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b. Defrost based on time
Time interval between defrosts is selected in the Func-
tional Parameters. The parameter value may be set to
1.5, 3, 6, or 12 hours. The factory default setting is 6
hours.
NOTE
The defrost timer will not count when the unit
is in defrost, the unit is in a Start/Stop off
cycle or the DTT is greater than 40.0°F
(4.4°C).
4.9.2 Defrost Modes
There are two Defrost Modes available:
NOTE
When the unit is in either defrost mode and
in Engine operation:
• the condenser fan will operate to supply air for
the radiator.
• the engine will operate in high speed when
HTCON1 and/or HTCON2 are energized (to
supply power for the heaters).
• the engine will operate in low speed when both
HTCON1 and HTCON2 are de−energized.
Normal Defrost Mode will be entered if RAT is less than
32.0°F (0.0°C) and SAT is less than 45.0°F (7.2°C).
Defrost will be accomplished by shutting down the
refrigeration system while the heaters (HTCON1 &
HTCON2) are energized.
Electric heaters will be used for all defrost with fans off
when temperatures are less than 32°C (89.6°F) for
RAT and less than 45°C (113°F) for SAT. For defrost
requests at temperatures higher (RAT > 35°C, SAT >
45°C, heaters will not be energized.
Natural Defrost Mode uses a combination of heat from
the product and the electric heaters to melt ice.
Natural defrost is available when:
• the Natural Defrost Configuration is set to YES
and
• defrost is initiated by closing of the DAS contacts
with RAT greater than or equal to 39.2°F (4.0°C)
and
• at least one normal defrost has been performed
since power up and
• this initiation will not result in more than 3 natural
defrosts between normal defrosts
When a natural defrost is initiated, the unit will operate
for the first five minutes with the evaporator fan and
HTCON2 de−energized, and only HTCON1 energized.
After 5 minutes the evaporator fan is energized while
both HTCON1 & HTCON2 are de−energized.
4.9.3 Defrost Termination
When in defrost mode, defrost will terminate when the
DTT rises to 55°F (12.8°C).
When operating in Natural Defrost mode, defrost will
continue for a minimum of 5 minutes, a maximum of 10
minutes, or until the SAT is equal to or higher than RAT.
During Defrost Termination, the heaters will de−ener-
gize, and the engine will go to low speed.
The EVXV will open, and the evaporator coil will begin
to cool down. The evaporator fans will not come on
right away, so that warm air is not blown into the refrig-
erated compartment, but will wait for the evaporator to
cool down, up to a maximum of 8 minutes.
If there is a DTT sensor alarm present, defrost will ter-
minate after 10 minutes.
NOTE
Defrost Mode uses a Defrost Duration Timer
that allows for a maximum of 45 minutes in
defrost. If defrost is not terminated during
the 45 minutes, the control system will end
the defrost cycle, and show the alarm
“DEFROST NOT COMPLETE”. When this
occurs, the Defrost Interval Timer is set for
1.5 hours, at which time the unit will go into
defrost again.
4.10 USER SELECTED OVERRIDE OPERATION
Three optional software override programs are avail-
able to the user. These programs include: Supply Air
Limit Control, Temperature Range Lock and Product
Shield. Information on how the unit operates under
these programs is provided in the following sub-para-
graphs.
4.10.1 Supply Air Limit Control
Supply Air Limit Control is available when the setpoint
is at or between 32 to 65°F (0 to 18.4°C) and the unit is
operating in cool.
Supply Air Limit is NOT a positive temperature control,
drift above and below the Supply Air Limit will be seen
as the system balances out.
Supply Air Temperature control will be accomplished
by varying the position of the CSMV.
Supply Air Limit Configuration values can be set to
allow for a different limit for Start-Stop than used in
Continuous Run.
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