16
maintenance
Blowdown Rate
Cooling Two Four
Range (°C)
Concentrations
Concentrations
3 0.7% 0.17%
4 1.1% 0.30%
6 1.5% 0.43%
Blowdown
A cooling tower cools water by continuously causing a portion of it to evapo-
rate. Although the water lost by evaporation is replenished by the makeup
system, it exits the tower as pure water – leaving behind its burden of dissolved
solids to concentrate in the remaining water. Given no means of control, this
increasing concentration of contaminants can reach a very high level.
In order to achieve water quality which is acceptable to the cooling tower (as
well as the remainder of your circulating water system), the selected water
treatment company must work from a relatively constant level of concentra-
tions. This stabilization of contaminant concentrations is usually accomplished
by blowdown, which is the constant discharge of a portion of the circulating
water to waste. As a rule, acceptable levels on which to base a treatment
schedule are in the range of 2–4 concentrations. The following table gives
approximate blowdown rates (percent of total water flow rate constantly
wasted) to achieve those concentrations at various cooling ranges*:
* Range is the difference between hot water temperature entering the tower and
cold water temperature leaving the tower.
When water treatment chemicals are added, they should not be in-
troduced into the circulating water system via the cold water basin
of the cooling tower. Water velocities are lowest at that point, which
results in inadequate mixing and may damage the cooling tower.
Note
