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6 TRG-TRC013-EN
notes
period one
Fan Performance
With the fan operating and the damper fully open, air moves through the duct
unimpeded. The impact of the moving air causes the vane to swing in the
direction of airflow. The pressure exerted on the vane is due to the velocity of
the air moving through the duct, not the static pressure exerted on the walls of
the duct.
At this point the outward, or static, pressure exerted on the duct walls is
negligible. Nearly all of the usable fan energy is being converted to velocity
pressure.
Partially closing the damper increases resistance to airflow. The fan generates
enough pressure to overcome this resistance (static pressure loss), but this
occurs at the expense of velocity pressure. Part of the fan’s usable energy is
now being devoted to generating enough static pressure to overcome the
resistance of the damper.
damper
damper
fully open
fully open
Velocity Pressure vs. Static Pressure
Figure 9
damper
damper
partially open
partially open
Velocity Pressure vs. Static Pressure
Figure 10