62-61753-21
Defrost based on coil condition will be initiated when:
blockage is sufficient to cause an air pressure dif-
ferential across the coil great enough to close the
contacts of the defrost air switch(DAS) .
OR
when the RAT - DTT temperature is greater than
20°C (36°F) for 15 continuous minutes. This meth-
od of defrost initiation will only be used for 3 con-
secutive defrosts. After three consecutive defrost
initiations by this method the counter will be reset
to allow this defrost initiation again after a defrost
cycle is initiated by another method (timer, air
switch or manual).
TIP
Ice is not the only thing that will cause the air
differential to increase across the evaporator
coil. Shrink wrap, paper, plastic bags, and oth-
er such items when caught in the return air
stream and pulled up against the evaporator
coil or the return air grille can also cause the
DAS contacts to close.
Time interval between defrosts is selected in the mi-
croprocessor Functional 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 engine is in an off cycle or
DTT is greater than 4.4°C (40.0°F).
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 0.0°C (32.0°F) and SAT is less than 7.2°C
(45.0°F). Defrost will be accomplished by shutting
down the refrigeration system while the heaters
(HTCON1 & HTCON2) are energized.
Natural Defrost Mode uses a combination of the
heat from the product and the electric heaters to
melt any ice.
Natural defrost is available when:
the Natural Defrost microprocessor Configura-
tion is set to YES and
defrost is initiation by closing of the DAS con-
tacts with RAT greater than or equal to 4.0°C
(39.2°F) and
at least one normal defrost has been performed
since power up and
this initiation will not result in more than 3 natu-
ral defrosts between normal defrosts
when a natural defrost is initiated, the unit will op-
erate 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.
3. Defrost Termination
When in either defrost mode, defrost will terminate
when the DTT rises to 12.8°C (55°F). When operating
in Return Air 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 the RAT.
During Defrost Termination, the heaters will de-
energize, and the engine will go to low speed. "dF" will
continue to be displayed in place of refrigerated com-
partment Temperature, and the Cool LED will come on
when the compressor starts.
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 re-
frigerated compartment, but will wait for the evaporator
to cool down, up to a maximum of 8 minutes. Once the
evaporator fan has started, "dF" will no longer be dis-
played, and refrigerated compartment Temperature
will again be shown in the main display. If there is a
DTT sensor alarm present, defrost will terminate after
10 minutes.
NOTE
Defrost Mode uses a Defrost Duration Timer
that allows for a maximum of 45 minutes in de-
frost. If defrost is not terminated during the 45
minutes, the microprocessor will end the de-
frost cycle, and show the alarm "DEFROST
NOT COMPLETE". When this occurs, the De-
frost Interval Timer is set for 1.5 hours, at
which time the unit will go into defrost again.
4.6 USER SELECTED OVERRIDE OPERATION
Three optional software override programs are availa-
ble 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-
paragraphs.
4.6.1 Supply Air Limit Control
Under supply air temperature control the compressor
suction modulating valve (CSMV) is modulated to
maintain supply air to a minimum in accordance with:
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