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T-318 3 − 9
3.3.16 Frozen Mode Cooling − Sequence of
Operation
NOTES
1 In the Frozen Mode the evaporator motors
run in low speed.
2 In low temperature ambients, the
condenser fan will be cycled by the
controller to maintain proper condensing
pressure.
a. When the return air temperature is above set point
and decreasing, the unit will be cooling with the con-
denser fan motor (CF), compressor motor (CH),
evaporator fan motors (ES) energized and the white
COOL light illuminated (see Figure 3-9).
b. When the return air temperature decreases to a pre-
determined tolerance above set point, the green IN-
RANGE light is illuminated.
c. When the return air temperature decreases to 0.2 C
(0.4 F) below set point, contacts TC and TN are
opened to de-energize the compressor and condens-
er fan motors. The white COOL light is also de-ener-
gized.
d. The evaporator fan motors continue to run in low
speed to circulate air throughout the container.
e. The green IN-RANGE light remains illuminated as
long as the return air is within tolerance of set point.
f. When the return air temperature increases to 0.2 ° C
(0.4 ° F) above set point and the three minute off time
has elapsed, relays TC and TN are energized to re-
start the compressor and condenser fan motors. The
white COOL light is also illuminated.
CONTROL TRANSFORMER
POWER TO
CONTROLLER
SIGNAL TO
CONTROLLER
SIGNAL TO
CONTROLLER
ENERGIZED
DE-ENERGIZED
FOR FULL DIAGRAM AND
LEGEND, SEE SECTION 7
Figure 3-9 Frozen Mode
3.3.17 Defrost
Defrost is initiated to remove ice buildup from the evap-
orator coil which can obstruct air flow and reduce the
cooling capacity of the unit. The defrost cycle may con-
sist of up to three distinct operations. The first is de-icing
of the coil, the second is a probe check cycle and the
third is snap freeze.
De-icing of the coil consists of removing power to the
cooling components (compressor, evaporator fans,
condenser fan), and turning on the heaters, which are
located below the evaporator coil. During normal opera-
tion, de-icing will continue until temperatures indicate
that the ice on the coil has been removed, proper air flow
has been restored, and the unit is ready to control tem-
perature efficiently.
After de-icing, and depending on unit configuration, a
Defrost/Probe Check and/or Snap Freeze may occur:
During Defrost / Probe Check, the evaporator fans
are allowed to run for a period of time after de-icing in
order to validate the accuracy of the temperature
sensors, and confirm that defrost was executed prop-
erly. Refer to Probe Diagnostics, Section 4.10 for ad-
ditional information.
Defrost / Snap Freeze allows the system to cool for a
period of time after de-icing, with the evaporator fans
turned off. This allows for the removal of latent
de-icing heat from the evaporator coils, and freezes
any remaining moisture that might otherwise be
blown into the container.
3.3.18 Automatic Defrost
In perishable mode, perishable-pull down mode, or
frozen mode, automatic defrost starts with an initial de-
frost set to three hours and then adjusts the interval to
the next defrost based on the accumulation of ice on the
evaporator coil. In this way, defrosts are scheduled to
occur only when necessary.
In frozen mode, once the frozen set point has been
reached, automatic defrost will set the time interval to 12
hours for the first two defrosts, and then adjust to 24
hours thereafter.
All defrost interval times reflect the number of com-
pressor run time hours since the last defrost cycle. The
minimum defrost interval in the automatic setting is
three hours while the maximum is 24, refer to Defrost In-
tervals for more information, Section 3.3.20.
In frozen mode the amount of actual time necessary to
accumulate defrost interval time will exceed the defrost
interval time by a factor of two to three depending on the
compressor duty-cycle. Defrost interval time is only ac-
cumulated when the compressor is running, and the De-
frost Termination Sensor (DTS) reads less than 10 C
(50 F), at which point the Defrost Termination Thermo-
stat (DTT) is considered closed.
If defrost does not terminate correctly and temperature
reaches the set point of the Heat Termination Thermo-
stat (HTT), the HTT will open to de-energize the heaters
(AL59 & AL60). If the HTT does not open and termina-
tion does not occur within two hours, the controller will
terminate defrost. AL60 will be activated to inform of a
possible DTS failure.