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Air Flow: Best possible CFM

  1. Nov 11, 2008 #1

    I was wondering if there was someone on this forum who could help me with this. Basically my question sums up to this: There are 2 fans with equal size/power/cfm. Air must exit a 6" hole. Which design would produce the best CFM (Cubic Feet per Minute) of air flow?

    I have attached a picture of the designs a colleague and I were conflicting on.

    We are also open to any other suggestions for best possible CFM.

    Thank you!

    Attached Files:

  2. jcsd
  3. Nov 11, 2008 #2


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    Staff: Mentor

    Welcome to PF.... Don't double-post.

    Fans are like pumps. If you put them in series (first picture), you keep the flow (CFM) constant, but add the pressure they can generate. If you put them in parallel (second picture), the pressure is constant and the flow rates are additive.

    Another way to look at it: if you take the second picture and split the two fans apart, giving them each their own duct, what have you really changed...?

    Caveat: If the duct system they are attached to (if they are attached to a duct system...) is long and/or undersized, you may need a lot more static pressure to force more air through the duct and you might make your fan less efficient so it has to work harder. So then which scenario is better depends on the particulars of the fan performance (google: fan curve) and duct resistance. If all you have is that little nozzle in your photo, then this isn't much of a concern.
    Last edited: Nov 11, 2008
  4. Nov 12, 2008 #3
    My apologies regarding my double post :shy:

    Thank you for your reply, it was very helpful :smile:

    Would you happen to know another model design that would be more efficient?

  5. Nov 12, 2008 #4


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    Staff: Mentor

    More efficient than what at what? I don't know what you are trying to do or what you've tried so far!
  6. Nov 13, 2008 #5


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    Homework Helper

    The combined induced wash efffects should increase air flow a bit. Also the pressure itself quickly translates into increased airspeed at the "exit" point where the affected air's pressure returns to ambient. If the series fans are too far apart, the second fan could end up producing little gain if it's rpm didn't increase significantly to compensate for the faster air being input.

    The parallel fans should double the air flow as mentioned (assuming that the 6 inch tube doesn't significantly restrict flow).

    Note you could always use a single fan with a higher output. There are 120mm (4.72 in) fans, intended for servers, that blow more than 200cfm, but they are loud.
    Last edited: Nov 13, 2008
  7. Nov 13, 2008 #6
    Thanks for the reply Jeff.

    I guess i should have included this in my orignal post:

    Fan Specs: 120mm, 1200rpm, 24.00 dBA, 68.54CFM, 2.86 CFM/dBA

    Sound was most definately an issue when the fans were purchased. Looked for decent amount of CFM with a low dBA level. Which is why we ordered 2 in hopes of doubling output.

    For the side by side fan design, we realized that the 45 degree angles would cause alot of turbulence. We are now designing a model with an "S" type curve in hopes of better air flow.

    Any more comments would be greatly appreciated

    Last edited: Nov 13, 2008
  8. Nov 13, 2008 #7


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    Staff: Mentor

    What is the application? Why the constraint on duct size and fan size? Since 6" is 152mm, why not use a 150mm fan? Here's one that generates 265 cfm: http://www.frozencpu.com/fan-88.html [Broken] It's loud, but the sound level drops with rpm, so you can slow it down until you are happy with the sound level.

    Or you can duct-in a much larger fan: http://www.newegg.com/Product/Product.aspx?Item=N82E16835705060
    Last edited by a moderator: May 3, 2017
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