# Sizing a vent?

Hello,
I have a box that needs to be force vented. I have a woodwork dust extractor and I want to size the vent for 450cfm throughput.
I have no idea of the speed of the air flow with the extractor other than it is rated at 550cfm and uses a 4" pipe.
Anyone know where I might find a formula for calculating the vent area?
Dave
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Dave, I can't do that wrote:

Velocity is easy, making assumptions that the air is not shearing near the boundary with the pipe.
450 CFM, and a 4" pipe. 4" = Pi 2^2 = 12.56 Sq In. or .087 Sq Ft.
450 / .087 = 5172 ft/min, or just about 60 MPH.
Jon
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5200' per minute is really howling. Most dust collector systems are sized to run at around 4000' per minute.
Jon Elson wrote:

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Hi Jon,

Holy crap Batman, that is cranking along.
Thanks Jon
Dave
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"Dave, I can't do that" wrote:

How long is the pipe? In the final analysis, its going to come down to pressure drop for most reasonable velocities.
--
Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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Hi Paul,

The 4" hose is 10' and it vents to air as there are no particulates in the flow, it is just fume extraction. However the extractor is on the outside end if that makes a difference. It is so bloody noisy I am putting it outside.
Dave
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The noise and the small pipe and the high velocity all go together. Suction line (fan outside) is much less efficient than a pressure line (fan inside).
You need to consider a different setup with larger tube, lower static pressure, lower speed fan. Another option is to use a fume hood type of box arrangement, you can get by with much lower CFM needs.
Dave, I can't do that wrote:

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Dave, I can't do that wrote:

"hose"? If you mean the hose that is a wire helix covered with cloth/vinyl, it will have a LOT more resistance than smooth pipe. E.g., aluminum clothes dryer exhaust pipe. Also, elbows add a lot to flow resistance: keep the run as direct as possible.
But, you knew all that, Bob
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