We are running Bitzer CSH9571 screw compressors on our 280 ton R134a chiller. We using their infinite load control strategy where the loading and unloading solenoids are pulsed to increase or decrease load. Even when no load or unload pulses are given we see the compressor load up, indicated by current increase and suction pressures dropping slightly. This continues until the evaporator outlet temperature falls below setpoint and the logic fires the unloading solenoid to reduce capacity. This unloads the compressor and we see reduced current, reduced cooling capacity and suction pressure goes up a little. The cycle then starts over again.

We have replaced the loading solenoid valve, thinking that the valve might be leaking and allowing oil pressure past the seat and allowing the slider valve piston to increase load slowly. No change was observed with a new loading solenoid valve in place.

We used to have significant liquid flood back issues that were believed to contribute to the automatic loading due to liquid refrigerant expanding in the slider valve piston chamber causing compressor loading. We have since installed much better electronic expansion valves controlling to much higher superheat and we are 100% no liquid is being returned to the compressor. This did not seem to impact the uncontrolled loading at all.

We have checked to make sure that the loading solenoid valves do not have any residual voltage on the coils or unintended PLC control actuation that could cause the valve to be open when it shouldn’t or lift it off the seat enough to allow for leakby. We even removed all the solenoid coils and observed no change. The compressor would still load from a fully unloaded state to a fully loaded state with no control input.

The auto loading behavior is less severe at lower loads (<50%) but gets pretty aggressive as you load the compressor above 70%.

What else could cause a compressor to automatically load up like this?