Question

Explain Step by step how to do the standard design procedure for a heat exchanger and how to critically assess the shortcomings of using a certain method.

Answer 1

The design procedure for heat exchanger is divided into two parts: Thermal Design and Mechanical Design

Thermal Design is discussed here.

Kern's method has been widely followed for a long time to design shell and tube heat exchangers. The main steps of the procedure are given below:

Step-1: Gather the required thermophysical properties data of hot and cold fluids at the arithmetic mean temperature.

Step-2: Perform the energy balance and calculate the heat duty (Q) of the exchanger.

Step-3: Assume an appropriate value of the overall heat transfer coefficient. It can be taken from the relevant literature available.

Step-4: Decide tentative values for number of shell and tube passes. Calculate the LMTD and the correction factor F_T . The correction factor should normally be greater than 0.75 for steady state operations. Otherwise the number of passes need to be increased.

Step-5: Calculate heat transfer area (A) required:

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

Step-7: Calculate tube side fluid velocity,

If u< 1 m/s, fix n_p` so that

Step-8: Decide the type of shell and tube exchanger (fixed tubesheet, U-tube, etc.). Select the tube pitch (p_T), determine inside shell diameter (D_s) that can accommodate the calculated number of tubes. Standard tube counts table can be used for this.

Step-9: Assign the fluid to tube side or shell side. Select the type of baffle, its size (percentage cut), spacing and number. The baffle spacing is usually chosen to be within 0.2 D_s to D_s .

Step-10: Determine the tube side film heat transfer coefficient (h_i ) using the suitable form of Sieder-Tate equation in laminar and turbulent flow regimes. Estimate the shell-side film heat transfer coefficient (h₀) from:

Select the outside tube dirt factor (R_do ) and inside tube dirt factor (R_di )

Calculate the overall heat transfer coefficient (U_0,cal) based on the outside tube area including dirt factors:

Step-11: If % go to the next step. Otherwise go to step 5, calculate heat transfer area (A) required using U_0,cal and repeat the calculations starting from step 5.

Step-12: Calculate the % overdesign. Typical value of 10 % or less is acceptable.

Step-13: Calculate the tube side pressure drop.

Step-14: Calculate the shell-side pressure drop.

Step-15: If the pressure drop criteria is fulfilled, go for mechanical design.