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20 TRG-TRC014-EN
notes
period two
Components of a VAV System
In the cooling mode, the primary airflow is reduced as the cooling load in the
space decreases. The total supply airflow to the space remains constant, a
combination of cool primary air and warm plenum air. If the space cooling load
drops below the minimum primary airflow setting for the unit, or if space
heating is required, a heating coil can be used to further warm the supply air to
the space.
The maximum airflow for the airflow modulation device typically equals the
terminal fan airflow. This means that at the design cooling load, primary airflow
equals supply airflow.
Series, fan-powered units are generally considered the premium VAV system
because while the central system operates as a variable-volume system, the
spaces receive constant supply airflow. Increased air motion in the space
improves comfort at all load conditions and the constant airflow simplifies
diffuser selection.
Series, Fan-Powered
100%
0%
primary air
primary air
plenum air
plenum air
space load
design
cooling load
design
heating load
% airflow to space
minimum
primary
airflow
maximum
primary
airflow
heating
heating
tempering
tempering
Figure 31
▲ Constant airflow to the space
▲ Acoustics
▲ Fan energy consumption
Parallel Versus Series Fan-Powered
Figure 32