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Something to think about
The rules may be changing on mixing POE and mineral oils together. The original reason for removing the mineral oil from the system had little to do with the mineral oil itself, and more to do with the paraffin waxes that used to be used in the motor windings of (semi)hermetic compressors. The mineral oil would dissolve the wax and circulate it in the system, and unless the mineral oil was removed from the system the waxes were still there. The HFC refrigerants couldn't dissolve much paraffin wax, and it dropped out in the first cold spot in the system (expansion device) and could plug up the system. The good news is that the waxes have been eliminated from the motor windings since the mid 1990s, so the original reason for removing the mineral oil is gone.
The other good news is that a typical line set doesn't hold much oil, so even if there were waxes used in the motor windings, there won't be enough remaining in the line set to cause problems. Blowing out any excess oil with nitrogen makes any potential problems even smaller. I really doubt you will find any problems from residual mineral oil in a system that reused the line set, whether it was flushed or not, or just blown out with nitrogen.
We have been doing a lot of work looking at what happens with oil circulation in systems when mixing mineral oil and POE together while using HFC refrigerants, and have given a series of technical papers at ASHRAE meetings. As little as 15% of POE added to mineral oil allows the mineral oil to mix into liquid HFC refrigerants. Our work was the basis for the recommendation to add some POE to R-422 systems if the oil circulation was not good enough. We are still doing more work in this area, and are learning even more about what the true limits of residual mineral oil needs to be to make a system running on HFC refrigerants have good oil circulation.
Eventually this new information will filter through the system and be picked up by the compressor and equipment manufacturers, and then make it into the service training programs. Until then, you may well hear people recommend that all the mineral oil should be removed when converting from mineral oil to POE. There is nothing wrong with this, it just may not be necessary any longer.
Rob Yost
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