In recent years, due to the development of new technologies and the development and utilization of new energy, various types of heat exchangers have received more and more attention from the industry, and heat exchangers are more critical equipment in energy conservation measures. Therefore, whether from industrial Development, or from the effective use of energy, the rational design, manufacture, selection and operation of heat exchangers are of great significance. Energy is one of the most important issues facing humanity. Energy development and conversion have become an important issue in all countries. Heat exchangers are indispensable equipment in the process of energy utilization. They are used in almost all industrial fields, chemical, metallurgical, The power, transportation, aerospace and aerospace sectors are particularly widely used.

1 Classification of heat exchangers

1.1 Direct heat transfer heat exchanger. A heat exchanger that does not require a heat transfer wall and is in direct contact with a hot fluid for heat exchange. Such heat exchangers are commonly used in industrial production.

1.2 Wall-to-wall heat transfer heat exchanger. The heat exchanger for heat transfer operation of heat exchange between the cold and hot fluid through the wall of the pipe, plate, etc. is a very common and very common heat exchanger. The cold and hot fluids are all fluids, which can be air, smoke, Steam, water. This is the type of heat exchanger that is discussed in this article.

1.3 Regenerative heat exchangers. Intermittent heat transfer is a practical and effective way to recover waste heat in the waste heat regenerator. It is often used as a means of regulating the waste heat of the combustion gas and the uneven amount of steam.

2 Characteristics and use of several heat exchangers In the actual design and selection, it is often known that the inlet and outlet temperatures of both high-temperature fluids and low-temperature fluids are considered to be as small as possible when designing the process design. Under the heat exchange area, there is as much heat exchange rate as possible, as well as lower equipment cost and construction cost. In addition, considering the design and heat transfer basic equation of the heat exchanger under the premise of convenient operation and maintenance and cleaning, it can be seen from the above formula that at a certain flow rate, the larger the Reynolds number, the larger the heat transfer coefficient. At the same time, the pressure drop is also greater.

2.1 Shell and tube heat exchangers. The shell-and-tube heat exchanger is a common common structure, which includes: fixed tube plate heat exchanger, U-tube shell heat exchanger, expansion tube heat exchanger, floating head heat exchanger, sectional heat exchanger , casing heat exchangers, etc. The fixed tube plate heat exchanger has the advantages of simple structure, light weight and low cost; the disadvantage is that the tube is bent due to thermal expansion. U-tube-and-tube heat exchanger overcomes this shortcoming and makes the tube into a "U" shape. The one end is fixed at the other end, so that the heat exchanger is not affected by the expansion. The structure is simple and light, and the disadvantage is that it cannot be mechanically cleaned. The pipe is inconvenient to be replaced, and the heat transfer capacity per unit capacity and unit mass is low. It is suitable for occasions where the temperature difference is large and the fluid medium in the pipe is relatively clean. The expansion joint heat exchanger can solve the expansion problem, and the structure of the expansion joint is used, so the temperature difference is large. Fluids and high-pressure fluids can handle fouling fluids because they can be removed for cleaning, but are not suitable for low-pressure gases, but have the disadvantage of being complicated to manufacture. The floating head shell heat exchanger has a floating head which is not connected to the outer casing and can be freely stretched and contracted, so that the problem of thermal expansion is solved, and the cleaning is convenient, and the core can be taken out during maintenance, and the tube sheet, the tube and the sleeve can be fixed. When the volume of each outer casing is 1m3, the heat transfer area is about 30-40m3. When the volume of each shell of the U-shaped shell-and-tube heat exchanger and the floating-head heat exchanger is 1 m3, the heat transfer area is about 70 m2.

2.2 Plate heat exchanger. Since the heat transfer surface of the plate heat exchanger can press the concave-convex discharge tank, the turbulent flow state can occur under the low Reynolds number condition, so the heat transfer coefficient is high, generally up to 3000-5000Kcal/m2. h. °C, compared with the shell-and-tube heat exchanger at the same flow rate, this value is about 3 to 5 times that of the shell-and-tube heat exchanger, although the resistance of the plate heat exchanger will be large. Some, if compared to the same power-consuming conditions, the heat release coefficient of the plate heat exchanger is about twice as high as that of the shell-and-tube type. Due to the compact structure and small gap of the plate heat exchanger, the heat transfer area per unit volume is increased, and the installation area is about 1/2 to 1/3 of the shell-and-tube type, which can save floor space and construction cost. When the volume of the outer casing is 1m3, the heat transfer area is about 80m2. In addition, the plate heat exchanger is easy to increase or decrease the heat exchange area. For the shell-and-tube heat exchanger, when the liquid processing capacity needs to be increased, the original heat exchanger is transmitted. The heat area is almost impossible to increase, but the heat transfer area of the plate heat exchanger is easily increased, thereby increasing the processing capacity. In addition, the plate heat exchanger only exposes the outer plate of the heat transfer plate to the atmosphere, so the heat loss is negligible. No, no insulation measures are required.

