Specifications

Engineering Data
Zephyr Fans
PENNBARRY
20
Effects Of Errors In Estimating
System Resistance
Higher System Resistance. Actual Duct System B in
Figure 4 shows a situation where an actual system has
more resistance to flow than was calculated. This condition
is generally the result of an inaccurate estimate of system
resistance to flow. All losses must be considered when
calculating system pressure losses or the final system will
be more restrictive than designed; the actual flow rate will
be less than expected, (Point 2). If the actual duct system
pressure loss is greater than design, an increase in fan
speed may be necessary to achieve Point 5, the design
volume flow rate.
NOTE: Direct drive fans can only change speed through the use of a speed
controller and can not increase speed beyond catalog maximum. Fans must
operate on single phase in order to change speed.
Lower System Resistance
Actual duct system C in Figure 4 shows a situation where
a system has less resistance to flow than was expected;
the actual flow rate will be more than expected, (Point 3)
unless changed by the use of a speed controller.
System Effect
Figure 5 illustrates deficient fan/system performance
resulting from one or more undesirable flow conditions. It
is assumed that the system pressure losses, shown in
system curve A, have been accurately determined, and a
suitable fan selected for operation at Point 1. However, no
allowance has been made for the effect of the system
connections on the fan’s performance. To compensate for
this System Effect it will be necessary to add a System
Effect Factor (SEF) to the calculated system pressure
losses to determine the actual system curve. The SEF for
any given configuration is velocity dependent and will,
therefore, vary across the range of flow volumes for the
fan.
In Figure 5 the point of intersection between the fan
performance curve and the actual system curve B is Point
4. The actual flow volume will, therefore, be deficient by
the difference from 1-4. To achieve design flow volume a
SEF equal to the pressure difference between Point 1 and
2 should have been added to the calculated system
pressure losses and the fan selected to operate at Point 2.
Note that because the System Effect is velocity related,
the difference represented between Points 1 and 2 is
greater than the difference between Points 3 and 4.
The SEF includes only the effect of the system
configuration on the fan’s performance.
System Effect Factor
A System Effect Factor is a pressure loss which
recognizes the effect of fan inlet restrictions, fan outlet
restrictions, or other conditions influencing fan
performance when installed in the system.
Figure 3
Figure 4
Figure 5