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.
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.
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 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.
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.
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.
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 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.
- In Oil refineries, Petrochemical Industries and other Chemical Plants for cooling the circulating water.
- In Thermal Power Stations, Nuclear Power Plants and HVAC systems.