1.3. Process (thermal) design procedure

 

Shell and tube heat exchanger is designed by trial and error calculations. The main steps of design following the Kern method are summarized as follows:

Step #1. Obtain the required thermophysical properties of hot and cold fluids at the caloric temperature or arithmetic mean temperature . Calculate these properties at the caloric temperature if the variation of viscosity with temperature is large. The detailed calculation procedure of caloric temperature available is in reference [3] (page 93-99) .

Step #2. Perform energy balance and find out the heat duty (Q) of the exchanger.

Step #3. Assume a reasonable value of overall heat transfer coefficient (U _o,assm ). The value of (U _o,assm ) with respect to the process hot and cold fluids can be taken from the books ([3] page 840 Table 8; [4] page 297 Table 8.2.)

Step #4 . Decide tentative number of shell and tube passes (n_p). Determine the LMTD and the correction factor F_T­ ([3] page 828-833 Figs. 18-23; [4] page 292 Figs. 8.10a & 8.10b). F_T normally should be greater than 0.75 for the steady operation of the exchangers. Otherwise it is required to increase the number of passes to obtain higher F_T values.

Step #5 . Calculate heat transfer area ( A ) required:           (1.1)

Step #6 . Select tube material, decide the tube diameter (ID= d_i, OD =d_o ), its wall thickness (in terms of BWG or SWG) and tube length (L). Calculate the number of tubes (n_t) required to provide the heat transfer area ( A ) calculated: .                                                        (1.2)

Calculate tube side fluid velocity, .                                                           (1.3)

If u <1 m/s, fix n_p so that, .                                                          (1.4)

where m, ρ and μ are mass flow rate, density and viscosity of tube side fluid. However, this is subject to allowable pressure drop in the tube side of the heat exchanger.