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Cooling tower basic knowledge.
What is a (wet, atmospheric) cooling tower?
A cooling tower is a heat rejection device, which extracts waste heat to the atmosphere though the cooling of a water
stream to a lower temperature. The type of heat rejection in a cooling tower is termed “evaporative” in that it allows a small
portion of the water being cooled to evaporate into a moving air stream to provide signicant cooling to the rest of that
water stream. The heat from the water stream transferred to the air stream raises the air’s temperature and its relative
humidity to 100%, and this air is discharged to the atmosphere. Evaporative heat rejection devices such as cooling towers
are commonly used to provide signicantly lower water temperatures than achievable with “air cooled” or “dry” heat rejec-
tion devices, like the radiator in a car, thereby achieving more cost-effective and energy efcient operation of systems in
need of cooling. Think of the times you’ve seen something hot be rapidly cooled by putting water on it, which evaporates,
cooling rapidly, such as an overheated car radiator. The cooling potential of a wet surface is much better than a dry one.
Common applications for cooling towers are providing cooled water for air-conditioning, manufacturing and electric power
generation. The smallest cooling towers are designed to handle water streams of only a few gallons of water per minute
supplied in small pipes like those might see in a residence, while the largest cool hundreds of thousands of gallons per
minute supplied in pipes as much as 15 feet (about 5 meters) in diameter on a large power plant.
The generic term “cooling tower” is used to describe both direct (open circuit) and indirect (closed circuit) heat rejection
equipment. While most think of a “cooling tower” as an open direct contact heat rejection device, the indirect cooling
tower, sometimes referred to as a “closed circuit cooling tower” is nonetheless also a cooling tower.
A direct, or open circuit cooling tower is an enclosed structure with internal means to distribute the warm water fed to
it over a labyrinth-like packing or “ll.” The ll provides a vastly expanded air-water interface for heating of the air and
evaporation to take place. The water is cooled as it descends through the ll by gravity while in direct contact with air that
passes over it. The cooled water is then collected in a cold water basin below the ll from which it is pumped back through
the process to absorb more heat. The heated and moisture laden air leaving the ll is discharged to the atmosphere at a
point remote enough from the air inlets to prevent its being drawn back into the cooling tower.
The ll may consist of multiple, mainly vertical, wetted surfaces upon which a thin lm of water spreads (lm ll), or several
levels of horizontal splash elements which create a cascade of many small droplets that have a large combined surface
area (splash ll).
An indirect, or closed circuit cooling tower involves no direct contact of the air and the uid, usually water or a glycol
mixture, being cooled. Unlike the open cooling tower, the indirect cooling tower has two separate uid circuits. One is
an external circuit in which water is recirculated on the outside of the second circuit, which is tube bundles (closed coils)
which are connected to the process for the hot uid being cooled and returned in a closed circuit. Air is drawn through the
recirculating water cascading over the outside of the hot tubes, providing evaporative cooling similar to an open cooling
tower. In operation the heat ows from the internal uid circuit, through the tube walls of the coils, to the external circuit
and then by heating of the air and evaporation of some of the water, to the atmosphere. Operation of the indirect cooling
towers is therefore very similar to the open cooling tower with one exception. The process uid being cooled is contained
in a “closed” circuit and is not directly exposed to the atmosphere or the recirculated external water.
In a counter-ow cooling tower air travels upward through the ll or tube bundles, opposite to the downward motion of the
water. In a cross-ow cooling tower air moves horizontally through the ll as the water moves downward.
Cooling towers are also characterized by the means by which air is moved. Mechanical-draft cooling towers rely on
power-driven fans to draw or force the air through the tower. Natural-draft cooling towers use the buoyancy of the exhaust
air rising in a tall chimney to provide the draft. A fan-assisted natural-draft cooling tower employs mechanical draft to
augment the buoyancy effect. Many early cooling towers relied only on prevailing wind to generate the draft of air.
