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PERFECT HUMIDITY™ ADAPTIVE DEHUMIDI-
FICATION SYSTEM
Units with the factory-equipped Perfect Humidity option are capa-
ble of providing multiple modes of improved dehumidification as
a variation of the normal cooling cycle. See Fig 15. The design of
the Perfect Humidity system allows for two humidity control
modes of operation of the rooftop unit, utilizing a common sub-
cooling/reheat dehumidification coil located downstream of the
standard evaporator coil. This allows the rooftop unit to operate in
both a dehumidification (Subcooling) mode and a hot gas (Reheat)
mode for maximum system flexibility. The Perfect Humidity
package is factory-installed and will operate whenever there is a
dehumidification requirement present.
The Perfect Humidity system is initiated based on an input from a
discrete input from a mechanical space or return air humidistat.
Perfect Humidity Modes
NORMAL COOLING FOR UNITS K17 - K28
During the Normal Cooling mode, the liquid refrigerant flows
from the outdoor condenser through the normally open (NO)
Cooling System Valve (CSV) to the expansion device. Both the
Reheat1 (RH1.x) and Reheat2 (RH2) valves are closed during the
normal cooling mode.
During the Normal Cooling mode, the refrigerant flows from the
outdoor compressor through the condenser coil. The Reheat2
(RH2.x) is closed, preventing the refrigerant from bypassing the
condenser coil. The refrigerant then flows through the open Re-
heat2 (RH1.x) 3-way valve to the TXV Metering Device, bypass-
ing the Perfect Humidity coil, and finally passing through the
evaporator coil before returning to the outdoor compressor. See
Fig 15.
REHEAT1 (SUBCOOLING MODE) FOR UNITS K17 - K28
The Reheat1 or Subcooling mode will be engaged to satisfy part-
load type conditions when there is a space call for cooling and de-
humidification. Although the temperature could have dropped and
decreased the sensible load in the space, the outdoor and/or space
humidity levels could have risen. A typical scenario could be
when the outside air is 85°F (29°C) with 70% to 80% relative hu-
midity (RH). Desired Sensible Heat Ratio (SHR) for equipment in
this scenario is typically from 0.4 to 0.7. The Perfect Humidity
unit will initiate the Dehumidification mode when both the space
temperature and humidity are above the temperature and humidity
setpoints while attempting to meet both setpoint requirements.
Once the humidity requirement is met, the unit can continue to op-
erate in normal cooling mode to meet any remaining sensible ca-
pacity load. Alternatively, if the sensible load is met and humidity
levels remain high the unit can switch to Hot Gas Reheat mode or
Reheat2 mode to provide neutral, dehumidified air.
During the Reheat1 or Subcooling mode, the liquid refrigerant
flows from the outdoor compressor through the condenser coil to
the Reheat1 (RH1.x) 3-way valve and on to the Humid-MiZer
coil. The Reheat2 (RH2.x) valve is closed. The liquid refrigerant
then passes through the Humid-MiZer coil and then a metering de-
vice or Thermostatic Expansion Valve (TXV). From the TXV, the
liquid refrigerant passes through the evaporator coil and back to
the outdoor compressor. See Fig 16.
REHEAT2 (HOT GAS REHEAT MODE) FOR K17 - K28
The Reheat2 or Hot Gas Reheat mode is used when dehumidifica-
tion is required without a need for cooling, such as when the out-
side air is at a neutral temperature, but high humidity exists. This
situation requires the equipment to operate at a low SHR of 0.0 to
0.2. With no cooling requirement calling for dehumidification, the
Perfect Humidity adaptive dehumidification system will energize
both compressors, opening the two hot gas bypass valves, allow-
ing refrigerant flow to the Perfect Humidity coil to reheat the
unit's supply air to a neutral temperature.
The hot bypassed refrigerant liquid (gas or two-phase mixture) ex-
its the outdoor compressor and passes through the open Reheat1
(RH1.x) at the same time it passes through the condenser coil to
the open Reheat2 (Rh2.x) to the Perfect Humidity coil. After the
refrigerant passes through the Perfect Humidity coil, it enters a
TXV metering device, decreasing the air pressure, and on to the
evaporator coil. The refrigerant is subcooled in this coil to a tem-
perature approaching the evaporator leaving air temperature. The
liquid refrigerant then returns to the outdoor compressor. See
Fig. 17.
The refrigerant enters the TXV and evaporator coil at a tempera-
ture lower than the temperature in the standard cooling operation.
This lower temperature increases the latent capacity of the evapo-
rator. The refrigerant passes through the evaporator turning it into
a superheated vapor. The air passing over the evaporator coil be-
comes 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 compres-
sor(s), evaporator coil and Round Tube-Plate Fin (RTPF) con-
denser coil. Additional refrigeration system hardware includes a
subcooler/reheat coil and control solenoid valves. On some mod-
els, 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 system controls include a
reheat relay mode, and evaporator coil freezestat, and secondary
low pressure switch.
OPERATING SEQUENCES
The Perfect Humidity system provides three sub-modes of opera-
tion: Normal Cooling, Reheat1 and Reheat2.
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 both cooling demand (thermostat Y1 demand) and
dehumidification demand, circuit 1 will operate in Subcooling
(Reheat1) mode. See Fig. 16 for system refrigerant flow.
When there is only a single cooling demand, one or both circuits
will operate in Hot Gas (Reheat2) mode. The DSV solenoid valve
is open and the CSV solenoid is closed. See Fig. 17 for system re-
frigerant flow.
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.
TROUBLESHOOTING
See Table 4.
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Troubleshooting
