3-- 5
T -334
However , pressure and current limit functions may
restrict the valve if either exceeds the preset value.
In perishable-pulldown mode, if return temperature is
above 16C (60.8F), frequency average is below 53 Hz,
and CnF56 is set to “1”, the controller will open the
expansion bypass solenoid valve (XBSV) to allow
additional refrigerant to bypass the TXV for increased
pulldown capability. The XBSV is disabled when the
temperature drops below 15C (59F) or frequency
average is above 55 Hz.
Once set point is reached, the unit will transition to the
perishable steady state mode. This results in unloaded
operation with some restriction of the SMV. The SMV
will continue to close and restrict refrigerant flow until the
capacity of the unit and the load are balanced.
If the SMV is at minimum modulation, the controller has
determined that cooling is not required, or the controller
logic determines suction pressure is at the low pressure
limit, the unit will transition to the perishable idle mode.
The compressor is turned off and the evaporator fans
continue to run to circulate air throughout the container.
If temperature rises above set point +0.2C, the unit will
transition back to the perishable steady state mode
If the temperature drops to 0.5C (0.9F) below set point,
the unit will transition to the perishable heating mode
and the heaters will be energized. The unit will transition
back to the perishable idle mode when the temperature
rises to 0.2C (0.4F) below the set point and the heaters
will de-energize.
3.3 .8 Pe r is ha ble Mode - Econom y
The economy mode is an extension of the conventional
mode. 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 run in high speed for three minutes.
They will then be switched to low speed any time the
supply air temperature is within +/- 0.25C (0.45F) of the
set point and the return air temperature is less than or
equal to the supply air temperature +3C (5.4F). 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. If bulb mode is
active, the economy fan activity will be overwritten.
3.3 .9 Pe r is ha ble Mode - De hum idific a t ion
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 unit is in the perishable steady state mode and
supply air temperature is less than 0.25C above set
point.
3. The heater debounce timer (three minutes) has
timed out.
4. Heater termination thermostat (HTT) is closed.
If the above conditions are true 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
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.
Power is applied to the defrost heaters in the
dehumidification mode. 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.
T wo 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.
3.3.10 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%).
In addition, if bulb mode is active, function code Cd37
may be set to override the previous defrost termination
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