Design methods and type selection for shell-and-tube heat exchangers

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Design methods and type selection for shell-and-tube heat exchangers: Shell-and-tube heat exchangers are a traditional and standard type of heat exchange equipment. It has the advantages of easy manufacturing, a wide range of available materials, strong adaptability, high processing capacity, easy cleaning, reliable operation, and the ability to withstand high temperatures and pressures. Shell and tube heat exchangers are widely used in many industrial sectors, especially in those such as petroleum, chemicals, thermal power, and power generation, where they play a dominant role. Given the wide range of applications for shell and tube heat exchangers, relevant authorities have established a series of standards for them to facilitate design, manufacturing, installation, and use. Example: (1) BIU 800—2.5—245—6/19-41 header tube box, with a nominal diameter of 800 mm; the pressure in both the tube side and shell side is 2.5 MPa. The nominal heat exchange area is 245 square meters. It features high-quality cold-drawn heat exchange tubes with an outer diameter of 19 mm and a length of 6 m. It is a U-tube type heat exchanger with 4 tube sides and 1 shell side. (2) BIU 600—1.6—90—6/25-2 II head tube box, with a nominal diameter of 600 mm; the pressure in both the tube side and shell side is 1.6 MPa. The nominal heat exchange area is 90 square meters. It features cold-drawn heat exchange tubes of ordinary grade, with an outer diameter of 25 mm and a tube length of 6 m. It is a U-tube type heat exchanger with 2 tube sides and 1 shell side. Unit conversion relationships for pressure: a) 1 kgf/cm2 = 98066.5 Pascals; b) 1 MPa = 10^6 Pascals; c) 1 bar = 0.1 MPa = 10^6 dyn/cm2; d) 1 atm = 760 mmHg = 101325 Pascals. The design and selection of shell-and-tube heat exchangers involve using calculations to determine an economically reasonable heat transfer area as well as other relevant dimensions of the heat exchanger, in order to fulfill the heat transfer requirements of production processes. 1. Basic principles of design (1) Selection of fluid flow paths refers to determining which fluid will flow in the tube side and which in the shell side. This decision is influenced by various factors. Taking a fixed-tube-sheet heat exchanger as an example, some selection principles are outlined below: a. Fluids that are dirty or prone to scaling should flow in the tube side, as it is easier to clean that side. b. Corrosive fluids should be passed through the tube side, to prevent both the tubes and the shell from being corroded; moreover, the tube side facilitates maintenance and replacement. c. Fluids with high pressure should also be routed through the tube side, to avoid stress on the shell and thus save on the amount of metal required for the shell. d. The fluid to be cooled should flow through the shell side, allowing the heat dissipation capability of the shell to enhance the cooling effect. e. Saturated steam should flow through the shell side, to facilitate the timely removal of condensate; it remains neat and clean, and generally no cleaning is required. f. Toxic and polluting fluids should be routed in the tube side to reduce leakage. g. Fluids with low flow rates or high viscosity are suitable for flow in the shell side; as the fluid flows in the shell side equipped with baffle plates, the continuous changes in flow velocity and direction lead to turbulence at low Re values (Re>100), thereby improving the heat transfer coefficient. h. If there is a large temperature difference between the two fluids, it is advisable to arrange the fluid with a higher convective heat transfer coefficient to flow in the shell side, as this allows the wall temperature to be closer to that of the fluid with a high α value, thereby reducing the temperature difference between the tube wall and the shell wall and minimizing thermal stress. The above principles are not absolute; for specific fluids, these principles may be contradictory to each other. Therefore, when selecting the flow path for the fluid, it is necessary to determine it based on the specific circumstances by focusing on the main contradictions. (2) Selection of fluid tassels involves aspects such as heat transfer coefficient, flow resistance, and heat exchanger design. Flow rate: An increased flow rate raises the convective heat transfer coefficient, reduces the formation of fouling, and thus increases the overall heat transfer coefficient. But at the same time, it increases flow resistance and raises power consumption ； Choosing a high flow rate reduces the number of tubes; for a given heat exchange area, longer tubes or more passes are required. Tubes that are too long make cleaning difficult, and changing from a single pass to multiple passes reduces the average heat transfer temperature difference. Therefore, an appropriate flow rate must be selected through comprehensive trade-offs. Tables 1–3 list the common flow velocity ranges; when selecting a flow velocity, try to avoid flow beneath the layer as much as possible. Table 1 Common flow rate ranges for shell-and-tube heat exchangers: Fluids prone to scaling, Gases. Flow rate in m/s: Inside the tubes, 0.5–3.0; >1.0. 5.0–30. Outside the tubes, 0.2–1.5; >0.5. 3.1–15. Table 2 Common flow rates for liquids with different viscosities in shell-and-tube heat exchangers: Liquid viscosity in mPa·s: >1500, 1500–500, 500–100, 100–35, 35–1