Operating instructions

TM 11574A-OI
2-53
(1) Sequence Of Operation – Perishable Mode Cooling
NOTE
In the conventional Perishable Mode of Operation, the evaporator motors run in high
speed. In the economy Perishable Mode, the fan speed is varied.
NOTE
In low temperature ambient, the condenser fan will be cycled by the controller to maintain
proper condensing pressure.
(a) With supply air temperature above set point and decreasing, the unit will be cooling with
the condenser fan motor (CF), compressor motor (CH), evaporator fan motors (EF)
energized, and the COOL light illuminated (see Figure 2-20).
Figure 2-19. Perishable Mode Cooling.
(b) When the air temperature decreases to a predetermined tolerance above set point, the
in-range light is illuminated.
(c) As the air temperature continues to fall, modulating cooling starts at approximately 4.5 F
(2.5 C) above set point (see Figure 2-20).
(d) The controller monitors the supply air. Once the supply air falls below set point and 0%
SMV position is reached, the controller periodically records the supply air temperature,
set point, and time. A calculation is then performed by subtracting the set point reading
from the supply air and multiplying the result by the time reading. The result is negative
number.
When the calculation reaches -250, contacts TC and TN are opened to de-energize.