# Dust collection flex tubing, what's good?

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• posted on July 23, 2004, 12:49 pm
Whoa! Slow down, everyone. Let's back up.
The most important governing equations here are the incompressible Navier-Stokes equations. The Bernoulli equation, as noted, is for frictionless, incompressible fluids (n.b., both liquids and gases are classified as fluids). The Euler equations are for frictionless, compressible gases, but air under these conditions is nearly incompressible, so we can make that simplification (if you want to get an anser down to the <1% error range, use the full compressible N-S).
As pointed out elsewhere, Boyle's Law is just a simplification of compressible gas laws, and isn't appropriate here.
Now, the solids in the airstream don't substantially affect the flow. That means that we can "decouple" the system and calculate how "pure" air would flow and then throw the wood dust/small chips in and simply track them through the ducts, using our solution for pure air. (again, a prefect model would account for the fact that the wood chips can _cause_ turbulence, but this is a secondary effect).
Now, as for the important answer of which is more important for moving chips: turbulence effects vs. friction effects? I can't say. But if you work through the calculations, you find that the "recommended" flow speed usually works out to the transition region between laminar and turbulent flow. Coincidence? I suspect (and this is pure conjecture) that some amount of turbulence is necessary to keep dust from sticking to the sides of the duct. Obviously, though, the bulk motion of the air is what moves the dust from A to B.
Greg
Todd Fatheree wrote:

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• posted on July 23, 2004, 5:18 pm
SNIP

This didn't seem quite right to me so I took a look at the numbers. IIRC, recommended duct velocities are 3000 to 4000 fpm.
Reynolds number = Re = (density)(velocity)(diameter)/(viscosity)
At 70 deg F: density = 0.075 lbm/cu ft viscosity = 0.044 lbm/ hr ft
A lower limit could be 3000 ft/min in a 4 inch duct.
Re = (0.075 lbm/ cu ft) (3000 ft/min) (60 min/hr) 4 in) / (0.044 lbm/hr ft) (12 in/ ft)
Re = 102,273
Since transition from laminar to turbulent flow (in internal duct flow) is in the range 2,000 to 10,000, this is clearly turbulent. Higher values for the flow rate and/or duct diameter will yield higher Re numbers.
I would expect you would want to stay away from laminar flow, and certainly stay away from transition for good performance.
Bill Leonhardt
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• posted on July 23, 2004, 7:28 pm
Damn. I'm sure I calculated a much lower Re once, but I can't find my notes to see where I made the mistake (I assume it was me, but I'll check yours). Probably got screwed up on the whole lbm/lbf thing...
G
Bill Leonhardt wrote:

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• posted on July 23, 2004, 7:49 pm
In metric:
V = 3000 ft./min. X 1 m/3 ft. X 1 min. / 60 sec. = 16.7 m/s D = (1/3) ft X 1 m / 3 ft. = 0.11 m nu = 1.46 E-5 m^2/s
Re = VD/nu = 16.7 X 0.11 / (1.46 E-5) = 125,000
A little higher than yours, but I rounded. So, you're correct.
G
Bill Leonhardt wrote:

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• posted on July 23, 2004, 1:00 pm
Todd Fatheree wrote:

I guess it depends on the definition of nationality. I have no idea what citizenship rules were like then, or if it's relevant. His dad, Johann, was Swiss and was working in the Netherlands at the time of Daniel's birth. Were I to move to, say, Sweden and have a child, I would still consider my child an American. Would it be Swedish? Technically, I suppose.
Let's just say that he was a member of the Axis of Fine Chocolate Producing Countries (not sure what the third would be)...
G
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• posted on July 22, 2004, 5:01 pm
DarylRos wrote:

I beg to disagree but at the velocities common in dust collection systems the flow of air is assumed to be incompressible and Boyle doesn't enter into the calculation. It's not until you have velocities approaching Mach 1 that you start having to consider compressibility.

Maybe so, but Boyle's Law applies to static pressures, not dynamic.

You've got it backwards. Reduce the size of the pipe or duct and you decrease the pressure and increase the velocity.

That may be _your_ idea but gases don't behave that way in ducts.

I'd like to see a reference to that.

If highways behaved like air ducts then you'd see people going 180 MPH though construction zones.
--
--John
Reply to jclarke at ae tee tee global dot net
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• posted on July 16, 2004, 4:15 am
On 15 Jul 2004 15:19:48 -0700, snipped-for-privacy@emenaker.com (Joe Emenaker) wrote:

Maybe. It really depends upon distance and the size of your dust collector. 6" may be *too* good and reduce velocity to the point where chips settle out. You really need to do the velocity/static pressure computations to be sure. In my case, 6" was too large, 5" was the ideal size. Unfortunately, that meant I could not use cheap PVC from the Borg.

Yep

Again, yep

Check out the various web sites like http://cnets.net/~eclectic/woodworking/cyclone / it has an Excel spreadsheet: http://billpentz.com/woodworking/cyclone/StaticCalc.xls that is invaluable in sizing your system. Pay attention as well to sizing the ductwork for each machine.
I just went through this late last year and have been very satisfied with the results. I wound up buying metal spiral pipe from a local fabricator along with Y's, T's and elbows. I can now see plumes of dust being sucked into the table saw through the insert and the shaper table chips are sucked into the system with few residuals left behind.