Cooling Tower Theory

Cooling Towers

Cooling Tower can be catagorized 2 systems as follows
1. Natural draft system
2. Mechanical draft system
Natural draft system uses natural motion of the air when heated. Hot air will naturally and vertically move upward as shown in figure 1. Hot water is sprayed from the top. The more water distributed over exchange surface, the more heat exchange and heat transfer will happen. The water distributuion depends upon design of water spary nozzles and air velocity (or wind speed). Hence efficiency of this type of cooling tower is very much reliance on wind speed. Generally this cooling tower is always installed in open field where wind blows well. This type of cooling tower is for required huge heat transfer or removal such as power plant etc.

Figure 1

Mechanical draft system requires fan to create air motion. It can make more heat transfer and reduce cooling tower sizes. There are two catagories as follows
  • Forced draft cooling tower which fan installed at air inlet (Dry stream side)
  • Induced draft cooling tower which fan installed at air outlet (Wet stream side) 
See figue 2 and 3 respectively

Figure 2


Figure 3

Since this type of cooling tower is with fan, it can be installed in restricted area (Unneccessory to be open field). If compare same size of cooling tower, mechanical draft system will have higher efficiency than natural draft system. Mechanical draft system can be catagorized into two types as per flow direction of cooling water and air stream.
1. Crossflow Cooling Tower
Cooling water moves vertically downward along the filler while air moves horizontally and perpendicularly against water direction. It creates crossflow interaction. Benefit is less air flow resistance creating low pressure drop. Lower fan horse power is required. However heat transfer efficiency is moderate
See Figure 4

Figure 4

2. Counterflow Cooling Tower
Cooling water is sprayed downward and air flow is upward. it creates counterflow interaction. Since air stream moves against water direction, air flow resistance is relatively higher than crossflow. Hence air pressure drop is also higher and fan horse power is required more. But efficiency is better.
See Figure 5

Figure 5

Preventive Maintenance

Spare Parts Description
 Fan Blade & Hub →Check if any nuts loosened out
→Check fan blade condition if any crack, bend or twist
 Gear Reducer →Check if any abnormal noise 
→Check if any oil leak at drive shaft, motor flange or gear housing
→Check oil level and see if any water content
 Filler →Clean with high pressure water jet if dirty
 Water Basin →Clean with plastice brush if moss or algae found
 Tower Wall →Clean with plastice brush both inside and outside if found dirty
 Tower Structure →Structure is made of hot dip galvanized steel, it can rust after some time. Regular check may required. If rust found, it should be cleaned and repainted. Check also bolts and nuts. Replace them if rusted.