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  1. #1
    Join Date
    Dec 2011
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    louisiana
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    58

    testing for non-condensables

    Can someone explain this "comparison between the liquid refrigerant pressure and corresponding saturation temperature at ambient conditions " can show if non-condensables or foreign gases are present???
    I mean excessive high side pressure does not always mean moisture in system, could be over charge, no fan (ok, some things are obvious), but exactly how can I use these tests to pin down moisture?
    Thanks
    Greg

  2. #2
    Join Date
    Dec 2004
    Location
    Chicago area
    Posts
    1,439

    The test

    add a pressure tap to the inside of the condenser, if there is not one already.

    Close the refrigeration service valves. Turn on ONLY THE FAN.

    let it run for a few minutes. Test the pressure and compare to the corresponding temperature on the gauge. That temperature should match the outdoor ambient.

    I am not aware of this test being accurate when the system is running.

    This test can also be useful in finding what refrigerant is in the system on refrigeration units.

  3. #3
    Join Date
    Dec 2011
    Location
    louisiana
    Posts
    58
    Could you explain the science behind this? I guess I never thought the refrigerant was in a liquid state in the condensor coil when the unit was off. So if moisture is present, corresponding temp would be off(higher?), fan is to ...?
    Thanks for your help
    Greg

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,750
    The fan is to coll the refrigerant to ambient temp. Without the fan running, solar heat could raise the temp and pressure of the refrigerant above the ambient temp.
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    How many times must one fix something before it is fixed?

  5. #5
    Join Date
    Apr 2005
    Posts
    472
    see below>>>
    Jason J Saylor
    Pinellas County Schools
    HVAC Tech/Part Time HVAC Instructor Fortis College
    Pinellas County Florida

    "You will encounter many distractions and many temptations to put your goals aside: The security of a job, a wife who wants kids, Whatever. But if you hang in there, always following your vision, I have no doubt you will succeed.
    Larry Flynt quote

  6. #6
    Join Date
    Apr 2005
    Posts
    472
    Quote Originally Posted by thecarpenter View Post
    Could you explain the science behind this? I guess I never thought the refrigerant was in a liquid state in the condensor coil when the unit was off. So if moisture is present, corresponding temp would be off(higher?), fan is to ...?
    Thanks for your help
    Greg
    The Science behind it is Every freon has a Temp/Pressure Chart and The pressure is Relative to the ambiant air. For example I can put a pressure gauge on a Can of R22 freon and compare that to a PT chart and tell you the ambiant air temp. Now IF i know the AMbiant temp, and Know the pressure of the Gas and have a chart I can
    Jason J Saylor
    Pinellas County Schools
    HVAC Tech/Part Time HVAC Instructor Fortis College
    Pinellas County Florida

    "You will encounter many distractions and many temptations to put your goals aside: The security of a job, a wife who wants kids, Whatever. But if you hang in there, always following your vision, I have no doubt you will succeed.
    Larry Flynt quote

  7. #7
    Join Date
    Nov 2011
    Location
    Charleston, Wv.
    Posts
    1,436
    Dalton's Law....the law of partial pressures, states: that the total pressure of a mixture of gasses is equal to the sum of their individual pressures.

  8. #8
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,400
    Quote Originally Posted by 71CHOPS View Post
    Dalton's Law....the law of partial pressures, states: that the total pressure of a mixture of gasses is equal to the sum of their individual pressures.


    Pure refrigerant will reflect the pressures on the PT chart. Those pressures are saturation pressures. Yes, it means that liquid can be present, but vapor can also be present at those temperatures. It's called saturation.

    Add non-condensible gasses to the container (system) and you will see higher pressures.

  9. #9
    Join Date
    Jun 2011
    Posts
    229
    Nice test. Thanks!

  10. #10
    Join Date
    Mar 2003
    Posts
    1,234
    Quote Originally Posted by jpsmith1cm View Post


    Pure refrigerant will reflect the pressures on the PT chart. Those pressures are saturation pressures. Yes, it means that liquid can be present, but vapor can also be present at those temperatures. It's called saturation.

    Add non-condensible gasses to the container (system) and you will see higher pressures.
    And the only place a P/T chart is accurate is when there is a mixture of liquid and gas i.e. the cond. or evap. coils.

  11. #11
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,400
    Quote Originally Posted by woody19 View Post
    And the only place a P/T chart is accurate is when there is a mixture of liquid and gas i.e. the cond. or evap. coils.
    Yes.

    Where it gets exciting is when dealing with blends and the temperature glide.

  12. #12
    Join Date
    Jun 2009
    Location
    Pompano Bch, S Fla
    Posts
    57
    Came across this thread and I don't know what exactly a pressure tap is. Is it a line piercing valve? If so, don't those have a reputation for leaking after a while? Any info would be greatly appreciated. Thanks in advance.

  13. #13
    Join Date
    Jun 2012
    Location
    Great lakes
    Posts
    41
    Quote Originally Posted by Pete954 View Post
    Came across this thread and I don't know what exactly a pressure tap is. Is it a line piercing valve? If so, don't those have a reputation for leaking after a while? Any info would be greatly appreciated. Thanks in advance.
    Self piercing valves are for DIY home owners, under bid jobs, or lazy techs. They do not exist in refrigeration systems.

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