The materials of the heat exchange tubes and shell of a shell-and-tube heat exchanger must be selected separately according to the corrosiveness of the media. A common misconception is to select them uniformly based only on the main material, which leads to pitting corrosion of the tube bundle under working conditions containing chloride ions. It is recommended that during design and selection, the chemical composition, temperature, and flow rate of the tube-side and shell-side media be clearly defined, and when necessary, a combined structure of stainless steel and carbon steel be adopted to balance corrosion resistance and economy.
Fixed tubesheet structures are suitable for working conditions where the temperature difference between the tube wall and shell wall is not greater than 50°C. When the temperature difference exceeds this range, floating head or U-tube structures with expansion joints should be selected. Designers need to calculate the maximum temperature difference and thermal expansion to avoid tube sheet cracking or tube pull-out caused by thermal stress. Although floating head structures can alleviate thermal expansion, additional consideration must be given to the sealing of the floating head cover and maintenance space.
Baffle spacing is usually taken as 0.2 to 1 times the shell inner diameter. Although too small a spacing can increase shell-side flow velocity and heat transfer coefficient, it significantly increases pressure drop and easily forms dead zones at baffle cutouts, aggravating fouling. For media with higher viscosity, it is recommended to appropriately increase the spacing and combine it with a longitudinal flow structure. During selection, a balance must be achieved between heat transfer efficiency and operating energy consumption, and process simulation software can be used for multi-scheme comparison.
During the design of a shell-and-tube heat exchanger, fouling thermal resistance should be reserved according to the fouling tendency of the media. The side prone to fouling should preferably be arranged on the tube side to facilitate mechanical or chemical cleaning. If the shell-side medium is prone to fouling, a removable structure or additional cleaning connections may be selected. In actual operation, if the fouling factor is taken too low, the heat exchange area will be insufficient; if too high, it will cause equipment redundancy. It is recommended to refer to operating data of similar installations and determine a reasonable value in combination with the maintenance cycle.
The flow velocity of liquid on the tube side is generally 0.5–3.0 m/s, and on the shell side 0.2–1.5 m/s. Too low a flow velocity will accelerate fouling deposition, while too high a velocity will increase erosion corrosion and pressure drop. Gas flow velocity can be appropriately increased, but the risk of tube bundle vibration must be checked. During selection, a comprehensive evaluation should be made in combination with the energy consumption of pumps or fans, avoiding the pursuit of high flow velocity alone while ignoring long-term operating costs.
