October 2012
6-9
a cooling tower is typically equal to approximately 1 percent of the rate of recirculat-
ing water flow for every 10°F in temperature drop that the cooling tower achieves.
6
Blowdown or Bleed-Off
When water evaporates from the tower, dissolved solids (e.g., calcium, magnesium,
chloride, silica) are left behind. As more water evaporates, the concentration of total
dissolved solids (TDS) increases. If the concentration gets too high, the TDS can cause
scale to form within the system or can lead to corrosion. The concentration of TDS is
controlled by removing (i.e., bleeding or blowing down) a portion of the water that
has high TDS concentration and replacing that water with make-up water, which has
a lower concentration of TDS. Carefully monitoring and controlling the quantity of
blowdown provides the most significant opportunity to conserve water in cooling
tower operations. Blowdown can be conducted manually using a batch method, in
which blowdown is initiated, and make-up water is fed to the system for a preset
time to decrease the concentration of TDS. It can also happen automatically through
a control scheme that initiates blowdown and make-up when the TDS concentration
reaches a preset point.
Drift
A small quantity of water can be carried from the tower as mist or small droplets
known as “drift.”Drift loss is small compared to evaporation and blowdown and is
controlled with baffles and drift eliminators.
Drift can vary from 0.05 to 0.2 percent of the
flow rate through the cooling tower.
7
Modern
drift eliminators can reduce this loss to less
than 0.005 percent, which would be negli-
gible.
Leaks or Overflows
Properly operated towers and associated pip-
ing should not have leaks or overflows. How-
ever, an overflow drain is provided within the
tower in case of malfunction and subsequent
overflow. Most green codes require overflow
alarms.
The water used by the cooling tower is equal
to the amount of make-up water that is added
to the system. The amount of make-up water needed is dictated by the amount of
water that is lost from the cooling tower through evaporation, drift, blowdown, and
leakage, as illustrated by Equation 6-3.
6
Schultz Communications. July 1999.
A Water Conservation Guide for Commercial, Institutional and Industrial Users
.
Prepared for the New Mexico Office of the State
Engineer. Page 60.
7
Ibid
.
6.3
Cooling Towers
Cooling tower
