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Nitrogen

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I am using nitrogen pressure to check a leak in the cold storage system. 
In the room, there are four evaporator units and the one is faulty, the room temp. Is -35 degree Celsius.
So my question is - I am charging nitrogen pressure 350psi to the system and after 24 hours, i check the pressure is 280psi. Is it because of -35 temp.? Or is there a leak
Please help 

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  • pecmsg
    pecmsg Member Posts: 4,886
    edited November 2023
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    I’m confused. You’re adding N2 to only 1 coil?

    https://www.oceanhvac.com/nitro.php
  • jesmed1
    jesmed1 Member Posts: 569
    edited November 2023
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    Kpit said:

    I am using nitrogen pressure to check a leak in the cold storage system. 

    In the room, there are four evaporator units and the one is faulty, the room temp. Is -35 degree Celsius.
    So my question is - I am charging nitrogen pressure 350psi to the system and after 24 hours, i check the pressure is 280psi. Is it because of -35 temp.? Or is there a leak
    Please help 
    Most or all of the pressure drop could be due to the temperature change, depending on what temperature your nitrogen started at.

    According to the perfect gas law, for a given quantity of gas in a fixed volume, the volume and mass terms cancel out, and the pressure is solely dependent on temperature, in which case:

    Pa/Ta = Pb/Tb

    Where
    P=absolute pressure
    T=absolute temperature
    and the subscripts "a" and "b" designate the two different conditions.

    Assuming you started with nitrogen at 70 deg F, that's 294 deg Kelvin (K), which is an absolute temperature scale. And you say the cold room temp is -35 C, which is 238 Kelvin.

    And you started out at 350 psi gauge, which is about 365 psi absolute. So according to the perfect gas law, you should end up with a pressure of

    Pb = Pa*Tb/Ta = 365*238/294 = 295 psi absolute

    Then subtracting 15 psi for atmospheric pressure to get the gauge pressure,

    295 psia - 15 psi = 280 gauge pressure.

    You said you're seeing 280 psi gauge pressure, which is exactly what the perfect gas law predicts. So if there is a leak, it's so small as to be swamped by the temperature effect on pressure, assuming you started with nitrogen that was around 70 deg F.
  • mattmia2
    mattmia2 Member Posts: 9,747
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    Probably a leak, but wait another fraction of that time and see if it goes down more.
    ethicalpaul
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,623
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    Just leave the pressure in it . If there is no leak it will stop dropping.
    ethicalpaulmattmia2
  • ratio
    ratio Member Posts: 3,652
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    If I'm looking at big temp differences, I'll go back a few times in the first hour or so & top it off. Then, depending on the size, it'll need to sit for some time. The last system I did dropped from 300# to 292# in 24 hours. Leak. But it was a small one on a 50 ton circuit, so it took a while to show up.