Trane TRG-TRC014-EN manual Pressure-DependentControl, Pressure-IndependentControl, period two

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.

Pressure-Dependent Control

▲Space sensor controls position of the modulating device

▲Airflow to space depends on SP in upstream duct system

Pressure-Independent Control

▲Space sensor controls desired airflow

▲Airflow to space is controlled directly, independent of SP in upstream duct system

Figure 41

A pressure-dependentVAV 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-independentVAV 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.

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