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Water Treatment
BLOWDOWN–Blowdown, or bleed-off, is the continuous removal of a
portion of the water from the circulating system. Blowdown is used to
prevent the dissolved solids from concentrating to the point where they
will form scale. The amount of blowdown required depends upon the cool-
ing range (the difference between the hot and cold water temperatures)
and the composition of the make-up water (water added to the system to
compensate for losses by blowdown, evaporation and drift). The follow-
ing table shows the amount of blowdown required to maintain different
concentrations with various cooling ranges:
BLOWDOWN–% OF CIRCULATING RATE
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X5.1 X0.2 X5.2 X0.3 X0.4 X0.5 X0.6
)C°87.2(F°5 87.83.52.81.11.80.60.
)C°65.5(F°01 85.187.15.83.52.81.41.
)C°33.8(F°51 83.2 81.187.85.83.82.22.
)C°11.11(F°02 81.3 85.1 50.187.15.83.03.
)C°98.31(F°52 89.3 89.1 23.189.46.84.83.
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EXAMPLE: 7000 GPM circulating rate, 15° cooling range. To maintain 4
concentrations, the required blowdown is .38% or .0038 times 7000 GPM
which is 26.6 GPM.
If tower is operated at 4 concentrations, circulating water will contain four
times as much dissolved solid as the make-up water, providing none of
the solids form scale or are otherwise removed from the system.
CHEMICAL TREATMENT–In some cases chemical treatment of the circu-
lating water is not required if adequate blowdown is maintained. In most
cases, however, chemical treatment is required to prevent scale formation
and corrosion. Sulfuric acid or one of the polyphosphates is most gener-
ally used to control calcium carbonate scale. Various proprietary materi-
als containing chromates, phosphates or other compounds are available
for corrosion control. When water treatment chemicals are required, the
services of reliable water treating companies should be obtained.
Slime, a gelatinous organic growth, and algae, a green moss, may grow
in the cooling tower or heat exchangers. Their presence can interfere
with cooling efficiencies. Proprietary compounds are available from
water treating companies for the control of slime and/or algae; however,