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Pressure at different points in vacuum system

  1. Oct 22, 2015 #1
    Hi all,

    I have a vacuum chamber which is consistently pumped out by a mechanical pump at the downstream end and is fed nitrogen at the upstream end. The path of the nitrogen is as follows: A mass flow controller regulates 150 SCCM flowing at all times. The N2 flows through several feet of 1/4 inch tube. The N2 then enters the reactor which is a 2.75 inch tube. The pressure is read by a gauge in the 2.75 inch tube. The N2 then flows to the pump.

    Like this:

    MFC (150 SCCM) --> 1/4" tube --> 2.75" tube (w/ pressure gauge) --> pump

    My question is, lets say the pressure is read to be 1 torr in the 2.75 inch tube, is the pressure the same in the 1/4 inch tube? I am familiar with the bernoulli equation but unsure if that applies here,

    Thanks for any feedback!
  2. jcsd
  3. Oct 22, 2015 #2


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    Staff: Mentor

    It applies here, and you can use it to calculate the pressure.
  4. Oct 22, 2015 #3
    Don't I need to know the velocity of the gas inside the two tubes in order to do the calculation? All I know is 150 SCCM of N2 are being fed into the 1/4" tube
  5. Oct 22, 2015 #4


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    Yes...so you have the volumetric flow rate (standard and pressure) and pipe size...

    The main x-factor I see is if the velocity is high enough, there may be losses.
  6. Oct 22, 2015 #5


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    Hi. If you're reading 1 torr in the main tube, that's awfully close to the Knudsen regime. It might already be Knudsen flow - probably you should check.

    If it is in the Knudsen regime Bernoulli's equation is no longer applicable and you should use Knudsen's equation, instead.
  7. Oct 22, 2015 #6


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    Staff: Mentor

    You also need the temperature to convert volumetric flow rate to flow velocity, but that should not be a major problem.

    Mean free path length should be of the order of 50 micrometers in the bigger tube. Didn't calculate it for the smaller tube.
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