I had a thought today, no it's OK it didn't hurt at all, regarding pump down operation on larger systems.

My experience of them is in relation to shell and tube water chillers using semi-hermetics, I am not including the smaller refrigeration units in this.

Re-reading papers concerning compressor losses due to oil starvation or dilution I began pondering the effects of repeated pump downs after pressure increases due to temperature rise within the evaporator. I appreciate that leaking solenoid valves will make the problem worse but that is another matter.

With these repeated pump downs does, or will, the oil level be lowered to a dangerous degree prior to the next start proper, bearing in mind that most oil safety switches have a 2 minute time delay before activation.

Also, of course the repeated starts do not help the state of the electric motor or motor starting components.

On a large system and assuming that a continual pump down is required, as against a once-only pump down, would it make sense to install say a small hermetic compressor across the main compressor purely to maintain the required pump down pressure?

Is this type of protection already in use or can anyone see a use for it?