Perishable Mode -- Conventional; Perishable Mode -- Economy; Perishable Mode -- Dehumidification; Perishable, Dehumidification -- Bulb Mode - Carrier 69NT40--541--200 TO 299 Operation And Service

3.3.8 Perishable Mode - - Conventional
The unit is capable of maintaining supply air
temperature to within ¦0.25_C (¦0.5_F) of set point.
Supply air temperature is controlled by positioning of the
suction modulation valve (SMV), cycling of the
compressor and cycling of the heaters.
When pulling down from a temperature that is more than
5_C (9_F) above set point, the SMV will open to reduce
the pulldown time. However, pressure and current limit
functions may restrict the valve, if either exceeds the
preset value.
The Operational Software is designed so the SMV will
begin to close as the set point is reached. The SMV will
continue to close and restrict refrigerant flow until the
capacity of the unit and the load are balanced.
If the temperature drops below the set point, the
compressor will remain running for a few minutes. This
is to accommodate any initial undershoot which might
occur. After this time has expired and the temperature is
0.2_C (0.4_F) or greater below the set point, the
compressor will be turned OFF.
If the temperature drops to 0.5_C (0.9_F) below set
point, the heaters will be energized . The heaters will
de-energize when the temperature rises to 0.2_C
(0.4_F) below the set point. The compressor will not
restart until the temperature rises to 0.2_C (0.4_F)
above the set point and three minutes have elapsed
since the last compressor turn off.
3.3.9 Perishable Mode - - Economy
The economy mode is an extension of the conventional
mode and is applicable to units with two speed
evaporator fan motors. The mode is activated when the
setting of function code Cd34 is "ON". Economy mode is
provided for power saving purposes. Economy mode
could be utilized 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 indicator that economy mode has
been activated. To check for economy mode, perform a
manual display of code Cd34.
In order to achieve economy mode, a perishable set
point must be selected prior to activation. When
economy mode is active, the evaporator fans will be
controlled as follows:
At the start of each cooling or heating cycle, the
evaporator fans will be run in high speed for three
minutes. They will then be switched to low speed any
time the supply air temperature is within ¦ 0.25_C
(0.45_F) of the set point and the return air temperature
is less than or equal to the supply air temperature + 3_C
(5.4_F). The fans will continue to run in low speed for
one hour. At the end of the hour, the evaporator fans will
switch back to high speed and the cycle will be repeated.
3.3.10 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 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:
1. Heater debounce timer (three minutes).
2. 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 are satisfied.
The out-of-range timer is started to maintain heater
operation during a temporary out-of-range condition. If
the 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 the
temperature exceeds the in-range tolerance value set
by function code Cd30.
3.3.11 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 configuration 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, this gives the user
the additional capability of selecting dehumidification
set points from 60 to 95% (instead of the normal 65 to
95%).
3-5
T-317