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TRG-TRC014-EN 29
period two
Components of a VAV System
notes
A pneumatically-controlled terminal unit has an airflow-modulation device
operated by a pneumatic actuator consisting of an inflatable bladder and a
return spring. The return spring positions the damper to a “normal” state (the
position it assumes with no pressure applied in the bladder), typically fully
open. If the pneumatic pressure increases in the bladder, the return spring
compresses and the modulation device begins to close. If pneumatic pressure
is allowed to bleed from the bladder, the force of the return spring expanding
begins to open the modulation device.
A pneumatic volume regulator (PVR) provides pressure-independent control of
the VAV terminal unit. The PVR is connected to the airflow-measurement device
and the pneumatic space thermostat. The PVR directs the actuator to position
the modulation device to deliver the required airflow to the space. The
pneumatic thermostat signal is used to reset this airflow set point, and it
directly controls the terminal fan and heat source.
Minimum and maximum airflow settings are physically adjusted on the PVR.
Compressed air is required to operate the modulation device and to power the
PVR and the space thermostat.
Pneumatic Control
▲ Pneumatic Volume Regulator (PVR) provides
pressure-independent control
▲ Pneumatic thermostat directly controls terminal-
unit fan and heat source
▲ Minimum and maximum airflow settings
adjusted physically on PVR
▲ Compressed air operates modulation device,
PVR, and space thermostat
Figure 44