Page 438 - Handbook of Energy Engineering Calculations

important variables include the physical properties of the streams, the
location of fluids in an exchanger, and the piping arrangement of the fluids as
they enter and leave the exchanger. (See design features in Table 5.)
Selection of optimum pressure drops involves consideration of the overall
process. While it is true that higher-pressure drops result in smaller

exchangers, investment savings are realized only at the expense of operating
costs. Only by considering the relationship between operating costs and
investment can the most economical pressure drop be determined.

Available pressure drops vary from a few millimeters of mercury in
vacuum service to hundreds of pounds per square inch in high-pressure
processes. In some cases, it is not practical to use all the available pressure
drop because resultant high velocities may create erosion problems.
Reasonable pressure drops for various levels are listed below. Designs for

smaller pressure drops are often uneconomical because of the large surface
area (investment) required.

In some instances, velocities of 10 to 15 ft/s (3 to 4.6 m/s) help to reduce
fouling, but at such velocities the pressure drop may have to be from 10 (68.9
2
kPa) to 30 lb/in (206.7 kPa).
Although there are no specific rules for determining the best temperature
approach, the following recommendations are made regarding terminal
temperature differences for various types of heat exchangers; any departure
from these general limitations should be economically justified by a study of

alternate system designs:

• The greater temperature difference should be at least 20°C (36°F).

• The lesser temperature difference should be at least 5°C (9°F). When heat is
being exchanged between two process streams, the lesser temperature

difference should be at least 20°C (36°F).

• In cooling a process stream with water, the outlet-water temperature should