60
FAXA-SVX01B-EN
Operation
general
information
Figure O-GI-4. Direct hall discharge with fan-coil or PTAC units
Figure O-GI-3. Indirect discharge to fan-
coil units
Figure O-GI-2. Direct discharge to
conditioned space
Overview
Fresh Air Unit Functions
The Packaged Fresh Air Unit (FAU)
provides conditioned outdoor air suitable
for mechanical ventilation or make-up air.
The FAU conditions outdoor air as
necessary to meet system performance
requirements by filtration, cooling,
dehumidification, and/or heating. The FAU
may deliver ventilation air in a number of
ways. See the System Configurations
section and Figures O-GI-2 through O-GI-4
for more information.
Filtration
Filtration is necessary for the FAU to
maintain system operating efficiency, meet
occupant comfort and health needs, and
provide adequate building filtration to aid
indoor air quality. The Packaged FAU offers
a variety of filter options to meet most
application needs.
Cooling
When the outdoor air dry-bulb temperature
is warmer than space conditions, the unit
cools the outdoor air to the an appropriate
supply air dry-bulb temperature. The
Packaged FAU provides cooling using an
air-cooled, direct-expansion (DX)
refrigeration system with scroll
compressors.
Dehumidification (Drying)
The Packaged FAU dehumidifies the
outdoor air when it is more humid than the
required space target conditions. This
helps prevent high relative humidity levels
in the space. Occasionally, the outdoor air
may need to be sub-cooled to remove
excess moisture. When necessary, the FAU
reheats discharge air to meet space
comfort conditions. The Packaged FAU
recovers energy from the cooling process
to reheat the sub-cooled air as necessary.
Heating
When the outdoor air dry-bulb temperature
is colder than space conditions require, the
FAU heats the outdoor air to meet space
temperature requirements. The Packaged
FAU has electric or gas heat options
available.
System Configurations
Dedicated outdoor air systems can deliver
conditioned outdoor air in one of the
following ways:
1. Directly to the occupied space, with the
local terminal unit controlling the space
dry-bulb temperature. See Figure O-GI-2.
2. Directly to local terminal units, which
deliver a mixture of the conditioned
outdoor air and (conditioned)
recirculated return air to the space. See
Figure O-GI-3.
3. Directly to a single space and control the
space temperature directly. For example,
this application will provide temperature
control of a hotel hallway and ventilation
of the adjacent rooms. See Figure O-GI-4.
Supply Air Temperature
Give careful consideration when selecting
the FAU supply air temperature. Many
current system designs control the fresh air
unit discharge air to a “space neutral” dry-
bulb condition, typically about 75°F. At
some operating conditions, reheating the
FAU supply air will add cooling load to the
space, thus requiring the local terminal
units to “re-cool” the conditioned outdoor
air to meet the space setpoint. Therefore, to
help maintain lower energy costs, use
reheat only when required to maintain
occupant comfort and space relative
humidity. The Capacity Control section
discusses this issue in more detail.
Airside Economizers
Often fresh air units are sized to deliver the
minimum ventilation air required for the
space it serves. However, the ventilation air
is typically only a portion of the space
supply air. Airside economizing can provide
up to 100% of the space supply air come
from outdoors when outdoor air
conditions are suitable for “free cooling”
capability.
Hallway
OA delivered
to hallway
Summary of Contents for FADA
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