13
➠
maintenance
Water Quality and Blowdown
Maintaining Water Quality:
The materials used in the QuadraFlow tower are selected to offer long, corro-
sion-free service in a “normal” cooling tower environment, defined as follows:
• Circulating water with a pH between 6 and 8; a chloride content (as NaCl)
below 750 mg/L; a sulfate content (SO
4
) below 1200 mg/L; a maximum
inlet water temperature not to exceed 52°C; no significant contamina-
tion with unusual chemicals or foreign substances; and adequate water
treatment to minimize scaling.
• Chlorine (if used) shall be added intermittently, with a free residual not to
exceed 1 mg/L – maintained for short periods. Excessive chlorine levels
may deteriorate sealants and other materials of construction.
• An atmosphere surrounding the tower no worse than “moderate indus-
trial,” where rainfall and fog are no more than slightly acid, and they do
not contain significant chlorides or hydrogen sulfide (H
2
S).
Blowdown:
A cooling tower cools water by continuously causing a portion of it to evaporate.
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 will be in the range of 2-4 concentrations. The following table gives
approximate rates of blowdown (percent of total water flow rate constantly
wasted) to achieve those concentrations at various cooling ranges.*
*
Range = Difference between hot water temperature entering the tower and cold water tempera-
ture leaving the tower.
Blowdown Rate
Cooling
Two
Four
Range (°C)
Concentrations
Concentrations
6
0.7%
0.17%
8
1.1%
0.30%
11
1.5%
0.43%
