Dust collection flex tubing, what's good?

[George]

Italian fellow name of Bernoulli, I believe, has some good words to say on the subject.

Consider the original force per unit area I mentioned. That's where the term PSI comes in. You can haul more air through a larger pipe, but the pressure drops, because you're not capable of real compression through the open sides of the impeller. This means that what's being carried along with the air will also drop. Reverse is also pretty true. Take your 4" hose, as I often do, and use a standard shop-vac adapter to 2", and notice you can pick up pencils, chunks of scrap, and even the bolt you dropped, and were looking for. Don't be frustrated and think you'll have to rummage through the cyclone, those things are just upstream of the adapter, if they made it that far, where there is no longer enough force/unit to carry them into the bin. I rely on this when looking for dropped objects in my shop.

As mentioned, the "standard" unit now moves 1200/CFM at (some PSI) or in reality, at some vacuum, measured in feet of water, inches/millimeters of mercury or furlongs per fortnight. Now since the old 650 CFM @ 8 types were the standard which spawned the 4" hose, I'm speculating that a 5" hose may be best for the 1200, because the impellers are still pretty leaky, if you read the mfrs specs. A 6" hose, as mentioned, would be 2 1/3 or so times the area of a 4, negating the additional chip-carrying power.

Oh yes, don't ask about 2" hose and 2" sanding discs for the lathe. Makes me veeery angry.

[G. Lewin]

No, the system is not limited by the narrowest pipe. It's not a "weakest link" analogy. The reason is that the speed of the air will vary inversely to the area of the duct. So the 4 inch restriction will just accelerate the air. There is some (small) loss with restrictions, mostly because it is easy to accelerate flow efficiently but hard to slow it down.

Others have properly noted the fact that while large ducts can pass a lot of air, the speed of the air drops so that dust can settle out in the pipe.

Greg

Fly-by-Night CC wrote:

[G. Lewin]

This is, well, to be blunt, kind of gobbletygook. Well, the conclusions are more or less valid (big pipe => high flow, low speed), but the physical explanation is not correct.

In short, larger pipes can pass more air because the wall friction per unit length of pipe is less (because the airspeed is lower). Less friction means the pressure losses in the pipe are less and the impeller has less head to work against. Since it's working against less head, it can pull more air.

The lower airspeed leads to less friction (on the dust particles) and less turbulence, which allows dust to settle out.

(Of course, there are a few caveats involved above, but for practical purposes, this is the basic principle).

For those who care, there are many references to explain fluid flow in pipes and DC's; I think the FAQ has some decent references. I wrote a primer once, and if I ever get some web space again, I'll gladly post it.

By the way, Bernoulli wasn't Italian. He was Swiss. And Bernoulli's principles aren't really valid in this context (duct flow) because the viscous forces are too large.

Greg

George wrote:

[igor]

Next you'll try to tell us that Columbus was from Brooklyn. Mama mia!!!

PS: The comments you made about a temp. reduction in pipe diam. was helpful. I don't understand much of the physics, but it has proven out in practice - e.g., putting a 4" pipe on machine's 2" duct fitting is better than putting a 2" pipe on it to the DC. In fact, this *suggests* a reason why my Dewlat TS has a small fitting - increased air speed perhaps improves dust capture over what it would otherwise be. Just a thought. -- Igor

[George]

Well, fluid dynamics was not my prime concern. The concern was with the "carry", which of course is related to the flow rate. You are concerned with the fluid, I with the solid, which, at least to me, is the reason for having a collector, not to move air around.

"Bernoulli's principle can be explained in terms of the law of conservation of energy (see conservation laws, in physics). As a fluid moves from a wider pipe into a narrower pipe or a constriction, a corresponding volume must move a greater distance forward in the narrower pipe and thus have a greater speed. At the same time, the work done by corresponding volumes in the wider and narrower pipes will be expressed by the product of the pressure and the volume. Since the speed is greater in the narrower pipe, the kinetic energy of that volume is greater. Then, by the law of conservation of energy, this increase in kinetic energy must be balanced by a decrease in the pressure-volume product, or, since the volumes are equal, by a decrease in pressure."

Will you go this? Lower vacuum (large pipe), pieces drop - higher vacuum (narrower pipe) , pieces move.

[G. Lewin]

OK, you seem to have two contradictory statements: The first paragraph (correctly) states that the "carry" is related to the flow rate. But then in the last paragraph (and the quote about Bernoulli) suggests that the pressure _itself_ is responsible for carrying the particles. The correlation that high speed == low pressure and vice versa (Bernoulli's principle) is not really relavent, and for a ducted system, only marginally applicable.

Yes, the pressure and flow rate do change (and you can use Bernoulli's principle on a limited basis at the junction) when you change duct size. But pressure is just a means to an end (in that pressure differences are what move the air, of course). It is air speed that is responsible for carrying the particles (turbulence and particle friction, in particular). So when you say "Lower vacuum (large pipe), pieces drop" it should really be "Lower speed...".

Greg

George wrote:

[George]

Kinetic energy, as stated. Seems that demands some consideration of mass or force.

I think Owen already realizes that air through a tube is not the same as trying to put 3# of the proverbial solid into a 2# bag, which answers his question. So here's my question. If I've a 4" flex hose (standard), and the current "standard" 1200CFM @ 11 ft of water static pressure impeller, what percentage of my potential chip-carrying energy will I lose between equal lengths of 6,5, or 4" inside diameter transport pipe? I figured it would be in approximate proportion to the difference in cross-section. So or not?

