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Boiling point in a vacuum

  1. May 6, 2010 #1
    Boiling point in a vacuum....

    Gidday, I am as new here as it gets, and I have a question to post.

    I have been having a (heated) discussion with regards to the temperature that water boils in a vacumm. Given: @ 10in Hg water will boil at 192F. If you had a pressure reading of 29.7in of mercury water will boil at 32F The pressure on the water has been reduced and the boiling point has lowered. Standard evacuation process of any AC/ Heat Pump maintenance.
    Question: In an Air Conditioning, closed system, when a vacuum is created, moisture is pumped out during evacuation or Deep Vacuum 29in Hg. Will this lowered pressure be enough to boil off any mineral oil. Given that most MSDS states that mineral oil boils @ 500-625F. Will the pressure be low enough to boil/vaporise the mineral oil in this system?

    Thank you

  2. jcsd
  3. May 7, 2010 #2


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    Staff: Mentor

    Re: Boiling point in a vacuum....

    Boiling point at 1 atm doesn't tell anything about the behavior of the liquid at lower pressures. You need more data for that, I bet it can be found in one of these engineering handbooks with properties of everything.
  4. May 7, 2010 #3


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    Science Advisor

    Re: Boiling point in a vacuum....

    The (admittedly, very low vapour pressure) oil in a diffusion pump system does not boil off into the ultrahigh (at the very low end, 10^-10 Torr) vacuum it creates:
  5. May 7, 2010 #4


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    Science Advisor

    Re: Boiling point in a vacuum....

    EDIT: I should mention that the oil is boiled off (using a heater) in order to create the oil jets, but the gas is refluxed against the sides of the pump, returning to liquid form.
  6. May 8, 2010 #5
    Re: Boiling point in a vacuum....

    That is a little beyond the scope I am reaching for.

    My re-phrased question: In a contained pressurized system, such as, an Air Conditioning system found everywhere. When maintenance is performed, a vacuum pump is attached to remove the refrigerant (R12, R134A etc), it also removes all moisture inside the system. As you can appreciate water in high temps or below freezing could have costly effect. The problem I am having trouble explaining is, why the oil is removed as well as the water.
    I contend that the lower pressure has changed the vapour point of the oil, thereby pulling it all into the maint machine. I have my detractors lining up and I have been on a research binge since wednesday.

    ' vacuum is measured in inches of mercury. sea level is zero. water will boil at 212F. as negative pressure or elevation increases the boiling temp. of water decreases. if you had a pressure reading of 10 in" of mercury the water will boil at 192F. if you had a pressure reading of 29.7 in" of mercury water will boil at 32F '

    So can I use the all things being equal defence? If its flammable, it is, then it must give off vapour at X temp, if I can figure that out I can prove that oil will vapourise. How to prove it

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