Types of cooling towers used in power plants:-
Today we discussed about type of cooling towers are used in power plant . cooling towers are used for the cooling the water which is used in turbine condensate cooling purpose . Cooling towers are special direct-contact heat exchangers where the warm cooling water from the condenser is brought into direct contact with the relatively dry air. The heat-transfer rate depends on maximizing the contact area between the water and air and the length of time for this contact.
-All cooling tower designs have the following common features:
-An air circulation system.
-A water distribution or spray system.
-Packing or fill to maximize the contact between the water and air.
-A cooling water collection and discharge basin.
-Mist eliminators that minimize droplet carry-over and water loss.
Cooling towers are generally configured with the air and water in a counter flow or cross-flow arrangement. In counterflow units, water falls down through the fill while the air moves upward. In cross-flow units, water cascades downward while the air moves horizontally, which is perpendicular to the water flow. These cooling towers are
generally classified by the method used to move the air. These are commonly known as (1) mechanical-draft units and (2) natural-draft or hyperbolic units.
Mechanical-draft cooling towers.
Mechanical-draft cooling towers use either single or multiple fans to provide a known volume of air through the cooling tower. Therefore, their thermal performance is generally more stable and is affected by fewer air variables than with natural draft cooling towers.
Mechanical-draft cooling towers are either forced draft or induced draft:
1. Forced draft cooling tower. The fan is located in the ambient air stream entering the cooling tower, and the air is blown through the unit .
2. Induced draft cooling tower. The fan is located at the exit of the airflow and draws air through the tower.
Natural-draft hyperbolic cooling towers:-
The natural-draft hyperbolic:-
Cooling tower utilizes airflow that is produced by the density differential that exists between the heated air inside the tower, which is less dense, and the relatively cool ambient air outside
the tower, which is more dense.
Mechanical type cooling tower working much similar as natural type cooling tower. Its name indicates that in mechanical type cooling tower circulation is occurred mechanically. For circulation proper size fan or centrifugal fan are used
Mechanical type cooling tower is more beneficial than natural type cooling tower because of :-
• In the similar capacity steam power plant used, the mechanical type draft cooling towers are much smaller size than the natural draft cooling towers, in other word we can say mechanical type cooling tower is required less area for installation as compare to natural type cooling tower. the cooling capacity of mechanical type cooling tower is high because of more volume of air is drawn by the fan outside.
• in mechanical draft cooling tower capacity control is possible. By controlling the speed of the fan with the help of VFD , the volume of air can be controlled, which in turn controls the capacity. In other side we can’t control the capacity in natural type cooling tower these type of cooling tower are installed predetermined capacity during the power plant installation.
• For natural draft type cooling towers open area is required for installation. They do not depend upon the atmospheric air, but other hand the mechanical type draft cooling towers shall be located even inside the building.
Mechanical type draft cooling towers disadvantages:
1. For running the mechanical type cooling tower fan more electric power is required other than natural type cooling tower .
2. Maintenance cost is high for mechanical type cooling tower because cooling tower fan and motor running cost is high other than natural type cooling tower .
Classification of natural draft type cooling tower
1. Spray type natural draft type cooling tower
2. Splash deck type natural draft type cooling tower
1.Spray type natural draft type cooling tower
The whole system is located inside a housed box-shaped structure which are contains spray headers, spray nozzles, and louvers. The louvers (usually made of steel) are placed on the sides to enhance natural circulation of air inside the cooling tower. To prevent the carryover of water droplets to the atmosphere, the louvers are slanted towards the inside. Usually these types of cooling towers are located outside the building, so that the air can pass freely through the tower
2.Splash deck type natural draft type cooling tower
Condensate hot water from the condenser is fed into the spray header by means of a CEP ( condensate extraction pump ) pump . The spray header is located on top of the cooling tower. The spray nozzles spray the hot water inside the tower. Air from the atmosphere comes in contact with the hot water, so that causing some water droplets to evaporate. Latent heat gains by the evaporated water droplets from the surrounding water, which causes the remaining water to cool. Some amount of sensible heat is absorbed by passing air from the warm water. A make-up line continuous feed the water may be used to make up the loss of water due to evaporation. The cooled water taken back again into the condenser.
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