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Cooling Towers for Refrigeration
An important device used in any refrigeration or air conditioning system is a condenser. A condenser is used in the high pressure side of a refrigeration or air conditioning system to convert the high-pressure vapour refrigerant from the compressor into liquid refrigerant. The medium used in a condenser may be water or air, depending upon the application. In the case of water cooled condensers, the warm water being pumped by the condenser should be cooled with the help of cooling towers so that the same water may be re-circulated to the condenser.
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Principle of Operation for Cooling Towers
The principle of operation of cooling towers is very similar to that of the evaporative type of condensers, in which the warm water gets cooled by means of evaporation. Water evaporates as a result of the hot water droplet coming in contact with the air (which is being pumped out by means of a fan). This evaporating water also absorbs the latent heat from the water surrounding it. By losing latent heat, the water is cooled.
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According to the method adopted to circulate the air, cooling towers may be classified as:
- Natural draft cooling towers
- Mechanical draft cooling towers.
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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.
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Mechanical Draft Cooling Towers
The mechanical draft cooling towers are very much similar to that of the natural draft cooling towers. As the name indicates, air is circulated inside the tower mechanically instead of natural circulation. Propeller fans or centrifugal fans may be used.
Advantages of mechanical draft cooling towers over natural draft cooling towers:
- For the same capacity used, the mechanical draft cooling towers are much smaller than the natural draft cooling towers. This is because of the increase in cooling capacity due to increase in volume of the air being forced out by fan.
- Capacity control is possible in mechanical draft cooling tower. By controlling the speed of the fan, the volume of air can be controlled, which in turn controls the capacity.
- The natural draft cooling towers can be located only in open space. As they do not depend upon the atmospheric air, the mechanical draft cooling towers shall be located even inside the building.
Disadvantages of using mechanical draft cooling towers:
- More power is required to run the system,
- Increased running cost due to increase in maintenance of the fans, motors and its associated controls,
According to the location of the fan, they are further classified as:
- Forced draft cooling towers, and
- Induced draft cooling towers.
Forced Draft Cooling Towers
In this system, fan is located near the bottom and on the side. This fan forces the air from bottom to top. An eliminator is used to prevent loss of water droplets along with the forced air.
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Induced Draft Cooling Towers
In this system, a centrally located fan at the top, takes suction from the tower and discharges it to the atmosphere. The only between the induced draft cooling tower and forced draft cooling tower is that the fan is located at the top in the induced draft cooling tower.
US DOE/Virginia Tech - Forced draft cooling tower
Princeton Engineering Services - Induced draft cooling tower
Refrigeration and Air-conditioning by R.S. Khurmi, J.K. Gupta