Carrier 69NT40-561-300, Operation And Service Manual

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4–7 T-365
If the previously mentioned 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,
Section 4.3.9 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.
Two timers are activated during dehumidification to prevent rapid cycling and consequent contactor wear:
• Heater debounce timer (three minutes) - The heater debounce timer is started whenever the heater contac-
tor status is changed. The heat contactor remains energized (or de-energized) for at least three minutes
even if the set point criteria are satisfied.
• Out-of-range timer (five minutes) - The out-of-range timer is started to maintain heater operation during a
temporary out-of-range condition. If supply 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 temperature exceeds in-range tolerance value set by Cd30.
4.3.9 Perishable Dehumidification - Bulb Mode
Bulb mode is an extension of dehumidification which allows changes to the evaporator fan speed and/or defrost
termination set points.
Bulb mode is active when Cd35 is set to “Bulb.” Once bulb mode is activated, the user may then change dehumid-
ification 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 Cd36 from its default of “alt” to “Lo” or “Hi” as desired. If low speed
evaporator fan operation is selected, this gives the user the additional capability of selecting dehumidification set
points from 50 to 95%.
In addition, if bulb mode is active, Cd37 may be set to override the previous defrost termination thermostat (DTT)
settings. The temperature at which the DTT will be considered “open” may be changed [in 0.1°C (0.2°F) incre-
ments] to any value between 25.6°C (78°F) and 4°C (39.2°F). The temperature at which the DTT is considered
closed for interval timer start or demand defrost is 10°C (50°F) for “open” values from 25.6°C (78°F) down to a
10°C (50°F) setting. For “open” values lower than 10°C, the “closed” values will decrease to the same value as the
“open” setting. Bulb mode is terminated when:
• Bulb mode code Cd35 is set to “Nor.”
• Dehumidification code Cd33 is set to “Off.”
• The user changes the set point to one that is in the frozen range.
When bulb mode is disabled by any of the above conditions, evaporator fan operation for dehumidification reverts
to “alt” and the DTS termination setting resets to the value determined by CnF41.
4.3.10 Perishable Economy
Economy fan mode is an extension of the Perishable Mode, and is provided for power saving purposes. Economy
fan mode is activated when Cd34 (also used for Frozen Economy Mode) is set to “ON.” Economy fan mode is used
in the transportation of temperature-tolerant cargo or non-respiration items which do not require high airflow for
removing respiration heat.
There is no active display that indicates that economy fan mode has been initiated. To check for economy fan
mode, perform a manual display of Cd34.
In order to initiate economy fan mode, a perishable set point must be selected prior to activation. When economy
fan mode is active, the evaporator fans will be controlled as follows: