Selection of shell and tube heat exchangers

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There are many types of shell and tube heat exchangers, with various classification methods. The shell and tube heat exchangers that are widely used today are primarily classified based on the presence or absence of thermal compensation and the different methods of such compensation. Because in the operation of a shell-and-tube heat exchanger, the different temperatures of the cold and hot fluids cause uneven heating of the shell and the tube bundle, resulting in different degrees of thermal expansion. If the temperature difference between the two is greater than 50°C, it may cause the tube bundle to bend and deform, become loose from the tube sheet, and even lead to the deformation or destruction of the entire equipment. Therefore, structural improvements are necessary to eliminate or mitigate the adverse effects of thermal expansion. This structural improvement method is called thermal compensation or temperature difference compensation. Based on the presence or absence of such compensation and various compensation methods, shell-and-tube heat exchangers are mainly divided into the following three categories. 1. Fixed-tube-sheet heat exchangers; however, due to the lack of thermal compensation, they are only suitable for applications where the temperature difference between the shell and the tube bundle is small, and the material flowing outside the tubes is clean and not prone to scaling. If there is a large temperature difference between the two fluid streams, thermal compensation should be applied; that is, on the basis of a fixed tube sheet heat exchanger structure, a thermal compensation element, namely an expansion joint, is added. When there is a large temperature difference between the shell and the tube bundle, the elastic deformation of the compensation ring can be utilized to adjust the different thermal expansions between them, thereby reducing thermal stress. However, this structure is generally suitable for situations where the temperature difference between the shell and the tube bundle is below 60–70°C, and the pressure of the fluid in the shell side is less than 600 kPa. Because if the pressure in the shell side is too high, it becomes difficult for the compensation ring to expand and contract, and it will lose its function of thermal compensation. 2. Floating-head heat exchangers: In this type of heat exchanger, the tube sheet at one end is connected to the shell using flanges, while the tube sheet at the other end is not connected to the shell; it can expand and contract freely along the axis of the tubes. An end cover is attached to this tube sheet, and it is referred to as the floating head. When the shell and the tube bundle experience different thermal expansions due to temperature differences, the tube bundle together with the floating head can move freely along the axis of the tubes within the shell, thereby eliminating thermal stress. This structure is relatively complex and costly, but since the tube bundle can be pulled out of the casing as a whole, it is easy to clean and maintain; therefore, it remains a widely used structural design at present. 3. U-tube heat exchangers: These types of heat exchangers have only one tube sheet; there are at least two tube passes, and each tube is shaped like a U, with its ends fixed to the same tube sheet. In this way, each tube can stretch and contract freely along its length to address the issue of thermal compensation. This type of heat exchanger has a simple structure and low weight; its tube bundle can be removed, making maintenance and cleaning convenient. However, the inside of the tube is difficult to clean, so it is required that the fluid flowing through the tube be clean. Additionally, the tube sheet utilization rate is low, and it is difficult to replace the tubes. This type of heat exchanger is suitable for high-temperature and high-pressure applications.