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TRG-TRC013-EN 55
period four
Application Considerations
notes
System Effect
At the end of Period One, we discussed the effect of the air resistance through
the system being greater than predicted. This is often caused by failing to allow
for the effects of the fan connections to the duct system. This system effect
can be attributed to turbulence due to fan inlet and outlet restrictions, and to
nonuniform air distribution influencing fan performance when the fan is
installed in a system.
If unaccounted for during fan selection, the fan – system operating point will fall
at B instead of A, delivering a lower airflow than desired.
System effects related to the fan inlet or outlet generally occur when the air is
not allowed to establish a uniform velocity profile. If there is not enough
straight duct at its inlet or outlet, the fan will not be able to generate the static
pressure for which it was rated. If a diffuser that connects the fan to the duct
system, an elbow, a branch, turning vanes, or a damper is located too close to
System Effect
airflow
airflow
static pressure
static pressure
design system
resistance curve
design system
design system
resistance curve
resistance curve
actual system resistance curveactual system resistance curve
actual system resistance curve
system effect
pressure loss
system effect
system effect
pressure loss
pressure loss
desired
desired
airflow
airflow
&
$
%
Figure 79
System Effect
uniform
uniform
velocity
velocity
profile
profile
Figure 80