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?

Thanks in advance.

Dave

Dave, I can't do that wrote:
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?
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

5200' per minute is really howling. Most dust collector systems are
sized to run at around 4000' per minute.

Jon Elson wrote:
Dave, I can't do that wrote:
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?
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

On Oct 28, 7:20 pm, Jon Elson wrote:

Hi Jon,

450 / .087 = 5172 ft/min, or just about 60 MPH.

Holy crap Batman, that is cranking along.

Thanks Jon

Dave

"Dave, I can't do that" wrote:

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?

Thanks in advance.

Dave

How long is the pipe? In the final analysis, its going to come down to
pressure drop for most reasonable velocities.

--
Paul Hovnanian
------------------------------------------------------------------
Bureaucrat, n.: A person who cuts red tape sideways. -- J. McCabe

On Oct 28, 9:29 pm, "Paul Hovnanian P.E." wrote:

Hi Paul,

How long is the pipe? In the final analysis, its going to come down to
pressure drop for most reasonable velocities.

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

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:
On Oct 28, 9:29 pm, "Paul Hovnanian P.E." wrote:

Hi Paul,

How long is the pipe? In the final analysis, its going to come down to
pressure drop for most reasonable velocities.

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

Dave, I can't do that wrote:
The 4" hose is 10' ...

"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

On Oct 29, 8:14 am, Bob Engelhardt wrote:

Hi Bob,

But, you knew all that,

grin Yup, I did but I don't think the 400cfm is critical and the
extractor is actually rated at 660cfm I down rated it because of
anticipated losses and in case someone just gave me the area of a vent
required rather than the algorithm.

But thanks anyway.

Dave

Dave, I can't do that wrote:
... the area of a vent required ...

OK, back to the area of the vent. Where is this vent? Is it the outlet
of the extractor that has the 10' of hose on its inlet? Or is is the
outlet of the box that you're venting (not likely, I guess)?

If it's the extractor outlet, it doesn't need to be more that 4"
diameter. With 10' of 4" hose on the inlet, the outlet size is almost
irrelevant. I.e., all the restriction is in the input.

Same reasoning applies to the size of the box outlet. Size it to the hose.

Bob