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BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
FAN TYPES
Two main types of fans are used in airflow systems centrifugal
and axial:
Centrifugal Fans: A centrifugal fan (Fig. 4) has airflow within
the wheel that is substantially radial to the shaft (or
away from the axis of the shaft). The air from an in-
line centrifugal fan does not have to turn before being
expelled from the fan housing. Some centrifugal fan
designs are differentiated by the inclination of the
blades. Each blade design has a peculiar advantage:
Photo Courtesy of the Trane Company
Fig. 4. Centrifugal Fan.
— Backward inclined blades are generally larger
and more quiet than forward inclined blades.
They are more suitable for larger sizes.
— Forward inclined blades are suitable in small
packaged units and operate at a lower static
pressure.
— Air foil blades are backward inclined, and are
efficient and quiet due to an air foil shaped blade.
Generally these are used on the largest fans.
Axial Fans: An axial fan has airflow through the wheel that is
substantially parallel to the shaft (or along the axis of
the shaft). Various designs of axial fans are available
(Fig. 5), mainly differentiated by the duty of the fan.
Each design has a peculiar advantage:
A.
PROPELLER FAN
TUBEAXIAL FAN
Fig. 5. Types of Axial Fans.
— Propeller fans are low pressure, high airflow,
noisy fans. They work up to a maximum static
pressure of 190 Pa.
— Tubeaxial fans are heavy-duty propeller fans
arranged for duct connection. They discharge air
with a motion that causes high friction loss and
noise. They work up to a maximum static
pressure of 0.75 kPa.
— Vaneaxial fans are basically tubeaxial fans with
straightening vanes added to avoid spiraling air
patterns. They are space efficient, quieter than
tubeaxial fans, and work at static pressures up to
2.5 kPa.
FAN PERFORMANCE TERMS
The following are terms used when discussing fan
performance:
Fan volume: The airflow passing through the fan outlet.
Generally this fan outlet value is only slightly less
than the airflow at the fan inlet because specific
volume changes due to air compression are small.
Fan outlet velocity: The fan volume divided by the fan outlet
area. This velocity is a theoretical value because, in
reality, the velocity pattern at the outlet of a fan is not
easy to measure.
Fan Static Pressure (FSP): The fan total pressure minus the
fan velocity pressure (FSP = FTP – FVP). It can be
calculated by subtracting the total pressure at the fan
inlet from the static pressure at the fan outlet.
Fan Total Pressure (FTP): The difference between the total
pressure at the fan inlet and the total pressure at the
fan outlet. The FTP value measures the total
mechanical energy added to the air by the fan.
Fan Velocity Pressure (FVP): The velocity pressure
corresponding to the fan outlet velocity.
272
ENGINEERING MANUAL OF AUTOMATIC CONTROL
C.
B.
VANEAXIAL FAN
C2668
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