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Volume of compressed air question.

  1. Sep 6, 2011 #1

    Can anyone help me with a personal project I am working on?

    I wish to compress air in a container up to 100psi. I'm making a small single cylinder compressor to achieve that. The container is small, approx 0.05 cubic feet.

    I'm trying to work out how many strokes of the compressor it will take. I know the swept volume of the compressor. Am I right to assume that the number of strokes required against pressure is not linear? I have researched Boyles Law etc but I'm just getting confused!

    How do I work out how many strokes to get from say, 50 psi to 60 psi?

    Any help would be much appreciated.
  2. jcsd
  3. Sep 7, 2011 #2
    I was hopeful that one of the over a hundred people that have viewed my question might have been able to help. Any takers? I really need a bit of guidance here!

  4. Sep 7, 2011 #3


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    Staff Emeritus
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    Sorry man, I don't have an experience in working with something like this.
  5. Sep 8, 2011 #4


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    Staff: Mentor

    Each stroke adds the same volume of the gas. Calculate total volume added by n strokes (hint: no pressure changes, each volume is just of the atmospheric air entering th ecompressor). Apply Boyles law - P1V1 are atmospheric pressure and total volume of the gas put into tank, P2V2 are pressure and (known) volume of the main tank. There is only one unknown here.

    Note: This is a homework or homework type problem, so it should be posted in the HW section.
  6. Sep 8, 2011 #5
    Hi Borek,

    Thanks for the reply. I'm not sure I understand it though - yet. I'm good at many things but maths is not one of them!

    It might sound like a Homework question but I assure you I left school a good while ago! I placed the question here as I thought it was similar to many others being asked and thought it appropriate.

    I'm still in the dark here so perhaps I could rephrase it in the manner of a HW question i.e.

    QUESTION: Calculate number of cycles required to increase tank internal pressure from 0 to 50psi.

    If :

    X = compressor volume

    V2 = Tank Vol

    n = compressor cycles

    P1= Atmos. pressure (I assume because I want a relative increase, this will be 0 and not 14.7psi?

    P2 = Final pressure in tank

    Could it be presented as an eqauation i.e. n= ???

    If you still think I should put this in the HW section should I ask again there?

    Best regards

  7. Sep 8, 2011 #6
    http://www.egr.msu.edu/classes/me416/Compressor.pdf [Broken]

    This site might be helpful. Do you know inlet temperature and mass flow rate of the air?
    Last edited by a moderator: May 5, 2017
  8. Sep 8, 2011 #7
    You know the volume of your tank, you know what pressure you want to achieve assume that the final temperature in the tank will be the same as the atmosphere.
    Think about the tank at full pressure and calculate using Boyles law the volume that the air in the tank would occupy if you opened the valve and let it out to atmosphere.
    Calculate the volume of your pump
    Divide the first number by the second number for the number of strokes.
    When using compressed air (especially with home made kit) as a minimum wear a pair of safety glasses at all times.
  9. Sep 8, 2011 #8


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    Staff: Mentor

    No, you are intested in ratio of absolute pressures.

    I have already moved the thread.

    See Jobrag answer. While he states basically the same I did, he uses different angle, so perhaps it will be easier to catch for you.
  10. Sep 9, 2011 #9
    This is a good question and not too hard to visualize. Take a empty volume at a pressure of 14.7 psi absolute and raise it to 114.7 psia. This is a ratio of 7.8. Therefore the number of strokes is equal to 7.8 * Vtank/Vstroke where Vtank is the volume of the tank and Vstroke is the volume of the stroke. So, if the tank is 1 cu.ft and the stroke is 1 cu ft, then it would take 7.8 strokes to fill the tank.
    Last edited by a moderator: Sep 9, 2011
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