3--4
T-334
Figure 3--4 Control Module
The programming is divided into function codes. Some
of the codes are read only while the remaining codes
may be user configured. The value of the user
configurable codes can be assigned in accordance with
user desired mode of operation. A list of the function
codes is provided in Table 3--5.
To access the function codes, perform the following:
a. Press the CODE SELECT key, then press an arrow
key until the left window displays the desired code
number.
b. The right window will display the value of this item for
five seconds before returning to the normal display
mode.
c. If a longer time is desired, press the ENTER key to
extend the time to 5 minutes.
3.3 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 change.
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.
3.3.1 Temperature Control - Perishable Mode
With configuration variable CnF26 (Heat Lockout
Temperature) set to
-
10C the perishable mode of
operation is active with set points
above -10C (+14
F).
With the variable set to
-
5C, the perishable mode is
active
above -5C (+23F
). Refer to Table 3--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.
3.3.2 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 3--4.
3.3.3 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, 24 hours or AUTO. Some units may be configured to
allow defrost to be disabled altogether; in this case a
user-selected value of OFF will be available. The factory
default for defrost is AUTO. Refer to Table 3--5.
In perishable mode, perishable-pulldown mode, or
frozen-pulldown 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 3 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 10C (50F).
3.3.4 Failure Action
Function code Cd29 may be operator set to select
action the controller will take upon system failure. The
factory default is full system shutdown. Refer to
Table 3--5.
3.3.5 Generator Protection
Function codes Cd31(Stagger Start, Offset Time) and
Cd32 (Current Limit) may be operator set to control start
up sequence of multiple units and operating current
draw. The factory default allows on demand starting (no
delay) of units and normal current draw. Refer to
Table 3--5.
3.3.6 Compressor High Temperature, Low Pres-
sure Protection
The controller monitors compressor discharge
temperature and suction pressure. If discharge
pressure or temperature rises above the allowed limit or
suction pressure falls below the allowed limit the
compressor will be cycled off on a three minute timer.
Condenser and evaporator fans continue to operate
during the compressor off cycle.
1. If compressor dome temperature exceeds 136C
(276.8F) continuously for 5 seconds (high dome
temperature), the liquid injection solenoid valve will
open. When compressor dome temperature then
decreases to 121C (249.8F) or below, the liquid injection
solenoid valve will close.
2. If suction superheat exceeds a 55C range during
unloaded capacity mode operation (suction quench),
open liquid injection solenoid valve. When suction
superheat decreases below 20C range, or if unit leaves
unloaded capacity mode, the liquid injection solenoid
valve will close.
3. If conditions occur that would require the liquid
injection valve to open, valve will close when dome
temperature goes below 136C and suction superheat is
less than 20C range.
3.3.7 Perishable Mode - Conventional
The unit is capable of maintaining supply air
temperature to /-0.25C (+/-0.5F) 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 cooling from a temperature that is more than 2.5C
(4.5F) above set point, the system will be in the
perishable pulldown mode. It will be in economized
operation with a target SMV position of 100% open.
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