Bitzer screw oil return from accumulator problem

[Drew]

Hey all,

I have two bitzer screw package units running a -24 C freezer.
The systems are independant of each other mechanically. The one unit runs well , while the other im having issues with oil return from the accumulator.
The oil that gets trapped in the accumulator drains to the bottom of the SA and then, via an oil return drain at the bottom of the SA, which is then connected to the sump of the screw, is drawn back to the compressor. In the oil return line is a sight glass and oil filter. From what i can understand the slight pressure difference between the sump and SA drives the oil back to the sump.
I have changed the oil filter , but still, after a period of weeks the entire oil charge ( from the oil sep) collects in the accumulator and the sytem shuts down on low oil level. I have seen the oil return down the line watching the sight glass so the oil is returning , so the pipe isnt restricted , but still over time the oil return seems to stop , while the other system has never had this fault. Im thinking that somehow the pressure difference that drives the oil return diminishes, but cant see how.
Between the SA and the compressor in the main suction is a filter cartridge , which has had the cores removed after a few months after the install. Could this assist in creating a slightly bigger pressure difference that would drive the oil better? Should i refit them?

[750 Valve]

Yes that suction core would provide a little more PD in the suction aline and may be the difference between install and now however I think you need to look at the design of the accumulator (assuming you mean a DX system accumulator and are not talking about a low pressure vessel - sometimes referred to as an accumulator) and its oil return a little more. Did it come on the unit from Bitzer? Usually they have a design capable of allowing oil to be swept up off the bottom of the accumulator and drawn up into the suction line with the velocity of the refrigerant.

I have seen some home made accumulators with a design similar to the one you describe however they are ALWAYS mounted above the height of the compressor and rely on gravity to drain the oil back into the suction line before the compressor.

What sort of Bitzer screw? Most refrig application ones (HSK) rely on a high pressure oil circuit and do not have a sump, I am assuming you must have a CSH type unit if it has an oil sump.

[Drew]

This would be the style with the high pressure oil circuit. (with out the sump). When i mentioned the sump earlier i should be saying suction of the compressor body, because as you said it doesnt really have an oil sump. The system is the complette Bitzer made up condensing unit, no home made pieces. I understand the standard SA that sucks up oil and sends it back up the suction line, but this style seems to rather have a drain at the base of the SA , draining back based on pressre difference and not returning up the suction line or both. ?
Either way the oil eventually collects in the SA.