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TRG-TRC013-EN 31
period two
Fan Types
notes
The angle of the backward inclined blade causes the air leaving the wheel to
bend back against the direction of rotation. However, the speed of wheel
rotation (S) causes the air to assume a velocity (V) in the direction shown.
Comparing the performance of FC and BI fans, for a given wheel speed (S) the
air velocity (V) off the FC wheel is substantially greater than that off a
comparable BI wheel. Therefore, when a BI fan is selected to handle the same
airflow, it must be operated at approximately twice the speed of a similarly
selected FC fan. In spite of this, the input power requirement of the BI fan is
less, often making it a more efficient selection.
This higher speed requires that BI fans be built with a larger shaft and bearing,
and it places more importance on proper balance. Their rugged construction
makes them suitable for moving large volumes of air in higher static-pressure
applications.
Backward Inclined Fan
S
S
V
V
Figure 45
forward curved
backward inclined
S
S
V
V
S
S
V
V
FC vs. BI Fans
Figure 46