UFC 3-240-13FN
25 May 2005
4-2
Cooling Tower Water Calculations
4-2.1
Principles of Cooling Tower System Operations. The function of a
cooling tower is to dissipate heat from water-cooled refrigeration, air-conditioning and
industrial process systems. Water is typically the heat transfer medium used to dissipate
the heat. A cooling tower uses a combination of heat and mass transfer (evaporation) to
cool the water flowing through the tower. Conductive heat transfer accounts for 20 to
30% of the total heat dissipated. The remaining 70 to 80% of total cooling is the result of
evaporative cooling of about 1 to 2% of the recirculating water, depending on the
decrease in temperature across the tower. It takes approximately 2,326,000 joules to
evaporate 1 kilogram of water (1000 BTU per 1 pound of water). If this amount of heat is
extracted from 454 kilograms (1000 pounds) of water, approximately 0.45 kilogram (1
pound) of water will be evaporated and the temperature will drop 0.55 oC (1 oF). If 4.5
kilograms (10 pounds) of water are evaporated, the water temperature will drop 5.5 oC
added to replace water lost through tower drift (loss of water from the tower as a fine
mist), leaks in the system (unintentional blowdown), and water discharged as intentional
blowdown. Water that is added to the cooling tower to replace all of these losses is
known as cooling tower makeup water.
4-2.1.1
Relationship Between Evaporation, Blowdown, and Makeup. The
operation of cooling towers can be described by the relationship between evaporation,
blowdown, and makeup. Makeup water must equal blowdown water plus water
evaporation to maintain a constant operating water level in the system:
EQUATION
M=B+E
(19)
where
M = makeup water, liters/sec (gpm)
B = blowdown, liters/sec (gpm) (all sources)
E = evaporation, liters/sec (gpm)
NOTE: Blowdown (B) includes discharge to sewer, drift loss, and any leaks from the
system.
EXAMPLE:
M = 6.3 liters/sec (100 gpm)
B = 0.63 liters/sec (10 gpm)
E = 5.67 liters/sec (90 gpm)
4-2.1.2
Cycles of Concentration (COC). One of the common terms used in
describing the water use efficiency of cooling tower water systems is COC. COC
represents the relationship between the makeup water quantity and blowdown quantity.
COC is a measure of the total amount of minerals that is concentrated in the cooling
tower water relative to the amount of minerals in the makeup water or to the volume of
each type of water. The higher the COC, the greater the water use efficiency. Most
cooling tower systems operate with a COC of 3 to 10, where 3 represents acceptable
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