flooded or sprayed over the distributing deck and flows
by gravity to the water-collecting basin.
A completely open space is required for the natural
draft tower since its performance depends on existing
air currents. Ordinarily, a roof is an excellent location.
Louvers must be placed on all sides of a natural draft
tower to reduce drift loss.
Important design considerations are the wind
velocity and the height of the tower. A wind velocity of
3-miles per hour is generally used for a design of
natural draft cooling towers. The natural draft cooling
tower was once the standard design for cooling
condenser water in refrigeration systems up to about
75 tons. It is now rarely selected unless low initial cost
and minimum power requirements are primary
considerations. The drift loss and space requirements
are much greater than for other cooling tower designs.
An induced draft cooling tower is provided with a
top-mounted fan that induces atmospheric air to flow
up through the tower, as warm water falls downward.
An induced draft tower may have only spray nozzles
for water backup, or it may be filled with various slat
and deck arrangements.
There are several types of
induced draft cooling towers.
In a counterflow induced draft tower (fig. 7-21, C),
a top-mounted fan induces air to enter through the
bottom of the tower and to flow vertically upward as
the water cascades down through the tower. The
counterflow tower is particularly well adapted to a
restricted space as the discharge air is directed
vertically upward, and if equipped with a inlet on each
side, requires only minimum clearance for air intake
area. The primary breakup of water may be either by
pressure spray or by gravity from pressure-filled
A parallel-flow induced draft tower (fig. 7-21, A)
operates the same way as a counter-flow tower, except
the top-mounted fan pulls the air in through the top of
the tower and pushes it out the bottom. The airflow
goes in the same direction as the water.
Comparing counterflow and parallel-flow induced
draft towers of equal capacity, the parallel-flow tower
is somewhat wider but the height is much less. Cooling
towers must be braced against the wind. From a
structural standpoint, therefore, it is much easier to
design a parallel-flow than a counterflow tower, as the
low silhouette of the parallel-flow type offers much
less resistance to the force of the winds.
Mechanical equipment for counterflow and
parallel-flow towers is mounted on top of the tower and
is readily accessible for inspection and maintenance.
The water-distributing systems are completely open
on top of the tower and can be inspected during
operation. This makes it possible to adjust the float
valves and clean stopped-up nozzles while the towers
The cross-flow induced draft tower (fig. 7-21, B) is
a modified version of the parallel-flow induced draft
tower. The fan in a cross-flow cooling tower draws air
through a single horizontal opening at one end and
discharges the air at the opposite end.
The cooling tower is a packaged tower that is
inexpensive to manufacture and is extremely popular
for small installations. As a packaged cooling tower
Figure 7-21.Types of induced fan cooling towers.