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good question both are used mixed together and seperately. I never have heard a good reason for either other than PAG is better for compressing certain Hydrocarbons I think since it will not mix in them.

Otherwsie both give LONG >7,000 hours operation time with little or no sludge or Coke buildup.

Esters maygive better cooling due to inherent lubricity from the ester that the pure PAG will not have so perhaps for cooler running a Ester would be better.
bruce
http://www.machinerylubrication.com/article_detail.asp?...ookgroup=Lubrication

PAG has a very high viscosity index so it is important that you get the correct viscosity at operating temperature, if the OEM recommends a viscosity at 100 degrees F for a mineral oil with a VI of 95, you will have to calculate the viscosity at operating temperature, then use a similar viscosity PAG at that temperature.
http://www.eskoindustries.com/viscadv.php?Deg=C&Temp1=4...mp=60&Calc=Calculate

I am considering changing the oil in our screw air compressors from diester to PAG. The OEM recommends a minimum discharge temperature of 180 F, I assume this is to keep the water levels below 500 ppm (0.05%), If I switch to PAG, I am wondering if I can lower the discharge temperature to get longer oil life (reduce oxidation), which will raise the water content (because water soluable types of PAGs can handle up to 0.7% water)
Last edited by badnewsbare
thanks for the info Bruce381.

Badnewsbare, I've read the article in machinery lubrication before and do agree to a certain extend. However, I was told that PAG in general are not really a good lubricant except for it's better heat transfer and water solubility properties.

As I'm in contact with ester based products more, I do believe for air compressor ester are still a better option. If you're using an IR screw comp, changing to a PAG may/would reduce the discharge temp due to it's design.

Anyway, above is just my personal opinion.

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