25 May 2005
b.3) For a cooling tower system serving air conditioner and chiller operations,
the evaporation rate used depends on the type of chiller:
Approximately 20 liters per hour per kilowatt (1.5 gallons per hour per
ton) for centrifugal, reciprocating, and screw-type chillers.
Approximately 40 liters per hour per kilowatt (3 gallons per hour per
ton) for absorption-type chillers.
OBJECTIVES OF COOLING WATER TREATMENT. The primary
objectives of cooling water treatment are to maintain the operating efficiency of the
cooling water system and to protect the equipment that contacts the cooling water.
These objectives are accomplished by controlling or minimizing deposition, corrosion,
and microbiological growth on the cooling water equipment. Treatment programs must
also address requirements for environmental compliance, safety, water conservation,
and limitation of chemical costs. This paragraph reviews the requirements for, and
elements of, a water treatment program for cooling water systems.
Deposit Formation and Control. Deposits that occur in cooling water
systems are usually divided into two categories: scale and fouling. The presence of
either type of deposit in the heat exchangers or in the film fill can interfere with heat
transfer, thereby reducing the efficiency of operation. Deposits can also promote under-
deposit corrosion. Scale and non-biological fouling are described in this paragraph.
Scale. Scale is formed from minerals, formerly dissolved in water, that
were deposited from the water onto heat transfer surfaces or in-flow water lines. As
water is evaporated in a cooling tower, the concentration of dissolved solids becomes
greater until the solubility of a particular scale-causing mineral salt is exceeded. When
this situation occurs in an untreated cooling water system, the scale will form on any
surface in contact with the water, especially on heat transfer surfaces. The most
common scaling minerals are calcium carbonate, calcium phosphate, calcium sulfate,
and silica, usually in that order. Formation of magnesium silicate scale is also possible
under certain conditions. Most other salts, including silica, are more soluble in hot water
than in cold water; however, most calcium and magnesium salts, including calcium
phosphate and calcium carbonate, are more soluble in cold water than in hot water.
This is called "reverse solubility." The water temperature will increase as recirculating
water passes through the cooling system. As a result, calcium and magnesium scales
may form anywhere in the system, but most likely on heated surfaces such as heat
exchangers or surface condensers. Silica will form in areas having the lowest water
temperature, such as in the cooling tower fill.
Determining Scaling Potential. The maximum solubility limit for specific
dissolved minerals will determine the types of scale that can form under a given set of
conditions. To minimize water blowdown, the amount of dissolved materials in the
cooling water should be maintained as close as possible to the maximum solubility level.
This water quality parameter, TDS, is controlled by maintaining COC in the system at a