4–5
T-372
Figure 4.4 Controller Operation - Perishable Mode
4.3.8
Perishable Dehumidification
Perishable Dehumidification is provided to reduce the humidity levels inside the container. The dehumidification
setpoint range is from 50% to 95%. 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 Dehumidifica
-
tion is active, the controller will activate the heat relay to begin Dehumidification as long as the following conditions
are satisfied:
1. The humidity sensor reading is above the humidity setpoint (Cd33).
2. The unit is in Perishable Steady State Mode and supply air temperature is less than 0.25°C (0.45°F) above
setpoint.
3. The heater debounce timer (three minutes) has timed out.
4. The heater termination thermostat (HTT) is closed.
If the above conditions are 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 four conditions are met (see Bulb mode,
for different evaporator fan speed options).
If any condition except item (1) becomes false OR if the relative humidity sensed is 2% below the dehumidification
setpoint, 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 electronic expansion valve (EEV) to match the increased heat load while still holding the supply air tem
-
perature very close to the setpoint. Opening the EEV reduces the temperature of the evaporator coil surface, which
increases the rate at which water is condensed and removes water from the passing air. Removing water from the
air reduces the relative humidity. When the relative humidity sensed is 2% below setpoint, the controller de-ener
-
gizes the heat relay. The controller will continue to cycle heating to maintain relative humidity below the selected
setpoint. If dehumidification 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.
Controller Set Point ABOVE -10
q
C (+14
q
F),
or -5
q
C (+23
q
F) optionally
Air Circulation
Air Circulation
Heating
Cooling,
Enconomized
Pull Down
Perishable Mode
Set
Point
Set
Point
-0.20
q
C
-0.20
q
C
-0.50
q
C
+2.5
q
C
(+4.5
q
F)
-0.50
q
C
+0.20
q
C
-0.25
q
C
Falling
Temperature
Rising
Temperature
Cooling,
Unloaded
Cooling,
Unloaded
(Only Applicable to Perishable Mode)
Controller Set Point ABOVE -10
q
C (+14
q
F),
or -5
q
C (+23
q
F) optionally
Air Circulation
Heating
-0.20
q
C
