Carrier WeatherExpert 48N2 Controls, Start-Up, Operation, Service, And Troubleshooting page 92

The N Series Humidi-MiZer system controls allow for the dis-
charge air to be reheated to either the return air temperature mi-
nus a configurable offset or to a configurable Reheat set point
(default 70°F). The hot gas reheat mode will be initiated when
only the humidity is above the humidity set point, without a de-
mand for cooling.
System Control
The essential difference between the De-humidification mode and
the Reheat mode is in the supply air set point. In Dehumidification
mode, the supply air set point is the temperature required to pro-
vide cooling to the space. This temperature is whatever the cool-
ing control point would have been in a normal cooling mode. In
Reheat mode, the supply air set point will be either an offset sub-
tracted from return air temperature (D.V.RA) or the Vent Reheat
Set Point (D.V.HT). Both values are configurable. For both Dehu-
midification mode and Reheat mode, the unit compressor staging
will decrease the evaporator discharge temperature to the Dehu-
midify Cool Set Point (D.C.SP COOL) in order to meet the latent
load and reheat the air to the required cooling or reheat set point.
There is a thermistor array called Temperatures  AIR.T  CCT
connected to the RCB. This thermistor array serves as the evapo-
rator discharge temperature (EDT). See Fig. 14.
The N-Series Humidi-MiZer
ulation valves that provide accurate control of the leaving-air
temperature as the evaporator discharge temperature is de-
creased to meet the latent load. As the refrigerant leaves the
compressor, the modulating valves vary the amount of refriger-
ant that enters and/or bypasses the condenser coil. As the by-
passed and hot refrigerant liquid, gas, or 2-phase mixture passes
Airflow
system uses refrigerant flow mod-
®
EVAPORATOR
CCT
Evaporator Discharge Temperature
In Subcool or Reheat Mode, compressor staging
and increased subcooling drives evaporator
discharge temperature down to meet higher latent
loads
D.C.SP COOL
Fig. 14 — Humidi-MiZer
through the Humidi-MiZer coil, it is exposed to the cold supply
airflow coming from the evaporator coil. The refrigerant is sub-
cooled in this coil to a temperature approaching the evaporator
leaving air temperature. The liquid refrigerant then enters an
electronic expansion valve (EXV) where the refrigerant pressure
is decreased. The refrigerant enters the EXV and evaporator coil
at a temperature lower than in standard cooling operation. This
lower temperature increases the latent capacity of the evaporator.
The refrigerant passes through the evaporator and is turned into a
superheated vapor. The air passing over the evaporator coil will
become colder than during normal operation. However, as this
same air passes over the Humidi-MiZer reheat coil, it will be
warmed to meet the supply air set point temperature require-
ment. See Fig. 15.
During dehumidification with Humidi-MiZer (Configuration 
DEHU  D.SEL=4), If dehum results in over-cooling (If Humi-
dimizer capacity is not sufficient to heat it up enough), there will
be a transition from dehum to heat mode for Configura-
tion  HEAT  HT.CF=1 or 2 or 5 (2-stage electric heat or 2-
stage gas heat or SCR electric heat) and all compressors will turn
OFF. This requirement on heating after cooling is that compres-
sors must shut down and cannot be powered when electric heat
powers up. This large power draw can result in blown fuses and a
fire hazard.
During Heating Dehum mode with Humidi-MiZer (Configura-
tion  DEHU  D.SEL=4), control enables Staged Gas or Hy-
dronic heat sources (Configuration  HEAT  HT.CF=3 or 4)
as Reheat sources along with Humidi-MiZer reheat. This is called
supplemental reheat.
HUMIDI-MIZER ADAPTIVE
DEHUMIDIFICATION
SYSTEM COIL
SAT
Supply Air Temperature Control
Innovative algorithm to control supply air temperature
modulates flow bypass to meet desired supply air setpoint -
no overcooling or overheating of the space.
Subcooling Mode: Meet Cooling Mode Supply Air Setpoint
Reheat Mode: Meet Return Air Offset or Reheat Setpoint (configurable)
RAT-D.V.RA or
D.V.HT
System Control
®
92