Shell-and-tube Heat Exchanger: The Core Tool for Industrial Heat Exchange

Shell-and-tube Heat Exchanger: The Core Tool for Industrial Heat Exchange

^{1.Classification}

^{(1)  According to the structure form}

^{-Fixed tube and plate heat exchanger:}

^{Both ends of the tube bundle of this heat exchanger are fixed on the tube plate by welding and other means, and the tube plate is connected with the shell. It has simple structure and low cost. However, due to the different coefficients of thermal expansion of the tube bundle and the shell, when the temperature difference is large, it may generate thermal stresses in the tubes and the shell, leading to deformation or damage of the tubes.}

^{-Floating head type heat exchanger:}

^{The tube sheet at one end of the floating head type heat exchanger is fixed, and the tube sheet (floating head) at the other end can move freely in the shell. This structure can effectively compensate for the difference in thermal expansion between the tube bundle and the shell, and is suitable for occasions where there is a large temperature difference between hot and cold fluids.}

^{-U-shaped tube heat exchanger:}

^{The tube bundle is U-shaped, and both ends of the tubes are fixed on the same tube plate. This structure makes the tube bundle can be free to expand and contract, can also better adapt to the thermal expansion. Moreover, the U-shaped tube heat exchanger has only one tube plate, which reduces the connection points between the tube plate and the tubes and reduces the possibility of leakage. However, it is more difficult to clean the inside of the U-shaped tube because the tube is curved, making it difficult for cleaning tools to penetrate deeply. It is often used in the tube fluid is relatively clean, not easy to scale occasions, such as some high-temperature and high-pressure gas heat transfer process.}

^{(2) According to the way the fluid flows through the points}

^{-Single-pass shell and tube heat exchanger:}

^{Hot fluid and cold fluid in the heat exchanger only after a heat transfer process. This heat exchanger structure is simple, but the heat transfer efficiency is relatively low. Usually applies to the heat transfer efficiency requirements are not high or the fluid flow is small.}

^{-Multi-pass shell and tube heat exchanger:}

^{By setting partitions in the tube and/or shell process, the fluid flows back and forth several times, thus increasing the fluid flow rate and heat transfer time and improving the heat transfer efficiency. For example, in some large-scale chemical plants, in order to make full use of thermal energy, multi-stage shell and tube heat exchangers are used to handle a large amount of fluid.}

^{2. Design points}

^{(1) Calculation of heat exchange area:
The heat exchange area is determined according to the amount of heat exchange tasks. Factors such as the flow rate of the fluid, temperature change and the specific heat capacity of the two fluids need to be considered. In the actual design, but also consider a certain margin to cope with possible changes in working conditions.}

^{(2) Material selection:
For metal tube and shell heat exchangers, the choice of material for the tubes and shell is critical. Factors such as the corrosiveness of the fluid, operating temperature and pressure should be considered. If the fluid is highly corrosive, like chemical materials containing acids, it is necessary to choose corrosion-resistant metal materials, such as stainless steel, titanium alloys and so on. For high-temperature and high-pressure working environment, the strength and high-temperature resistance of the material is also a key consideration, for example, in the supercritical boiler heat exchanger, alloy steel materials may be used to meet the performance requirements of the equipment.}

^{(3) Pressure drop design:
Pressure drop occurs as the fluid flows through the tube and shell courses. Excessive pressure drop will increase the energy consumption of fluid transport and may affect the normal operation of the whole system. In the design, through a reasonable choice of tube diameter, arrangement and the form of folding plate and other factors to control the fluid flow rate, so as to control the pressure drop. For example, in the design of the tube range, the use of smaller tube diameter can increase the heat transfer area, but at the same time will also increase the fluid flow rate and pressure drop, so it needs to be considered in a comprehensive manner.}

^{3. Manufacturing process}

^{(1) Connection of tube and tube plate:}

^{This is a key link in the manufacture of metal tube and shell heat exchangers. Common connection methods are expansion and welding. Expansion is through mechanical or hydraulic ways to make the tube plastic deformation, so that it closely fits in the hole of the tube plate, this way is suitable for lower pressure, temperature is not high occasions. Welding, on the other hand, is to connect the tube and the tube plate together through the welding process, which can provide higher connection strength and is suitable for high pressure, high temperature and other harsh working conditions. In the actual manufacturing, sometimes also used expansion + welded composite connection, in order to give full play to the advantages of both.}

^{(2) Shell manufacturing:}

^{The shell is generally made of cylindrical shape through the process of rolled plate, welding and other processes. In the manufacturing process, to ensure that the roundness of the shell, straightness and wall thickness uniformity. For large shells, non-destructive testing, such as ultrasonic testing, ray testing, etc., is also required to ensure the quality of the shell and prevent welding defects and other problems.}

^{(3) Assembly process:}

^{When assembling the heat exchanger, firstly, the tube bundles should be mounted on the tube plate and fixed, and then the tube bundles and the tube plate should be mounted into the shell together. During this process, care should be taken to protect the tube bundle to prevent the tubes from being damaged. At the same time, components such as the folding plate should be installed, and it should be ensured that the installation position of each component is accurate to ensure that the fluid can flow according to the designed path.}

^{4. Operational and maintenance considerations}

^{(1) Operation:}

^{When starting up the heat exchanger, introduce the fluid slowly to prevent damage to the equipment due to thermal shock and other factors. For example, during startup, slowly open the cold fluid valve first, so that the cold fluid gradually fills the heat exchanger, and then introduce the hot fluid, and to control the flow rate of the hot fluid and the rate of temperature change. During operation, pay close attention to the fluid pressure, temperature, flow rate and other parameters to ensure that they are within the design range. If there are abnormalities, such as a sudden rise in pressure or a sharp drop in temperature, take timely measures, such as adjusting the valve opening or stopping the equipment operation.}

^{(2) Maintenance:}

^{Regular inspection and maintenance of the heat exchanger is the key to ensure its long-term stable operation. The main inspection includes whether there is leakage, corrosion, scaling, etc. of the tubes, and whether the connection parts of the tube plate and shell are firm. For the scaling problem, according to the nature and thickness of the scale layer, chemical cleaning or mechanical cleaning methods can be used. Chemical cleaning is the use of chemical reagents and scale layer of chemical reaction, the scale layer will be dissolved or stripped; mechanical cleaning is through physical methods, such as the use of brushes, high-pressure water gun and other tools to remove the scale layer.}