Wednesday, March 27, 2019


The cooling tower is a device used to cool down the working fluid with the aid of air. It is categorized into heat rejection devices, with the sole purpose of transferring heat from the working fluid to the cooling medium (air) through the thermodynamic heat transfer process (mainly convection).

The process of heat transfer takes place mainly by convection when the cold air passes over the warm water. Since the rate of heat transfer is proportional to the surface area of contact (Q=hAdT for convection), the working fluid is sprayed into fine droplets to increase the heat transfer by increasing the area of contact. This is done with the help of spray nozzles/sprinklers and fills. More fine droplets mean better heat transfer and better cooling. The other factor which affects the cooling rate is the time of contact. As the time of contact increases, better cooling is achieved. There are two methods of attaining proper airflow through the cooling tower. One is the natural method (Natural draught cooling tower) and the other is by forcing the air by mechanical means (Mechanical draught cooling towers).

Classification of Cooling towers:
1.   According to air to water flow
a.    Crossflow Cooling tower
Crossflow cooling towers have water flowing vertically downwards where the cooling air will be passing by the water horizontally. It allows the use of gravity flow basins where the gravity can be utilized for water flow, as the air flow direction is not opposing the water flow.
Identification: The fills will be packed on the sides of the cooling tower where the center portion will be left hollow for air flow.

b.   Counterflow cooling tower
As the name suggests, the air flows in the opposite direction of water flow. This necessitates the requirement of the pressurized water distribution system. Pipelines and spray nozzles are provided for the same.
Identification: The fills will be packed across the air flow, where the sides will be left open for air intake.
PS: Do not get confused with the fills and air filters. Air filters are kept at the air inlet side to filter out dust from the air. The fills are used to provide maximum contact time and area between water and air.
Cooling towers without or having damaged air filters will have a greater possibility of fill choking as the dust in the air will turn to mud when it comes in contact with water.

2.   According to draught
a.    Natural draught cooling tower
The draught (current of air) in natural draught cooling towers is obtained by the density difference between warm and cold air. The hot air flows upwards due to the lighter density which allows the cool air to enter inside through the space provided at the bottom.
The tall stack acts as a nozzle in maintaining a negative pressure at the top to facilitate better airflow. Taller the stack, more air velocity at the top, creating more negative pressure, aiding better airflow in the stack.

b.   Forced draught cooling tower
The air is forced into the cooling tower with the aid of the fan. The air forced inside flows through the fills and then, out of the cooling tower. A positive pressure acts inside the cooling tower. The fans will be mounted at bottom of the cooling tower.

c.    Induced draught cooling tower
The air is sucked through the cooling tower using a fan generally mounted at the top of the cooling tower. Negative pressure acts inside the cooling tower. The air after passing through the fills, sucked by the fan, is pushed out of the cooling tower.
The forced and induced draught are together called as mechanical draught cooling towers as the airflow is achieved by means of mechanical energy.

Major components of cooling tower
1. Fill: 
    Fill is one of the major components of the cooling tower. As we know, the cooling takes place when the heat from the water is carried away by air. The better cooling is achieved when the contact area and contact time between the two are more. The fill assists in this purpose.
There are two types of fills, splash type fill, and film type fill. As the name suggests, the water is broken into fine droplets in splash type fill and in film type, the water is spread into a thin film to facilitate the heat transfer process.

2. Nozzles: 
    These are used to form fine droplets of water to improve the heat transfer rate. Different arrangements are available for the various types of cooling towers.

3. Drift eliminators: 
  These are used to save the process water drifting out with the air. Drift eliminators act like an obstruction in the air flow path, assisting in separating the suspended water droplets from cooling air. The use of drift eliminators will help in reducing the consumption of make-up water, but the pressure drop incurred across the same will have a negative impact on cooling efficiency.

4.  Fans: 
   These are used only in mechanical draught cooling towers where mechanical energy imparted by the fan is used for getting the required air circulation. The fans are available in different designs. These fans will be driven by a motor, coupled either directly or through a gear reducer.

5. Cold water basin/sump: 
   The cold water basin/sump is where the cooled water is collected for further circulation. The sump designs are critical for the performance of cooling tower pumps. These basins are designed as per the Hydraulic Institute standards to avoid any adverse hydraulic effects on the pump. Pump bays will be a part of the basin design. The basin should be strong enough to withstand the water load and weight of tower support structures including drives.

6.  Header pipes and hot water distribution basins: 
    Header pipes are used in counterflow type cooling towers where the water to be cooled is supplied under pressure, sprayed through nozzles on to the fills.
Hot water distribution basins are used in cross flow type cooling towers where the water from the basin flows down through the nozzles by gravity.


No comments:

Post a Comment