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The Lag Unit Pump Select configuration must be set
consistently. If pump control is NOT being used, set Con-
figuration
RSET
LAGP to 1. If pump control IS being
used, set Configuration
RSET
default value. This must be set in both the master and slave
chillers, and it must be consistent in both.
The control system cycles com-
Capacity Control —
pressors and minimum load valve solenoids (if equipped) to
maintain the user-configured leaving chilled fluid temperature
set point. Entering fluid temperature is used by the main base
board (MBB) to determine the temperature drop across the
cooler and is used in determining the optimum time to add or
subtract capacity stages. Entering fluid temperature, space
temperature (requires additional sensor), or outdoor-air temper-
ature reset features can automatically reset the leaving chilled
fluid temperature set point. It can also be reset from an external
4 to 20-mA signal (requires energy management module).
The control has an automatic lead-lag feature built in for
circuit and compressor starts. If enabled, the control will deter-
mine which circuit (Configuration
compressor to start to even the wear. The compressor wear
factor (combination of starts and run hours) is used to deter-
mine which compressor starts.
Compressor Wear Factor = (Compressor Starts) + 0.1
(Compressor Run Hours)
In this case, the circuit with the lowest average compressor
wear factor (the average of the wear factors of all available
compressors in the circuit) is the circuit that starts first. The
compressor within the circuit with the lowest wear factor is the
first to start. If the automatic lead-lag function for the circuit is
not enabled [Configuration
leads), 2 (Circuit B leads), or 3 (Circuit C leads)], then the se-
lected circuit will be the first to start. Again, the compressor
with the lowest wear factor within the circuit will be the first to
start. If Minimum Load Control is enabled (Configuration
UNIT
HGBP=1), the valve will be operational only during
the first stage of cooling.
Once the lead compressor has been started, the lag compres-
sors will be determined by the wear factor and loading
sequence selected. If equal loading is selected, (Configura-
tion
OPTN
LOAD=0), the circuit with the lowest average
wear factor for the available compressors will start next, with
the compressor with the lowest wear factor starting. The con-
trol will attempt to keep all circuits at approximately the same
number of compressors ON. For this option to function proper-
ly, all circuits must have the same number of compressors
available. If a circuit compressor is not available due to an
alarm condition or demand limit, the capacity staging will
change to staged. If staged loading is selected, (Configura-
tion
OPTN
LOAD=1), the started circuit will continue to
turn on compressors according to the lowest wear factor until
all are on, then start the next circuit with the lowest average
wear factor. If Minimum Load Control is enabled for close
control (Configuration
UNIT
COMPRESSOR
A1
A2
A3
B1
B2
B3
C1
C2
C3
LAGP to 0, which is the
OPTN
LLCS=0) and
OPTN
LLCS=1 (Circuit A
HGBP=2), the valve will be
Table 15 — Compressor Starts and Run Hours
STARTS
RUN HOURS
25
249
22
237
26
128
41
453
38
138
35
297
93
103
57
98
61
99
available at all stages for better temperature control. If
Minimum Load Control is enabled for high ambient control
(Configuration
only when a high pressure override is active for that circuit.
The electronic expansion valves provide a controlled start-
up. During start-up, the low pressure logic in the lead circuit
will be ignored for 5 minutes to allow for the transient changes
during start-up. As additional stages of compression are
required, the processor control will add them. The following
example is based on a 30RB225 machine, which has three
25-ton compressors in each circuit. See Table 15.
Each example below has different configurations and is
intended to illustrate the loading sequences possible for normal
operation.
In Example 1 (Table 16), assume the following configurations
are in place:
Configuration
installed and enabled for Start-Up Only
Configuration
Configuration
Since Circuit A has the lowest average wear factor, it will be
the lead circuit. Within the circuit, compressor A3 has the
lowest wear factor and will start first with Minimum Load
Control ON. The next stage will turn OFF the minimum load
control. Stage 3 will start another circuit because of the Equal
Circuit Loading configuration. The next circuit with the lowest
wear factor is Circuit B, and the compressor with the lowest
wear factor is B2. The next stage will be a circuit C compres-
sor. The process continues until all compressors are ON. See
Table 16.
In Example 2 (Table 17), assume the compressor starts and
run hours are the same as in the previous example and the
following configurations are in place:
Configuration
installed and enabled for Start-Up Only
Configuration
Configuration
Since Circuit A has the lowest average wear factor, it will be
the lead circuit. Within the circuit, compressor A3 has the
lowest wear factor and will start first with Minimum Load
Control ON. The next stage will turn OFF the minimum load
control. Stage 3 will start a compressor in the same circuit be-
cause of the Staged Circuit Loading configuration. Compressor
A2 has the next lowest wear factor and will be started next.
Compressor A3 will be next to start. Since all compressors in
Circuit A are ON, the next stage will start another circuit. Of
the remaining circuits, Circuit B has the lowest wear factor, and
the compressor with the lowest wear factor is B2. All of the
Circuit B compressors will be started in the same manner as
Circuit A. Once all Circuit B compressors are ON, then Circuit
C will be started. The process continues until all compressors
are ON. See Table 17.
WEAR FACTOR
49.9
45.7
38.8
86.3
51.8
64.7
103.3
66.8
70.9
20
UNIT
HGBP=3), the valve will be used
UNIT
HGBP=1 . . Minimum Load Control
OPTN
LOAD=0. . . .Equal Circuit Loading
OPTN
LLCS=0 . .Automatic Circuit Select
UNIT
HGBP=1 . . Minimum Load Control
OPTN
LOAD=1. . . Staged Circuit Loading
OPTN
LLCS=0 . .Automatic Circuit Select
CIRCUIT AVERAGE
WEAR FACTOR
44.8
67.6
80.3
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