October 2012
6-11
Equation 6-4. Cooling Tower Cycles of Concentration
= Conductivity of BlowdownWater ÷ Conductivity of Make-UpWater
Where:
••
Conductivity of Blowdown Water (parts per million of TDS)
••
Conductivity of Make-UpWater (parts per million of TDS)
Equation 6-5. Cooling Tower Cycles of Concentration
= Make-UpWater ÷ BlowdownWater
Where:
••
Make-UpWater (gallons)
••
Blowdown Water (gallons)
To use water efficiently in the cooling tower system, the cycles of concentration must
be maximized. This is accomplished by minimizing the amount of blowdown re-
quired, thus reducing make-up water demand. The degree to which the cycles can be
maximized depends on the water chemistry within the cooling tower and the water
chemistry of the make-up water supply. As cycles of concentration are increased, the
amount of TDS that stays within the system also increases.
Facilities often employ a water treatment vendor to monitor the cooling tower, add
chemicals to the system to control scaling and chemical buildup, and maximize the
cycles of concentration. Critical water chemistry parameters that require review and
control include pH, alkalinity, conductivity, hardness, microbial growth, biocide, and
corrosion inhibitor levels.
8
Controlling these parameters allows water to be recycled
through the system longer, thereby increasing cycles of concentration. Controlling
blowdown using an automatic scheme allows a better opportunity to maximize
cycles of concentration, as the TDS concentration can be kept at a more constant set
point.
Equations 6-4 and 6-5 can also be used to determine if there is a leak, overflow, or
excessive drift. Since the equations assume that the water lost to drift and overflow
is negligible, if cycles of concentration are calculated using both equations and the
results from Equation 6-5 are higher than that from Equation 6-4 by more than 10
percent, the cooling tower might be losing water due to one of these malfunctions.
In addition to carefully controlling blowdown and checking for unexpected losses,
facilities can also reduce potable water demand from cooling towers. Water from
other equipment within a facility can sometimes be recycled and reused for cooling
6.3
Cooling Towers
8
North Carolina Department of Environment and Natural Resources, et al.,
op. cit
.,
Page 44.
