Carrier 69NT40-541-505, Operation And Service Manual

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4–9 T-368
4.3.16 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 con-
denser fan will be cycled by the control-
ler to maintain proper condensing
pressure, refer to
Section 4.4.4 .
a. When the return air temperature is above set
point and decreasing, the unit will be cooling
with the condenser fan motor (CF), compressor
motor (CH), evaporator fan motors (ES) ener-
gized and the white COOL light illuminated (see
Figure 4.9 ).
b. When the return air temperature decreases to a
predetermined tolerance above set point, the
green INRANGE light is illuminated.
c. When the return air temperature decreases to
0.2°C (0.36°F) below set point, contacts TC and
TN are opened to de-energize the compressor
and condenser fan motors. The white COOL
light is also de-energized.
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
(0.36°F) above set point and the three minute off
time has elapsed, relays TC and TN are energized
to restart the compressor and condenser fan motors.
The white COOL light is also illuminated.
Figure 4.9 Frozen Mode
4.3.17 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 depending upon
the reason for the defrost or model number configura-
tion. The first is de-icing of the coil, the second is
defrost due to a probe check cycle and the third is a
snap freeze process based on the unit model configu-
ration.
Defosting the coil consists of removing power to the
cooling components (compressor, evaporator fans,
and condenser fan), closing the SMV, and turning
on the heaters, which are located below the evapo-
rator coil. During normal operation, de-icing will con-
tinue 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 tempera-
ture efficiently.
If defrost was initiated by the probe check logic,
then a Probe Check is carried out after the comple-
tion of the defrost cycle. A Probe Check is initiated
only when there is an inaccuracy between the con-
troller temperature sensors. For more information
on Probe Check refer to Section 5.7 .
Snap Freeze allows the system to cool for a period
of time after de-icing, with the evaporator fans
turned off and is only carried out if configured by
model number. Snap-Freeze allows for the removal
of latent de-icing heat from the evaporator coils, and
freezes any remaining moisture that might other-
wise be blown into the container.
4.3.18 Defrost Operation
Initiation of defrost is dependent on the state of the
Defrost Temperature Sensor (DTS). When the (DTS)
senses a temperature less than 10°C (50°F) the defrost
options become active and the timer is engaged for the
initiation of the defrost cycle. The defrost time accumu-
lates when the compressor is running. In the perishable
mode this is the same as real time as the compressor
in general runs continuously. In frozen mode the actual
time necessary to count down to the next defrost will
exceed the defrost interval depending on the compres-
sor duty-cycle.
When the defrost mode is in the active state, defrost
can be initiated when any one of the following addi-
tional conditions become true:
1. Manually: A manual defrost is initiated by press-
ing the MANUAL DEFROST/INTERVAL key for
greater than 5 seconds.
2. Timer: The Defrost Interval Timer reaches the
user selectable Interval. The user-selected
intervals are (OFF), 3, 6, 9, 12, 24 hours, AUTO,
or PuLS; factory default is 3 hours. Refer to
code select CD27 (Table 3-5).
CONTROL TRANSFORMER
POWER TO
CONTROLLER
SIGNAL TO
CONTROLLER
SIGNAL TO
CONTROLLER
ENERGIZED
DE-ENERGIZED
FOR FULL DIAGRAM AND
LEGEND, SEE SECTION 7
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