MIL-HDBK-1165
5.4
HVAC Equipment
5.4.1
Cooling Towers, Evaporative Coolers, and Once-Through
5.4.1.1
Conventional. Cooling towers, evaporative coolers, and
once-through cooling systems are some of the largest users of
water in facilities with air conditioning or cooling loads.
Recall Figure 5, which showed how much water is used by office
building cooling and heating systems. Only restrooms consume
more water.
a) Once-through cooling. Once-through cooling systems
are used for evaporative coolers, ice-makers, hydraulic
equipment, and air compressors. They do not recirculate the
water, discharging it after using it once. Once-through cooling
wastes water.
b) Cooling towers. Cooling towers are the most common
type of cooling system for large cooling loads. Cooling towers
are used to cool equipment such as air conditioning systems.
They work by circulating a stream of water to the equipment and
back. The circulating water is cooled by evaporation. A water
spray travels against an air flow resulting in a portion of that
water evaporating. The water that is left is cooled by heat
exchange to the evaporated water droplets. The cooled water
travels to the equipment that requires it. There, heat is
transferred from the equipment to the water, and the warmed water
then returns to the cooling tower to be re-cooled. Figure 17
shows a typical cooling tower in operation.
c) Water losses. Water loss in the recirculating
system normally occurs in three ways: (1) evaporation, (2) bleed-
off and drift. Make-up water must be added to the cooling tower
to replace the lost water. Evaporation is the natural process by
which the tower cools the water. It can be estimated that the
evaporation rate in a cooling tower equals approximately 2.4 gpm
(9 lpm) per 100 tons of (1.2 million BTU's per hour) cooling.
Drift results when water droplets are carried away from the by
the air flow. Drift usually contains sediment material and is
considered part of bleed-off. Only 0.05 to 0.2 percent
of the cooling tower's water is lost through drift. Bleed-off
is the portion of recirculating water that is purposely released
from the tower to remove accumulated impurities. Suspended and
dissolved solids accumulate in the circulating water as
evaporation removes pure water vapor and increases the
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