Design steps for shell and tube heat exchangers

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Design steps: When designing heat exchangers, making simple estimates or blindly increasing the safety factor for the heat transfer area can lead to waste. Only through relatively detailed calculations can reliable guarantees be provided in terms of safety and economy for the heat exchanger put into operation. However, many factors in the design are interrelated, and the design process is complex; therefore, the design procedure should vary depending on the design task and the initial conditions. For example, in heat transfer calculations and resistance calculations, the structure is inevitably involved, so it is often necessary to initially select a heat transfer coefficient to obtain an estimated heat transfer area, thereby determining the structural layout. Subsequent heat transfer calculations are then carried out to determine the actual value of the heat transfer coefficient and the required heat transfer area. Generally, heat transfer calculations and structural calculations are successful when the heat transfer area determined by the structure has a margin of 10% to 20% compared to the required heat transfer area calculated accordingly. If there are still issues with resistance calculation, strength calculation, and vibration tuning, certain parts will need to be modified again, or even the design choice may have to be changed. The general design procedure is as follows: 01 Collect relevant preliminary data based on the design requirements, and determine the type of heat exchanger, etc. The preliminary data should include the physicochemical properties of the fluid (such as scaling tendency, corrosivity, explosiveness, chemical reactivity, etc.), the flow rate, pressure, temperature, and heat load of the fluid, constraints related to the installation location of the equipment, material limitations, pressure drop limits, and so on. 02 Determine the qualitative temperature and consult the property data ； 03 Calculate the heat load and the flow rate of hot or cold fluids based on thermal equilibrium ； 04 Select materials for the casing and pipes ； 05 Select the flow mode to determine the flow space for the fluid ； 06 Calculate the average temperature difference ； 07 Determine the preliminary heat transfer coefficient K', and calculate the preliminary heat transfer area F' ； 08 Design the structure of the heat exchanger (or select a standard model of heat exchanger), including: ① Selection of pipe diameter and fluid flow velocity in the tube side ； ②Determine the number of pipes per run, pipe length, and total number of pipes ； ③Determine the pipe arrangement, pipe spacing, inner diameter of the shell, and diameter of the connecting pipes, etc ； ④Determine the shell-side stage count and the number and size of longitudinal baffles, or the number, spacing, and size of baffles and other structural dimensions of the shell side. At this step, it is best to determine the relevant data and the heat transfer area F\" through sketches (F\" generally will not be exactly equal to F’). It should be noted that when determining the structural dimensions, many factors interact with one another, and these effects are reflected in the diameter and length of the shell. Generally, shells that are shorter have a larger diameter, while longer shells have a smaller diameter; usually, the latter option is more cost-effective. This is because: ① Small-diameter housings can potentially be manufactured using standard tubes ； ②For given operating conditions, a smaller shell diameter allows the thickness of components such as the shell, flanges, and end caps to be reduced as well ； ③The processing cost of the tube sheet is relatively high; if the shell diameter is small, the thickness and diameter of the tube sheet can be reduced accordingly, thereby lowering the manufacturing costs ； ④The cost per unit length of pipe is low. Of course, when selecting shells with a small diameter but long length, it is necessary first to meet the requirements regarding allowable pressure drops, while also taking into account the feasibility of arranging, installing, and maintaining the equipment within the available space. 09 Coefficient of heat transfer calculation and friction loss calculation in pipe systems. Only when the heat transfer coefficient is much larger than the initially selected value and the pressure drop is less than the allowable value can the next calculations be carried out; otherwise, K' must be reselected and the structure adjusted. 10 Shell-side heat transfer calculation. Based on the adopted structure, assume the wall temperature and calculate the heat transfer coefficient; if these are unreasonable, the shell-side structure should be adjusted until satisfactory results are obtained. 11 Verify the heat transfer coefficient and heat transfer area. Based on the heat transfer coefficients of the tube and shell sides, as well as the fouling thermal resistance and wall thermal resistance, the heat transfer coefficient K and the heat transfer area F are calculated. Considering various factors such as the uncertainties in the heat transfer calculation formulas, differences between operating conditions and design conditions, the need to block some tubes due to severe scaling or leaks over time, and the possibility that fluid parameters may change rapidly in emergency or abnormal situations, it is required that the heat transfer area F\" determined through structural calculations be 10% to 20% larger than the calculated required heat transfer area F in order to consider it satisfactory. 12 Calculate wall temperature. It is required to be consistent with the assumed wall temperature. 13 Calculate the shell-side resistance to ensure it is below the allowable pressure drop; if the pressure drop does not meet the requirements, adjust the flow rate or structural dimensions. 14 Perform strength calculations on the components of the heat exchanger. For example, the wall thickness of the shell, the thickness and dimensions of the tube sheet, head, and flanges, the type and size of supports, the size and quantity of screws, and so on. 15 Calculate the thermal stresses in the tubes and shells, as well as the pulling forces at the tube joints; consider thermal compensation measures and perform calculations to assess vibration. 16 Draw official drawings, prepare material lists, etc. Note: Some of the steps above can be adjusted as appropriate depending on the specific circumstances. The design results should be analyzed, and any unreasonable aspects require further revisions. For example, if a certain thermal resistance is dominant, measures should be taken to reduce it if possible. Similarly, the allowable pressure drop must be utilized as much as possible; if there is a significant difference between the calculated pressure drop and the allowable one, attempts should be made to adjust the design parameters or structural dimensions, or even change the structural layout. Sometimes, in order to save on investment, it is even necessary to compare several options, which shows that the design process is quite complex and time-consuming.