Hi guys,

I have a tricky problem that I can not seem to get my head around, please post some suggestion on what might be a cause to this problem.

I have a twin screw compressor rack, both compressors are Bitzer HSK7451-70 models.
My problem is that I have started the compressors up a month ago and they have both been running fine but only recently have had an oil fault issue where there is no oil in oil seperator and compressors have tripped on low oil level in oil seperator.I have checked the oil seperator as there is an upside down plate internally before the oil sep discharge pipe to catch the oil and cause it to fall back to the bottom of oil sep and this seems to be ok. Here are some operating details:

Medium Temperature,
R404A
Lead, Lag Operation/Demand
Air cooled condenser set to maintain head pressure of 260 PSI with VSD driven condenser fans,
Air cooled oil cooler with cooling fan set to cut in at 65 degrees Celsius,

Compressor 1 Cut In: 75 PSI, Cut Out: 40 PSI
" " Unloader 1 C.I: 60 PSI, C.O: 55 PSI
" " Unloader 2 C.I: 54 PSI, C.O: 41 PSI
Compressor 2 Cut In: 85 PSI, Cut Out: 50 PSI
" " Unloader 1 C.I: 60 PSI, C.O: 55 PSI
" " Unloader 2 C.I: 54 PSI, C.O: 51 PSI

The compressors are used for a couple of fast chillers, chilling freshly picked corn. Room temperature set points are +3 degrees C.

What I do know is that the compressors have a check valve in the discharge portion of the screw compressors to prevent compressor reversing in the off cycle. If one compressor is running and the other one is off, could there be a chance that if the check valve is faulty on the compressor thats off, could the oil be flowing past the faulty check valve and through the compressor and into the suction accumulator?

I have checked the oil solenoid to see if there is oil passing through it when the compressor is off but there is no flow through the sight glass which is after the oil solenoid which is a good thing I presume. If oil solenoid was leaking past when compressor is off I could imagine that the oil would flow through compressor and into low side accumulator.

Has anyone had this problem before? I am also wondering if the unloading sequence might be causing an issue but may need some technical help with that. Thanks