Natural Draft Cooling Tower
As the name indicates, the air is circulated inside the cooling tower by natural convection. The natural draft cooling towers are further classified as:
1. Natural draft cooling towers spray type, and
2. Natural draft cooling towers splash deck type.
The entire system is housed inside a box-shaped structure which also accommodates 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 carry over 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.
The sketch shown below explains about the spray type of cooling tower.
Warm water from the condenser is fed to the spray header by means of a pump. The spray header is located on top as shown in the sketch. The spray nozzles spray the warm water inside the tower. Air from the atmosphere comes in contact with the warm water, thereby causing some water droplets to evaporate. The evaporating water also absorbs some amount of latent heat from the surrounding water, which causes the remaining water to cool. The passing air also absorbs some amount of sensible heat from the warm water. A make-up line, which may be controlled by a simple float, may be used to make up the loss of water due to evaporation. The cooled water may be then taken back to the condenser.
The size of the spray plays a vital role. If the spray is too fine, a greater amount of water will be taken away by the air. On the other hand, if the size of the spray is too large, the area of contact of water with the air will be reduced.
SPLASH DECK TYPE
This type of cooling tower is very similar to that of the spray type. Instead of a spray header, a water box is used. The water box has small holes at the bottom. It also contains decking inside the tower. The hot water from the condenser enters into the water box and splashes via holes in the water box on the decking. The main objective of the decking is to increase the surface area of contact of air with the warm water. This type of cooling tower is 20-30% more effective than the spray type.