Carrier 69NT40-541-306 Operation And Service Download

Page: 43 / 154

4–7

T-354

4.3.10

Perishable Mode - Dehumidification

The dehumidification mode is provided to reduce the humidity levels inside the container. The mode is activated
when a humidity value is set at function code Cd33. The display module SUPPLY LED will flash ON and OFF every
second to indicate that the dehumidification mode is active. Once the mode 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 set point.

2. The supply air temperature is less than 0.25



C 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 to low speed
operation. The evaporator fan speed will switch every hour thereafter as long as all conditions are met (see Bulb
Mode section 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.

In the dehumidification mode, power is applied to the defrost and drain pan heaters. This added heat load causes
the controller to open the suction modulating valve to match the increased heat load while still holding the supply
air temperature very close to the set point.

Opening the modulating valve 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 the 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 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.

Two timers are activated in the dehumidification mode to prevent rapid cycling and consequent contactor wear.
They are:

• Heater debounce timer (three minutes)

• Out-of-range timer (five minutes)

The heater debounce timer is started whenever the heater contactor status is changed. The heat contactor
remains energized (or de-energized) for at least three minutes even if the set point criteria is satisfied.

The out-of-range timer is started to maintain heater operation during a temporary out-of-range condition. If the sup-
ply air temperature remains outside of the user selected in-range setting for more than five minutes, the heaters will
be de-energized to allow the system to recover. The out-of-range timer starts as soon as the temperature exceeds
the in-range tolerance value set by function code Cd30.

4.3.11

Perishable, Dehumidification - Bulb Mode

Bulb mode is an extension of the dehumidification mode, which allows changes to the evaporator fan speed and/or
defrost termination set points.

Bulb mode is active when function code Cd35 is set to “Bulb”. Once the bulb mode is activated, the user may then
change the dehumidification mode evaporator fan operation from the default (speed alternates from low to high
each hour) to constant low or constant high speed. This is done by toggling function code Cd36 from its default of
“alt” to “Lo” or “Hi” as desired. If low speed evaporator fan operation is selected, the user has the additional capa-
bility of selecting dehumidification set points from 60 to 95% (instead of the normal 65 to 95%).

In addition, if bulb mode is active, function code Cd37 may be set to override the previous defrost termination ther-
mostat settings (refer to

Section 5.10.4

) The temperature at which the defrost termination thermostat will be con-

sidered “open” may be changed [in 0.1



C (0.2



F) increments] to any value between 25.6



C (78



F) and 4