Carrier 69NT40-561-300, Operation And Service Manual

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T-365 4–12
4.3.20 Defrost
Defrost is initiated to remove ice buildup from the evaporator coil which can obstruct air flow and reduce the cooling
capacity of the unit. The defrost cycle may consist of up to three distinct operations depending upon the reason for
the defrost or model number configuration. 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 configuration.
Defrosting the coil consists of removing power to the cooling components (compressor, evaporator fans, and
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.
If defrost was initiated by the probe check logic, then the Probe Check is carried out after the completion of the
defrost cycle. A Probe Check is initiated only when there is an inaccuracy between the controller temperature
sensors. For more information on Probe Check refer to
Section 5.9 .
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 otherwise be blown into the container.
4.3.21 Defrost Operation
Initiation of defrost is dependent on the state of the Defrost Temperature Sensor (DTS). When then (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 accumulates 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 compressor duty-cycle.
When the defrost mode is in the active state, defrost can be initiated when any one of the following additional con-
ditions become true:
1. Manually: A manual defrost is initiated by pressing 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 4–2 ).
a. Automatic defrost starts with an initial defrost at three hours and then adjusts the interval to the next
defrost based on the accumulation of ice on the evaporator coil. Following a start-up or after termination
of defrost, the time will not begin counting down until the DTS reading falls below 10°C (50°F). If the
reading of DTS rises above termination setting any time during the timer count down, the interval is reset
and the countdown starts over. The Auto defrost time is reset to three hours start time after every PTI ini-
tiation or trip start interval.
b. Fan Pulsing Logic is used to help prevent ice formation in the drain gutter and drain cup and ice buildup
in supply air channel by using the evaporator fans to blow the warm air onto these areas during unit
defrost. When cooling at lower setpoints, evaporator fan pulsing can be used during Defrost/De-ice when
the “PuLS” option is selected in the Defrost Interval function select code. When enabled, evaporator fan
pulsing will occur based on the unit temperature setpoint and the Evaporator Fan Pulsing Temperature
Setting (Cd60). QUEST II also pulses the evaporator fans during Defrost/De-ice within a narrow perish-
able setpoint range. The logic for each evaporator fan pulsing feature is described below.
c. After a new Defrost Interval is selected, the previously selected Interval is used until the next defrost ter-
mination, the next time the DTS contacts are OPEN, or the next time power to the control is interrupted. If
the previous value or the new value is “OFF”, the newly selected value will be used immediately.
3. Probe Check: If defrost is initiated due to Probe Check immediately following the defrost cycle the evapora-
tion fans are started and run for eight minutes to stabilize the temperature throughout the container. A probe
check comparison is carried out at the end of the eight minute period if any sensor is found out of calibration.
At this time its alarm set is no longer used for control/reorder purposes.
4. Probe Check Logic: The logic determines that a Probe Check is necessary based on temperature values
currently reported by the supply and return probes.
5. Remote: An Initiate Defrost command is sent via communications.