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TRG-TRC014-EN 23
period two
Components of a VAV System
notes
As before, this graph indicates how the air supplied to the space by the terminal
unit varies as the space loads change. The vertical axis indicates the total
airflow (cool primary air plus warm primary air) supplied to the space. The
horizontal axis indicates the space load.
To deliver a variable volume of supply air to the space in a two-fan, dual-duct
system in the cooling mode, the dual-duct unit is controlled in the same manner
as the cooling-only unit. The cool primary airflow is reduced as the cooling load
in the space decreases. When the space cooling load drops below the minimum
cool primary airflow setting for the unit, overcooling the space, the second
modulating device begins to open. This allows the warm primary air to mix
with the cool primary air and provide warmer supply air to the space.
As the cooling load decreases further and the space requires heating, more
warm primary air is mixed with the minimum amount of cool primary air.
100%
0%
cool
cool
primary air
primary air
warm
warm
primary air
primary air
space load
design
cooling load
design
heating load
% airflow to space
minimum
primary
airflow
Variable Air Volume to the Space
heating
heating
tempering
tempering
maximum
primary
airflow
Figure 35