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FAN FUNDAMENTAlS
130
BELT DRIVE
One advantage of choosing a belt drive over a direct
drive is that it is capable of adjusting the fan rpm,
which enables the fan to move more air if necessary.
Motor pulleys are adjusted by loosening the set
screw and turning the top half of the pulley (see
illustrations at right). This causes the pulley diameter
to change, which results in changing the fan rpm.
Opening the pulley decreases fan rpm.
Closing the pulley increases fan rpm.
Belt
APPLICATIONS
Ventilating a building simply replaces stale or foul
air with clean, fresh air. Although the ventilation
process is required for many different applications,
the airflow fundamentals never change:
UNDESIRED AIR OUT, FRESH AIR IN
The key variables that do change depending on
applications are the fan model and the air volume
flow rate (cfm). Other considerations include the
resistance to airflow (static pressure or Ps) and
sound produced by the fan (Sones).
Occasionally, a customer will require a fan to perform
a particular function, yet does not know which model
to use or even what cfm is necessary. In this case,
some fan specification work must be done.
Fan specification is usually not a precise science
and can be done confidently when the fan
application is understood.
Based on the application, four parameters need to
be determined. They are:
Fan Model
Cubic Feet per Minute (cfm)
Static Pressure (Ps)
Loudness limit (sones)
MOTOR HORSEPOWER
The motor horsepower for direct drive fans is
always sized by Greenheck and does not require
further consideration. For belt drive models,
the catalog identifies which horsepower is
recommended. However, there are times when it
is wise to bump the horsepower one size. For
example, the hp recommended for the GB-180
at 810 rpm (2375 cfm @ .5” Ps) is 1/3 hp.
Although a 1/3 hp motor is recommended, it is
not necessarily a good motor selection for this
application. Our static pressure of 0.5 in. was only
an estimate. It may actually turn out to be .625 in.
If this is the case, we will need a 1/2 hp motor
because our fan will have to run at almost
810 rpm (refer to performance box - 2052 cfm at
0.625 in. Ps). Therefore, choosing a 1/2 hp motor
in this case is exercising good judgement.
The complete model designation for this
application is GB-180-5.
NOTE: The GB-180-5 has an rpm range of 700-940.
This means that if the static pressure is less than
estimated, say 0.25 in. Ps, the fan can be slowed down
to accommodate this condition.
The information that follows will help walk you
through this type of problem and enable you to
select the right fan for the job.
FAN MODEL
Fans all perform the basic function of moving air
from one space to another. But the great diversity of
fan applications creates the need for manufacturers
to develop many different models. Each model has
benefits for certain applications, providing the most
economical means of performing the air movement
function. The trick for most users is sorting through
all of the models available to find one that is
suitable for their needs.
PROPELLER vs. CENTRIFUGAL WHEEL
Propeller fans provide an economical method to
move large air volumes (5,000+ cfm) at low static
pressures (0.50 in. or less). Motors are typically
mounted in the airstream which limits applications to
relatively clean air at maximum temperatures of 110°F.
Centrifugal fans are more efficient at higher static
pressures and are quieter than propeller fans.
Many centrifugal fan models are designed with
motors mounted out of the airstream to ventilate
contaminated and high temperature air.
Introduction to FAN SELECTION