The process design engineer selects the tower design operating pressure as follows:
1. Determines the maximum cooling water or ambient air temperature that is typically expected on a hot summer day
in the locale where the plant is to be built.
2. Calculates the condenser outlet, or reflux drum temperature, that would result from the above water or air temperature.
3. Referring to Fig. 5.2, the designer calculates the pressure in the reflux drum, assuming that the condensed liquid is at its bubble point. Adding 5 or 10 psig to this pressure, for pressure loss in the overhead condenser and associated piping, the designer then determines the tower-top pressure.
Of course, the unit operator can physically deviate from this design pressure, but to what purpose?