Cooling Towers
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 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) which 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.
Blow-out - Water droplets blown out of the cooling tower by wind,
generally at the air inlet openings.
Water may also be
lost, in the absence of wind, through splashing or misting.
Devices such as wind
screens, louvers, splash deflectors and water diverters are used to limit these
losses.
Plume - The stream of saturated exhaust air
leaving the cooling tower.
The plume is visible
when water vapor it contains condenses in contact with cooler ambient air, like
the saturated air in one's breath fogs on a cold day.
Under certain
conditions, a cooling tower plume may present fogging or icing hazards to its
surroundings.
Note that the water
evaporated in the cooling process is "pure" water, in contrast to the
very small percentage of drift droplets or water blown out of the air inlets.
Blow-down - The portion of the circulating water
flow that is removed in order to maintain the amount of dissolved solids and
other impurities at an acceptable level.
Leaching - The loss of wood preservative chemicals
by the washing action of the water flowing through a wood structure cooling
tower.
Noise - Sound energy emitted by a cooling tower
and heard (recorded) at a given distance and direction.
The sound is
generated by the impact of falling water, by the movement of air by fans, the
fan blades moving in the structure, and the motors, gearboxes or drive belts.
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