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TRG-TRC014-EN 21
period two
Components of a VAV System
notes
Most designers also believe that series units offer improved acoustical
conditions in the space. The constant sound of the series unit, with the fan
operating whenever the space is occupied, is generally preferred to the on-off
sound generated by the cycling fan in the parallel unit.
However, because the fan runs continuously whenever the space is occupied, a
series fan-powered unit consumes more energy than a parallel fan-powered
unit. The development of high-efficiency motors has lessoned the energy
consumption difference. The fan in a series fan-powered unit also costs more
than an equivalent parallel fan-powered unit because it generally requires a
larger terminal fan.
Fan-powered terminal units without a heating coil are typically used to provide
tempering for those zones that require year-round cooling and have relatively
high minimum airflow settings, such as the densely occupied interior zones of a
building (i.e., a conference room). A fan-powered terminal unit with a heating
coil is typically used for spaces that require seasonal cooling and heating, such
as the exterior zones of a building. Units with heating coils are the most
common of the fan-powered terminal units.
The types of terminal units discussed so far are used in single-duct VAV
systems. Single-duct VAV systems use a central return-air path that allows the
air from the spaces to come back to the air handler. At the air handler, a portion
of this return air is recirculated and mixed with outdoor air (introduced for
space ventilation purposes). This mixture of outdoor and recirculated return air,
or primary air, is then conditioned and delivered to the VAV terminal units
through the supply duct system.
The remainder of the return air is exhausted from the building.
Single-Duct VAV Systems
OA
PA
single
single
-
-
duct
duct
VAV terminal units
VAV terminal units
central air handler
central air handler
EA
VSD
VSD
80°F
80°F
[26.7°C]
[26.7°C]
55°F
55°F
[12.8°C]
[12.8°C]
90°F
90°F
[32.2°C]
[32.2°C]
RA
Figure 33