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Thread: Dust collector question

  1. #16
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    Feb 2009
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    Cedar Park, TX - Boulder Creek, CA
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    832
    A 6" pipe has a 12% greater area than 2 4" pipes. 6^2=36, 4^2*2=32. So double 4's look pretty good.

    What I haven't seen mentioned, that I remember anyway, is the difference in surface area. The perimeter of a 6" pipe is 18", roughly (6*3 is good enough). For 2 4" pipes it's 24 (4*3*2). So 2 4's have 33% more area in the pipe creating drag.

    Extreme example, a couple thousand 1/8" tubes would flow basically nothing at these pressures.

  2. #17
    Join Date
    Feb 2013
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    Tippecanoe County, IN
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    Quote Originally Posted by Wes Grass View Post
    A 6" pipe has a 12% greater area than 2 4" pipes. 6^2=36, 4^2*2=32. So double 4's look pretty good.

    What I haven't seen mentioned, that I remember anyway, is the difference in surface area. The perimeter of a 6" pipe is 18", roughly (6*3 is good enough). For 2 4" pipes it's 24 (4*3*2). So 2 4's have 33% more area in the pipe creating drag.

    Extreme example, a couple thousand 1/8" tubes would flow basically nothing at these pressures.
    Excellent point about the "wetted periphery", it's often forgotten. You can quantify the effect of both by considering that pressure loss per unit length in a duct varies with the square of the flow rate and the inverse fifth power of diameter*. So, for two 4" ducts compared to one 6" duct the pressure drop for the two 4" is (1/2)^2 x (6/4)^5 = 1.9, or almost twice that of of the 6".

    * Neither are exact because of the variation of friction factor with Reynolds number, but close enough.
    Beranek's Law:

    It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
    L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.

  3. #18
    Join Date
    Nov 2007
    Location
    NW Indiana
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    3,078
    How much air you pull through a 4" pipe depends on your system. I use the 4" port on my SawStop PCS and am pulling 670 cfm.

    There is a huge difference depending on the hp and impeller diameter.

  4. #19
    Join Date
    Dec 2006
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    Mesa, Arizona
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    1,798
    Quote Originally Posted by David L Morse View Post
    Excellent point about the "wetted periphery", it's often forgotten. You can quantify the effect of both by considering that pressure loss per unit length in a duct varies with the square of the flow rate and the inverse fifth power of diameter*. So, for two 4" ducts compared to one 6" duct the pressure drop for the two 4" is (1/2)^2 x (6/4)^5 = 1.9, or almost twice that of of the 6".

    * Neither are exact because of the variation of friction factor with Reynolds number, but close enough.
    I'll just pretend like I followed the math.
    David Walser
    Mesa, Arizona

  5. #20
    Join Date
    May 2018
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    Lancaster, Ohio
    Posts
    1,346
    Originally Posted by David L Morse
    Excellent point about the "wetted periphery", it's often forgotten. You can quantify the effect of both by considering that pressure loss per unit length in a duct varies with the square of the flow rate and the inverse fifth power of diameter*. So, for two 4" ducts compared to one 6" duct the pressure drop for the two 4" is (1/2)^2 x (6/4)^5 = 1.9, or almost twice that of of the 6".

    * Neither are exact because of the variation of friction factor with Reynolds number, but close enough.



    I'll just pretend like I followed the math.


    It is amazing to me how much real, actual knowledge is available and shared on this board.
    YES there is a lot of people spouting off like they know something. Have to ignore that and pay attention to the real knowledge that comes forth.
    THANK YOU to the ones who really know and share whether it is by formal training or by actual, factual information gained from observation, measuring of doing and then reporting.
    Ron

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