Carrier 48TC 17 Series Service And Maintenance Instructions page 12

The Humidi−MiZerR system is initiated based on an
input from a discrete input from a mechanical space or
return air humidistat.
Humidi−MiZerR 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. Although 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
Humidi−MiZerR unit will initiate Dehumidification mode
when the space temperature and humidity are both above
the temperature and humidity setpoints and will attempt to
meet both setpoint requirements.
Once the humidity requirement is met, the unit can
continue to operate in normal cooling mode to meet any
remaining sensible capacity 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 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 Humidi−MiZerR adaptive dehumidifica-
tion system will turn on both compressors and open the
two hot gas bypass valves allowing refrigerant flow to the
Humidi−MiZerR 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 Humidi−MiZerR 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 Humidi−MiZerR Reheat Coil, it will be
warmed to the neutral supply air temperature.
from 0.4 to 0.7. The
Humidi−MiZerR System Components
The Humidi−MiZerR
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 Humidi−MiZerR
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. 16.
RH2.A
Fig. 16 − Humidi−MiZerR 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
12
uses the standard
RH1.B
RH1.A
RH2.B
C08057
unit