Operating instructions

TM 11574A-OI
2-45
f. Condenser Pressure Control.
When configuration variable CnF14 is set to “In,” the condenser pressure control logic is
activated to maintain discharge pressures above 130 psig when the ambient temperature is
low. The logic turns the condenser fan on or off in accordance with the condenser pressure
transducer reading (refer to Table 2-4). The function is enabled when the following
conditions are met:
1. The ambient sensor reading is less than or equal to 80 F (27 C), and
2. Voltage/Frequency ratio is less than or equal to 8.38.
When the above conditions are met, either pressures or timers may dictate a change of state
from OFF to ON or ON to OFF. If the condenser fan is OFF, it will be energized if saturated
condensing pressure is greater than 200 psig OR if the condenser fan has been OFF for a
variable time period of up to 60 seconds depending on the ambient temperature. As the
ambient temperature increases, the amount of time that the condenser fan is energized will
correspondingly increase towards the maximum. If the condenser fan is ON, it will de-
energize only if the saturated condensing pressure is less than 130 psig and the condenser
fan has been running for a minimum of thirty seconds depending on the ambient temperature.
g. Perishable Mode -- Conventional.
The unit is capable of maintaining supply air temperature to within +/- 0.5 F (+/- 0.25 C) of set
point. Supply air temperature is controlled by positioning of the suction modulation valve
(SMV), cycling of the compressor, and cycling of the heaters. When pulling down from a
temperature that is more than 9 F (5 C) above set point, the SMV will open to reduce the pull
down time. However, pressure and current limit functions may restrict the valve if either
exceeds the preset value. The operational software is designed so the SMV will begin to
close as the set point is reached. The SMV will continue to close and restrict refrigerant flow
until the capacity of the unit and the load are balanced. If the temperature drops below the
set point, the compressor will remain running for a few minutes. This is to accommodate any
initial undershoot which might occur. After this time has expired and the temperature is 0.4 F
(0.2 C) or greater below the set point, the compressor will be turned OFF. If the temperature
drops to 0.9 F (0.5 C) below set point, the heaters will be energized. The heaters will de-
energize when the temperature rises to 0.4 F (0.2 C) below the set point. The compressor
will not restart until the temperature rises to 0.4 F (0.2 C) above the set point and three
minutes have elapsed since the last compressor turn off.
h. Perishable Mode -- Economy.
The economy mode is an extension of the conventional mode and is applicable to units with
two speed evaporator fan motors. The mode is activated when the setting of function code
Cd34 is “ON.“ Economy mode is provided for power saving purposes. Economy mode could
be utilized in the transportation of temperature tolerant cargo or non-respiration items that do
not require high airflow for removing respiration heat. There is no active display indicator that
economy mode has been activated. To check for economy mode, perform a manual display
of code Cd34. In order to achieve economy mode, a perishable set point must be selected
prior to activation. When economy mode is active, the evaporator fans will be controlled as
follows:
At the start of each cooling or heating cycle, the evaporator fans will be run in high speed for
three minutes. They will then be switched to low speed any time the supply air temperature
is within +/- 0.45 F (/- 0.25 C) of the set point and the return air temperature is less than or
equal to the supply air temperature +5.4 F (+ 3 C). The fans will continue to run in low speed
for one hour. At the end of the hour, the evaporator fans will switch back to high speed and
the cycle will be repeated.