Heat exchanger design concept

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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 disadvantage is that mechanical cleaning cannot be performed outside the tube. 2. Shell-and-tube heat exchangers with temperature difference compensation devices: they allow the heated section to expand freely. This structural type can be further divided into: ① Floating-head heat exchanger: In this type of heat exchanger, one of the tube sheets can expand and contract freely, which is what is known as a “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 Packing box type heat exchangers: There are two variants of this type. In one variant, each tube at the end of the tube sheet is equipped with a separate packing seal to allow the tubes to expand and contract freely. This design is used when there are few tubes inside the heat exchanger; however, the tube spacing is larger than in conventional heat exchangers, and the structure is more complex. Another design involves creating a floating structure at one end of the tube bundle relative to the casing, with an integral stuffing box used for sealing at that floating point; this design is relatively simple, but it is not suitable for applications with large diameters and high pressures. Gland-type 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 (this must be provided as one of the criteria for determining the equipment category). ② Operating temperature in the tube side and shell side (inlet/outlet). ③ Metal wall temperature (determined through process calculations; provided by the user). ④ Name and properties of the material. ⑤ Corrosion allowance. ⑥ Number of passes. ⑦ Heat exchange area. ⑧ Specifications of the heat exchange tubes and their arrangement pattern (triangular or square). ⑨ Number of baffle plates or support plates. ⑩ Insulation material and thickness (to determine the height at which the nameplate should be mounted). 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 must be provided as one of the criteria for determining the equipment category. ② Material properties: If the user does not provide the name of the material, information on its toxicity level must be provided. Since the toxicity level of the medium is relevant to the non-destructive testing of equipment, heat treatment, and the grade of forgings for such equipment, it is also related to the classification of the equipment: a) According to GB150 10.8.2.1(f), containers holding media that are extremely or highly toxic must undergo 100% RT testing. b) As specified in 10.4.1.3, containers holding extremely or highly toxic media require post-weld heat treatment (welded joints of austenitic stainless steel do not need heat treatment). c) Regarding forgings, those used with extremely or highly toxic media must meet the requirements of grade III or IV. ③Tube specifications: Commonly used carbon steel tubes include φ19×2, φ25×2.5, φ32×3, φ38×5; stainless steel tubes include φ19×2, φ25×2, φ32×2.5, φ38×2.5. Arrangement patterns of the heat exchange tubes: triangular, angled triangular, square, 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