Page 232 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

8.3.

Example 8.3

Estimate the utility cost for producing a circulating cooling water stream using a mechanical draft cooling
tower. Consider a basis of 1 GJ/h of energy removal from the process units. Flow of cooling water
required to remove this energy kg/h.

An energy balance gives

Therefore,

Latent heat of water at average temperature of 35°C = 2417 kJ/kg

Amount of water evaporated from tower, W tower

This is (413.7)(100)/(23,923) = 1.73% of the circulating water flowrate.

Typical windage losses from mechanical draft towers are between 0.1 and 0.3% [9, 10]; use 0.3%.

To calculate the blowdown, we must know the maximum allowable salt (inorganics) concentration factor,

S, of the circulating water compared with the makeup water. The definition of S is given in the following
equation:

Typical values are between 3 and 7 [9]. Here a value of 5 is assumed. By performing a water and salt
balance on the loop shown in Figure 8.2, the following results are obtained:

W MU = W tower + W wind + W BD

s W MU = s loop W wind + s loop W BD
in

Because s loop = 5s , it follows that
in