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Blowdown
Blowdown, or bleed-off is the continuous removal of a portion of the
water from the circulating system. It is used to prevent dissolved solids
from concentrating to the point where they will form scale. The amount
of blowdown required depends upon the cooling range (design hot water
temperature minus design cold water temperature) and the composition
of the make-up water (water added to the system to compensate for
losses by blowdown, evaporation, and drift). The following table shows
the amount of blowdown (percent of total water flow) required to maintain
different concentrations with various cooling ranges:
Example: 150,000 gpm (9465 L/s) circulating rate, 28°F (15.56°C) cooling
range. To maintain 3 concentrations, the required blowdown is 1.1% or
.011 times 150,000 gpm (9465 L/s) which is 1650 gpm (104.1L/s).
If tower is operated at 3 concentrations, circulating water will contain three
times as much dissolved solid as the make-up water, assuming none of
the solids form scale or are otherwise removed from the system.
The use of corrosion and scale inhibitors is strongly recommended.
Chemical Treatment
In some cases chemical treatment of the circulating 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 generally used to control calcium
carbonate scale. Various proprietary materials containing 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.
The circulating water pH should be maintained between 7.0 and
8.5.
The Langelier index (calcium carbonate saturation index) has proven to be
an effective tool in predicting the aggressiveness of cooling tower water
toward concrete. The Langelier index relates the methyl orange alkalinity,
the calcium hardness, the total solids, the pH value, and the temperature
of the water. From these values it is possible to calculate the index and
Number of Concentrations
Cooling Range
1.5X
2.0X
2.5X
3.0X
4.0X
5.0X
6.0X
10°F ( 5.56°C)
1.58
0.78
0.51
0.38
0.25
0.18
0.14
15°F ( 8.33°C)
2.38
1.18
0.78
0.58
0.38
0.28
0.22
20°F (11.11°C)
3.18
1.58
1.05
0.78
0.51
0.38
0.30
25°F (13.89°C)
3.98
1.98
1.32
0.98
0.64
0.48
0.38
30°F (16.67°C)
4.78
2.38
1.58
1.18
0.78
0.58
0.46
Multipliers are based on drift of 0.02% of the circulating water rate.
Water Treatment
▲ Note
▲ Note