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How to calculate cfm through pipe

  1. Oct 23, 2008 #1
    I work at a cement distribution terminal. We load bulk powdered cement pneumatically into ships. Part of this operation is to remove the excess air from the ship by use of dust collectors. I need to know if my dust collectors are pulling enough cfm to keep up with the total cfm going into the ship.

    Is there an instrument I can take a reading with to calculate cfm? Or does this need to be crunched through a formula? Is there someone out there that can assist me with this? I can provide whatever info you need to make the calculation, ie pipe diameter, specs on the dust collector fan, length of pipe the collector is pulling through etc. Let me know what is needed.

  2. jcsd
  3. Oct 24, 2008 #2
    The easiest thing to do is just to put a nozzle with known dimensions somewhere in the duct and measure the pressure difference across of it. If you know the Cp, Ch, and Cq of your fan you could also just measure the pressure the across and correlate the data.
  4. Oct 24, 2008 #3


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    Science Advisor

    Another option is a pitot tube traverse to get the velocity in the pipe. Then it's simply

    [tex]Q = V*A[/tex]
  5. Oct 24, 2008 #4
    Pitot tubes typically don't like dust.
  6. Oct 24, 2008 #5


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    Science Advisor

    True. The OP didn't really specify how bad it was, but it stands to reason. However, one never knows until the question has been raised. Technically orifices and venturis are out of scope for two phased flows. It would have to be calibrated for that particular flow.
  7. Oct 24, 2008 #6


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    Staff: Mentor

    You could always perform the test when there is no dust...
  8. Dec 4, 2008 #7
    I work for an environmental engineering and testing firm. We frequently test at portland cement manufacturing locations. Its a relatively easy test to measure the flow of a dust collector. Typically if you traverse the duct with a Type-S pitot tube and an oil manometer to get an average pressure differential, use the stack diameter, moisture (typically ambient on a dust collector 2-5%), barometric pressure, pitot tube coefficient, etc you can get a very accurate flow measurement. Refer to EPA methods 1 through 4 in Title 40 of the Code of Federal Regulations Part 60.

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