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32 TRG-TRC013-EN
notes
period two
Fan Types
The application range of the BI fan is from approximately 40 to 85 percent
wide-open airflow. As before, an operating point below 40 percent wide open
may place the fan in surge and an operating point above 85 percent wide open
typically produces noise and inefficiency. The maximum static efficiency of the
BI fan is from approximately 65 to 75 percent and occurs at approximately
50 percent wide-open airflow.
Since the magnitude of surge is related to generated pressure, the surge
characteristic exhibited by the BI fan is greater than that of the FC fan. This is
primarily due to BI fans being used in higher static-pressure applications.
Unlike the FC fan, the input power lines of a BI fan are, for the most part, nearly
parallel to the fan performance curves. If the system resistance were to drop,
the actual system resistance curve would also drop, moving the operating point
(A) to a higher airflow (B). The fan input power requirement would change only
slightly. For this reason, BI fans are referred to as “nonoverloading” type fans.
40%
40%
85%
85%
airflow
airflow
static pressure
static pressure
Backward Inclined Fan
application
application
range
range
static efficiency
static efficiency
65 to 75%
65 to 75%
Figure 47
airflow
airflow
static pressure
static pressure
actual system
resistance curve
actual system
actual system
resistance curve
resistance curve
surge
line
surge
surge
line
line
7.5 hp
7.5 hp
[5.6 kW]
[5.6 kW]
10 hp
10 hp
[7.5 kW]
[7.5 kW]
5 hp
5 hp
[3.7 kW]
[3.7 kW]
$
%
Figure 48