3.2.5 Cooling Water
When a process stream requires cooling at high temperature, various heat recovery techniques
should be considered. These include transferring heat to a cooler process stream, raising steam, preheating
boiler feed water, etc., as discussed in Section 3.3.
When heat must be rejected at lower temperatures, below about 120 ºC (248 ºF) (more strictly,
below the pinch temperature), then a cold utility stream is needed. Cooling water is the most commonly
used cold utility in the temperature range 120 ºC to 40 ºC, although air cooling is preferred
in regions where water is expensive or the ambient humidity is too high for cooling water systems
to operate effectively. The selection and design of air coolers are discussed in Section 19.16. If a
process stream must be cooled to a temperature below 40 ºC, cooling water or air cooling would be
110 CHAPTER 3 Utilities and Energy Efficient Design
used down to a temperature in the range 40 ºC to 50 ºC, followed by chilled water or refrigeration
down to the target temperature.
Natural and forced-draft cooling towers are generally used to provide the cooling water required
on a site, unless water can be drawn from a convenient river or lake in sufficient quantity. Sea
water, or brackish water, can be used at coastal sites and for offshore operations, but if used directly
will require the use of more expensive materials of construction for heat exchangers (see Chapter 6).
The minimum temperature that can be reached with cooling water depends on the local climate.
Cooling towers work by evaporating part of the circulating water to ambient air, causing the
remaining water to be chilled. If the ambient temperature and humidity are high, then a cooling
water system will be less effective and air coolers or refrigeration would be used instead.