Trane TRG-TRC014-EN VAV Terminal Unit Controls, Upstream Pressure Affects Airflow, period two

period two

Components of a VAV System

notes

VAV Terminal Unit Controls

Figure 39

VAV Terminal Unit Controls

Next we will explore the control of VAV terminal units. Before discussing the commonly-available control technologies, it is important to first understand a fundamental control concept: pressure-dependentversus pressure- independent control.

Upstream Pressure Affects Airflow

0.70 in. H2O		1000 cfm
[175 Pa]
static pressure	VAV terminal unit	[0.47 m3/s]
in supply duct	(identical damper positions)
0.80 in. H2O		1300 cfm
[200 Pa]
static pressure		[0.61 m3/s]
in supply duct

Figure 40

The modulating device in a VAV terminal unit changes the air resistance of the supply duct path to the space. As terminal units modulate, the static pressure within the duct system changes. The airflow that passes through the terminal unit depends not only on the position of the modulation device but also on the static pressure at the inlet. With the modulation device in a fixed position, a higher static pressure in the upstream duct will “push” more air through the terminal unit. Because the inlet static pressure constantly changes due to the modulation of the terminal units in the system and the resulting variation of

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