Cooling Tower Types
Natural draft, utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential to the dry, cooler outside air. This moist air buoyancy produces a current of air through the tower.
Hyperboloid (aka hyperbolic) cooling towers have become the design standard for all natural draft cooling towers because of their structural strength and minimum usage of material. The hyperbolic form is popularly associated with nuclear power plants, however, this association is misleading, as hyperbolic natural draft cooling towers are often used at large coal-fired power plants as well.
Mechanical draft, which uses power driven fan motors to force or draw air through the tower.
Induced draft Cooling Tower Design: A mechanical draft cooling tower with a fan at the discharge which pulls air through tower. The fan induces hot moist air out the discharge. This produces low entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intake.
Forced draft Cooling Tower: A mechanical draft cooling tower with a blower type fan at the intake. The fan forces air into the tower, creating high entering and low exiting air velocities. The low exiting velocity is much more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. Another disadvantage is that a forced draft design typically requires more motor power than the equivalent induced draft design. The forced draft benefit is its ability to work with high static pressure.