Carrier 69NT40-561-201, Operation & Service Manual

Page: 38 / 117

3 − 9 T-364
24 VOLT POWER
ENERGIZED
DE-ENERGIZED
TN
ST
F
HPS
CH
IP-CM
TC
CF
TE
IP-EM1
EF
IP-EM2
HTT
TH
HR
ESV
TS
TV
EF
ES
Figure 3 − 9 Frozen Mode
NOTE
The EEV and DUV are independently operated
by the microprocessor. Complete schematics
and legends are located in Section 7.
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), closing the EEV, and turning on the
heaters, which are located below the evaporator coil.
During normal operation, 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 temperature 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 Section 4.8, Probe Diagnostics for addi-
tional 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 Section
3.3.19, Defrost Intervals for more information.
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.
3.3.19 Defrost Initiation
Initiation of defrost is dependent on the state of the De-
frost Temperature Thermostat (DTT). The functionality
of the DTT is controlled by the system software, based
on the temperature reading of the Defrost Termination
Sensor (DTS). In the case of a failed DTS, the RTS may
be used to determine the state of the DTT.
Defrost cannot be initiated when the DTT is considered
OPEN, because an open DTT indicates that the temper-
ature is too high for ice to build up. Defrost can only oc-
cur when the temperature sensed by the DTS is low
enough to CLOSE the DTT.
If the DTT is closed and the unit is in Perishable or
Frozen Mode, then defrost can be initiated when any
one of the following additional conditions become true:
1. Manual defrost is initiated by the operator. Request
for defrost is ignored during Pretrip, and the DTT
must be considered CLOSED for MDS activation to
work.
Manual defrost is initiated by pressing the MANUAL
DEFROST/INTERVAL key for greater than 5
seconds, or pressing the PRE TRIP key and the ALT
MODE key for greater than 5 seconds. If the two key
method is used the display may show “P” or “Dc”
while waiting for defrost to initiate.