Natural Draft Cooling Towers

A cooling tower is an open direct contact type heat exchanger where hot water from system or condenser gets cooled by direct contact with fresh air. Cooling towers use the principle of evaporation of water against the air flow. Hot water is sprayed from the nozzles for increasing the heat transfer surface area. The temperature and humidity of the air get increased after direct contact heat transfer between hot water and fresh air. The warm and moist air being less dense goes to the top of the tower, and cold water gets collected at the bottom of the tower. Fresh air is supplied from the bottom of the cooling tower due to the density difference between hot air inside the stack and atmospheric air outside the cooling tower.

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

1. Natural Draft Cooling Towers
2. Mechanical Draft Cooling Towers

Natural Draft Cooling Towers

In 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,
ρ_o is the density of outside air,
ρ_i 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 (ρ_o – ρ_i) 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

1. Cool and humid climates (low wet-bulb temperature and high relative humidity)
2. Heavy winter loads

   Working Principle of Natural Draft Cooling Towers

   The hot water that is to be cooled in the cooling tower is pumped to the top of the cooling tower at the hot water inlet. The hot water inlet is connected to a series of nozzles which spray this water over the fill material (provides a large contact surface area for heat transfer). Fresh air is induced by the open structure at bottom of the tower and then the air flows in the upward direction for direct contact heat transfer between warm water and air. The hot water liberates heat after direct contact with fresh air and some of the water gets evaporated and cold water gets collected at the bottom of the tower. Warm and moist air will be discharged from the top of the tower into the atmosphere.

   Advantages of Natural Draft Cooling Towers

   1. No electrical fan is installed so power saving
   2. No corrosion problems
   3. Maintenance is low
   4. No recirculation of air occurs due to high stack outlet

   Disadvantages Natural Draft Cooling Towers

   1. Huge water flow required.
   2. These require a large area.
   3. Its performance depends on wind velocity and direction.