It’s The Cool Thing to Do

The cooling tower; it’s the icon of the nuclear world. It’s also mistakenly thought to be spewing pollution. It’s actually water vapor (or steam).  Let’s take a look at how cooling towers produce the water vapor you see.

Thermoelectric power plants, whether fossil-fueled or nuclear, require cooling water systems. The fuel heats water, turning it into steam, which drives a turbine generator that produces the electricity. The steam, once used to turn the turbine, can be recycled back into water and reused. To reuse this steam, it is cooled and condensed back into water.

View animated image of a Pressurized Water Reactor

Most power plant use one of two types of cooling systems:

  • Once-through system – Water is drawn from a water source such as a lake, river, or pond. This water passes through a condenser where it absorbs the heat from the steam and is returned to where it came from, but at a higher temperature.
  • Closed loop cooling system – Condenser cooling water (in darker blue in the link above)  is circulated to remove the extra heat is has gained. The water is pumped to the top of the cooling towers and is allowed to pour down through the structure. At the same time, a set of fans at the top of each tower pulls air up through the condenser water. This lowers the temperature of the water by about 24 degrees. After it is cooled, the condenser water flows back into the turbine building to begin its work of condensing steam again.  Water from a lake, river or pond is added as needed to compensate for evaporation. 

How Cooling Towers Work

Cooling of the water is achieved through direct contact of the water with air. Water vapor is a byproduct of the cooling process within the cooling tower.  During this phenomenon, which is widely known as evaporative cooling, heat is transferred from water to air. This heat is then rejected to the atmosphere, either through the use of fans or natural convection. Here’s how it works:
 
Mechanical Draft Cooling Towers

With mechanical draft cooling towers, the incoming condenser cooling water is sprayed throughout the tower’s interior. The spray flows downward to baffles that maximize the time of water contact with the air.  Air enters the interior of cooling tower through properly designed “louvers.” Air is drawn through the baffled area by large fans which reduce the water temperature.  

Diagram of a mechanical draft cooling tower

Natural Draft Cooling Towers

In a natural draft cooling tower, air enters the shell of the tower through louvers at the tower’s base.  Water is typically sprayed around the periphery of the cooling tower and cascades to the bottom. As the air inside the tower is heated, it becomes lighter and starts rising through the tower. This process draws more air through the louvers along the tower’s base, creating a continuous natural air flow. Continuous air circulation is accomplished due to the density difference between the warmer air inside and the cooler air outside.

Natural draft or convection cooling towers have a much bigger footprint compared to mechanical draft cooling towers and also cost much more to build. Due to these reasons, natural draft cooling towers are built less frequently compared to other types.

Diagram of a natural draft cooling tower