Defrost Operation - Carrier PrimeLINE Operation And Service Manual

4.3.21

Defrost Operation

Initiation of defrost is dependent on the state of the Defrost Temperature Sensor (DTS). When the 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
conditions become true:
1. Manually: While in the Defrost screen, when the Manual Defrost soft key is selected, if conditions will allow
for a defrost, a manual defrost is initiated. The Defrost Indicator light is lit, and the user is brought back to
the Main / Default screen.
While in the Defrost screen, when Manual Defrost soft key is selected, and conditions will NOT allow a
defrost, a pop up message screen appears.
2. Timer: The Defrost Interval Timer reaches the user selectable Interval. The user-selected intervals are 3, 6,
9, 12, 24 Hours, Off, AUTO, Pulse, or AUTO2; factory default is AUTO. See Defrost Interval setting on the
Trip Settings screen.
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
initiation 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 set points, evaporator fan pulsing can be used during Defrost/De-ice
when the "Pulse" option is selected for the Defrost Interval setting on the Trip Settings screen. When
enabled, evaporator fan pulsing will occur based on the unit temperature set point and the Evap Fan
Pulsing Temp setting on the Trip Settings screen. QUEST II also pulses the evaporator fans during
Defrost/De-ice within a narrow perishable set point 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
termination, 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
evaporation 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. Delta T Logic: If the difference between return and supply air temperature (Delta T) becomes too great
indicating possible reduced airflow over the evaporator coil caused by ice buildup requiring a defrost.
a. In Perishable Pull Down - Delta T increases to greater than 12°C, and 90 minutes of compressor run
time have been recorded.
b. In Perishable Steady State – A baseline Delta T is recorded following the first defrost cycle after steady
state conditions are reached, (the unit is cooling, and the evaporator fans and heaters must remain in a
stable state for a period of five minutes). Defrost will be initiated if Delta T increases to greater than 4°C
above the baseline, and 90 minutes of compressor run time have been recorded.
c. In Frozen Mode - Defrost will be initiated if Delta T increases to greater than 16°C and 90 minutes of
compressor run time have been recorded.
When defrost is initiated, the controller closes the EEV, opens contacts TC, TN and TE (or TV) to de-energize the
compressor, condenser fan and evaporator fans. The controller then closes contacts TH to supply power to the
heaters. The orange DEFROST light and heat light are illuminated and the COOL light is also de-energized.
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