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TRG-TRC014-EN 27
period two
Components of a VAV System
notes
duct static pressure, airflow to the space can vary even when the immediate
modulation device does not change position.
A pressure-dependent VAV control scheme uses the space temperature
sensor to directly control the position of the modulating device. The actual
airflow delivered to the space is a by-product of this position and depends on
the duct system static pressure at the inlet of the terminal unit. Although the
space temperature sensor will continually correct the position of the
modulating device, the response can be sluggish and cause unacceptable
temperature variations within the space.
In contrast, a pressure-independent VAV control scheme directly controls the
actual volume of primary air that flows to the space. An airflow-measuring
device on the terminal unit makes this possible. The position of the modulation
device is not directly controlled and is basically a by-product of regulating the
airflow through the unit. Because the airflow delivered to the space is directly
controlled, it is independent of inlet static pressure.
Pressure-independent control increases the stability of airflow control, and
allows minimum and maximum airflow settings to become actual airflows
rather than physical positions of the modulation device. It is clearly the most
popular form of VAV terminal unit control.
Pressure-Dependent Control
Pressure-Independent Control
▲ Space sensor controls position of the
modulating device
▲ Airflow to space depends on SP in upstream
duct system
▲ Space sensor controls desired airflow
▲ Airflow to space is controlled directly,
independent
of SP in upstream duct system
Figure 41