Cycle of Concentration for a Cooling Tower

Cycle of Concentration(COC):
As water is circulating in a cooling tower and at the same time there is evaporation of water. During evaporation there will be loss of water in the system, hence the concentration of chlorides increases in the circulating water (as the chloride concentration is not affected but the water quantity decreased) and at the same time there is make up of water at certain concentration. But the concentration should not increase hence accordingly the make up is made. To know the quantity of make up required the term COC is defined.
M = Make-up water in m3/hr
C = Circulating water in m3/hr
D = Draw-off (i.e., blowdown) water in m3/hr
E = Evaporated water in m3/hr
W = Windage (i.e., drift) loss of water in m3/hr
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 of concentration = M / (D + W) = M / (M - E) = 1 + {E / (D
+ W)}

>From a simplified heat balance around the cooling tower:

(E) = (C) (delta T) (sp.ht.) / Hv

where:
Hv = latent heat of vaporization of water = ca. 2260 kJ / kg
delta T = water temperature difference from tower top to tower bottom, in
degrees C
sp.ht. = specific heat of water = ca. 4.184 kJ / kg / degree C

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 petroleum refinery cooling towers usually range from
3 to 7. In some large power plants, the cooling tower concentration cycles
may be much higher.

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