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TRG-TRC013-EN 23
period one
Fan Performance
notes
In a constant-volume system, where the fan is always delivering the same
airflow, the fan is generally selected to balance the airflow and static-pressure
requirements at a point on the fan curve that permits a certain margin of safety
before surge occurs.
As the coil and filters become dirty, the system resistance increases, causing
the system resistance curve to shift to the left. In response, the operating point
follows the intersection of the system resistance and fan performance curves to
a higher static-pressure condition. This margin of safety allows a certain
increase in system static pressure before the fan’s operating point reaches the
surge region. Of course, as the fan is called upon to generate more static
pressure to overcome this increased system resistance, its airflow is reduced
accordingly.
airflow
airflow
static pressure
static pressure
surge
region
surge
surge
region
region
design system
resistance curve
design system
design system
resistance curve
resistance curve
Constant-Volume System
Figure 34