Natural Draft Cooling Towers

Natural Draft Cooling Tower

Natural draft cooling towers are alternatives to the widely used mechanical draft cooling tower and are made of large concrete structures that use natural convective airflow to cool water. These systems are large, expensive and only used for water flow rates above 200,000 gallons per minute. Natural draft cooling towers are typically about 400 feet tall and instead of using fans, depend on rising warm air to create airflow. Sometimes referred to as hyperbolic cooling towers, they are cooling systems used mainly in large power utility companies. Their water source comes from a large body of nearby water such as a lake. These cooling towers make use of the temperature difference between ambient air and air inside the tower, since the warm, denser water vapor rises, leaving the cooler air at the bottom of the tower.

At the base of a natural draft cooling tower, thousands of nozzles spray water. Because warm, moist air naturally rises, the cool air stays at the bottom and is drawn into the tower through an inlet at the base of the concrete shell. There are two types of natural draft cooling systems—cross flow towers and counter flow towers. Cross flow towers draw the air across falling water and the fill is located outside the tower, while in counter flow towers, the air is drawn up through falling water and the fill is located inside the tower. Fills are made of plastic or wood and facilitate the heat transfer by maximizing water and air contact.