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In addition, the current heat pipe's index in heat transfer performance is high enough, and it is even higher for heat-generating heat pipes. The heat transfer rate of a single-tube heat pipe is very high. The thermal conductivity of metal and non-metal depends on the thermal conductivity of the material, temperature gradient, and the cross-sectional area orthogonal to the temperature gradient. For example, metallic silver has a value of 415 W/m2. About K, the "water-heating pipe" is hundreds of times higher than that of silver, and the energy-increasing heat pipe is about 7 times that of the "water-heating pipe". For a single tube heat pipe to have such a high conduction rate, the external structure design of the shell is very important, that is, there must be enough convection heat transfer flow rate and a large enough expansion area, that is, a very high heat transfer coefficient, or high enough Conduction rate is very difficult to achieve. Specifically, the flow rate of the heat-cooled fluid relative to the outer fins of the heating section and the cooling section outside the tube and the size of the fin surface area depend on the size of the fin surface area. Therefore, the outer surface of the heat tube is generally a finned surface, that is, the tube is used outside the tube. In the form of finned tubes, heat pipes are designed with a large finning ratio as much as possible. The heat pipe heat exchanger is composed of the above-mentioned heat pipe, so it also has a long working life and a large amount of heat transfer.
2 Work environment of heat pipe heat exchanger Because the heat pipe heat exchanger has the following main characteristics compared with the conventional heat exchanger: (1) high heat transfer coefficient; (2) large heat output or small volume, light weight, compact structure; ( 3) The heat pipe has good isothermality; (4) It is not easy to corrode, and it is not easy to accumulate ash and scale; (5) It can be detached, installed and replaced and maintained; (6) Each heat pipe is independent in the heat pipe heat exchanger. The small heat exchanger, so one or two bad heat pipe does not affect the overall heat pipe heat exchanger, but will not affect the heat pipe heat exchanger related systems. This is especially important for on-site operations.
Since the heat pipe heat exchanger has the above characteristics, it is widely used in practice, and is used in the fields of petroleum, chemical, natural gas, metallurgy, electric power, and small nitrogenous fertilizer plants for waste heat recovery and heat conversion. In the above-mentioned fields, the environmental conditions of heat pipe heat exchangers are generally poor, such as dust accumulation, fouling, large thermal resistance, and affect heat transfer; wear greatly affects the working life of the heat pipe, and the thermal contact resistance greatly affects the heat transfer and high temperature. Corrosion and low-temperature corrosion affect the safety and life of heat pipes. If the heat pipe heat exchanger can not adapt to the poor working conditions, its work, safety and life can not be guaranteed, which is a great waste of heat pipe heat exchanger. The manufacturing of heat pipes and their heat exchangers consumes a lot of materials, equipment, working hours, and energy. If the working life of heat pipe heat exchangers is reduced by a factor of two, then the material, working hours, energy, and costs will have to be doubled. It is a great waste. If the working life is doubled, all of the above items are doubled. That is the real energy-efficient product. After long working practices and investigations and practical examples, heat pipe heat exchangers capable of working long hours in the above environment have only nickel-based brazed heat pipe heat exchangers.
3 Nickel-base brazing heat pipe heat exchanger A nickel-base brazing device capable of processing a nickel-based brazing heat pipe is hardware in the processing heat pipe hardware. The nickel-based brazing heat pipe heat exchanger is a heat exchanger composed of many nickel-based brazing heat pipes. Its performance and characteristics are determined by a nickel-based brazing heat pipe. The tube shell is generally a carbon steel pipe, and the outer tube is a carbon steel wing. Sheets, typically spiral wound fins, are then sintered at high temperatures by nickel-based brazing equipment, which has a high thermal conductivity and a high working life.
4 Ni-base brazing heat pipe and its characteristics After nickel-base brazing of the heat pipe base pipe and the outer fins: (1) the welding rate of the base pipe and the fins is 100%; (2) the welding part is smooth; (3) There is no residue in welding; (4) The finned tube has a small thermal resistance; the contact thermal resistance is 0, which is obviously less than the high-temperature welding and copper alloy welding finned tube thermal resistance; (5) Its heat transfer ability is higher than that of high frequency welding. The finned tube is about 20% higher, which is equivalent to the finned tube of the overall rolled steel. Nickel-based brazing tubes are made of special equipment and techniques. Uniform nickel-chromium alloys are sprayed on the surface of carbon steel. The alloy is melted in a protective atmosphere and penetrates into the carbon steel matrix 0.013 to 0.024 mm. The deposit on the surface of the substrate is about 0.025 mm. The overall formation of nickel-chromium alloy layer thickness of about 0.05mm, its main features: (1) the alloy infiltrated layer is uniform, bright, dense; (2) and the coating, the coating is fundamentally different, there is no peeling or shedding Possible; (3) surface hardness is about 4 times that of ordinary steel pipe; (4) high temperature and low temperature dew point corrosion; (5) erosion and wear, not easy to accumulate ash and scaling; (6) can contain acid and alkali, Salt and other corrosive atmosphere environment in the long-term work; (7) The price is far lower than the stainless steel pipe is generally lower than 1/4 of the stainless steel pipe. According to the above related characteristics, the speed of convection heat transfer can also be increased to increase the heat transfer coefficient, thereby further increasing the heat exchange capacity or reducing the size of the structure.
5 Nickel-based welding of special equipment and process applications Nickel-based brazing equipment and processes are not only suitable for nickel-based brazing heat pipes, but also for steel and copper alloy finned tubes, stainless steel finned tubes, and various forms of carbon. Brazing of steel finned tubes can also be applied to corrugated tubes, nickel-based brazing of steel tubes for special occasions, infiltration of carbon steel substrates, deposition of layers on the surface of the substrate, and overall thicknesses, all of which can achieve nickel-based brazing. Heat pipe features.
Characteristics and Application of Ni-based Brazing Heat Pipe Heat Exchanger
1 heat pipe and heat pipe Heat pipe heat exchanger is mainly composed of the shell and the working medium inside the pipe. First of all, these two substances are required to be chemically compatible, that is, no chemical reaction, no new substances can be generated. This is mainly Guaranteed by the performance index of the working medium. Heat pipe work media generally have more than ten requirements, of which chemical compatibility is the primary performance, in addition, the heat pipe working media must also have safety performance indicators, that must be non-toxic, non-flammable and non-explosive Such requirements, these are indicators of heat pipe working life and safety.