Page 116 - Chemical process engineering design and economics

100 Chapter3

For a final oxygen concentration of 1.0 % at a nitrogen flow rate of 180 1/s
3
or0.180m /s,
40000 0.01
=_ ———hi —— = 6.766xl0 5 (3.1.17)
t
0.180 0.21
s
The purging time is 6.766 x 10 s (188 h).

Example 3.2 Cooling-Tower Analysis_______________________

Water, from lakes, rivers, and the sea, is a common coolant. Because of water
shortages or the environmental effects of discharging heated water, air may also be
use as a coolant, either directly or indirectly. In the direct method, called the dry
system, a fan blows air directly over a heat exchanger surface. Because of the low
heat capacity of air, a large quantity is required. In the indirect method, called the
wet system, water is the primary coolant. Air cools the water by evaporating a
small fraction of the water in a tower. The cooled water is then returned to the
process. A process engineer will have to choose either the dry or wet method.
Cooling water is not a main part of the process but an "offsite" operation, i.e., it is
generally located off to one side of the process area. We may consider cooling and
treating the water to remove dissolved salts as a sub-process.
Figure 3.2.1 shows the mechanical-draft crossflow tower, which is the most
commonly used cooling tower [11]. Water enters the top of the tower and flows
downward over packing, called fill. The fill increases the surface area for mass
transfer by breaking up the water into droplets or spreading it into a thin film. A
cooling tower, like a packed bed absorber or stripper, must provide good contact
between air and water to promote rapid evaporation. Good contact reduces the size
of the tower and also the pressure drop, called "draft" by cooling-tower design
engineers. A fan, located at the top of the tower and shown in Figure 3.2.1, draws
air into the tower. Louvers distribute incoming air, which then flows across the
tower, removing evaporated water.
During the operation of the tower, water is lost by evaporation, water drop-
lets entrained in the outgoing air, and in a water purge, called blowdown. To re-
duce carry-over of water droplets the air flows across drift eliminators. The water
droplets impinge on the drift eliminators and then flows down to the bottom of the
tower. The droplet water loss is about 0.2% of the incoming water [11]. After
leaving the drift eliminators, air flows up and out of the tower. Evaporation of
water into air transfers heat from the water to the air. Cooling the water requires
about 1.0 % evaporation for every 5.56 °C (10.0 °F) drop in the water tempera-
turefl 1]. To reduce scale formation in the tower because of dissolved calcium or

Copyright © 2003 by Taylor & Francis Group LLC