on.



According to the installation method, the electric heating pipe can be divided into screw fastening, flange fastening, clamp fastening and other types. In addition, it can also be classified according to the electrical protection method, sealing method, wiring method, temperature detection, surface treatment method, etc.



4. General characteristics of electric heating tube

From the mechanical professional point of view, the electric heating tube is a static functional component with low manufacturing accuracy, strong specificity, wide operating temperature range, and unique manufacturing process and material selection. From the electrical professional point of view, the electric heating tube is a slightly inductive resistance element, the power factor is very high, the characteristic parameters in the work will have a great change amplitude, the start has a shock, the risk of short circuit, open circuit, leakage is relatively high. From the perspective of control professionals, electric heating tube is a control object with important process status, large temperature control inertia, obvious individual differences and performance attenuation, and high precision control difficulty.



From the application point of view, the electric heating tube electric conversion efficiency is very high, but its economy in the normal temperature range is usually far lower than the heat pump technology, in the high temperature range is usually far lower than the flame heating technology, mainly suitable for capacity is not too large, space, weight, structure limited, the pursuit of convenience, reliable, easy to control, quick effect occasions. In general, electric heating tube is the preferred technology in daily appliances and transportation vehicles, and usually does not have economic advantages in the field of HVAC, and needs to be demonstrated in the field of high temperature heating [6].



From the manufacturing point of view, electric heating tube is a kind of product that is not complicated in structure, difficult in process, slow in technological progress, difficult in automatic production, and difficult in product quality control.



From a business point of view, electric heating tube is a simple entry, a huge market, fierce competition, complex segmentation, involving more professional knowledge, not easy to become bigger and stronger, but full of challenging charm of the industry.



5. Technical challenges of electric heating tubes

The internal stress mechanism of the electric heating tube by reducing the diameter of the protective tube, the electric heating tube achieves the compaction of the insulating filler in it. In this process, the diameter of the protective tube decreases, the wall thickness increases, the length increases, the insulation filler is elastoplastic compacted, the spiral diameter of the heating wire decreases, and the diameter of the wire increases, which will form a complex internal stress, and the cold hardening effect may also be formed in the protective tube and the heating wire. When the reducing pressure is discharged, the electric heating pipe will still have various forms of residual stress.



For example, due to the elastic action of the insulating packing particles, the diameter and perimeter of the protective tube will be forced to rebound, and a large circumferential tensile stress will remain in the protective tube wall.



Theoretically, due to the natural existence of element diffusion and tissue recovery effect inside the material, when the electric heating tube falls from the reducing roller press, the creep relaxation process of the electric heating wire, protective tube, insulating filler and other materials begins, so that the residual stress of the material is continuously released, and this stress release will accelerate significantly with the increase of temperature, as shown in Figure 3.



For the electric heating tube, when the temperature rises, the thermal stress and residual stress of the protective tube are positively superimposed, which further accelerates the creep relaxation process, releases part of the internal stress of the insulation packing, alleviates the cold work hardening of the metal material, and makes the performance parameters tend to be stable. However, this will also increase the diameter of the protective tube and reduce the binding force on the insulation filler, resulting in a reduction in the compactness and thermal conductivity of the insulation filler. If the temperature continues to rise, the yield strength of the protective tube material is lower than the sum of the residual stress and temperature stress, and the yield plastic deformation will be formed, and the diameter of the protective tube will increase faster, which should usually be avoided. When the electric heating tube is repeatedly put into and out of operation, the internal stress and strain value and direction are constantly changing, which will also lead to thermal fatigue and embrittlement of the material, thereby endangering the reliability, safety and service life of the electric heating tube. Although the above discussion can roughly describe the stress mechanism inside the electric heating tube, due to the difficulty of stress detection, the complexity of strain occurrence conditions, and the difficulty of determining the physical property parameters of the material, it is still possible to roughly respond with the help of experiments and product backtracking, and the scientificity, reaction speed and implementation effectiveness are difficult to guarantee.



The breaking mechanism of the electric heating wire The breaking of the electric heating wire is usually the main reason for the damage of the electric heating tube.



There may be organizational defects and mechanical defects in the processing and manufacturing process of electric heating wire, and mechanical defects may also be formed in the operation of handling, uncoiling, winding, charging, and compacting. The stress and creep effect, alternating thermal stress and thermal fatigue effect during the working process of electric heating tube, and oxidation and ablation caused by the weak sealing of insulating filler are also important reasons for the breaking of electric heating wire.



