14
maintenance
Water Treatment and Blowdown
Maintaining Water Quality:
The materials used in an NC Fiberglass tower are selected to offer long,
corrosion-free service in a “normal” cooling tower environment, defined
as follows:
• Circulating water with a pH between 6.5 and 8; a chloride content
(as NaCl) below 500 mg/L; a sulfate content (SO
4
) below 250 mg/L;
total alkalinity below 500 mg/L; calcium hardness (as CaCO
3
) above
50 mg/L.
• 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).
Unless you purchased an NC Fiberglass tower with stainless steel
structure, the structure of your tower consists primarily of galvanized
steel, therefore your water treatment program must be compatible
with zinc. In working with your water treatment supplier, it is impor-
tant that you recognize the potential effects on zinc of the specific
treatment program you choose.
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.*
Note
