We have trane oil 22 sent for analysis. The new oil viscosity is 56 cSt. The used oil (from a Trane chiller compressor) is 20 cSt. Based on viscosity only, does that mean the oil needs to be changed immediately? We don't have the lube hours.

It most likely means one of two possible things happened:

The oil was mixed with another lower viscosity oil, or

The refrigerant was not removed from the oil prior to viscosity testing.

The new oil would have never been exposed to refrigerant, so would not have been diluted. Ask the lab how they prepare the samples for viscosity testing to see if they remove the dissolved refrigerant. The 20 cSt viscosity is much lower than it should be if it was mixed with another refrigeration mineral oil. The industry does not use anything lower than a 32 cSt mineral oil. I have never seen a sample of Trane 22 at less than 40 cSt in field samples.

If the lab tested for sulfur, then it is easy to check for oil mixing because Trane 22 is very low in sulfur content (less than 10 ppm). Most other refrigeration mineral oils are at least 100 ppm sulfur or more.

Not too many labs specialize in testing refrigeration lubricants, so doing internal checking of the results to see if they make sense can be difficult.

I had the oil retested for refrig. viscosity. The viscosity result is 28cSt. Not much difference. Can it be anything else than wrong oil mix-up?

If the refrigerant was completely removed from the Trane 22 prior to testing for viscosity, and you got only 28 cSt, then it is likely mixed. The Trane compressors do not normally shear the oil, and if it were under that much stress there would be signs of other major problems in the oil analysis (high acidity, very dark color, high levels of wear metals etc.) If you have any more information that supports severe chemical degradation of the oil, then the oil may have been chopped up to lower viscosity.

I still have doubts about the test lab's removal of dissolved refrigerant prior to viscosity testing. Trane 22 is used with one of three refrigerants (R-11, R-22 and R-123). R-11 and R-123 are very difficult to remove from the oil since they have lower volatility than most other refrigerants. R-22 is fairly easily removed compared to the other two refrigerants.

FTIR can be used to tell if there are significant amounts of dissolved refrigerant.

May I know what is the way to remove dissolved refrigerant in oil, be it R-11, R-22 or R-123? Any literature on this, I'd be grateful.