Page 217 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

7.2 Cooling tower 215

The evaporation loss can also be estimated from heat balance across the cooling tower, i.e., the
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amount of heat to be removed from circulating water (Q ¼ C m hr C p R) and the amount of
heat removed by evaporative cooling (Q ¼ E l)
On equating the two, we get:

C C p R
E ¼ (7.7)
l
Where, l ¼ latent heat of vaporization of water ¼ 2260 kJ/kg
C p ¼ specific heat of water ¼ 0.238 kJ/kg C;

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C is water recirculation rate in m hr and R is in C
A more rigorous way of calculating E based on humidity difference of inlet and exit air and its flow
rate is illustrated in the design example at the end of this chapter.
Drift loss (D) is usually estimated as a percentage of recirculation (C). Splash fill towers tend to
have higher drift rates than film fill towers. Drift eliminator design, unit maintenance and airflow also
have an influence on the amount of drift from a cooling system. In the absence of manufacturer’s data,
D may be assumed to be:
(i) 0.3%e1.0% C for a natural draft cooling tower
(ii) 0.1%e0.3% C for an induced draft cooling tower
(iii) about 0.01% C or less, if the cooling tower has drift eliminator
Makeup water (M) is estimated from a water balance around the entire system

M ¼ E þ B þ D þ L (7.8)
Ideally with negligible leakage,
M ¼ E þ B þ D (7.9)
Cycles of Concentration e The quantities of blowdown and makeup water are optimized by the
Cycles of Concentration (COC).
Any of the following ion concentration or even the conductivity can be used to calculate COC.
(i) COC ¼ Silica in Recirculating Cooling Water/Silica in Makeup Water
(ii) COC ¼ Calcium hardness in Cooling Water/Calcium hardness in Makeup water
(iii) COC ¼ Conductivity of Cooling Water/Conductivity of Makeup water
From a chloride balance around the system and considering that the evaporated water (E) has no
salts,
M E
COC ¼ X C =X M ¼ ¼ 1 þ (7.10)
ðB þ DÞ ðB þ DÞ
Where,
X M ¼ Concentration of chlorides in makeup water (M), in ppm by weight
X C ¼ Concentration of chlorides in circulating water (C), in ppm by weight