Operating instructions

TM 11574A-OI
2-44
V. NORMAL OPERATING PROCEDURES
2-16. Modes of Operation
The operational software responds to various inputs. These inputs come from the temperature
and pressure sensors, the temperature set point, the settings of the configuration variables, and
the function code assignments. The action taken by the operational software will change if any
one of the inputs changes. Overall interaction of the inputs is described as a “mode” of operation.
The modes of operation include perishable (chill) mode and frozen mode. Descriptions of the
controller interaction and modes of operation are provided in the following sub paragraphs.
a. Temperature Control -- Perishable Mode.
With configuration variable CnF26 (Heat Lockout Temperature) set to +14F (-10 C), the
perishable mode of operation is active with set points above +14 F (-10 C). With the variable
set to +23 F (-5 C), the perishable mode is active above +23 F (-5 C) (refer to Table 2-4).
When in the perishable mode, the controller maintains the supply air temperature at set point,
the SUPPLY indicator light will be illuminated on the display module, and the default reading
on the display window will be the supply temperature sensor reading. When the supply air
temperature enters the in-range temperature tolerance (as selected at function code Cd30),
the in-range light will energize.
b. Evaporator Fan Operation.
Opening of an evaporator fan internal protector will shut down a unit with Normal Evaporator
Fan Operation. (CnF32 set to 2EFO). On units equipped with Single Evaporator Fan
Capability (CnF32 set to 1EFO), additional relays are installed to allow the unit to continue to
operate on a single fan (refer to Table 2-4).
c. Defrost Interval.
Controller function code Cd27 sets two modes for defrost initiation -- either user-selected
timed intervals or automatic control. The user-selected values are 3, 6, 9, 12, or 24 hours
and the factory default for the timed interval is 12 hours (some units may be configured to
allow defrost to be disabled altogether; in this case a user-selected value of OFF will be
available) (refer to Table 2-6). In perishable mode, perishable-pull down mode, or frozen-pull
down mode, automatic defrost starts with an initial defrost set to three hours and then adjusts
the interval to the next defrost based on the accumulation of ice on the evaporator coil. In
this way, defrosts are scheduled to occur only when necessary. Once set point has been
reached in frozen operation, the automatic selection will set the time interval to 12 hours for
the first two defrosts once the return probe is reading below the frozen set point and then
adjust to 24 hours thereafter. All defrost interval times reflect the number of compressor
runtime hours since the last defrost de-ice cycle. The minimum defrost interval under the
automatic setting is three hours, while the maximum is 24. In frozen mode, the amount of
wall-clock time necessary to accumulate a given amount of defrost interval time will exceed
the defrost interval time by a factor of two to three depending on the compressor duty-cycle.
Defrost interval time is not accumulated in any mode until the defrost termination sensor
reads less than 50 F (10 C).
d. Failure Action.
Function code Cd29 may be operator set to allow continued operation in the event the control
sensors are reading out of range. The factory default is full system shutdown (refer to Table
2-6).
e. Generator Protection.
Function codes Cd31 and Cd32 may be operator set to control start-up sequence of multiple
units and operating current draw. The factory default allows on demand starting of units and
full current draw (refer to Table 2-6).