General Information
Fixed Sequence – Compressor 2 / Compressor 1
If the chiller is in the Auto mode and all interlocks have been satisfied, compressor 2 will be started based on the leaving water temperature rising above the “Differential to Start” setting. Compressors 1 will stage on when the overall chiller average capacity exceeds Stage on Load point for 30 seconds. The stage on load point is adjustable up to 50%. The default is 40% which means that a single compressor would have to
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 shut down 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.
load to 80% (the average would be 40%) before the second compressor
Figure 13. CDHE/F/G sequence of operation: lead 2 lag 1
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