Carrier 69NT40-541-300, Operation And Service, Page: 37 / 152

T-318 3 − 5
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Controller Set Point ABOVE − 10 ° C (+14 ° F),
or − 5
°
C (+23
°
F) optionally
− .20
°
C
− .50 ° C
+.20 ° C
− .20
°
C Air Circulation
Heating
Cooling,
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Controller Set Point ABOVE − 10 ° C (+14 ° F),
or − 5 ° C (+23 ° F) optionally
− .20
°
C
− .50 ° C
− .20 ° C
Air Circulation
Heating
Cooling
Pull Down
+5 ° C
(+9 ° F)
Cooling,
Modulating
Falling
Temperature
Rising
Temperature
Perishable Mode
(Only Applicable to Perishable Mode)
Set
Point
Modulating
Figure 3-5 Controller Operation − Perishable Mode
3.3.6 Perishable Mode Dehumidification
Dehumidification is provided to reduce the humidity
levels inside the container. Dehumidification is activated
when a humidity value is set at Cd33. The yellow
SUPPLY LED will flash ON and OFF every second to
indicate that dehumidification is active. Once
dehumidification is active and the following conditions
are satisfied, the controller will activate the heat relay to
begin dehumidification.
1. The humidity sensor reading is above the humidity
set point (Cd33).
2. The unit is in the perishable steady state mode, and
the supply air temperature is less than 0.25 C
(0.45 F) above set point.
3. The heater debounce timer (three minutes) has
timed out.
4. Heater termination thermostat (HTT) is closed.
If the above conditions remain true for at least one hour,
the evaporator fans will switch from high speed to low
speed. Evaporator fan speed will then switch every
hour, as long as the 4 conditions are met (see Bulb
Mode, Section 3.3.7 for different evaporator fan speed
options).
If any condition except for item (1) becomes false OR if
the relative humidity sensed is 2% below the
dehumidification set point, the high speed evaporator
fans will be energized.
During dehumidification, power is applied to the defrost
heaters. This added heat load causes the controller to
open the SMV to match the increased heat load while
still holding the supply air temperature very close to the
set point.
Opening the SMV reduces the temperature of the
evaporator coil surface, which increases the rate at
which water is condensed from the passing air.
Removing water from the air reduces the relative
humidity. When the relative humidity sensed is 2%
below set point, the controller de-energizes the heat
relay. The controller will continue to cycle heating to
maintain relative humidity below the selected set point.
If the dehumidification mode is terminated by a condition
other than the humidity sensor, e.g., an out-of-range or
compressor shutdown condition, the heat relay is
de-energized immediately.