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28 TRG-TRC014-EN
notes
period two
Components of a VAV System
Accurate measurement of primary airflow is required to enable pressure-
independent control. Most terminal units accomplish this with a multipoint
airflow sensor mounted on the inlet.
This sensor measures a pressure difference between the ports that face the
airflow and the ports that face downstream. The result is a pressure difference
signal that relates to the airflow rate passing through the modulation device.
Now that we understand pressure-dependent versus pressure-independent
control, we will look at the technologies available to perform this control.
Three control technologies are generally available in VAV terminal units. They
are pneumatic, electronic, and direct digital control (DDC).
Primary Airflow Measurement
measured
measured
pressure
pressure
difference
difference
airflow sensor
airflow sensor
airflow
airflow
Figure 42
Terminal-Unit Control Technologies
▲ Pneumatic control
▲ Electronic control
▲ Direct digital control (DDC)
Figure 43