Author Topic: Material balances for large evaporative cooling towers  (Read 3576 times)

mbeychok

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Material balances for large evaporative cooling towers
« on: December 27, 2007, 03:02:36 AM »
Here are the governing material balance relationships for the makeup flow rate, the evaporation and windage losses, the draw-off rate, and the concentration cycles in large evaporative cooling tower system systems as used in many oil refineries.
 

 
M = Make-up water in m3/hr
C = Circulating water in m3/hr
D = Draw-off water in m3/hr
E = Evaporated water in m3/hr
W = Windage 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 = M / (D + W) = M / (M E) = 1 + {E / (D + W)}
 
From a simplified heat balance around the cooling tower:
 
(E) = (C) (∆T) (sp.ht.) / Hv
 
where:
Hv = latent heat of vaporization of water = ca. 2260 kJ / kg
∆T = water temperature difference from tower top to tower bottom, in C
sp.ht. = specific heat of water = ca. 4.184 kJ / kg / 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.
 
(Note: Draw-off and blowdown are synonymous. Windage and drift are also synonymous.)
« Last Edit: December 27, 2007, 03:08:23 AM by mbeychok »

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Material balances for large evaporative cooling towers
« on: December 27, 2007, 03:02:36 AM »