The history of heat exchanger development
Plate heat exchangers appeared in the food industry in the 1920s. The heat exchanger made of the plate is compact and has a good heat transfer effect, so it has gradually developed into various forms. In the early 1930s, Sweden made spiral plate heat exchangers. Then the UK used brazing to produce a plate-fin heat exchanger made of copper and its alloy materials for heat dissipation in aircraft engines. In the late 1930s, Sweden produced shell-and-shell heat exchangers for pulp mills. In the meantime, in order to solve the heat exchange problem of highly corrosive media, attention has been paid to heat exchangers made of new materials. In the 1960s, due to the rapid development of space technology and science, various high-efficiency and compact heat exchangers were urgently needed. Together with the development of technologies such as stamping, brazing and sealing, the heat exchanger manufacturing process was further improved. Promoted the development and wide application of compact plate heat exchangers. In addition, since the 1960s, in order to meet the needs of heat exchange and energy saving under high temperature and high pressure conditions, the typical shell-and-tube heat exchanger has been further developed. In the mid-1970s, in order to enhance heat transfer, heat pipe heat exchangers were created on the basis of research and development of heat pipes. Heat exchangers can be divided into three types: hybrid, regenerative and partition. Hybrid heat exchangers are heat exchangers that exchange heat through direct contact and mixing of cold and hot fluids, also known as contact heat exchangers. Since the two fluids must be separated in time after the heat exchange, the heat exchanger is suitable for the heat exchange between the gas and the liquid. For example, in the cooling towers used in chemical plants and power plants, hot water is sprayed from top to bottom, while cold air is sucked from bottom to top, on the surface of the water film of the filler or on the surface of droplets and water droplets, hot and cold air. The heat is exchanged with each other, the hot water is cooled, the cold air is heated, and then the density difference between the two fluids is separated in time. The regenerative heat exchanger is a heat exchanger that uses a cold and hot fluid to alternately flow through the surface of the regenerator (filler) in the regenerator to exchange heat, such as a regenerator that preheats the air under the coke oven. This type of heat exchanger is mainly used to recover and utilize the heat of high temperature exhaust gas. The same kind of equipment for the purpose of recovering the cold amount is called a regenerator, and is mostly used in an air separation device. The heat exchangers of the partition wall heat exchanger are separated by solid partitions and exchange heat through the partition walls. Therefore, they are also called surface heat exchangers. These heat exchangers are widely used. The wall-mounted heat exchanger can be divided into a tube type, a plate surface type and other types according to the structure of the heat transfer surface. The tubular heat exchanger uses the surface of the pipe as a heat transfer surface, including a coiled-tube heat exchanger, a tube-type heat exchanger, and a shell-and-tube heat exchanger; the plate-surface heat exchanger uses the plate surface as a heat transfer surface, including Plate heat exchangers, spiral plate heat exchangers, plate-fin heat exchangers, plate-and-shell heat exchangers and umbrella plate heat exchangers; other types of heat exchangers are heat exchangers designed to meet certain special requirements Such as scraped surface heat exchangers, rotary heat exchangers and air coolers. The relative flow direction of the fluid in the heat exchanger is generally both forward and reverse. When flowing downstream, the temperature difference between the two fluids at the inlet is the largest, and gradually decreases along the heat transfer surface, and the temperature difference to the outlet is minimized. In the case of countercurrent flow, the temperature difference distribution of the two fluids along the heat transfer surface is relatively uniform. Under the condition that the inlet and outlet temperatures of the cold and hot fluids are constant, when the two fluids have no phase change, the average temperature difference in the countercurrent is the largest downstream. Under the condition of completing the same amount of heat transfer, the countercurrent can be used to increase the average temperature difference, and the heat transfer area of the heat exchanger is reduced; if the heat transfer area is constant, the consumption of heating or cooling fluid can be reduced by using the reverse flow. The former can save equipment costs, the latter can save operating costs, so countercurrent heat transfer should be used in design or production. When both or one of the cold and hot fluids has a phase change (boiling or condensation), since only the latent heat of vaporization is released or absorbed during the phase change, the temperature of the fluid itself does not change, so the inlet and outlet temperatures of the fluid are equal. The temperature difference between the two fluids is independent of the flow direction of the fluid. In addition to the forward flow and the reverse flow, there are flow directions such as cross flow and baffle flow. In the heat transfer process, reducing the thermal resistance in the partition wall heat exchanger to increase the heat transfer coefficient is an important problem. The thermal resistance mainly comes from a thin layer of fluid (called a boundary layer) stuck to the heat transfer surface on both sides of the partition wall, and a fouling layer formed on both sides of the wall in the use of the heat exchanger, and the thermal resistance of the metal wall is relatively small. Increasing the flow rate and the perturbation of the fluid can reduce the boundary layer and reduce the thermal resistance to increase the heat transfer coefficient. However, increasing the fluid flow rate will increase the energy consumption, so the design should be reasonably coordinated between reducing the thermal resistance and reducing the energy consumption. In order to reduce the thermal resistance of the dirt, it is possible to delay the formation of the dirt and periodically clean the heat transfer surface. Generally, heat exchangers are made of metal materials, among which carbon steel and low alloy steel are mostly used to manufacture medium and low pressure heat exchangers; stainless steel is mainly used for different corrosion resistance conditions, and austenitic stainless steel can also be used as resistance. High and low temperature materials; copper, aluminum and their alloys are mostly used in the manufacture of low temperature heat exchangers; nickel alloys are used in high temperature conditions; non-metallic materials have been used to make non-metallic materials in addition to gasket parts. Etch heat exchangers, such as graphite heat exchangers, fluoroplastic heat exchangers and glass heat exchangers.