Page 214 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

212 Chapter 7 Industrial cooling systems

It is important to note that the specified wet-bulb temperature is generally the ambient air wet-bulb
temperature, but in reality, this is often affected by the discharged moist air recirculating into the tower.
Hence the ambient wet-bulb temperature (the temperature in the cooling tower area) may not be same
as the inlet wet-bulb temperature (the wet-bulb temperature of the air entering the tower) and the
designer may adjust the wet-bulb temperature used for sizing the tower upward to account for any
potential recirculation. In general, the design wet-bulb temperature selected is close to the average

maximum wet-bulb temperature in summer. The design WBT (T amb;WBT , C) should not be exceeded
for more than 5% of the time, and it is necessary to evaluate the effects of increased wet-bulb tem-
peratures on the tower performance.
Approach to wet-bulb temperature (A) is the difference between T c and T amb;WBT .

Að CÞ¼ T c T amb;WBT (7.3)
The lower the approach, the better is the cooling tower performance, but this leads to designing a

more expensive cooling tower due to its increased size. An approach lower than 2.8 Cisnot
economical, nor will it be certified by the Cooling Technology Institute (https://www.cti.org/ and see
7.2.5). Cost-effective selections are based on a criterion using an approach close to 4 C. This is not

because towers are unable to achieve lower than this, but any error in measurement becomes very

significant when performance is calculated at the design point. Approach temperature above 4 C
results in higher cooling water temperature in process plants and reduced efficiency of heat exchange
using cooling water without much savings on cooling tower.
While both range and approach should be monitored, “Approach” is a better indicator of cooling
tower performance. When the size of the tower has to be chosen, the approach is the most important
parameter, closely followed by the flow rate. The range and wet-bulb temperature are of lesser
importance.
As the water gets cooled during its downflow in contact with air, the required depth of fill over
which the contact takes place is governed by the range of cooling while the plan area of the fill section
increases with increasing water flow rate. In simple words, the tower height is decided by the cooling
range, whereas the tower cross-section is decided by the water flow capacity.
Liquid/Gas ðL=GÞ ratio of a cooling tower is the ratio of the mass flow rates of inflow water ðLÞ
and dry air (G).
From enthalpy balance, heat removed from water must be equal to the heat absorbed by sur-
rounding air or

ðh vo h vi Þ
ðL=GÞ¼ (7.4)
R
Where h vo and h vi are the enthalpy (kcal/kg dry air) of air-water vapor mixture at exhaust wet-bulb

temperature and inlet wet-bulb temperature respectively in ( C).
Effectiveness ðE f Þ is the ratio (expressed as %) between the actual range and the maximum range.
ðT h T c Þ R
¼ (7.5)
E f ð%Þ¼ 100
T h T amb;WBT R þ A
As discussed above, a cooling tower can never be 100% effective. Moreover, in summer, the
ambient wet-bulb temperature is higher as compared to winter, and this limits the CT efficiency.