3. AIR FLOW GENERATION METHODS
Cooling towers vary on how air flow is introduced in the system, this can be by natural, mechanical, cross flow or counter flow towers:
As their name implies, natural draft cooling towers rely on natural convection to circulate air throughout the tower, with no mechanical drivers to create the air flow.
They use the design and shape of the tower to move up the air naturally and takes advantage of the difference in ambient air densities below and above the tower.
Air movement occurs due to differences in density between the entering air and the internal air within the tower. Warm, moist air, which is more dense than cool air, will naturally rise through the tower, while the dry, cool air from outside will fall, creating a constant cycle of airflow.
These towers can only be installed outdoors and have a lower reliability than others as they are affected by wind and temperature changes.
Mechanical Draft towers tend to use a fan to circulate air inside the tower, common fans used include propeller or centrifugal fans.
These types of cooling tower are much smaller than natural draft tower and so can be located anywhere inside the building. Capacity control is easy in these types of towers as the speed of the fan can be controlled.
While mechanical draft towers are more effective than natural draft towers, they do consume more power than natural draft cooling towers and cost more to operate as a result. Mechanical draft cooling towers can be further classified as forced or induced 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. Forced draft cooling towers are used in indoor applications where high static pressure is a concern.
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. Differing 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.
The towers can also be classified as cross-flow and counter-flow cooling towers. Their difference is the method on how water comes into contact with the air stream.