(C)ooling Tower
COOLING
TOWER:
Cooling towers
are a special kind of heat exchanger
devices where air and hot water
are brought into direct contact
with each other in order
for the evaporative cooling to take place and the temperature of the water to drop due to it.
During evaporative cooling process the air has to be unsaturated so that it can store the evaporated water vapours and for the water to change its phase from liquid to gas, latent energy is required which the water absorbs from the surrounding.
Common applications include cooling the circulating water used in oil
refineries, petrochemical and other
chemical plants, thermal power stations, nuclear power stations and HVAC systems for cooling buildings etc.
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Working of a cooling tower:
The working principle of a cooling tower
is evaporative cooling.
The hot water is input in the equipment from the top of the cooling tower.
The hot water is flows
through a pipe and is made to pass through spray nozzles. The water flows
in the tower as droplets. The advantage of doing this is that
the surface area of contact
of air water increases. The water falls down due to the influence of gravity.
The air is input in the equipment
through the bottom of the cooling tower.
The water falls on the fill
material. The fill materials are the zone where the intimate contact
of air and hot water occurs. The fill materials
are also sometimes
called packings or baffles.
The purpose of the fill material is to increase the effective surface area
which is responsible for heat transfer.
After the hot water
and air come
in contact, evaporative cooling takes place
which results in decrease in temperature of the water
and increase in temperature of the
air.
The cooled water is collected from the bottom of the cooling tower and the warm air rises and moves out from the top of
the cooling tower.
Types of
cooling towers:
Air Draft cooling towers are classified as
Atmospheric towers, Natural draft
towers and Mechanical draft towers.
The Mechanical draft towers are further
classified into Forced draft type and Induced draft type.
1.
Atmospheric Cooling Towers
This is the simplest type of cooling tower. It consists of a rectangular chamber which has louvered walls on the opposite end. The louvers are devices which act as barrier for splash out and sunlight, also the louvers give a direction to the flowing wind. The atmospheric air enters the cooling tower depending on the speed of the wind guided by the louvers. The tower is filled with fill material on which contact of air and water takes place. They are cheap but inefficient because the performance is largely driven by wind velocity and direction.
2.
Natural Draft Cooling Towers
A natural draft tower is also called hyperbolic
tower because it is a large concrete structure of hyperbolic shape. The tower
is called so because the movement of air happens naturally inside the tower.
The tower consists of a spray mechanism beneath
which alternative layers
of battens are stacked along with the packing material.
The air enters from the bottom and the water gets
splashed when it touches battens and forms the shape of droplets. These
droplets result in a good heat transfer area. The rise in temperature of the air and humidity
reduces its density due to which it becomes relatively
more buoyant and rises naturally. Drift
eliminators are placed in the tower to catch the
liquid droplets which may be entrained with the air.
3. Forced Draft Cooling Towers
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4.
Induced Draft Cooling Towers
Induced draft towers are designed to eliminate the re-circulation effect which happens in forced draft towers. The fans in induced draft towers are placed at the top of the cooling tower. The space for air inlet is made at the bottom of the tower. When the fan operates, suction is created at the bottom of the tower due to which the flow of air is induced.
In the counter
flow mechanism of the process,
the power required
to move air will be high because the air entry to the packing will face
the maximum resistance due to the liquid exiting the packing. Induced draft
towers can be operated in concurrent flow, counter current flow and cross flow.
Cooling
towers in industries:
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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 for other industrial facilities
such as in condensers of distillation columns, for cooling
liquid in crystallization, etc.
References: -
1. Chemical
Engineering World
2. Wikipedia
3. YouTube
https://www.youtube.com/watch?v=ZzEHoMvzErY
https://www.youtube.com/watch?v=Gouq8epjX5Y
https://www.youtube.com/watch?v=15gy525eDhE
Written
by:
- Nipen kumar sahoo
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