SYS-APG001A-EN Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit 7
Dedicated OA System Configurations
classrooms may never reach the point when overcooling occurs during
occupied hours, especially if demand-controlled ventilation is used to
reduce outdoor airflow when zone population decreases. These
applications are typically well-suited for delivering the conditioned OA at
a cold temperature.
• In applications that require low dew points
If an application has very high indoor latent loads the outdoor air may
need to be dehumidified to a very low dew point. In this case, the
corresponding dry-bulb temperature of the air leaving the cooling coil
may be colder than the HVAC design engineer is willing to discharge
directly into an occupied zone—below 45°F (7°C), for example. In this
case, the dehumidified OA could be reheated to a more traditional supply-
air temperature—55°F (13°C), for example—but not reheated all the way
to neutral.
• To avoid condensation when conditioned OA is delivered to the
ceiling plenum
In some applications, the dedicated OA system delivers the conditioned
outdoor air (CA) to the ceiling plenum, near the intake of each local HVAC
terminal (see Figure 5, p. 4). The outdoor air mixes with recirculated air
(RA) in the plenum before being drawn in through the intake of the local
unit. In this configuration, the dedicated OA unit should reheat the
dehumidified OA to a dry-bulb temperature that is above the expected
dew-point temperature of the air within the ceiling plenum. If cold air is
dumped into the ceiling plenum, it could cool surfaces (structural beams,
electrical conduit, ceiling framework). At night, when the dedicated OA
unit is off, wind or operating exhaust fans may cause humid outdoor air
to leak into the plenum, which may lead to condensation on these cold
surfaces.
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