SPX Cooling Technologies 800 user manual Water Treatment, Blowdown, Chemical Treatment

▲Note

▲Note

Water Treatment

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:

Cooling Range		1.5X	2.0X	Number of Concentrations			5.0X	6.0X
				2.5X	3.0X	4.0X
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.

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

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