Cooling Towers in Thermal Power Plants

After air, water is the most important requirement for life on this planet. With fresh water resources depleting and increased population pressure, water sources have become very precious. The primary priority for fresh water is for human consumption and agriculture. Power plant requirements are only secondary. This necessitates the need for thermal power plants that require less water. Cooling Towers help by reusing the cooling water, making power plants economical and more environmentally friendly.

Environmental regulations in most countries require that fresh water sources like lakes or rivers cannot be used anymore for an open cycle system. This makes using cooling towers the only option. Open cycle systems can only use seawater.

Cooling towers can be of two types.

• First is the natural draft-cooling tower with a large hyperbolic tower, which pulls in air due to the stack effect. Even though the capital costs are high, operating costs are less.This is because there is no fan to create the air flow.
• However, most commonly used is the Mechanical or forced cooling tower. A fan forces or sucks air through the cooling tower where the water falls through a packed heat transfer media. Operating costs are high for operating this, but they are simple and quick for construction.
  • The fans can be induced fan or forced draught fan.
  • The airflow can be parallel or cross flow to the water flow.

Cooling towers work on the principle of psychometric properties of air. A part of the cooling water evaporates taking in Latent Heat from the water reducing its temeprature. A properly sized Cooling tower can cool the incoming water to temperatures up to 3 °C more than the ambient wet bulb temperature. The relative humidity of the ambient air is an important deciding factor.

This makes it very important that the location and direction of the towers is such as to prevent re-entrainment of the plume exiting the tower and interference from the adjacent towers. Wet air if it reenters the tower reduces the cooling capacity of the towers.

Losses in cooling Towers.

The loss of water in a cooling tower is due to three different reasons and has to be made up during the operation. The makeup percentage in modern towers is around 1 %.

• Evaporation loss. A part of the water evaporates, this is what creates the cooling effect. This depends on the ambient temperature and Relative humidity or the ambient wet bulb temperature.
• Drift loss. The water particles carried away through the flowing air. Drift eliminators and detail design have reduced this largely. In modern towers, this could be in the range of 0.02 % of the water flow.
• Blowdown losses. The evaporated water leaves behind the salts, which over time accumulates increasing the TDS levels. This requires to be blowdown occasionally. This constitutes a loss which has to be made up.

Biocides and chemical controls are required to eliminate bacterial growth and eliminate scales that are harmful and at the same time a performance reduction factor.