Handbook of air conditioning and refrigeration / Shan K

10.34 CHAPTER TEN
10.9 COOLING TOWERS
A cooling tower is a device in which recirculating condenser water from a condenser or cooling
coils is evaporatively cooled by contact with atmospheric air. Most cooling towers used in refrigerating
plants for commercial buildings or industrial applications for air conditioning are mechanical
draft cooling towers. A mechanical draft cooling tower uses fan(s) to extract atmospheric air. It consists
of a fan to extract intake air, a heat-transfer medium (or fill), a water basin, a water distribution
system, and an outer casing, as shown in Fig. 10.13.
According to the location of the fan corresponding to the fill and to the flow arrangements of air
and water, currently widely used mechanical draft cooling towers for HVAC&R can be classified
into the following categories:
● Counterflow induced-draft
● Crossflow induced-draft
● Counterflow forced-draft
Counterflow Induced-Draft Cooling Towers
In a counterflow induced-draft cooling tower, as shown in Fig. 10.13a, the fan is located downstream
from the fill at the air exit. Atmospheric air is drawn by the fan through the intake louver or,
more simply, an opening covered by wire mesh. Water from the condenser or recirculating water
from the coil, or a mixture of the two, is evenly sprayed or distributed over the fill and falls down
into the water basin.
Air is extracted across the fill and comes in direct contact with the water film. Because of the
evaporation of a small portion of the condenser water, usually about 1 percent of the water flow,
the temperature of the water gradually decreases as it falls down through the fill countercurrent to
the extracted air. Evaporated water vapor is absorbed by the airstream. Large water droplets
entrained in the airstream are collected by the drift eliminators. Finally, the airstream and drift are
discharged at the top exit. Drift, or carryover, is the minute water droplets entrained in the airstream
discharged out of the tower. The evaporatively cooled water falls into the water basin and flows to
the condenser.
In a counterflow induced-draft cooling tower, the driest air contacts the coldest water. Such a
counterflow arrangement shows a better tower performance than a crossflow arrangement. In
addition, air is drawn through the fill more evenly by the induced-draft fan and is discharged at
a higher velocity from the top fan outlet. Both higher exhaust air velocity and even velocity
distribution reduce the possibility of exhaust air recirculation. Compared with the crossflow
induced-draft cooling tower, the vertical height from the installation level to the inlet of the
water-spraying nozzles in a counterflow tower is greater and, therefore, requires a higher pump
head.
Crossflow Induced-Draft Cooling Towers
In a crossflow induced-draft cooling tower, as shown in Fig. 10.13b, the fan is also located downstream
from the fill at the top exit. The fill is installed at the same level as the air intake. Air enters
the tower from the side louvers and moves horizontally through the fill and the drift eliminator. Air
is then turned upward and finally discharged at the top exit. Water sprays from the nozzles, falls
across the fill, and forms a crossflow arrangement with the airstream.
The crossflow induced-draft cooling tower has a greater air intake area. Because of the crossflow
arrangement, the tower can be considerably lower than the counterflow tower. However, the risk of
recirculation of tower exhaust air increases.