View Full Version : Cooling tower evaporation, blowdown, makeup and cycles

13-10-2005, 05:18 AM
Here are the governing relationships for the makeup flow rate, the evaporation and windage losses, the draw-off rate, and the concentration cycles in an evaporative cooling tower system:


M = Make-up water in gal/min
C = Circulating water in gal/min
D = Draw-off water in gal/min
E = Evaporated water in gal/min
W = Windage loss of water in gal/min
X = Concentration in ppmw (of any completely soluble salts … usually chlorides)
XM = Concentration of chlorides in make-up water (M), in ppmw
XC = Concentration of chlorides in circulating water (C), in ppmw
Cycles = Cycles of concentration = XC / XM
ppmw = parts per million by weight

A water balance around the entire system is:

M = E + D + W

Since the evaporated water (E) has no salts, a chloride balance around the system is:

M (XM) = D (XC) + W (XC) = XC (D + W)

and, therefore:

XC / XM = Cycles = M / (D + W) = M / (M – E) = 1 + {E / (D + W)}

From a simplified heat balance around the cooling tower:

(E) = (C) (delta T) (cp) / HV

HV = latent heat of vaporization of water = ca. 1,000 Btu/pound
delta T = temperature difference from tower top to tower bottom, in degrees F
cp = specific heat of water = 1 Btu/pound/degree F

Windage losses (W), in the absence of manufacturer's data, may be assumed to be:

W = 0.3 to 1.0 percent of C for a natural draft cooling tower
W = 0.1 to 0.3 percent of C for an induced draft cooling tower
W = about 0.01 percent of C if the cooling tower has windage drift eliminators

Concentration cycles in industrial cooling towers usually range from 3 to 7. In some very large units, the cooling tower concentration cycles may be much higher.

(Note: Draw-off and blowdown are synonymous. Windage and drift are also synonymous.)

13-10-2005, 04:49 PM
Welcome Milton,

From a waste standpoint, how is the blow down water most efficiently recovered / cleaned, so the water can be reused and so all the chlorides won’t go to the waste water plant. By filtering, evaporation, chemical means or other?



13-10-2005, 05:14 PM

Very few industrial plants attempt to recover and reuse their cooling tower blowdown. However, a fair number of large power plants do so.

The most widely used technology is reverse osmosis (RO). Other technologies are: electrodialysis reversal, and evaporators plus crystallizers. It should be noted that recovering and reusing cooling tower blowdown incurs a high capital cost as well as high ongoing operating costs.

Milt Beychok
(Visit me at www.air-dispersion.com)

13-10-2005, 07:47 PM
Are we selling something here Milt?

Are you signed in with more than 1 user name?

13-10-2005, 11:22 PM

As far as I know, I am only signed in with one user name, which is "mbeychok". However, I sign some of my posts/replies as Milton Beychok and some as Milt Beychok and sometimes I add on the (Visit me at ...) and sometimes I don't ... with no specific rhyme or reason. Is there anything wrong about doing so? If so, please let me know so that I can correct whatever I have done wrong.

Milt Beychok

14-10-2005, 06:34 PM
In my last job, they used the blowdown water to flush through waste water collection tanks. The food waste collected in the tank tended to push up the waste alkalinity beyond contracted limits so we used the tower water to dilute it a bit.

We looked at other ways of reducing water usage but as stated earlier, the capital cost was too high given the bean counters that ran the company never looked further forward than a few months....:mad: