Natural Draft Cooling Towerson 24/2/2012 & Updated on 30/7/2018
Generally, we use cooling towers in power stations, oil refineries, petrochemical plants, and natural gas plants to remove heat from circulating water system. The below-shown diagram represents the process of cooling towers. Based on the type of air flow into the cooling tower they are classified into two types
- Natural Draft Cooling Towers
- Mechanical Draft Cooling Towers
Natural Draft Cooling TowersIn natural draft cooling towers, air flow is obtained by pressure difference obtained from its structure i.e. chimney effect. Warm and moist air (less dense) after heat transfer will go out of the cooling tower to the atmosphere, creating to flow in fresh air (denser). The flow of air occurs due to the density difference between the warm (less dense) air inside the cooling tower and relatively cool (denser) ambient air outside the cooling tower. Pressure head developed due to the density difference between the cold air and warm air is given by formula, Where H is the height of the tower above the fill, ρo is the density of outside air, ρi is the density of inside air, g is the gravity constant.
In order to provide sufficient airflow, the height of the cooling tower H must be large as the density difference (ρo - ρi) is not quite large. These towers tend to be very large both in height (around 200 meters) and in cross-section so that the amount of water to flow in each tower is also large. Cooling towers are in generally cylindrical (made of wood) or hyperbolic (made of concrete) shape. The hyperbolic shape of the cooling tower made of a concrete shell which is critical in heat transfer process occurring within the cooling tower. These hyperbolic shape cooling towers offer superior structural strength and resistance to high ambient wind loadings than the cylindrical shape. Natural draft cooling towers are generally preferred in
- Cool and humid climates (low wet-bulb temperature and high relative humidity)
- Heavy winter loads
- No electrical fan is installed so power saving
- No corrosion problems
- Maintenance is low
- No recirculation of air occurs due to high stack outlet
- Huge water flow required.
- These require a large area.
- Its performance depends on wind velocity and direction.