How many people are having flow loss issues with the Liebert XDC Chillers that use R-407C to cool the pumped R-134a? Also, is there anyone who has NEVER had this occur?

We've determined multiple times that when an additional compressor stages on, the chilled R-134a pressure drops so fast that it loses subcooling at the pump inlet and flashes, even with a vessel half filled with liquid a few feet above the pumps. This loss of subcooling (flashing) causes the differential pressure switch to sense loss of flow. When this occurs and nobody is around, the server room temp gets out of control. When a technician is there, the SMA-12 test instrument is used to take control of the hot gas valves (Sporlan SDR valves) to prevent abrupt changes in pressure after an additional compressor stages on. It is a slow tedious process to keep the chiller running, by only moving the SDR valve for 1 to 4 seconds at a time, about once every minute or so. If the change happens too quickly, you lose flow and the compressors shut off, then you start all over again.

The plate & frame heat exchanger is so efficient that it makes the R-134a flow become very sensitive to changes in the R-407c temperature changes.

Does anyone have any suggestions as to the root cause of the problem?

Has anyone come up with a solution to this problem?

We have the XDC 160 (46 ton) chiller with 7 overhead XDO cooling coils. We have 2 bypass flow regulators (Griswold valves) and have tried all of the different combinations of closing the ball valves at one or both or neither bypass regulators. We don't have a pump curve or any information to verify the actual gpm of flow, but we're told that our flow is good because we have about 35 psig increase through the pump.