Types of Cooling Towers

Natural Draft: Natural draft cooling towers utilize no mechanical drivers or fans to create air flow through the cooling tower. This cooling tower takes advantage of the difference in ambient air densities below and above the tower. Air flow is created as the denser air at the bottom of the tower travels to a lower pressure area above the tower. These towers are inexpensive but can only be installed outdoors. Also, these towers have lower reliability as they are more affected by ambient wind and temperature changes.
Mechanical Draft: These towers use either single or multiple fans to generate air flow through the tower. Mechanical draft cooling towers are more reliable and stable than natural draft towers since air flow can be manipulated according to the cooling load required. Mechanical draft cooling towers can be further classified as forced or induced draft.
Forced Draft: As the name suggests, this type of cooling tower uses fans or blowers to force air into the cooling tower. Air flow has high entrance velocity as it is being forced by the blower. As it passes through the tower, air flow slows down. Thus, performance is less stable compared to induced draft towers due to recirculation. Forced draft cooling towers are used in indoor applications where high static pressure is a concern.
Induced Draft: These have their fans located at the top that draws (or induces) air from the air intake louvers at the bottom or sides of the tower. Contrary to forced draft cooling towers, this arrangement has low entrance and high exit velocity, which results in reduced recirculation. These types of cooling towers are widely used in industrial plants requiring stable performance.
Hybrid Draft: Its operation is the same as natural draft towers, but equipped with fans to augment air flow. Hence, they are also referred to as fan-assisted natural draft cooling towers. The fans in this setup have lower horsepower compared to forced and induced draft fans. Because of the additional draft, there is no need to construct a tall tower which may be economically impractical for a given application.
Air-to-Water Flow: This classification divides cooling towers into cross-flow and counter-flow cooling towers. Their difference is the method of how water comes into contact with the air stream.
Cross-Flow: In cross-flow configuration, air flows horizontally through the fills across the downward fall of water. A distribution basin distributes the hot water to evenly fall into the fill by gravity through nozzles or orifices. The action of gravity eliminates the need for a pressurized-spray system. Maintenance is easier for cross-flow cooling towers since the distribution system can be sectionalized and serviced separately, eliminating downtime.