Design steps for shell and tube heat exchangers

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I. Classification of heat exchangers: Shell-and-tube heat exchangers can be divided into the following two categories based on their structural characteristics. 1. Shell-and-tube heat exchangers with rigid structures: These heat exchangers are also known as fixed-tube-sheet types, and can generally be divided into single-pass and multi-pass types. Its advantages are a simple and compact structure, low cost, and wide range of applications ； The drawback is that mechanical cleaning cannot be performed outside the tube. 2. Shell-and-tube heat exchanger with temperature difference compensation device: It allows the heated portion to expand freely. This structural form can be further divided into: ① Floating-head heat exchangers: In this type of heat exchanger, the tube sheet at one end can expand and contract freely; this is what is known as the “floating head”. It is suitable for situations where there is a large temperature difference between the tube wall and the shell wall, and the tube bundle area needs to be cleaned frequently. However, its structure is complex, and the cost of processing and manufacturing it is high. ② U-tube heat exchanger: It has only one tube sheet, so the tubes can expand and contract freely when heated or cooled. This type of heat exchanger has a simple structure, but the work required to manufacture the bent tubes is considerable. Moreover, since the tubes need to have a certain bending radius, the utilization efficiency of the tube sheet is low; mechanical cleaning inside the tubes is difficult, and it is also not easy to replace the tubes. Therefore, it is necessary for the fluid flowing inside the tubes to be clean. This type of heat exchanger can be used in applications with large temperature differences, as well as in high-temperature or high-pressure environments. ③ Packed joint heat exchangers: There are two types of these. In one type, each tube end on the tube sheet is equipped with a separate packing seal to allow for free expansion and contraction of the tubes. This design is used only when the number of tubes in the heat exchanger is very small. However, the tube pitch in this case is larger than that in ordinary heat exchangers, and the structure is more complex. Another form involves making one end of the tube bundle and the shell into a floating structure; at the floating joint, an integral stuffing box seal is used. This structure is relatively simple, but it is not suitable for applications involving large diameters and high pressures. Packed-bed heat exchangers are now rarely used. II. Review of Design Conditions 1. For the design of the heat exchanger, the user should provide the following design conditions (process parameters): ① Operating pressure in the tube side and shell side (one of the criteria for determining the equipment category; provided by the user or the process). ② Operating temperature in the tube side and shell side (inlet/outlet); provided by the user or the process. ③ Metal wall temperature (calculated based on process requirements; provided by the user or the process). ④ Name and properties of the fluid; provided by the user or the process. ⑤ Corrosion allowance. ⑥ Number of passes; provided by the user or the process. ⑦ Heat exchange area; provided by the user or the process. ⑧ Specifications of the heat exchange tubes and their arrangement pattern (triangular or square); provided by the user or the process. ⑨ Number of baffle plates or support plates; provided by the user or the process. ⑩ Insulation material and thickness (to determine the height at which the nameplate will be mounted); provided by the user or the process. ⑾ Paint: I. If the user has special requirements, they should specify the brand and color. II. If there are no special requirements, the designer will choose accordingly. 2. Key design conditions: ① Operating pressure: This is one of the criteria for determining the equipment category, so it must be provided. ② Properties of the fluid: If the user does not provide the name of the fluid, information regarding its toxicity level must be provided. Because the toxicity level of the medium is related to the non-destructive testing of equipment, heat treatment, and the grade of forgings for such equipment; it is also relevant to the classification of the equipment: a. According to GB150 10.8.2.1(f), containers that hold media with extremely high or high toxicity must undergo 100% RT testing. b. Per 10.4.1.3, containers holding media with extremely high or high toxicity require post-weld heat treatment (welded joints of austenitic stainless steel do not need heat treatment). c. Forgings: Forgings that use media with extremely high or high toxicity must meet the requirements of grade III or IV. ③Tube specifications: Commonly used ones include carbon steel tubes with dimensions of φ19×2, φ25×2.5, φ32×3, and φ38×5; stainless steel tubes with dimensions of φ19×2, φ25×2, φ32×2.5, and φ38×2.5. Arrangements of the heat exchange tubes: triangular, angled triangular, square, and angled square. ★ When mechanical cleaning is required between the heat exchange tubes, a square arrangement should be used. 1. Design pressure, design temperature, weld joint factor 2. Diameter: DN