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How to calculate Air flow rate...?

  1. Dec 9, 2015 #1
    Sir, Kindly let me know how to calculate flow rate (cfm or Litre/m) of air flowing through a pipe of 12 mm dia from a reservoir at pressure 7 kg/cm2 to atmosphere.

    i.e ;- how much volume of compressed air could pass within a minute through a "X"mm dia pipe from a constant pressure (7 kg/cm2)

  2. jcsd
  3. Dec 9, 2015 #2
    What level of accuracy are you going for?

    If you don't feel the need to calculate the pressure drop in the pipe, or the velocity profile, you can get an approximation with Bernoulli's. Find the kinetic energy of the air as it leaves, solve for the velocity, multiply by the crossectional area and you've got a volume flow rate.
  4. Dec 9, 2015 #3


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    You'll also need to know the length of pipe through which the air travels from the reservoir to atmosphere, along with any fittings, elbows, valves, etc. to calculate the minor losses in the system.

    Since the reservoir is maintained at a pressure of 7 kg/cm2(is this gauge or absolute pressure?), which is approximately 7 atmospheres, choking of the flow is a distinct possibility, which would render use of the Bernoulli equation invalid, since in the choked condition, the flow has become supersonic and one can no longer ignore the effects of compressibility on the flow of the air inside the pipe.
  5. Dec 9, 2015 #4
    You also need to consider viscous/turbulent friction loss in the pipe.

  6. Dec 9, 2015 #5


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    Those were my reasons for asking for more information on the exit piping.
  7. Dec 9, 2015 #6
    thanks for your Suggestions,
    @jackwirl - i need to know basic Methodology for calculating air flow capacity of a System at a specified pressure, For Recommendation of Pipe Sizing for a required Air Flow.

    @SteamKing - Sir 7 kg/cm2 is gauge pressure & assume with no choking of flow in pipe line and the pipe is open to atmosphere.

    i need to know(calculate) what would be the max Flow a 12mm pipe (10m length) could provide from a Air Receiver at 7kg/cm2.

  8. Dec 9, 2015 #7
    See Example 6.2-2 "Flow Rate for a Given Pressure Drop" in Bird, R.B., Stewart, W.E., and Lightfoot, E.N., Transport Phenomena, John Wiley, 2002, p. 183.

    This example shows how to solve for the case of an incompressible fluid. Based on this, it is easy to figure out how to handle a compressible fluid.

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