Page 223 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

7.3 Design illustration 221

the design operating point for the tower for the type of fill selected. The ðL=GÞ at the operating point is
used to find the airflow ðGÞ to be provided by the fan.
Further steps of cooling tower design are illustrated in the design illustration (Section 7.3).

7.2.7 Notes on design and operation

Effect of altitude/ambient pressure: The standard psychrometric chart given in Fig. 7.6 is drawn for
atmospheric pressure of 1000 mbar. When the atmospheric pressure differs from this, the chart loses
accuracy. For small changes in pressure, the error is small but for appreciably lower pressures, as at
high altitudes, it is necessary to apply a correction. This is because although the enthalpy of air at a
particular dry-bulb temperature and absolute humidity is independent of barometric pressure, the
moisture carrying ability of air is increased with reduced pressure and this alters the composition of the
air/water vapor mixture at saturation. The enthalpy at saturation, therefore, increases with altitude. The
effect of this increase in enthalpy improves the driving force and tends to reduce the size of the tower
needed for a particular duty. However, this is counteracted by the fan, a nearly constant volume
machine, delivering a lower mass flow rate due to the reduced density of air. CTI has an elaborate
procedure to make pressure deviation corrections to the cooling tower performance. The details are not
included in this text.
Good practices: Cooling water treatment to control suspended solids and algal growth is
mandatory for any cooling tower irrespective of fill media. With increasing costs of water, efforts to
increase COC by cooling water treatment would help to reduce makeup water requirements signifi-
cantly. In large industries and power plants, improving the COC is often considered a key area for
water conservation.
Drift loss is a perennial concern in cooling towers and nowadays, most of the end-user specifi-
cations assume a 0.02% drift loss. However, improved design and material (mostly PVC) being
employed have improved drift eliminators with loss as low as 0.001%e0.003%.
Operation of cooling tower needs to be energy efficient. Energy is spent to run the circulating
cooling water system, i.e., for pumping the water and in the fans, the sum total of which should be
minimized. During cold weather months, the plant engineer should maintain the design water flow rate
and heat load in each cell of the cooling tower. If less water is needed due to temperature changes (i.e.,
the water is colder), one or more cells should be turned off to maintain the design flow in the other
cells. It is a practice to run the fans at half speed or turn them off during colder months to maintain the
desired temperature range.

7.3 Design illustration
3
Design a cooling tower to cool 6000 m /hr of warm cooling water returned at 45 C from a process

plant. Cold water from the tower is circulated to the supply header at 33 C. The maximum ambient

wet-bulb temperature during summer in the area where the tower is to be installed does not exceed
29 C for more than 5% of the days.