One of the characteristics of the plate heat exchanger in terms of operation and maintenance is that it is convenient to assemble and disassemble, and it is not necessary to completely disassemble it. Only the loosening bolt can be loosened to remove the plate to clean, replace the gasket, and even replace the plate. This is especially important for materials that are prone to deposits in the heat exchange medium. Allowable temperature and pressure: For the plate heat exchanger, the gasket between each two plates is used to prevent material leakage, so the total length of its sealing perimeter is very long. Preventing gasket leakage is an important factor in plate heat exchangers. The temperature, pressure and chemical stability that the gasket can withstand are often the temperature and pressure limits used in plate heat exchangers and the range of materials allowed. In addition, since the gap between the heat transfer surfaces is small, the heat transfer surface has irregularities, so the pressure loss is larger than the conventional smooth pressure, and the plate heat exchanger is subjected to a lower working pressure.

2.3 Plate-fin heat exchanger. Due to the special structure of the fins, the fluid forms a strong turbulence in the channel, which causes the boundary layer of the thermal resistance to be continuously destroyed, thereby effectively reducing the thermal resistance and improving the heat transfer efficiency. Generally, the heat transfer coefficient of boiling water is 1500 to 30000 Kcal/m2.h.°C. In addition, the structure of the plate-fin heat exchanger is relatively compact, and the heat transfer area per unit volume is generally more than 5 times higher than that of the tubular heat exchanger. When the volume of each shell is 1 m3, the heat transfer area is about 160 m2. Plate-fin heat exchangers are lighter and stronger. Due to the thin fins, they are generally 0.2-0.3mm. Due to their compact structure and small size, they are generally made of aluminum, so the weight is very light. At the same time, the fins are the main pass. The hot surface is supported by two partitions, so the strength is high. The plate-fin heat exchanger is highly adaptable, allowing 2 to 9 kinds of medium heat exchange in the same equipment, and can be used for heat exchange between gas-gas, gas-liquid, liquid-liquid. Main disadvantages: Due to the narrow flow path, it is easy to cause blockage and increase the resistance drop. When the heat exchanger is fouled, cleaning is very difficult, and because the separator and fin of the heat exchanger are very thin aluminum plates (foil) Therefore, it is required that the medium does not corrode aluminum, and if it is corroded and causes internal cross-leakage, it is difficult to repair.

3. Heat pipe heat exchanger The heat pipe is a heat transfer device that utilizes the evaporation and condensation process of the liquid enclosed in the closed container to efficiently transfer heat. A heat exchanger assembled with a plurality of heat pipes as heat exchange elements is called a heat pipe heat exchanger. The heat pipe heat exchanger is mostly used for gas-gas heat exchange. At this time, the heat receiving section and the heat releasing section at both ends of the heat pipe are provided with fins to improve the heat transfer effect. The advantage of the heat pipe heat exchanger is that the heat transfer surface is basically isothermal, and the heat transfer area per unit volume is large, and different working working materials are selected, so that the heat pipe can be used under different temperature conditions, the structure is relatively simple, and the heat expansion is performed. The problems caused by etc. are small. However, the primary problem of the heat pipe heat exchanger is that the working fluid in the heat pipe as the heat transfer element has a certain critical heat transfer amount and working temperature limit for the working environment used, and if it exceeds the limit temperature, it continues to work. , the evaporation section will "burn dry" and stop working.

4 Conclusion

In the operation with heating or cooling, there are always various heat transfer processes. The narrow heat exchanger only pays attention to the meaning of heat recovery. It only sees the heat transfer device, the difference in heat recovery and the effective recovery. The importance of degree, etc. However, the recovery of waste heat by the heat exchanger improves the heat enthalpy of the cold fluid, thereby further effectively utilizing the waste heat, which makes the heat exchanger have new value and has a new understanding of others. Generally, it is advantageous for the heat exchanger to have a higher temperature of the cold fluid outlet, but it is not required to have an infinite heat transfer area. At the same time, because of the two fluid flow ratios of cold fluids, fluid state, presence or absence of heat transfer walls, equipment structure, operation methods and other factors, not only affects the heat transfer equipment, but also affects the heat transfer process, so these factors are not only explicitly It is more difficult to distinguish and is also subject to the range of heat exchanger selection.