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TRG-TRC014-EN 25
period two
Components of a VAV System
notes
the respective duct system to the modulation devices in the dual-duct VAV
terminal units.
While this single-fan configuration requires only one air handler, it is very
complicated to control efficiently. In this example, the economizer is controlling
the mixed air temperature to 55°F [12.8°C], thus saving the energy to operate
the cooling coil. However, the heating coil must warm the air from 55°F [12.8°C]
to the 105°F [40.6°C] primary air temperature. If the economizer was not
activated, the mixed air temperature would be 75°F [23.9°C], requiring less
heating energy, but now the cooling coil must operate. Optimizing the energy
use of a single-fan, dual-duct system requires a very complicated control
system and does not operate as efficiently as a two-fan, dual-duct system.
Dual-duct systems can be very energy efficient when the mixing of cool and
heated air is avoided and two supply fans are used – one for heating, the other
for cooling. This arrangement eliminates the inefficient or wasteful reheating of
mixed air in a single-fan, dual-duct system and allows an airside economizer to
provide cool primary air whenever possible.
At the cooling air handler, a portion of the return air is recirculated and mixed
with outdoor air (introduced for space ventilation purposes). This mixture of
outdoor and recirculated return air is then cooled and delivered as cool primary
air to one of the modulation devices in the dual-duct VAV terminal units through
the cooling supply duct system.
Air delivered by the heating air handler contains only recirculated air, no
outdoor air. This air is heated and delivered as warm primary airflow to the
other modulation device in the dual-duct VAV terminal units through the
heating supply duct system.
The remainder of the return air is exhausted from the building. The cooling air
modulation device typically has a minimum setting to accommodate the
ventilation requirements of the space.
Two-Fan, Dual-Duct VAV System
RA
heating
heating
air handler
air handler
dual
dual
-
-
duct
duct
VAV
VAV
terminal
terminal
units
units
cooling
cooling
air handler
air handler
EA
PA
PA
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 38