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  1. #1
    Join Date
    Dec 2004
    Posts
    440
    Nitrogen pressure changes with temperature

    Posted by someone on another HVAC message board:

    The company I work for just changed out a 15-ton split
    system. We left it pressured with nitrogen overnight to
    make sure we didn't have any small leaks. It was
    about 80 F ambient when it was pressured, and the
    temp dropped to about 50 F that night. The next day
    we had lost 10 psig. Leak check all welds. After finding
    no leak we contacted the supplier. They said it was
    possible that with that much of a temperature difference
    it could have changed the pressure of the nitrogen. It
    was something we had never heard, and has never been
    brought up before. I was just wondering if anyone had
    some insight.


    My response:
    Re: Nitrogen pressure change with temperature

    You want to use Gay-Lussac's Gas law:
    P1 / T1 = P2 / T2 (constant Volume and moles)
    P = Pressure T = Temperature P1 is first temperature, etc.
    or
    Second Pressure = (first pressure x second temperature) divided by (first temperature)

    The night temperature doesn't matter. The only thing that
    matters is the temperature of the gas at the time you
    measured the second pressure.

    So the second pressure should equal the first pressure
    multiplied by the second temperature all divided by the
    first temperature.

    In order for the equation to work, you have to convert from Fahrenheit to
    some temperature scale that is zero at absolute zero. You can convert
    to Kelvin degrees or Rankine degrees, then back to Fahrenheit. If you
    have Java on your computer here is a website that will do the conversion
    for you.

    http://www.csgnetwork.com/tempconv.html

    To convert from Fahrenheit to Rankine subtract 459.67

    To convert from Rankine to Fahrenheit add 459.67

    Here is another site that will do the conversion:

    http://www.metric-conversions.org/te...fahrenheit.htm

    Cheers,
    Michael

  2. #2
    Join Date
    May 2004
    Location
    Rapid City, SD
    Posts
    7,415
    Better blow some N2 through the lines before you pressure check too, you want to get all the air and moisture out that you can because it will throw your reading's off too.
    "If you call that hard work, a koala’s life would look heroic."

  3. #3
    Join Date
    Dec 2004
    Posts
    440
    Hi amickracing,

    Originally posted by amickracing
    Better blow some N2 through the lines before you pressure check too, you want to get all the air and moisture out that you can because it will throw your reading's off too.
    You don't need to get air out of the lines or
    for that matter any gas. So long as there is
    just gas in the line pressure will vary in
    direct proportion to temperature increase or
    decrease. Liquid moisture turning to vapor
    could theoretically influence pressure, but
    unless there is standing water, I doubt that
    mere moisture turned to water vapor would do
    much to increase internal pressure.

    If you are vaccuming a system with moisture,
    you are not going to see the moisture create
    a system pressure of even 1 psig.

    Small amounts of moisture in refrigerant oil
    are not going to change the pressure reading
    even 1 psig.

    Cheers,
    Michael

  4. #4
    nitrogen pressure does not change with temp change, you have a leak, even if it changed 1lb you have a leak.
    the only way the pressure changed is if there was another gas in the system.

  5. #5
    Join Date
    Feb 2006
    Location
    USA
    Posts
    4,381
    Originally posted by airworx
    nitrogen pressure does not change with temp change, you have a leak, even if it changed 1lb you have a leak.
    the only way the pressure changed is if there was another gas in the system.
    not true .. it will change .. the other day we did a coil & cond unit change out .. i did od unit & when it was time to pressurize .. i allowed it to hold so i can spray my fittings & so he can inside ... well i went & took a dump & came back & my suction pressure was up real high ... i left it at 90 .. it was past 100 .. i was like wtf .. lol then i immediately knew he disregarded my standing pres. test & either put on the fan or the heat .... anyway it will change

  6. #6
    Join Date
    Dec 2004
    Posts
    440
    Dear airworx,

    Originally posted by airworx
    nitrogen pressure does not change with temp change, you have a leak, even if it changed 1lb you have a leak.
    the only way the pressure changed is if there was another gas in the system.
    Nitrogen pressure does change with temperature
    change as does the pressure of every gas in this
    universe. You might want to review the Perfect
    gas law. PV = nRT Then Charles gas law. And
    Boyle's gas law.

    I'd invite you to review my inlaws gas, but I
    wouldn't be that crass.

    There is a direct mathematical relationship
    between the pressure of every single gas in
    the universe and its temperature so long as
    gas volume remains constant. With volume constant,
    increased pressure leads to increased temperature
    and vice versa. The increased temperature of a
    compressed gas is called heat of compression.
    Compressors add heat to refrigerant by
    compressing refrigerant gas.

