T-340
4–12
4.3.21
Automatic Defrost
In perishable mode, perishable-pull down mode, or frozen mode, automatic defrost starts with an initial defrost 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 compressor run time hours since the last defrost cycle. The minimum
defrost interval in the automatic setting is three hours while the maximum is 24. See
, Defrost Inter
-
vals 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 accu
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mulated when the compressor is running, and the Defrost Termination Sensor (DTS) reads less than 10°C (50°F),
at which point the Defrost Termination Thermostat (DTT) is considered closed.
If defrost does not terminate correctly and temperature reaches the set point of the Heat Termination Thermostat (HTT),
the HTT will open to de-energize the heaters (AL59 & AL60). If the HTT does not open and termination does not occur
within two hours, the controller will terminate defrost. AL60 will be activated to inform of a possible DTS failure.
4.3.22
Defrost Initiation
Initiation of defrost is dependent on the state of the Defrost 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 tempera
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ture is too high for ice to build up. Defrost can only occur when the temperature sensed by the DTS is low
enough to CLOSE the DTT.
If the DTT is closed, the unit is in Perishable or Frozen Mode, and the Controlled Atmosphere Vent is closed (if
applicable), then defrost can be initiated when any one of the following additional conditions become true:
1. Manual defrost is initiated by the operator. The Manual Defrost Switch (MDS), located on the front of the
unit, is a momentary toggle switch that must be held closed until defrost is initiated. The MDS is ignored
during Pretrip, and the DTT must be considered CLOSED for MDS activation to work.
Manual defrost can also be 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.
2. The Defrost Interval Timer reaches or exceeds the Defrost Interval.
3. During Pretrip, defrost may occur during tests P-8, and P-10. Defrost is forced during Pretrip test P-9.
4. Temperature Probe Diagnostics logic determines that a Probe Check is necessary based on temperature
values currently reported by the supply and return probes.
5. An Initiate Defrost command is sent via communications.
6. The microprocessor can determine if defrost is required by calculating the difference between return air tem
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perature and supply air temperature (Delta T). If the temperature difference between return air and supply
air is too great, it indicates reduced airflow over the evaporator coil, and a defrost cycle may be required:
a. In Perishable Pull Down - Defrost will be initiated if Delta T increases to greater than 12°C, DTT is
closed, and 90 minutes of compressor run time have been recorded.
b. In Frozen Mode - Defrost will be initiated if Delta T increases to greater than 16°C, DTT is closed, and 90
minutes of compressor run time have been recorded.
c. In Perishable Steady State - A baseline Delta T is used to determine when defrost should be activated.
The baseline is recorded after completion of an initial defrost cycle. In order to record a baseline Delta T,
the unit must be cooling, and the evaporator fans and heaters must remain in a stable state for a period
of five minutes. Defrost will then be initiated if Delta T increases to greater than 4°C above the baseline,
the DTT is closed, and 90 minutes of compressor run time have been recorded.
NOTE
During defrost, the Defrost Relay is energized, the orange DEFROST light is illuminated, and the orange
HEAT light is illuminated.
