Step #12 . Calculate % overdesign. Overdesign represents extra surface area provided beyond that required to compensate for fouling. Typical value of 10% or less is acceptable.

                                                                                  (1.7)

A= design area of heat transfer in the exchanger; A_reqd = required heat transfer area.

Step #13 . Calculate the tube-side pressure drop (ΔP_T ): (i) pressure drop in the straight section of the tube (frictional loss) (ΔP_t) and (ii) return loss (ΔP_rt) due to change of direction of fluid in a ‘multi-pass exchanger'.

Total tube side pressure drop:ΔP_T = ΔP_t+ ΔP_rt                                                            (1.8)

Step #14 . Calculate shell side pressure drop (ΔP_S): (i) pressure drop for flow across the tube bundle (frictional loss) (ΔP_s) and (ii) return loss (ΔP_rs) due to change of direction of fluid.

Total shell side pressure drop: ΔP_S = ΔP_s+ ΔP_rs                                                             (1.9)

If the tube-side pressure drop exceeds the allowable pressure drop for the process system, decrease the number of tube passes or increase number of tubes per pass. Go back to step #6 and repeat the calculations steps.

If the shell-side pressure drop exceeds the allowable pressure drop, go back to step #7 and repeat the calculations steps.

Step #15 . Upon fulfillment of pressure drop criteria, go mechanical design. Refer module # 2 for the details of mechanical design.

1.4. Design problem

The above design procedure is elaborated through the calculation of the following example