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TRG-TRC013-EN 47
period three
Fan Capacity Control
notes
As the VAV terminal units modulate shut, the system resistance curve shifts
upward. The fan begins to “ride up” its current vane position curve toward B,
from the design operating point A, trying to balance with this new system
resistance curve. As a result, the fan delivers a lower airflow at a higher static
pressure.
The system static-pressure controller senses this higher static pressure and
sends a signal to the inlet vanes, instructing them to begin closing. When the
inlet vanes are closed, the performance curve for the fan shifts downward until
the balance point C falls along the VAV system modulation curve and the fan
satisfies the system static-pressure controller.
The advantage that inlet vanes provide over discharge dampers is that the fan
approaches the surge region at a much lower airflow and static pressure.
The low end of the fan’s modulating range is either the intersection of the
modulation curve with the surge line, or the leakage rate through the fully
closed inlet vanes, whichever is larger. Typically, a satisfactory modulating
range is achieved by selecting a fan that is two or three sizes (fan wheel
diameter) smaller than a fan that would be selected for a constant-volume
system. Then the design operating point will fall farther to the right on the
curve, permitting a larger range of modulation.
It should be noted that the addition of vanes to the inlet of a fan introduces a
pressure drop that must be overcome by the fan. Inlet vane performance curves
are established through fan testing, and manufacturers typically publish fan
performance data with the effect of the inlet vanes included.
Inlet Vanes
airflow
airflow
static pressure
static pressure
$
&
inlet vane
inlet vane
position (%)
position (%)
surge
line
surge
surge
line
line
%
VAV system
VAV system
modulation
modulation
curve
curve
sensor
sensor
set point
set point
design system
design system
resistance curve
resistance curve
Figure 70