SPX Cooling Technologies MARLEY CLASS 800 - Manual (Page 18)

18
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
Water Treatment
Note
Note
Cooling Range
Number of Concentrations
1.5X 2.0X 2.5X 3.0X 4.0X 5.0X 6.0X
5ºF (2.78ºC) .78 .38 .25 .18 .11 .08 .06
10ºF (5.56ºC) 1.58 .78 .51 .38 .25 .18 .14
15ºF (8.33ºC) 2.38 1.18 .78 .58 .38 .28 .22
20ºF (11.11ºC) 3.18 1.58 1.05 .78 .51 .38 .30
25ºF (13.89ºC) 3.98 1.98 1.32 .98 .64 .48 .38
Multipliers are based on drift of 0.02% of the circulating water rate.