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24 TRG-TRC014-EN
notes
period two
Components of a VAV System
To deliver a constant volume of supply air to the space in a two-fan, dual-duct
system as the cooling load in the space decreases, the amount of cool primary
air is reduced and the amount of warm primary air is increased, maintaining a
constant total supply airflow to the space.
Because of this constant airflow to the space, no fan energy savings is realized
at part-load conditions.
Dual-duct VAV systems also have a central return-air path that allows air from
the spaces to come back to the air handler. Dual-duct systems may, however,
have either one or two central air handlers.
At the single air handler, a portion of the return air is recirculated and mixed
with outdoor air. This mixture of outdoor and recirculated return air is then
diverted through either the cooling coil or the heating coil and delivered down
100%
0%
warm
warm
primary air
primary air
space load
design
cooling load
design
heating load
% airflow to space
minimum
primary
airflow
Constant Air Volume to the Space
cool
cool
primary air
primary air
maximum
primary
airflow
Figure 36
Single-Fan, Dual-Duct VAV System
RA
cooling
cooling
coil
coil
heating
heating
coil
coil
EA
VSD
VSD
central air handler
central air handler
dual
dual
-
-
duct
duct
VAV
VAV
terminal
terminal
units
units
OA
75°F
75°F
[23.9°C]
[23.9°C]
55°F
55°F
[12.8°C]
[12.8°C]
40°F
40°F
[4.4°C]
[4.4°C]
55°F
55°F
[12.8°C]
[12.8°C]
105°F
105°F
[40.6°C]
[40.6°C]
Figure 37