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Natural Draft Cooling Tower

Natural Draft Cooling Tower

The natural draft cooling tower is an open, direct-contact system

It works using a heat exchanger, allowing hot water from the system to be cooled through direct contact with fresh air. To increase the heat transfer surface area (and optimize the cooling process), hot water is sprayed from nozzles within the tower. This increases both the temperature and humidity of the air in the tower. The warmer, moister air moves to the top of the tower, while the cold water is collected at the bottom. The fresh air supply is located in the bottom of the natural draft cooling tower to take advantage of the difference in density between the hot air at the top and the atmospheric air outside the cooling tower. 

Industries using the natural draft cooling tower

This "natural draft" type of cooling tower is mostly used in power stations, but is also found in energy-intensive facilities like oil refineries, petrochemical plants and natural gas plants. They are installed to remove heat from the circulating water system. These structures use the chimney effect, which means that they are shaped in such a way as to draw natural draft into the tower. Their structure is generally concrete or sometimes steel.  

Principle of the natural draft cooling tower

Air flow is obtained in natural draft cooling tower systems by way of the chimney effect of the cooling tower’s actual structure, which uses the natural pressure difference. Warm and moist air is less dense, which causes it to rise out of the cooling tower into the atmosphere and draw in denser fresh air. The difference between the warm air inside the tower and the cooler air outside creates the perfect air flow. For sufficient air flow to occur, a specific mathematical formula is used to calculate the height of the cooling tower to ensure it is almost as large as the density difference. This means cooling towers using this system tend to be large: around 200 meters tall and 150 meters in width. There is also a significant amount of water flowing in the towers. The shell itself is typically made from concrete in a hyperbolic shape. The natural draft cooling tower is the preferred choice for cool and humid climates and for heavy winter loads. 

Hot water that needs cooling in the natural draft cooling tower is pumped in via the hot water inlet. The inlet is connected to nozzles that spray the water over the fill material, which provides a large surface area for heat transfer. At the bottom of the tower, the structure is open to draw in fresh air, which then flows upward and allows for direct-contact heat transfer between the warm water and the air. The hot water releases heat after coming into direct contact with the fresh air, and some of the hot water is evaporated. Cold water is collected at the bottom of the tower. 

The warm and moist air is discharged from the top of the tower into the atmosphere.

Advantages of a natural draft cooling tower

Some advantages of natural draft cooling tower systems include power savings due to the absence of an electrical fan; no corrosion problems; low maintenance; and no recirculation of air because the stack outlet is located high up. This is useful in vertical plant situations where space is an important consideration. Hamon will do a full assessment of the site plot to determine the best use of the available land when helping to engineer, design, and build each unique cooling tower.


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