General Information
Duplex Compressor Sequencing Four methods (Two fixed sequence methods, a balanced start and hour’s method, and a no staging method) are provided for order of a compressor sequencing on CTV Duplex chillers. The desired method is selectable at startup via the service tool. The application can decide to either balance the wear burden among the unit’s compressors, to start the most efficient compressor, or to simultaneously start and stop both compressors to minimize startup pull down time. Each method has specific applications were it can be used advantageously.
If one compressor is locked out, in restart inhibit, or generally not ready to start, the available compressor will be started.
Fixed Sequence – Compressor 1/ Compressor 2 (Default mode)
If the chiller is in the Auto mode and all interlocks have been satisfied, compressor 1 will be started based on the leaving water temperature rising above the “Differential to Start” setting. Compressors 2 will stage on when the overall chiller average capacity exceeds Stage ON Load point for 30 seconds. The stage on load point is adjustable (via service tool) up to 50%. The default is 40% which means that a single compressor would have to load to 80% (the average would be 40%) 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 (via service tool) (default = 30%, range from 0 to 50%). Compressor 2 will be shut down and compressor 1 will run until water temperature drops below the differential to stop. Before shutting down, compressor 2 will be unloaded and compressor 1 will be loaded to maintain the same average capacity command.
When running chilled water temperature at selected conditions, the downstream compressor usually will be the most efficient compressor to operate at part load because compressors on Duplex chillers are not sized exactly the same.
Note: The following description assumes compressor 1 is the down stream compressor.
Figure 12. CDHF/G sequence of operation: lead 1/lag 2
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