A cooling tower is a heat rejection device, which extracts waste heat to the atmosphere
through 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 significant
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
significantly lower water temperatures than achievable with "air cooled" or "dry" heat
rejection devices, like the radiator in a car, thereby achieving more cost-effective and
energy efficient 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 "fill." The fill
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 fill 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 fill from which it is pumped back through the process to
absorb more heat. The heated and moisture laden air leaving the fill 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 fill may consist of multiple, mainly vertical, wetted surfaces upon which a thin film
of water spreads (film fill), or several levels of horizontal splash elements which
create a cascade of many small droplets that have a large combined surface area (splash
fill).
An indirect, or closed circuit cooling tower involves no direct contact of the air and
the fluid, usually water or a glycol mixture, being cooled. Unlike the open cooling
tower, the indirect cooling tower has two separate fluid circuits. One is an external
circuit in which water is recirculated on the outside of the second circuit, which is
tube bundles (closed coils) that are connected to the process for the hot fluid 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
flows from the internal fluid 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 fluid being cooled is contained in a "closed"
circuit and is not directly exposed to the atmosphere or the recirculated external
water.
In a counter-flow cooling tower, air travels upward through the fill or tube bundles,
opposite to the downward motion of the water. In a cross-flow cooling tower, air moves
horizontally through the fill 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.
If cooled water is returned from the cooling tower to be reused, some water must be added
to replace, or make-up, the portion of the flow that evaporates. Because evaporation
consists of pure water, the concentration of dissolved minerals and other solids in
circulating water will tend to increase unless some means of dissolved-solids control,
such as blow-down, is provided. Some water is also lost by droplets being carried out
with the exhaust air (drift), but this is typically reduced to a very small amount by
installing baffle-like devices, called drift eliminators, to collect the droplets. The
make-up amount must equal the total of the evaporation, blow-down, drift, and other
water losses such as wind blowout and leakage, to maintain a steady water level.
Some useful terms, commonly used in the cooling tower industry:
Drift - Water droplets that are carried out of the cooling tower with
the exhaust air. Drift droplets have the same concentration of impurities as the water
entering the tower. The drift rate is typically reduced by employing baffle-like
devices, called drift eliminators, through which the air must travel after leaving the
fill and spray zones of the tower.
