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Fixed-tube-sheet heat exchangers are relatively common heat exchangers in fields such as petroleum refining, coal chemical industry, and pharmaceuticals; they are also a type of shell-and-tube heat exchanger. A fixed-tube-sheet heat exchanger is composed of a tube bundle, tube sheet, baffle plates, and tube boxes. Both ends of the tube bundle are fixed to the end tube sheets by welding or expansion joining; the tube sheets are then welded directly to the shell, creating a rigid connection that cannot be disassembled. The shell is the outer casing that surrounds the tube bundle; it is generally made by rolling steel plates. The fluid flows within the shell, while the fluid in the tubes flows in the opposite direction, thereby achieving heat exchange. While providing support for the tube bundle, the baffle changes the direction and turbulence level of the fluid in the shell side, thereby increasing the heat exchange area and enhancing the heat transfer rate ； The tube box is located on the outside of the tube sheet to distribute the fluid in the tube side and determine its flow direction. The advantages are a simple structure, few components, and relatively low cost ； The heat exchange tubes are arranged quite closely, resulting in a high heat exchange efficiency ； The maintenance costs are low, it is easy to clean, and the tube side can withstand relatively high pressures; it is suitable for applications with low pressure fluctuations and high pressures. The downside is that it is relatively difficult to clean. If the material chosen is not compatible with the fluid medium, it can lead to blockages in the heat exchange tubes; in severe cases, corrosion and leakage may occur, requiring the replacement of both the tubes and the shell, which incurs high replacement costs ； If scaling in the shell side causes blockage that cannot be cleaned or is difficult to clean, the tube bundle cannot be replaced alone; it must be replaced together with the shell. It is applied in a process framework where the temperature difference is small and pressure remains essentially stable; the fluid in the shell side needs to be clean and should not form scale easily. In cases where there is a large temperature difference between the tube side and the shell side, it is necessary to install expansion joints (which are usually U-shaped or Ω-shaped expansion joints) to compensate for the expansion differences caused by thermal expansion, thereby ensuring the safe use of the tubular heat exchanger. For expansion joints, it is recommended that the temperature difference between the tube side and the shell side be ≤50°C ； With expansion joints, it can be increased to 80~100°C. Pressure, temperature difference, and tube sheet stiffness must be taken into consideration; it is usually designed in accordance with ASME or GB/T 151 standards. During design, it is necessary to specify the fluid type, flow rate, inlet and outlet temperatures, allowable pressure drop, fouling coefficient, design pressure, design temperature, temperature range, corrosivity, tendency to scale, and vibration risk. The heat transfer area is determined based on the heat load (Q=m⋅Cp⋅ΔT); A=Q/(U⋅ΔTlm⋅Ft). When designing the shell side, factors such as tube diameter, tube length, number of tubes (determined based on the heat transfer area), number of shell sides, shell diameter, type of baffle plates, and their spacing must be taken into account. When designing the tube sheet, the thickness must be calculated by taking into account pressure, temperature differences, and the reduction factor due to tube holes; refer to ASME or GB/T 151 for guidelines. The spacing between tube holes is generally 1.25 times the outer diameter of the tubes. When checking for thermal stress, the expansion difference between the tube side and the shell side must be calculated (ΔL=α⋅L⋅ΔT). If the temperature difference exceeds 50°C and there are no expansion joints, it is necessary to verify whether the stresses on the tube sheet, shell, and pipe ends are within acceptable limits. Material selection for the tube side and shell side: chosen based on the corrosivity of the medium ； Tube sheet material: Must match the thermal expansion coefficient of the heat exchange tubes ； Sealing gaskets: graphite, metal-wound gaskets, etc. What the users haven’t mentioned is that they may be facing difficulties when making their selection decisions; for example, they might wonder whether fixed-tube-sheet heat exchangers can be used in situations with large temperature differences, or they may need to know how to calculate thermal stress. In such cases, more detailed technical information or recommendations regarding relevant calculation formulas and standards would be useful.