T-340
4–6
Figure 4.5 Controller Operation - Perishable Mode
4.3.8
Perishable 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 dehumidifi
-
cation is active. Once dehumidification is active and the following conditions are satisfied, the controller will activate the
heat relay to begin dehumidification.
• The humidity sensor reading is above the humidity set point (Cd33).
• The unit is in perishable steady state and supply air temperature is less than 0.25°C (0.45°F) above set point.
• The heater debounce timer (three minutes) has timed out.
• 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 4 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
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 EEV to match the increased heat load while still holding the supply air temperature very close to the set point.
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 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 dehumidification is termi
-
nated 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
°
C (+14
°
F),
or−5
°
C (+23
°
F) optionally
−.20
°
C
−.50
°
C
+.20
°
C
−.20
°
C
Air Circulation
Heating
Cooling,
Unloaded
Controller Set Point ABOVE
−10
°
C (+14
°
F),
or−5
°
C (+23
°
F) optionally
−.20
°
C
−.50
°
C
−.20
°
C
Air Circulation
Heating
Cooling,
Economized
Pull Down
+2.5
°
C
(+4.5
°
F)
Cooling,
Unloaded
Falling
Temperature
Rising
Temperature
Perishable Mode
(Only Applicable to Perishable Mode)
Set
Point
