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Category: Industrial Utilities
Question: How can I estimate the blowdown flow rate that should be used on a cooling tower?
Keywords: cooling,tower,water,blowdown,drift,losses,evaporation,blow,down
Answer: Start by examining the feed water to the tower and determine the concentration of the following: chloride, sulfate, sodium bicarbonate, and calcium based salts.
As rule-of-thumb limits, try to keep the tower water below the following limits: 750 ppm chlorides, 1200 ppm sulfates, 1200 ppm calcium salts, 200 ppm sodium bicarbonate.
With these limits in mind, it should be fairly clear via the feedwater examination which of the species will determine the blowdown rate (more often than not, it's the chloride levels...but well waters can contain significant levels of calcium based salts).
Now, use the following procedure:
First, we?ÇÖll define the blowdown flow rate as:

BLOWDOWN = Blowdown flow rate in GPM
EVAP = Rate of evaporation in GPM
DRIFT = Rate of drift losses in GPM
CONC = Number of allowable concentrations
Since evaporation and drift losses are very difficult to measure, the following estimates can be used:
EVAP (GPM) = Total Water Flow (GPM) x Cooling Range (??F) x 0.0008
DRIFT (GPM) = Total Water Flow (GPM) x 0.0002

Considering an example, suppose that a tower is cooling 20,000 GPM of water from 115 ??F to 88 ??F (27 ??F cooling range). The make up water contains 150 ppm chlorides and the tower water should not exceed 750 ppm chlorides. Calculate the following values:
CONC = 750 / 150 = 5 conc cycles
EVAP = 20,000 GPM x 27 ??F x 0.0008 = 432 GPM
DRIFT = 20,000 GPM x 0.0002 = 4 GPM
and the blowdown flow rate is calculated as:

Links: Cooling Towers: Design and Operation Considerations