With respect to drawing
air through cooling tower, there are three types of cooling towers:
1) 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.
2) Mechanical
Draught: Uses power-driven fan motors to force or
draw air through the tower.
a)
Induced Draught: A
mechanical draft tower with a fan at the discharge (at the top) which pulls air
up through the 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. This fan/fin arrangement is also known as draw-through.
b) Forced Draught: 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 as blow-through.
3)
Fan Assisted Natural Draught: A
hybrid type that appears like a natural draft setup, though airflow is assisted
by a fan.
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