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Air in compressor crankcase oil...how bad is it, what to do?

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  • Air in compressor crankcase oil...how bad is it, what to do?

    We've got a Quincy QT-10 compressor that's about eighteen months old. Not too long after we started using it, the crankcase oil turned milky white. I replaced it with a different brand of compressor oil, and that turned milky white, too.

    I thought the problem was moisture getting in the oil because the compressor is outside where the relative humidity is pretty high. I added a crankcase heater to keep the compressor well above the dewpoint at about 105 degrees F, but that didn't help.

    When I heated a sample of the milky oil, I noticed bubbles forming and coming to the surface. I put a cold mirror above the sample, but it didn't fog. After the temperature of the oil reached about 130 degrees F for several minutes, the bubbling stopped, and the oil was no longer milky. It appears that air, not moisture, is getting into the oil while the compressor is running, and it doesn't get hot enough to boil it off.

    How bad is it for the oil to have air in it? What can I do to keep the oil clear?
    Any products mentioned in my posts have been endorsed by their manufacturer.

  • #2
    I still think your problem is moisture getting in the oil because the compressor is outside where the relative humidity is pretty high & it's sucking the moisture out of the air when compressed & heated & running into the oil. Is there a place it could draw drier air from? It like old cars that only run a few miles at a time never warm up enough to heat the water out of the oil. Maybe open the tank drail once a month & let it run till hot once a month.
    "Let me recommend the best medicine in the
    world: a long journey, at a mild season, through a pleasant
    country, in easy stages."
    ~ James Madison

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    • #3
      The shop I worked at when I was a teen had a problem with excessive humidity. We solved it by piping the intake to inside where the humidity was lower. Did not like the noise, but it provided a solution.

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      • #4
        Is there another way to check for moisture in the oil? The cold mirror seemed like a pretty good test.

        There's no place nearby where we can get air with a significantly lower relative humidity, and we can't move the compressor.
        Last edited by winchman; 08-04-2017, 08:54 AM.
        Any products mentioned in my posts have been endorsed by their manufacturer.

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        • #5
          Perhaps the you put too much oil in the crankcase causing the oil to foam. I don't know about the Quincy's oil system but mine is splash-lubed like a lawnmower engine. Too much oil in those will cause foaming and/or a broken crank.

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          • #6
            When in doubt call Quincy,however the QT-10 is a splash lubed compressor,odds are it won't hurt anything.
            I just need one more tool,just one!

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            • #7
              The next time you see it milky, maybe send it out for analysis?

              http://www.machinerylubrication.com/...103/cloudy-oil

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              • #8
                Originally posted by flylo View Post
                I still think your problem is moisture getting in the oil because the compressor is outside where the relative humidity is pretty high & it's sucking the moisture out of the air when compressed & heated & running into the oil. .............
                Under compression and heating the water vapor will be LESS prone to condensing out of the air.

                Condensation would form upon expansion and lowering of pressure and/or temperature.

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                • #9
                  Southwest Georgia --- yeah I bet your wipping up some humidity in the oil, even highly agitated oil is not "milky white" it's a froth of bubbles with the light color of the oil itself,

                  and those bubbles should dissipate fairly fast esp. when heated some, not so with humidity and water in the oil - takes allot longer... if you brought the oil to over water boiling temp in a pan that might tell you something, the air will have no reaction but to go to the top faster and escape, but water in the oil --- well we all know what happens in that combination - wear some safety glasses...

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                  • #10
                    After the temperature of the oil reached about 130 degrees F for several minutes, the bubbling stopped, and the oil was no longer milky. It appears that air, not moisture, is getting into the oil while the compressor is running, and it doesn't get hot enough to boil it off.
                    It sounds very much like air entrainment to me rather than moisture. It would take a much higher temperature to remove moisture, 130°F simply reduces the viscosity enabling the oil to release the air bubbles faster. I'm sure ambient temps would allow the release as well but would take longer. How does the oil look after a long period of inactivity, say first thing in the morning or after the weekend?

                    A couple of resources from Quincy. Also a link to their contact page, perhaps a quick email will clear the air, no pun intended.

                    Why Mixing Air Compressor Fluids Reduces Performance


                    http://www.quincycompressor.com/contact-quincy/


                    Home, down in the valley behind the Red Angus
                    Bad Decisions Make Good Stories

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                    • #11
                      Originally posted by lynnl View Post
                      Under compression and heating the water vapor will be LESS prone to condensing out of the air.

                      Condensation would form upon expansion and lowering of pressure and/or temperature.
                      Only half correct because compressing the gas increases relative humidity. Thats why compressor tanks get filled with water.

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                      • #12
                        It may speak of bad rings too, compressing air also compresses the already high moisture content into and even denser area, if the rings are not holding and some of this densely packed charge is then allowed to go directly into the crankcase and expand it will leave behind it's moisture content...

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                        • #13
                          I kept the oil that was removed from the compressor in a closed jug, and it's still milky white six months later.

                          The compressor does have a splash lube system. I refilled the crankcase to the same level it was at when we got it, about 3/4 up on the round sight glass, and slightly below the oil refill opening.

                          If the entrained air isn't going to hurt the compressor, I'll just not worry about it. It's just that other compressors around the school in similar situations have oil that's dark and clear, just like what's in a newly-opened jug.
                          Any products mentioned in my posts have been endorsed by their manufacturer.

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                          • #14
                            6 months later? ! ? !!!

                            there's no way in hell it's just air...

                            Have you rotated the compressor by hand and listened for bleed-down past the rings? remove the oil fill cap and listen while you do this - normal to hear very little just getting past the ring end-gap and the compression should hold pretty solid whilst doing this, should give up the stroke very slowly...
                            Last edited by A.K. Boomer; 08-04-2017, 11:36 AM.

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                            • #15
                              You could try heating the milky sample in an old saucepan to see if in returns to normal oil colour, that would likely point to water. Some types of oil had a detergent in the formula especially to emulsify condensation and reduce rusting. It was not uncommon to see white oily gunge inside the rocker covers of older cars if they were used for short hops.

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