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Operating Principles
The default is 40 percent which means that a single
compressor would have to load to 80 percent (the average
would be 40 percent) before the second compressor starts.
Both compressors will run until chiller average capacity
drops below stage-off load point for 30 seconds. The stage-
off load point is also adjustable. Compressor 1 will be
shutdown and Compressor 2 will run until water
temperature drops below the differential to stop. Before
shutting down, Compressor 1 will be unloaded and
Compressor 2 will be loaded to maintain the same average
capacity command. If chilled water reset is used, the
upstream compressor usually will be the most efficient
compressor to operate at part load. If the leaving water
temperature is reset and the chiller only needs one
compressor, then the upstream compressor would be
running closer to its selection point and will be the most
efficient compressor to operate.
Figure 54. Duplex™ chiller sequence of operation:
lead 2 / lag 1
Evap LWT Falls
Below
Differential to
Stop
Running
(Circuit 2)
Chiller Capacity
Less than 30%
(De-Energize
Cprsr with Most
Starts and Hours)
Running
(Circuit
1 and 2)
Sequencing – Balanced Starts and Hours
When desired to balance the wear between the
compressors, this method will extend the time between
maintenance on the lead compressor. When balanced
starts and hours is selected, the compressor with the
fewest starts will start. If that compressor is unavailable to
start due to a circuit lockout (including restart inhibit) or a
circuit diagnostic, then the other compressor will be started.
The second compressor will stage on when chiller capacity
exceeds the stage-on load point for 30 seconds. When
chiller capacity falls below stage-off load point for
30 seconds, the compressor with the most hours will be
shut off.
78
Auto
Evap LWT Rises
Above Diff to
Start (Start Cprsr
with Fewest
Starts and Hours)
Single Comp
Capacity Greater
than 80% (No
Time Delay)
Figure 55. Duplex™ chiller sequence of operation:
equalize starts and hours
Evap LWT Falls
Below
Differential to
Stop
Running
(Single Circuit)
Chiller Capacity
Less than 30%
(No Time Delay)
Running
(Both Circuits)
Chiller Start Initial Capacity Command
If the Chiller Start Initial Capacity Command is set greater
than the Staging On Boundary, both compressors will cycle
on similar to a Combined Staging Sequence and both
compressors will load up to the initial capacity command. If
there is a reduction in load, the lag compressor will shut off
based on the staging sequence chosen. The simultaneous
start feature only applies when both compressor are off.
This feature is intended to allow a Duplex™ CenTraVac™
chiller to start and load quickly for customers that require
quick restart after a power failure.
Figure 56. Duplex™ chiller sequence of operation:
ready to serve
Evap LWT Falls
Below
Differential to
Stop
Running
(Single Circuit)
Chiller Capacity
Greater than
Staging Off
Boundary
Setpoint
Running
(Both Circuits)
Auto
Evap LWT Rises
Above
Differential to
Stop
Compressor A
Capacity Greater
than 80%
(No Time Delay)
Auto
Evap LWT Rises
Above
Differential to
Stop
On Start, if Initial
Capacity Command
Greater than Staging
On Boundary, then
Start First
Compressor, and
Immediately Start
Second Compressor
Chiller Capacity
Greater than
Staging On
Boundary
Setpoint
CDHH-SVX003C-EN
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