FAXA-SVX01B-EN
61
Operation
general
information
Subcooler
Receiver
and reheat coils. Air leaves the evaporator
coil saturated at the preset dew point
condition (h2) and is reheated by the
reheat condenser to the preset reheat
temperature setpoint (h3). The reheat
evaporator transfers energy to the reheat
condenser. A liquid solenoid valve
effectively modulates the reheat
condenser capacity. The reheat outdoor
condenser rejects surplus heat. The
reheat evaporator circuit is first on and
last off, so reheat energy is available at full
and part load conditions. For those
conditions where the reheat condenser
capacity is insufficient to meet reheat
demand, optional electric or gas heat can
provide additional reheat capacity. Since
both the dew point setpoint and
discharge temperature setpoint are fully
adjustable, the desired supply air
conditions are maintained at all load
conditions.
Cooling or Heating
Consider Figure O-GI-6. If the outdoor air
dew point or zone RH is equal to or below
the drying setpoint, the FAU will heat or
cool the outdoor air to separate and
adjust cooling or heating setpoints. At t1
or t3, 100% outdoor air enters the FAU.
The FAU filters, and cools or heats the air
as it is drawn through the evaporators
and heating section. The air leaves the
FAU at the cooling or heating setpoint (t2
or t4) and equal to or below the dew point
setpoint.
Figure O-GI-7. Refrigeration system diagram with reheat
It’s important to remember that airside
economizer capability can be lost if
dedicated outdoor air systems are sized
for minimum ventilation air only. To
provide economizer capability on a
dedicated outdoor air system, the terminal
units must have the ability to introduce
100% outdoor air during the economizer-
cooling mode. The terminal unit’s outdoor
air damper should modulate between fully
closed and fully open to allow the FAU to
deliver the ventilation air design volume to
the space. The terminal unit should
enable economizing only if the outdoor
dry-bulb and dew point conditions are
sufficiently low to justify economizing.
Packaged FAU Operation
The Packaged FAU can use either DX
cooling, condenser reheat, electric or gas
heat to condition the outdoor air. The unit
controls cooling and heating capacity in
stages with the exception of the
modulated heat options. Cooling and
heating staging will result in supply air
temperature swings. The unit controls will
deliver the desired supply air conditions
on a time–based, weighted average.
FAU with Reheat
Figure O-GI-7 shows the Packaged Fresh
Air system with a DX refrigerant circuit
design using reheat.
Dehumidification
Consider Figure O-GI-5. If the outdoor air
dew point is above the drying setpoint (or
in the case of zone control, the zone RH is
above the RH setpoint), the FAU will:
• cool the outdoor air to remove required
moisture and
• reheat to meet the discharge
temperature setpoint.
At h1, 100% outdoor air enters the FAU.
The FAU filters, cools, and dehumidifies
the air as it moves through the evaporator
Figure O-GI-5. Psychrometric chart with
dehumidification and reheat
Figure O-GI-6. Psychrometric chart with
cooling and heating only
h1
h3
h4
h2
Dry Bulb Temperature
Dewpoint T
emperature
W
et Bulb T
em
pe
rature
t3
t4
heating
t1
t2
cooling