Air flow generation methods

With respect to drawing air through the tower, there are three types of cooling towers:

• Natural draft — Utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential compared to the dry, cooler outside air. Warm moist air is less dense than drier air at the same pressure. This moist air buoyancy produces an upwards current of air through the tower.
• Mechanical draft — Uses power-driven fan motors to force or draw air through the tower.
  • Induced draft — A mechanical draft tower with a fan at the discharge (at the top) which pulls air up through the tower. The faninduces hot moist air out the discharge. This produces low entering and high exiting air velocities, reducing the possibility ofrecirculation in which discharged air flows back into the air intake. This fan/fin arrangement is also known as draw-through. (see Image 3)
  • Forced draft — A mechanical draft 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 horsepower than an equivalent induced draft design. The benefit of the forced draft design is its ability to work with high static pressure. Such setups can be installed in more-confined spaces and even in some indoor situations. This fan/fill geometry is also known asblow-through. (see Image 4)
• Fan assisted natural draft — A hybrid type that appears like a natural draft setup, though airflow is assisted by a fan.

Hyperboloid (sometimes incorrectly known as hyperbolic) cooling towers (Image 1) have become the design standard for all natural-draft cooling towers because of their structural strength and minimum usage of material. The hyperboloid shape also aids in accelerating the upward convective air flow, improving cooling efficiency. These designs are popularly associated with nuclear power plants. However, this association is misleading, as the same kind of cooling towers are often used at large coal-fired power plants as well. Conversely, not all nuclear power plants have cooling towers, and some instead cool their heat exchangers with lake, river or ocean water.