Bryant 581J-17-28 Service And Maintenance Instructions page 12

Reheat Mode
This mode is used when dehumidification is required
without a need for cooling, such as when the outside air is
at a neutral temperature, but high humidity exists. This si-
tuation requires the equipment to operate at a low SHR of
0.0 to 0.2. With no cooling requirement calling for dehu-
midification, the Perfect Humidity™ dehumidification
system will turn on both compressors and open the two
hot gas bypass valves allowing refrigerant flow to the Per-
fect Humidity™ coil to reheat the unit's supply air to a
neutral temperature.
As the hot bypassed refrigerant liquid (gas or two−phase
mixture) passes through the Perfect Humidity™ coil, it is
exposed to the to the cold supply airflow coming from the
evaporator coil. The refrigerant is subcooled in this coil to
a temperature approaching the evaporator leaving air
temperature. The liquid refrigerant then enters a
Thermostatic Expansion Valve (TXV), decreasing the air
pressure.
The refrigerant enters the TXV and evaporator coil at a
temperature lower than the temperature in the standard
cooling operation. This lower temperature increases the
latent capacity of the evaporator. The refrigerant passes
through the evaporator turning it into a superheated vapor.
The air passing over the evaporator coil becomes colder
than it would during normal operation. As this same air
passes over the Perfect Humidity™ Reheat Coil, it will be
warmed to the neutral supply air temperature.
Perfect Humidity] System Components
The Perfect Humidity™ system uses the standard unit
compressor(s), evaporator coil and Round Tube−Plate Fin
(RTPF) condenser coil. Additional refrigeration system
hardware includes a subcooler/reheat coil and solenoid
valves. On 50HC models, the evaporator coil includes a
TXV as a standard feature. Units with Perfect Humidity™
FIOP also include a factory−installed head pressure
control system (Motormaster I) to provide proper liquid
pressure during reheat modes. Unique controls include a
Reheat Relay Board (RHB), evaporator coil freezestat,
secondary low pressure switch and a low outdoor
temperature lockout switch (LTLO). Units with two
refrigeration circuits include a solenoid valve, TXV,
freezestat and low−pressure switch for each circuit. See
Fig. 15.
Subcooler/Reheat Coil
The Subcooler/Reheat Coil is mounted across the leaving
face of the unit's evaporator coil. The coil is a one−row
design with two separate circuits.
— 3−Way Solenoid Valve
— Hot Gas Bypass Solenoid Valve
— Relay Reheat Board (RHB)
— Evaporator Freeze Protection Thermostat
— Low Outdoor Temperature Lockout Switch
— Secondary Low Pressure Switch
— Head Pressure Control
RH2.A
Fig. 15 − Perfect Humidity] Valve Locations
Operating Sequences
The Perfect Humidity™ system provides three sub−modes
of operation: Normal Cooling (see Fig. 16), Subcooling
Reheat1 (see Fig. 17) and Hot Gas Reheat2 (see Fig. 18).
The Reheat1 and Reheat2 modes are available when the
unit is not in a heating mode and when the Low Ambient
Lockout Switch is closed.
When there is only a single cooling demand (thermostat
Y1 alone or with thermostat Y2), one or both circuits will
operate in Reheat2 mode. Both solenoids are energized in
both circuits. See Fig. 18, Hot Gas Reheat Schematic for
system refrigerant flow.
When there is both cooling demand (thermostat Y1
demand) and dehumidification demand, circuit 1 will
operate in Reheat1 mode (Subcooling, Fig. 17) and circuit
will operated in Reheat2 mode (Reheat, Fig. 18). In
Reheat1 mode, the 3−−way solenoid valve is energized,
opening the reheat coil to the refrigeration flow path
providing sub−−cooling to the liquid before it enters the
TXV.
Refer to Tables 2 and 3 for input/output parameters. See
Table 4 for Perfect Humidity™ troubleshooting.
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RH1.B
RH1.A
RH2.B
C08057