COOLING
TOWERS
WHAT IS A
COOLING TOWER ??
In simple words as the name indicates
Cooling Towers are towers used for cooling purposes, usually cooling of hot
water. Cooling towers are tall and large diameters cylindrical or hyperbolic
concrete towers with an open top mainly used for cooling hot water with the
help of cold air. This cold water from cooling towers is then used for cooling other
industrial hot process fluids.
PRINCIPLE :-
In a Cooling Tower, the process hot
water is sprayed through the nozzles from the top of the tower where as the
cold air enters into the tower from the bottom. The hot water and cold air
comes in to contact with each other due to the fill or packings present inside
the Cooling Tower just above the air entry. These packing provide a large interfacial area
for heat transfer from hot water to cold air.
CONSTRUCTION
:-
A Cooling Tower may be Cylindrical or
Hyperbolic in shape. Hyperbolic shape offers superior structural strength and
resistence to ambient wind loadings.
A COOLING TOWER consists of following main
parts :
HOT WATER
INLET
The hot water inlet is generally provided at
the top of the tower, below the Drift Eliminators and above the fill. It is
connected to a series of nozzles that sprays the water downwards on the fills
present.
NOZZLES
Nozzles are connected to Hot Water Inlet and are
present above the fill. Uniform water distribution at the top of the fill is
very much essential to achieve proper wetting of the entire fill surface. Nozzles sprays water from the top of the fill
to uniformly wet the entire fill.
COLD AIR
INLET
The cold air inlet is generally provided at
the bottom of the tower, above the cold basin and below the fill. The cold air
enters into the towers through this.
LOUVERS
The Louvers are present at the Cold Air
Inlet. The purpose of louvers is to equalise the air flow into the fill. Also
they help in retaining the water within the Coling Towers. They are mainly used
when the flow of water and air is crossflow. Many counter flow towers do not
require Louvers.
FILL
Fills are present inside the tower below the
nozzles to provide a large surface area and time for air and water contact . Most
towers have fills made of plastic or wood as it facilitate good heat transfer
by maximising water and air contact.
Fill can either be splash or filmtype. In splash fill,
water falls in successive layers and is continuously broken it into smaller
droplets, thereby wetting the entire fill surface.
Plastic splash fill provides better heat transfer
than the wood splash fill.
Film fill consists of thin, closely spaced plastic
surfaces(PVC) over which the water spreads uniformly forming a thin film which
is in contact with the air. These surfaces may be flat, corrugated, honeycombed,
or other patterns. The film type of fill is more efficient than the splash fill
and provides same heat transfer in a smaller volume.
DRIFT
ELIMINATORS
Sometimes it happens that the droplets of
water are carried out of the cooling tower along with the hot exhaust air. The concentration
of impurities in these droplets is same as that in the water entering the tower.
These may lead to environmental pollution. To reduce this baffle-like devices, called as
Drift Eliminators are installed above the nozzles. Drift Eliminators allow only
the air to pass through it thereby removing water droplets from the air.
COLD WATER BASIN
The cold water basin, is located at the bottom of the
tower. The cooled water that flows down
through the tower and fill is collected by the Cold Water Basin. These
collected cold water is then discharged and used for cooling other hot process
fluids .
FAN
Fan may be located at the top or bottom of the
tower. Both axial (propeller type) and centrifugal fans are used in Cooling
towers. Fan present at the top of the tower sucks the air out of the tower
whereas the fan present at the bottom of the tower blows the air into the tower.
WORKING OF COOLING TOWERS :-
The water that is to be cooled in the Cooling Tower
usually has a temperature between 40 and 60°C. These hot water is pumped to the
top of the cooling tower at the hot water inlet. The hot water inlet is
connected to a series of nozzles which sprays this water over the fills. Simultaneously
air is introduced from the bottom of the tower which flows in upward direction.The
fills provide a large surface area and time for air and water contact .
The water then flows down through plastic or wooden fills. This causes drop
formation. While flowing down, the water emits heat which mixes with the air
flowing upward, causing it to cool down to 10 to 20 ˚C. Part of the water
evaporates, causing it to emit more heat. Water vapor can sometimes be observed
over the cooling tower. The cooled water falls down into the cold water basin which
is discharged and can be then used for cooling various hot process fluids.
TYPES
OF COOLING TOWERS :-
There are two types of Cooling Towers, they are :
- Natural Draft Cooling Towers
- Mechanical Draft Cooling Towers
- INDUCED Draft Cooling Towers
- FORCED Draft Cooling Towers.
APPLICATIONS
:-
- In Oil refineries, Petrochemical Industries and other Chemical Plants for cooling the circulating water.
- In Thermal Power Stations, Nuclear Power Plants and HVAC systems.