The inconsistency between the expansion coefficient of the heating wire and the insulating filler is the formation and accumulation of the defects of the heating wire, the formation of weak points in the length direction, and finally the breakage under the superposition of the current shock, thermal shock, mechanical shock, etc., which becomes the most common damage mode of the heating wire. Under the condition that the expansion coefficient of the heating wire is less than the expansion coefficient of the insulation filler, the stress change process of the heating wire during temperature rise is shown in Figure 4. Obviously, there is not only creep relaxation but also possible yield deformation. In addition, the welding point between the electric heating wire and the wire rod may not only have virtual welding and de-welding defects in the operation, but also may form a break due to intergranular corrosion of the welding point at high temperature and large current.



Another mechanism of heating wire breakage may be that the working current forms local high temperature at the defect point, accelerates the oxidation and ablation of the defect point, and further deepens and expands the defect, so that the vicious cycle eventually leads to breakage. If we consider that the electric heating wire usually has a certain positive temperature coefficient, it can also be inferred that the electric heating wire will form a certain multiple of the impact current when starting, and the defect point is also the most likely to break at this time.



Although the electric heating wire is a key component of the electric heating tube, the research on its breaking mechanism is still limited to the inspection and post-breaking analysis of the product, and there is still a lack of sufficient technical theory and design method.



The surface corrosion of the protective tube is another main reason for the failure of the electric heating tube.



The surface of the protective tube of the electric heating tube is directly in contact with the working atmosphere, and even directly in contact with the heated material, which may be damaged by chemical corrosion, metal corrosion, material abrasion and other reasons, so that the seal of the electric heating tube is damaged, harmful substances are intruded, electrical break, and electrical short circuit. For example, the electric heating tube used for the heating and insulation of the electroplating bath will dissolve corrosion under the ionization of acidic substances; The surface of the electric heating tube used for liquid metal heating and insulation will form melting corrosion due to the alloying of liquid metal. The surface of the electric heating tube used in the flow particle furnace will be abraded under the impact and friction of the flow particles.



Although the above erosion mechanism can provide a theoretical framework, there will be new research results in related fields that can also provide reference. However, the use condition, interaction and erosion mechanism of electric heating tube are very complicated, so it must be deeply studied. For example, in an atmosphere containing hydrogen, there may be a brittle microstructure weakening due to the infiltration of hydrogen into the metal surface of the electric heating tube; Electric heating pipes used to heat air in coastal areas may corrode due to the salt in the air sticking to the surface. The auxiliary heating of the electric heating tube of the air conditioner may corrode the outer surface because of the cold wind that blows out in summer and contains water. There are a lot of theoretical and technical problems to be solved.



In addition, there is also a lot of research work to be done on how to use new materials and how to coat, coat and strengthen the surface of the protective tube to solve the corrosion problem on the surface of the protective tube.



According to experience, with the extension of the use time, the electrical performance, thermal performance and mechanical strength of the electric heating tube will continue to decay, and the reliability and safety of the electric heating tube will be affected to a certain extent, although it can still be powered up. The existing research shows that there are many factors affecting the performance attenuation process of electric heating tube, but it can be roughly summarized as chemical factors, material factors, mechanical factors and electrical factors. The performance attenuation mechanism of qualified electric heating tube is shown in Table 2.



Table 2. Performance attenuation mechanism analysis table of qualified electric heating tube

sort

cause

Influence mechanism

Chemical factor

Hot wire oxidation

Hot wire corrosion corrosion outside pipe corrosion outside pipe scaling seal failure terminal oxidation

The effective diameter of the hot wire is reduced, the resistance is increased, the heating power and the electrothermal conversion efficiency are reduced, and the embrittlement and ablation may be associated.

The oxide formed on the surface of the electric heating wire is easy to have mineral reaction, sintering and corrosion with the insulating filler. Increase the thermal resistance of the tube wall, increase the thermal resistance between the tube wall and the fin, increase the operating temperature of the electric heating wire, and accelerate the attenuation. Increase the thermal resistance of the tube wall, increase the working temperature of the electric heating wire, and accelerate the attenuation. Water vapor and other harmful substances invade, accelerate the heating wire oxidation, embrittlement, ablation, increase resistance, reduce insulation.