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Industrial
cooling towers can be used to remove heat from
various sources such as machinery or heated
process material. The primary use of large,
industrial cooling towers is to remove the heat
absorbed in the circulating cooling water
systems used in power plants, petroleum
refineries, petrochemical plants, natural gas
processing plants, food processing plants,
semi-conductor plants, and other industrial
facilities. The circulation rate of cooling
water in a typical 700 MW coal-fired power plant
with a cooling tower amounts to about 71,600
cubic metres an hour (315,000 U.S. gallons per
minute)[1] and the circulating water requires a
supply water make-up rate of perhaps 5 percent
(i.e., 3,600 cubic metres an hour).
If that same plant had no cooling tower and used
once-through cooling water, it would require
about 100,000 cubic metres an hour [2] and that
amount of water would have to be continuously
returned to the ocean, lake or river from which
it was obtained and continuously re-supplied to
the plant. Furthermore, discharging large
amounts of hot water may raise the temperature
of the receiving river or lake to an
unacceptable level for the local ecosystem. A
cooling tower serves to dissipate the heat into
the atmosphere instead and wind and air
diffusion spreads the heat over a much larger
area than hot water can distribute heat in a
body of water.
Some coal-fired and nuclear power plants located
in coastal areas do make use of once-through
ocean water. But even there, the offshore
discharge water outlet requires very careful
design to avoid environmental problems.
Petroleum refineries also have very large
cooling tower systems. A typical large refinery
processing 40,000 metric tonnes of crude oil per
day (300,000 barrels per day) circulates about
80,000 cubic metres of water per hour through
its cooling tower system.
With respect to the heat transfer mechanism
employed, the main types are:
Wet cooling towers or simply cooling towers
operate on the principle of evaporation.
Dry coolers operate by heat transfer through a
surface that separates the working fluid from
ambient air, such as in a heat exchanger,
utilizing convective heat transfer. The do not
use evaporation.
Fluid coolers are hybrids that pass the working
fluid through a tube bundle, upon which clean
water is sprayed and a fan-induced draft
applied. The resulting heat transfer performance
is much closer to that of a wet cooling tower,
with the advantage provided by a dry cooler of
protecting the working fluid from environmental
exposure.
In a wet cooling tower, the warm water can be
cooled to a temperature lower than the ambient
air dry-bulb temperature, if the air is
relatively dry. (see: dew point and
psychrometrics). As ambient air is drawn past a
flow of water, evaporation occurs. Evaporation
results in saturated air conditions, lowering
the temperature of the water to the wet bulb air
temperature, which is lower than the ambient dry
bulb air temperature, the difference determined
by the humidity of the ambient air.
To achieve better performance (more cooling), a
media called fill is used to increase the
surface area between the air and water flows.
Splash fill consists of material placed to
interrupt the water flow causing splashing. Film
fill is composed of thin sheets of material upon
which the water flows. Both methods create
increased surface area. |