[G. Lewin]

Hmmm...not quite sure on your question, so I'll answer it two ways:

If you have two otherwise identical systems, one with say 4" ducts and one with 5" ducts, the airspeed will go [to a rather gross first order] like 1/AREA. With the reduced resistance of the 5" duct, however, that system will have a higher flow rate, and so the airspeed will be higher than said 1/AREA back of the envelope analysis. By how much depends on many factors, as you well can guess, including the impeller design, roughness, duct layout, etc.

If you have ONE system, with both 4" and 5" ducts connected in series, obviously the mass flow rate is the same in each duct. Since the volume changes little at these pressure differences, the volumetric flow rate is nearly unchanged. Then the airspeed will go almost exactly like

1/AREA for each section of pipe. Of course, the 4" will cause greater pressure losses; for equal sections of pipe, the narrower pipe will be more "lossy." Geez, I wish I could say how much; off the top of my head I think pressure loss goes like 1/RADIUS^3, but don't quote me on that. When all my textbooks get out of "storage" (read: the moving van blew its transmission), I can look it up.

When it comes to "chip carrying energy," if you mean kinetic energy, well, you know how to find that. If you mean "chip carrying _ability_," we'll have to define ability first. Good luck on that one. The best I've seen is a relationship between airspeed and maximum particle size, but I can't remember where I saw it. I seem to remember 3000 ft/min. is a good rule of thumb for wood dust, chips, and fingers.

Greg

[George]

OK, pretty much as advertised. Lower velocity (sqroot) lower the pull, I guess.

[GerryG]

Greg, while I agree with your statements, per se, I'd like to toss in one more item. Specifically, the intake bypass in a 2-bag DC. We have a single fan (impeller), and if the air line to that was fully (or even mostly) blocked for some reason, the upper bag would collapse. To avoid this, there appears to be a partial intake bypass. The air movement would then split between the main duct and the bypass by the relative resistance of the two paths.

Now, I imagine a pressure limit valve could be used in the bypass, but I doubt they do this.

Haven't seen this mentioned before in discussions. But it explains why a 2-hp DC cannot match the static vacuum of even a medium shop vacuum, no matter how much you restrict the opening. It would also impact some of your conclusions (by degree, not type), in that moving from a 5- to 4-inch hose would be worse than expected since more air would flow though the intake bypass.

Does this make sense, or am I miss>No, the system is not limited by the narrowest pipe. It's not a "weakest

[G. Lewin]

When I'm lucky (wealthy?) enough to have a two-bag DC, I'll let you know. OK, really, there are a lot of caveats that are important in practice, and not quite knowing what you're describing, I'll just chalk it up as "it's quite possible."

There is one thing I'd like to point out and that is that the reason a shopp-vac has much higher static pressure is that the impeller speed is much higher. Pressure rise at zero flow goes something like [rotation speed * radius]^2 (I think--again, don't quote me). Despite the larger diameter of DC's, the high speed of the shop-vac is more than enough to compensate. Obviously, when there is airflow, things change, but you get the idea.

Greg

GerryG wrote:

[DarylRos]

It's not Bernoulli mainly that factors into this, it's Boyle.

Pressure dynamics is the same whether it's for gas, liquid or even traffic patterns.

Reduce the size of the pipe, duct or road and you increase the pressure and reduce the velocity.

So the idea that a reduction at one point (be it the smaller pickup at a saw or a roadblock in the middle of the road), the pressure increases, the dust, car, water, whatever slows, but then, as the pressure decreases with the increase in the roadwork, the speed increases.

The traffic analogy was not mine, but worked out by some highway engineers. They were surprised to learn that traffic flow basically obeys Boyle's law.

Which is why you want large main ductwork, this is your freeway. The smaller gates are your on ramps.

The speed cannot be the same throughout. Just as traffic picks up after a slowdown. Sometimes when you hit traffic and then it speeds up, you wonder why. Well, there was a stoppage a while ago, and the system is simply recovering. It does not stay slow the entire way.

[George]

Nope, Newton.

We're moving solids, hopefully. That's Newton. Thus the concept passage cited.

[Dave Hinz]

Solids suspended in air perform as a fluid, do they not?

[George]

Nope, they behave as masses acted upon by outside forces.

The fluid is a lube to reduce fricti> > Nope, Newton.

[Dave Hinz]

I'm not sure that that's how a suspension behaves. "lube" would indicate that it forms a film between the thing being transported, and the plenum it's being transported in. Seems to me you're moving both the air _and_ the sawdust suspended in the air.

[George]

Chunks, man, think chunks.

[Todd Fatheree]

I guess we need to find out which laws apply to a non-colloidal suspension. By the way...the Bernoulli equation is for frictionless, incompressible flow. It works well enough for fluids, but it's out for gases. A cursory look over my fluid mechanics info says that we might have better luck with the Euler equation.

Also, someone here pointed out that Bernoulli was Swiss (after someone else said he was Italian). He lived much of his life in Switzerland, but he was, in fact, Dutch.

todd

[J. Clarke]

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.

[George]

Guess we need to think about how much energy we want to waste in turbulent flow to get things in suspension versus what we'd like to have to get them flowing in a more laminar pattern toward the impeller.

That is what velocity is, is it not? Motion in a direction?

says Swiss, but with Netherlands

Ethnicity of the name? Probably Italian. Wrote in Latin, so what's the diff?

Swiss are by language German or Italian, with a bit of French.

suspension.