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 FIGURE 2-8
Impeller Types Used in Centrifugal Fans
The BI and the AF fans are more efficient, and more expensive than the
FC wheel. The rpm for these fans is much higher than for the FC fans. Note
the direction of rotation indicated. It is opposite from the usual
direction. You must be able to recognize the correct rotation direction to
be backwards when troubleshooting a new installation. A fan that is
rotating backwards will not move air in a backwards direction. It will move
air in the proper direction through the ductwork, but at a drastically
reduced flow rate.
The radial design fan blade is not generally used in normal air condi-
tioning applications. It is designed for high-strength, rugged
It is more commonly used in material handling applications.
construction.
Axial fan airflow is in the same direction as the fan shaft. Common
types of axial flow fans are:
1.
Propeller
Tubeaxial
2.
Vaneaxial
3.
The propeller fan has relatively low efficiency, inexpensive construc-
tion, and is not capable of producing static pressures in excess of a few
tenths of an inch. Propeller fans can move lots of air with a free
discharge but they have a very flat fan curve. As the static pressure
requirement increases, the cfm decreases very dramatically (Figure 2-9).
The tubeaxial fan (Figure 2-10) consists of a propeller-type fan blade
inside a cylindrical tube. The clearance between the blades and the tube is
close, increasing the efficiency. The tubeaxial is capable of developing
enough static pressure to move air through low resistance duct systems.
2-8
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