2 replies to this topic

[mbeychok]

Harry:

For a circulating cooling water system with an evaporative cooling tower, here are the pertinent equations:

A water mass balance of the system is:
M = E+D+W

Combining the water balance with a chloride balance of the system:
Cycles = M/(D+W)
Cycles = 1 + E/(D+W)
Cycles = M/(M-E)

where:
Cycles = cycles of concentration
Cycles = (Chlorides concentration in C) / (Chlorides concentration in M)
M = make-up water in volume/minute
E = evaporation in volume/minute
D = draw-off or blowdown in volume/minute
W = windage loss in volume/minute

The evaporation, E. can be obtained by a heat balance:
E = C(dT)(Cp)/H

where:
E = evaporation in gallons/minute
C = circulating water in gallons/min
H = heat of vaporization of water = approx. 1000 Btu/pound
Cp = specific heat of water = 1 Btu/pound/°F
dT = delta T = water temperature drop from top to bottom of tower

The windage loss, W, can be approximated by using:
W = 0.3 to 1.0 percent of C for a natural draft tower
W = 0.1 to 0.3 percent of C for an induced draft tower
W = approx. 0.01 percent of C if tower has windage drift eliminators

To use the above equations, you must first obtain:

(1) The circulating rate of water, C, in volume units/minute.
(2) The concentration of chlorides in the circulating water, C, and in the draw-off, D.
(3) The design windage rate from the tower manufacturer. If that is not possible, refer to the above approximations.

The typical range of cycles of concentration in cooling towers used in petroleum refineries is from 3 to 7. Cooling towers in large power plants will have much higher cycles.