I was thinking again about "first compression stage". My choice was Copeland R22 compressor or York 210 AC compressor. Both have pros and cons.
Copeland performs great, has motor built-in reliable but suction pressure is low and gas is "warm" compare to refrigerant. Also Refrigeration oil is very hygroscopic (mineral oil is less but still).
The problem with this oil is it becomes acidic and this acid eats motor copper (not good at all). When this compressors used in closed refrigeration system you remove as much water as possible by pulling deep vacuum for few hours and you have filter/drier which kills the rest of water. In my application I will always have "fresh gas" I have no idea how dry it is.
To put just a compressor oil in it? I don't know how scroll will like it and again motor windings are cooled by this oil and I don't know how wire insulation will like it or not....
Also I would like to keep electric parts out of gas as much as possible. With this compressor gas goes around windings. May be I am just a frick but I saw enough burned out compressors....
York is on other hand is just a bare compressor. It accepts w30 non detergent oil which is cheap and non hygroscopic (even engine oil can be used). This compressor is a high volume compressor. I need to turn it only about 700 RPM to get 2 GGE /hour. At this speed it spits less oil about 1 oz /hour. This oil will help to lubricate "second stage compression". The only one problem I can see right now is I will need to find sealed electric motor.....
I bought one York 210 compressor on eBay today, I will need to determine power of electric motor for it. I have few electric motors lying around I know compressor RPM, Max pressure, so I will need to check motor amperage and this will give me power. Easy enough. When I know motor power I will buy a sealed one....
Here is great info about York 210 compressor:
http://www.worldaire.com/pdf/Worldai...ompressors.pdf