Carrier 48TC series, Service And Maintenance Instructions, Page: 11 / 112

11
HUMIDI-MIZER
®
ADAPTIVE DEHUMIDIFICA-
TION SYSTEM
Units with the factory-equipped Humidi-MiZer option are capable
of providing multiple modes of improved dehumidification as a
variation of the normal cooling cycle. The design of the Humidi-
MiZer system allows for two humidity control modes of operation
of the rooftop unit, utilizing a common subcooling/reheat dehu -
midification coil located downstream of the standard evaporator
coil. This allows the rooftop unit to operate in both a dehumidifi -
cation (Subcooling) mode and a hot gas Reheat Mode for maxi -
mum system flexibility. The Humidi-MiZer package is factory-in -
stalled and will operate whenever there is a dehumidification re -
quirement present.
The Humidi-MiZer system is initiated based on an input from a
discrete input from a mechanical space or return air humidistat.
Humidi-MiZer Modes
DEHUMIDIFICATION MODE (SUBCOOLING)
This mode will be engaged to satisfy part-load type conditions
when there is a space call for cooling and dehumidification. Al -
though the temperature could have dropped, decreasing levels of
the sensible load in the space, the outdoor and/or space humidity
levels can be higher. A typical scenario could be when the outside
air is 85°F (29°C) with 70% to 80% relative humidity (RH).
Desired sensible heat ratio (SHR) for equipment in this scenario is
typically from 0.4 to 0.7. The Humidi-MiZer unit will initiate De -
humidification mode when the space temperature and humidity
are both above the temperature and humidity setpoints and will at -
tempt 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 to
provide neutral, dehumidified air.
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 tem -
perature, 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 Humidi-MiZer
adaptive dehumidification system will turn on both compressors
and open the two hot gas bypass valves allowing refrigerant flow
to the Humidi-MiZer coil to reheat the unit’s supply air to a neutral
temperature.
As the hot bypassed refrigerant liquid (gas or two-phase mix -
ture) passes through the Humidi-MiZer 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 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 Humidi-MiZer Reheat Coil, it will be warmed
to the neutral supply air temperature.
HUMIDI-MIZER SYSTEM COMPONENTS
The Humidi-MiZer uses the standard unit compressor(s), evapora
-
tor coil and Round Tube-Plate Fin (RTPF) condenser coil. Addi -
tional refrigeration system hardware includes a subcooler/reheat
coil and solenoid valves. On 48TC models, the evaporator coil in -
cludes a TXV as a standard feature. Units with Humidi-MiZer
FIOP also include a factory-installed head pressure control system
(Motormaster I) to provide proper liquid pressure during reheat
modes. Unique components 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. 16. See Table 2 for
Reheat Control input/output (I/O) connections.
Fig. 16 — Humidi-MiZer Valve Locations
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
OPERATING SEQUENCES
The Humidi-MiZer system provides three sub-modes of opera
-
tion: Normal Cooling (see Fig. 17), Subcooling Reheat1 (see
Fig. 18) and Hot Gas Reheat2 (see Fig. 19).
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 Re -
heat2 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. See
Table 3 for further details on Humid-MiZer system operating
modes. See Table 4 for details on troubleshooting the Humidi-
MiZer system.
RH1.A
RH2.A
RH1.B
RH2.B