Plate heat exchangers appeared in the 1920s and were used in the food industry. The heat exchanger made of plate instead of tube has compact structure and good heat transfer effect, so it has gradually developed into various forms. In the early 1930s, Sweden made the spiral plate heat exchanger for the first time. Then the British used brazing to produce a plate-fin heat exchanger made of copper and its alloy materials for the heat dissipation of aircraft engines. At the end of the 1930s, Sweden produced the first plate and shell heat exchanger for pulp mills. During this period, in order to solve the heat exchange problem of strong corrosive media, people began to pay attention to heat exchangers made of new materials. Around the 1960s, due to the rapid development of space technology and cutting-edge science, a variety of high-performance and compact heat exchangers were urgently needed. Coupled with the development of stamping, brazing and sealing technologies, the heat exchanger manufacturing process was further improved. This has promoted the vigorous 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, typical shell and tube heat exchangers have also 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 type, heat storage type and partition type according to different heat transfer methods. Hybrid heat exchanger is a heat exchanger that exchanges 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 they are mixed and exchanged, this type of heat exchanger is suitable for heat exchange between gas and liquid. For example, in the cooling water towers used in chemical plants and power plants, hot water is sprayed from top to bottom, while cold air is sucked in from bottom to top, on the surface of the water film of the filling or on the surface of droplets and water droplets, hot water and cold air Contact with each other for heat exchange, the hot water is cooled, the cold air is heated, and then separated in time by the density difference between the two fluids. Regenerative heat exchanger is a heat exchanger that uses cold and hot fluid to alternately flow through the surface of the regenerator (filler) in the regenerator to exchange heat, such as the regenerator for preheating 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 similar equipment for the purpose of recovering cold is called cold storage, which is mostly used in air separation plants. The cold and hot fluids of the partition wall heat exchanger are separated by solid partition walls and exchange heat through the partition wall. Therefore, it is also called surface heat exchanger. This type of heat exchanger is the most widely used. Partitioning wall heat exchangers can be divided into tube type, plate type and other types according to the structure of the heat transfer surface. Tubular heat exchangers use the surface of the tube as the heat transfer surface, including coiled tube heat exchangers, double-pipe heat exchangers and shell-and-tube heat exchangers, etc.; plate surface heat exchangers use the plate surface as the heat transfer surface, including Plate heat exchangers, spiral plate heat exchangers, plate-fin heat exchangers, plate-shell heat exchangers and umbrella plate heat exchangers, etc.; other types of heat exchangers are heat exchangers designed to meet certain special requirements , Such as scraped surface heat exchangers, turntable heat exchangers and air coolers. The relative flow of the fluid in the heat exchanger generally has two types: forward flow and reverse flow. When flowing downstream, the temperature difference between the two fluids at the inlet is the largest, and gradually decreases along the heat transfer surface until the temperature difference at the outlet is the smallest. In countercurrent, the temperature difference between the two fluids along the heat transfer surface is more evenly distributed. 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 between the countercurrent and the downstream is the largest. Under the condition of completing the same heat transfer, the use of counterflow can increase the average temperature difference and reduce the heat transfer area of ​​the heat exchanger; if the heat transfer area remains unchanged, the use of counterflow can reduce the consumption of heating or cooling fluid. The former can save equipment costs, and the latter can save operating costs, so countercurrent heat exchange should be used as much as possible in design or production. When there is a phase change (boiling or condensing) of both or one of the cold and hot fluids, 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. At this time, the temperature difference between the two fluids has nothing to do with the choice of fluid flow direction. In addition to the downstream and countercurrent flows, there are also flow directions such as cross-flow and baffle flow. In the heat transfer process, it is an important issue to reduce the thermal resistance in the dividing wall heat exchanger to improve the heat transfer coefficient. The thermal resistance mainly comes from the thin layer of fluid sticking to the heat transfer surface on both sides of the partition wall (called boundary layer) and the dirt layer formed on both sides of the wall during the use of the heat exchanger. The thermal resistance of the metal wall is relatively small. Increasing the flow velocity and turbulence of the fluid can thin the boundary layer, reduce the thermal resistance and increase the thermal coefficient. However, increasing the fluid flow rate will increase energy consumption, so a reasonable coordination should be made between reducing thermal resistance and reducing energy consumption during design. In order to reduce the thermal resistance of dirt, try to delay the formation of dirt and clean the heat transfer surface regularly. Generally, heat exchangers are made of metal materials. Among them, carbon steel and low-alloy steel are mostly used to manufacture medium and low-pressure heat exchangers. In addition to being mainly used for different corrosion resistance conditions, austenitic stainless steel can also be used as a 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 are used in the production of non-metallic materials except for gasket parts. Corrosion heat exchanger, such as graphite heat exchanger, fluoroplastic heat exchanger and glass heat exchanger, etc.