6 Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001A-EN
Dedicated OA System Configurations
Compared to a neutral-air system, a dedicated OA system that delivers cold
air directly to each zone or to the supply-side of each local HVAC unit:
• Requires less overall cooling capacity
The required capacity of the dedicated OA unit is the same for both
configurations, but the required cooling capacity of each local unit is less
in a cold-air system than in a neutral-air system.
• Requires less overall cooling energy for much of the year
By taking advantage of the sensible cooling already done by the
dedicated OA unit, the cold-air system requires less cooling energy at
each local unit. The neutral-air system throws away this sensible cooling
benefit by reheating the air to approximately zone temperature.
• Requires less overall fan airflow and, therefore, less fan energy
The airflow delivered by the dedicated OA unit is the same for both
configurations, but for those zones that require seasonal cooling and
heating, the supply airflow delivered by the local HVAC unit is less in a
cold-air system than in a neutral-air system. (For zones that require year-
round cooling, the local HVAC equipment may not be able to be
downsized as much, since it may need to be sized based on the warmest
temperature expected to be delivered by the dedicated OA unit.)
While the conditioned outdoor air should be delivered cold whenever
possible, there are situations when the dedicated OA unit should reheat the
dehumidified outdoor air:
• To avoid overcooling at part-load conditions
As explained earlier, delivering the conditioned OA at a dry-bulb
temperature colder than the zone temperature offsets part of the sensible
cooling load in the zone. As the zone sensible cooling load decreases—
due to changes in outdoor conditions, solar heat gain, and/or internal
loads—it is possible that the cold, conditioned OA may provide more
sensible cooling than the zone requires. As a result, the temperature in
the zone begins to drop. At these conditions, depending on the type of
local HVAC equipment being used, it may be desirable to heat (or reheat)
the outdoor air before delivering it directly to the zones.
For many applications, a better approach to avoid overcooling is to
implement demand-controlled ventilation. This control strategy reduces
the quantity of outdoor air delivered to a zone when there are fewer
people in that zone. This often avoids overcooling altogether, and reduces
the energy used to condition and deliver that air.
• In applications where zone sensible cooling loads differ greatly
at any given time
In hotel guest rooms or dormitories, the sensible cooling loads can be
drastically different from zone to zone. The result is that, if the
conditioned OA is delivered cold, it may be more likely that some zones
will experience overcooling. For these applications, it may be simpler to
deliver the conditioned OA at a neutral dry-bulb temperature because the
benefit of delivering the air cold occurs less frequently.
In classrooms or offices, however, sensible cooling loads in the zones are
relatively high during daytime hours. In fact, for some climates,
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