Issues to consider in the design calculations of heat exchangers

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1. Calculation of shell wall thickness: This includes the calculation of the wall thicknesses for the tube box joints, end caps, and the shell-side cylinder. The wall thicknesses of the tubes and the shell-side cylinder must comply with the minimum thickness requirements specified in GB/T151. For carbon steel and low-alloy steel, the minimum thickness is determined based on a corrosion allowance of C2 = 1 mm; when C2 is greater than 1 mm, the minimum wall thickness of the shell must be increased accordingly. 2. Opening reinforcement calculation: For shells made of steel pipes, it is recommended to use overall reinforcement (by increasing the wall thickness of the cylinder or using thick-walled pipes) ； For making large holes in relatively thick tube boxes, considering overall economic efficiency, the following requirements must be met without additional reinforcement: ① Design pressure ≤ 2.5 Mpa; ② The distance between the centers of adjacent holes should be at least twice the sum of their diameters; ③ The nominal diameter of the connections should be ≤ 89 mm; ④ The minimum wall thickness of the connections should meet the specified requirements (with a corrosion allowance of 1 mm for the connections). 3. Flanges: When standard flanges are used for equipment, attention must be paid to the compatibility between the flanges, gaskets, and fasteners; otherwise, calculations for the flanges must be carried out. For example, for type A flat welding flanges, the gasket specified in the standards as a match is a non-metallic soft gasket ； The flange must be recalculated when using wound gaskets. 4. For the tube sheet, the following points need attention: ① Design temperature of the tube sheet: According to GB/T150 and GB/T151, it should be set at a level not lower than the metal temperature of the components. However, it is impossible to take into account the effects of the media in the tube side and shell side during the calculation of the tube sheet, and it is also difficult to determine the metal temperature of the tube sheet. Therefore, generally, the design temperature of the higher-side is adopted as the design temperature for the tube sheet (as defined by standards). ② Multi-tube-side heat exchangers: In the area where the tubes are arranged, there is an area that cannot be supported by the heat exchanger due to the need for partition grooves and tie rod structures. For triangular or square arrangements, the calculation is carried out using the formulas specified in GB/T151. ③ Effective thickness of the tube sheet: The effective thickness of the tube sheet refers to the thickness of the tube sheet at the bottom of the partition grooves on the tube side, minus the sum of the following two values: a) The portion of the corrosion allowance on the tube side that exceeds the depth of the partition grooves; b) The maximum value between the corrosion allowance on the shell side and the depth of the structural grooves on the shell-side of the tube sheet. 5. Installation of expansion joints: In heat exchangers with fixed tube sheets, since there is a temperature difference between the fluid in the tube side and other fluids, and since the heat exchanger and the shell are fixedly connected to the tube sheet, an expansion difference exists between the shell and the tubes during operation, resulting in axial loads on both the shell and the tubes. To prevent damage to the shell and heat exchanger, instability of the heat exchanger, and the pulling apart of the heat exchange tubes from the tube sheet, expansion joints should be installed in order to reduce the axial loads on the shell and heat exchanger. Generally, when there is a large temperature difference between the shell and the heat exchanger walls, it is necessary to consider installing expansion joints. In the calculations related to the tube sheet, values for δ, τ, and q are determined based on various scenarios with temperature differences; if any of these values do not meet the required standards, additional expansion joints must be installed. δ – Axial stress on the heat exchange tubes; τ – Axial stress on the shell side cylinder; q – Force acting to pull the heat exchange tubes away from the tube sheet. This article is from the Design Institute website: www.shejiyuan.com