Trane TRG-TRC014-EN manual Pneumatic Control, Components of a VAV System, period two

period two

Components of a VAV System

notes

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

Apneumatically-controlledterminal 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.

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