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Air Compressor Pressure vs CFM vs SCFM

  1. Jan 2, 2017 #1
    So my boss hook me up with the Compressed Air Calculations which we never did previously at our workplace.
    So started researching about the subject and designed a Compressed air system.
    The only thing i am not sure about now here is the relation between CFM vs SCFM vs Pressure.

    Let us take an Example:
    An Air compressor of 250 CFM @ 8 bar(FAD) pressure outlet roughly translates to 2114.93 SCFM at 30 deg Air Temp.

    Now if i want the Air at 1 bar atmospheric pressure at the outlet for some reason, will i get 2114.93 CFM from the Compressor if we use PRV to reduce the pressure.

    I used TLV toolbox for the Air Compressor Calculations.
  2. jcsd
  3. Jan 2, 2017 #2


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    Staff: Mentor

    Welcome to PF!

    You didn't actually ask any questions there, but it looks like you discovered by accident that the "S" in SCFM stands for "Standard" (as in standard temperature and pressure). SCFM is useful because:
    1. The inlet and outlet CFM of a compressor are different.
    2. The pressure at which a compressor is rated affects the CFM but not (necessarily) the SCFM. So using SCFM makes it easier to compare different compressors.
  4. Jan 2, 2017 #3
    Thanks for the reply russ_watters.

    The only thing that is giving me hard time here is will the air compressor give me 2114.93 CFM at the outlet if we use a PRV to reduce the pressure from 8 bar to 1 bar(a).
    I mean to say will the CFM increase if we reduce the pressure at outlet
  5. Jan 2, 2017 #4


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    Staff: Mentor

    Certainly: when you pressurize air, you reduce its volume and therefore when you de-pressurize air you are allowing it to expand and increase its volume.

    An important caveat is that the air is treated as isothermal (constant temperature) in these problems, but the actual path the air takes does involve temperature changes. That may have to be taken into account depending on the specifics of your application.
  6. Jan 2, 2017 #5

    Ranger Mike

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    Science Advisor
    Gold Member

    also adding to what Russ said ( he is always the top notch advisor here)
    make sure the quality of the compressed air is what you want. If you have a high precision instrument the compressed air is running, and you have a lot of moisture in the air, you will have longevity problems. You may need an air dryer added to the system as well.
  7. Jan 2, 2017 #6
    Thank you so much Russ and Mike.
    It was indeed very helpful.
    I shall update you'll about my project when i complete the entire design.
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