Operating instructions

TM 11574A-OI
2-15
2-7. Perishable Mode Heating
If the air temperature decreases 0.9 F (0.5 C) below the set point, the system enters heating
mode. The controller closes the contacts to energize the heaters. The HEAT light is illuminated.
The evaporator fan circulates the warm air throughout the container. When the temperature
reaches 0.4 F (0.2 C) the contact opens to de-energize the heaters. The HEAT light is also de-
energized. The evaporator fan continues to circulate air throughout the container.
2-8. Frozen Mode Cooling
With supply air temperature above set point and decreasing, the unit will be cooling with the
condenser fan motor, compressor motor, evaporator fan motors energized and the COOL light
illuminated. When the air temperature decreases to a predetermined tolerance above set point,
the in-range light is illuminated. Contacts are opened to de-energize the compressor and
condenser fan motors when the air temperature decreases to 0.4 F (0.2 C) below set point. The
COOL light is also de-energized. The evaporator fan motors continue to run to circulate
throughout the container. The in-range light remains illuminated as long as the return air is within
tolerance of set point. When the return air temperature increase to 0.4 F (0.2C) above set point
and the three minute off time has elapsed, relays are energized to start the compressor and
condenser fan motors. The cool light is also illuminated.
2-9. Defrost
The processing of the defrost request is controlled by software that acts as a thermostat. The
defrost cycle may consist of up to three distinct operations. The first is de-icing of the coil, the
second is a probe check cycle, and the third is snap freeze.
Figure 2-12. Defrost Cycle.