Label viscosity versus working viscosity
An ISO viscosity grade such as ISO VG 68 means the oil has a kinematic viscosity of about 68 mm²/s (cSt) at 40°C, measured on neat oil at atmospheric pressure. Inside a refrigeration compressor the oil is never neat. It is saturated with refrigerant at the pressure and temperature of the crankcase, sump or oil separator, and that dissolved refrigerant acts as a very low-viscosity diluent.
The viscosity that actually separates metal surfaces in a bearing is the working viscosity: the viscosity of the oil and refrigerant mixture at the real temperature and pressure where the oil film forms. Depending on the refrigerant, pressure and oil temperature, dissolved refrigerant can reduce viscosity to a small fraction of the label value. Select on label viscosity alone and you may be running bearings on a film a fraction as thick as the designer intended.
What controls dilution
- Solubility. The more soluble the refrigerant is in the oil, the more of it dissolves at a given pressure. The same miscibility that helps oil return also increases dilution. This is the central trade-off in refrigeration lubrication.
- Pressure. Higher pressure in the oil reservoir drives more refrigerant into solution. High-pressure-side oil separators on screw compressors run at discharge pressure, so dilution there can be significant.
- Oil temperature. Warmer oil holds less refrigerant. This is why crankcase heaters are fitted and why oil temperature should be held well above the saturation temperature of the refrigerant at reservoir pressure. Many OEMs specify a minimum margin (commonly in the region of 15 to 20 K); always follow the compressor manufacturer's figure.
- Refrigerant type. R-744 (CO2) and R-32 behave very differently from R-134a in the same ester, so a grade that works on one refrigerant is not automatically correct for another.
Using pressure-viscosity-temperature (Daniel) charts
For a serious selection, engineers use pressure-viscosity-temperature diagrams, often called Daniel charts, which plot mixture viscosity and refrigerant solubility against oil temperature and pressure for a specific oil and refrigerant pair. The procedure is:
- Establish the oil reservoir pressure (suction pressure for most reciprocating and scroll machines, discharge pressure for high-side screw separators).
- Establish the actual oil temperature at that point, at the worst operating condition, not the design point.
- Read the refrigerant concentration and mixture viscosity from the chart.
- Compare the mixture viscosity against the compressor manufacturer's minimum working viscosity.
- If the margin is too small, move up a grade, raise the oil temperature, or choose an oil chemistry with lower solubility for that refrigerant.
The AA Lubricants POE ladder
We offer our polyolester range in closely spaced grades so a selection can be matched to the working-viscosity requirement rather than forced into the nearest available product. Typical data from our technical data sheets:
| Grade | Viscosity @ 40°C (mm²/s) | Flash point (°C) | Pour point (°C) | Typical application |
|---|---|---|---|---|
| RF POE 32 | 33.6 | 245 | -48 | Small hermetic reciprocating and rotary compressors, domestic and light commercial |
| RF POE 55 | 52.4 | 250 | -39 | Commercial hermetic and semi-hermetic compressors, air conditioning |
| RF POE 68 | 68.1 | 250 | -40 | Semi-hermetic reciprocating and scroll compressors on R-134a, R-404A, R-507 |
| RF POE 170 | 175.3 | 260 | -30 | Screw compressors and heavily diluted high-side reservoirs |
| RF POE 220 | ISO VG 220 | Data sheet on request | Large industrial screw compressors in commercial and industrial chillers | |
For HCFC duty, RF MO (grades 32 to 100) and RF AB (grades 68, 100 and 150) provide an equivalent ladder in mineral and alkylbenzene chemistry.
Why higher is not automatically safer
Going up a grade increases film thickness, but it also:
- raises viscous friction and power draw, reducing COP;
- makes oil return harder in cold evaporators, because the oil that reaches the evaporator is thicker;
- can slow oil drainage in separators and increase carry-over if the separator was sized for a lighter oil.
The correct grade is the lowest viscosity that maintains the required working viscosity with margin at the worst operating condition. That is an engineering calculation, not a guess.
Field checks that point to a viscosity problem
- Bearing wear or scoring found at overhaul, with oil that otherwise tests clean.
- Foaming in the sight glass at start-up, a sign of refrigerant flashing out of heavily diluted oil.
- Low oil pressure differential on force-lubricated machines after long off cycles.
- Oil temperatures running close to the saturation temperature at reservoir pressure.
If you are seeing any of these, send the compressor model, refrigerant, oil temperature and reservoir pressure to sales@aalubricants.co.za. We will check the working viscosity against the OEM requirement and recommend the right grade.
