Thermo King 098203, Maintenance Manual

Page: 130 / 264

Operating Theory
128
Probe Test
The controller constantly monitors t supply sensor, return sensor and evaporator coil sensor to determine
when to initiate a demand defrost. If a demand defrost is requested and defrost has occurred within last 90
minutes, the controller initiates a probe test to check for a defective sensor.
During a Probe test, the Display shows “PROBE TEST PLEASE WAIT”. The controller operates the unit
on high speed evaporator fans only for 5 minutes. All sensor temperatures are then compared:
• Sensors with large temperature differences are discarded from the control algorithm. The controller
then activates the appropriate Alarm codes to identify the defective sensor(s).
• If no sensors are found defective, controller display shows “RUNNING WITH HIGH SUPPLY
DIFFERENCE” warning.
Sensor errors recorded during a probe test are cleared when the next Defrost is initiated or U
NIT
O
N
/O
FF
switch is turned O
FF
.
NOTE: A manual probe test can be performed by a technician by selecting “SENSOR CHECK” from
the Manual Test Function menu.
Dehumidify Mode
During Chill mode operation, a dehumidification system is available to reduce the relative humidity in the
container to the desired humidity setpoint. The Dehumidify mode option is turned on from Setpoint menu
of the controller. The relative humidity setpoint can be set from 60 to 99 percent from the Setpoint menu.
NOTE: The use of the Dehumidify mode should be established by the shipper.
Changing the humidity control from off to DEHUM in the setpoint menu activates the dehumidify control
algorithm. When the Dehumidify mode is on, the supply air temperature must be in-range to activate
dehumidification:
• When the humidity level is 2 percent or more above setpoint and the Digital Control valve has
reduced the unit cooling capacity to 85 percent, the controller pulses the electric heaters on and off.
This increases the cooling load on the evaporator coil, thereby causing the coil to become even colder
and condense more moisture from the container air.
Continuous Temperature Control Operation
Chill Loads (Controller Setpoint at -9.9 C [14.1 F] and Above):
The controller regulates the compressor, Digital Control valve and electric heaters based on a Control
Temperature Differential (see “General Theory of Operation” in this chapter for more detail). This means
the unit operating mode can
not be predicted based only on the setpoint and supply air temperature.
At setpoints of -9.9 C (14.1 F) and above, the controller operates the unit on:
• Cool mode with Modulation.
• Controller energizes the vapor injection valve continuously when the cool capacity is 100 percent.
• Heat mode (electric heaters pulse on and off on a 60 second duty cycle).
• Defrost mode (electric heaters on, evaporator fans off).