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Air flow in an open system

  1. Jan 14, 2012 #1
    Hi, I have a question relating to normalizing air flow data in an open system…

    The system comprises of a regulator feeding 2barg line pressure to a valve (controlling the downstream flow), a variable restriction, a second valve (normally open) and a flow meter. Pressure readings are taken upstream and downstream of the restriction – these generate the line pressure and differential pressure readouts.

    Because of the layout, the restriction in the pipe work generates backpressure as the flow increases – as such, a ‘0 barg’ test can end up with 1 barg of line pressure…

    We apply a calculation to the data to ‘normalize’ it at the generated temperatures and pressures.
    This is the equation to normalize the data used: QN = Q × (Pa / Pref) × (Tref / T) - sourced from
    The problem is that we wish to compare the data to other data sets on a uniform scale (set temperature and set pressure)…
    As the generated data is produced with a variable line pressure, I presume that another calculation is needed to remove this variability before entering the uniform values into the normalizing calculation…??

    For example:
    I wish to compare the following data at 0.5barg line pressure and 15°C to create a flow curve
    0 l/min, 0mbar dP at 0barg line pressure, 10°C
    20 l/min, 226mbar dP at 0.2bar line pressure, 9°C
    42 l/min, 502mbar dP at 0.38bar line pressure, 9°C
    The equation can be applied to each line – generating a ‘normalized’ value.
    The desired conditions can be applied to each line – generating ‘normalized’ values… but would these really be accurate for comparison as the dP was generated with different line pressures?

    (I'm finding this a little difficult to explain exactly what I mean...)

    Thanks for your time :smile:
  2. jcsd
  3. Jan 15, 2012 #2
    Hopefully this will explain a little better...

    http://img268.imageshack.us/img268/5466/query1.jpg [Broken]
    (Illustrative results)
    The actual data shows an increase in line pressure and a decrease in temperature during the test where the flow rate and differential pressure are recorded.
    The ideal conditions are set at a static temperature and line pressure.
    The Nm3/hr in each data table shows the difference changing these values makes.

    My query is: because the temperature and line pressure recorded are not static, are the generated results accurate? Or does the calculation work regardless as each point is individually converted? - the fact that the data is part of a data set and across different temperatures and pressures does not matter...
    Last edited by a moderator: May 5, 2017
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