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

"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

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

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

Hi Bob,

ASCII Art, check it out with Notepad.exe if you are not using a fixed
width font...

+-----------+
| 6c-ft |
| Box |
| +-------+ |
| | VENT | |
| +-------+ |
+-----------+
Front view ^
OUT
Front - +-----------+ - Back | |
| 6c-ft | +--+ +--+
Inlet ++ Box +-------------+ Exhaust |
Vent || to fan -- Fan |
Here -|| +-------------+ |
++ | 4" hose +---------+
+-----------+
Side view

I just bought some 4" PVC pipe to smooth the flow a little so there
will only be about a foot of flex hose each end.

If I were to use just a 4" vent then the velocity across the box would
be about 60mph according to earlier calcs so I am trying to make the
turbulence inside as light as possible but still get a good
extraction.

Box is 24" x 18" x 30" Thanks for the help

Dave

Dave, I can't do that wrote:
ASCII Art, check it out with Notepad.exe if you are not using a fixed
width font...


Nice drawing! Makes it all very clear. Did you do the drawing on your
keyboard, or do you have an ASCII-drawing app?


If I were to use just a 4" vent then the velocity across the box would
be about 60mph according to earlier calcs so I am trying to make the
turbulence inside as light as possible but still get a good
extraction.
....

Well, the velocity through the 4" *vent* would be 60 mph, but it will
slow down considerably through the box, but unevenly - that's your
turbulence. The *average* speed through the box will depend upon the
cross section: 24 x 18 would give less than 2 mph
(450cfm/(2x1.5)sq-ft=150 ft/min). 24 x 30 & 18 x 30 even less. But the
average is pretty meaningless with a 60 mph flow through the 4" inlet.

Looking at it another way: your box being 6 cu-ft means a flow of 450
cfm will give an air change every .8 sec.

My suggestion would be to not have a front on the box at all. You will
have a nice gentle 1-2 mph air flow with little turbulence. With an air
change every .8 sec, you will get very good extraction.

My opinion only,
Bob

Bob Engelhardt wrote:
....
My suggestion would be to not have a front on the box at all. You will
have a nice gentle 1-2 mph air flow with little turbulence.

Wait - that's not quite right. Through the outlet at the back of the
box, the air speed will be 60 mph. So the air speed inside the box will
vary according to the distance from the outlet. Very non linear, but
worst in front of the outlet.

With an air change every .8 sec, you will get very good extraction.

That's still true.

What I would do is this: take off the front, turn on the
blower/extractor, and use a smoke stick to see the air flow. You should
be able to tell if the extraction is good enough & if the air speeds in
the box are acceptable for your process.


My opinion only,

That is still very true.

Bob

On Oct 30, 6:00 pm, Bob Engelhardt wrote:

Hi Bob,

Thanks for all the deliberations, suggestions and compliment, I did
the ASCII art with the keyboard. Only took me about 2 minutes.

I can't leave the front panel off as it is part of holding it all
together, but I can make a fairly large hole. I will leave the front
off as a temporary test and check as you suggested.

I routed 3/4" a slot 18" wide across the front and the breeze fairly
howls through there. grin I will open that up in 2" increments and
see how that works. I was concerned about making it too big and not
getting any flow across the table and having to make a new front
panel. That would mean a 60 mile round trip to the hardware store for
more MDF. Ahhh, country living.

Thanks very much for your help.

Dave