    On the other hand if you raise the temperature
    of a gas, while its volume remains, that means
    the atoms and/or molecules of gas will move
    with greater velocity and exert a greater force
    on the sides of the container. Temperature
    is really just a measure of speed. As atoms
    of a gas move around faster, they strike the
    walls of their container with greater force,
    and this exerts greater pressure on the container
    walls.

    My college major was mathematics and physics.
    I took second year college chemistry at Wayne
    State University during the summer following
    ninth grade.

    Cheers,
    Michael


  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,074
    Doubt you would see a 10PSI drop in pressure with just a 30° temp change.

    Nitrogen has a very small thermo expansion rate.
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    How many times must one fix something before it is fixed?

  8. #8
    Join Date
    Apr 2002
    Posts
    11,808
    Well they are going from 80 to 50 F, be about a 5.6% drop in absolute pressure.

    (50 + 460)/(80 +460)=0.9444444444444444444

    so the pressures would drop in a similar manner.

    10 psi is a big drop though.

    10/.056=178.6 psia absolute or about 164 pounds of gauge pressure. So maybe it could drop from 164 to 154 psi because of the temperature. They pump it up that high in the first place?





  9. #9
    Join Date
    Apr 2002
    Posts
    11,808
    Originally posted by cobaltfjord
    Dear airworx,

    Originally posted by airworx
    nitrogen pressure does not change with temp change, you have a leak, even if it changed 1lb you have a leak.
    the only way the pressure changed is if there was another gas in the system.
    Nitrogen pressure does change with temperature
    change as does the pressure of every gas in this
    universe. You might want to review the Perfect
    gas law. PV = nRT Then Charles gas law. And
    Boyle's gas law.

    I'd invite you to review my inlaws gas, but I
    wouldn't be that crass.

    There is a direct mathematical relationship
    between the pressure of every single gas in
    the universe and its temperature so long as
    gas volume remains constant. With volume constant,
    increased pressure leads to increased temperature
    and vice versa. The increased temperature of a
    compressed gas is called heat of compression.
    Compressors add heat to refrigerant by
    compressing refrigerant gas.

    On the other hand if you raise the temperature
    of a gas, while its volume remains, that means
    the atoms and/or molecules of gas will move
    with greater velocity and exert a greater force
    on the sides of the container. Temperature
    is really just a measure of speed. As atoms
    of a gas move around faster, they strike the
    walls of their container with greater force,
    and this exerts greater pressure on the container
    walls.

    My college major was mathematics and physics.
    I took second year college chemistry at Wayne
    State University during the summer following
    ninth grade.

    Cheers,
    Michael

    So you quit your day job at NASA to fix window shakers?

  10. #10
    Join Date
    May 2000
    Posts
    338
    NASA... too funny



  11. #11
    Join Date
    Jun 2004
    Location
    Maryland's Eastern Shore
    Posts
    892
    WTF... Nitrogen is what is referred to as an inert gas... Pressures don't change regardless of temperature... For the people that have seen pressures rise, have you replaced your manifold set recently, if you have an old set and close them off with 250+ pounds of bottled nitrogen on one side and 100+ pounds system nitrogen on the other, you will bleed through... Secondly, if you put nitrogen into the vapor side of the system, it may take time to show up on the liquid side of the system depending on the TXV... So, if you put in 100+ pounds, you leave for the night, you come back and you have lost pressure and you don't have a leak, it probably took some time to bleed through the remainder of the system...

  12. #12
    Join Date
    Mar 2006
    Location
    SW Colorado
    Posts
    161
    Sorry, but I gotta disagree. Inert gas means that it is not reactive under normal circumstances. Hence, nitrogen is a popular choice to flush a system prior to brazing to prevent the oxidation reaction. Also good for food preservation, etc.

    I gotta go with Mr. Science, too.

    See http://en.wikipedia.org/wiki/Gas_laws

  13. #13
    Join Date
    Nov 2000
    Location
    Lancaster, Pennsylvania
    Posts
    885

    If

    you do not "chase" the air out of the refrigerant lines and indoor coil with nitrogen and just dump it in, the pressure will flucuate in the system you are testing. Trust me, I was faced with this same scenario this weekend on a Tempstar 4 ton install. Pressure dropped 3psi when the attic dropped 20 degrees overnight. Next day, conditions were identical and magically I got my 3psi back. Vacumned the system down to 500 microns and opened the service valves.
    KX500......the original big green meanie

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