Rectangular DC Ducting?

[Eric DeSilva]

As I think about redoing my DC franken-pipe and read up on the topic some more, if it is all about CFM and cross sectional area, anyone use rectangular ducting for their DC system? People talk about PVC or spiral metal pipe all the time; has anyone dug into the pro's and con's of using normal air handling ductwork? 6" pipe has a cross section of about 28 in^2, seems like placing and hanging 100mm x 200mm rectangular ducting might be easier and give you a cross section of 32 in^2...

So, has anyone gone square?

[Rod Sheridan]

I think if you calculate the ratio of flow area to wall surface area, round has the lowest frictional loss.

Regards, Rod.

[Bill Arnold]

My first thought is the sides of the rectangular ducting will collapse, at least in part, with the suction of the DC. Round pipe of the right gauges will not collapse.

[Mitchell Andrus]

There is an inherent strength in round pipe. I've seen round pipe sucked flat.... I'd imagine it would be quite a bit easier to collapse a pipe with flat side walls. Don't do it except perhaps right at the tool in order to spread the area of at-source collection such as the floor sweep.
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[Eric DeSilva]

Hmm. Maybe there is a good reason I don't see anyone using it.

The one place I'm still thinking about it is under my TS--a Unisaw w/the long fence. I've used 4" flex hose to go under the extension wing and connect to the 4" port. I'm now thinking it might make sense to get rid of the 4" port (just open up the side of the saw there), build a square box that connects to the side of the saw, lays flat under the extension, and connects to a 6" pipe. Flex pipe has poor flow characteristics, so I'm guessing I'll get much better fine dust collection if I use--say--a melamine box with a taper to a 6" outlet.

[Chris Padilla]

Eric,

Since you are considering that at a dust collection point, I think it is fine. I think a lot of us were thinking you might be running several feet of rectangular duct.

My 1023Z Griz had a 4" opening...I just hogged it out to a 6" opening and bolted in a 6" flange to accept some 6" flex.

Yeah, flex has loss but there are places where it is just needed and the best solution.

Also, the corners of square/rectangular duct have "different" air-flow characteristcs that likely lead to lower-efficiency over round.

If you want to capture the most dust, you still need above-the-table collection in addition to your below-the-table collection.

[Dan Friedrichs]

I think Pentz's website has some mention of people building ducts out of melamine or something. I agree with others that regular HVAC would probably get sucked flat, but if you had a ton of scrap plywood and time...

[Eric DeSilva]

I was actually thinking of more extensive use of square ducting, but am abandoning that. I was thinking that it seems easier to fab rectangular parts than round ones. But, the TS use intrigues me the more I think about it...

If I do this, I can make something that looks like a toe-kick underneath the long wing side of the TS. Right now, the 4" flex hose I've got means it isn't a great place to put stuff. If I open the TS side of it to the interior of the saw, I'll definitely get less restriction at the machine. I can then put some diagonal walls inside the toe kick unit to neck it down to a 6" pipe outlet where my DC pipe currently ends. I'm thinking it might be amusing to build it with a gate, and have part of the back open to the floor to use as a floor sweep pickup.

I've got a Bies overarm guard/DC on the top of the machine. Converting that to 4" hose seems like an awful task, so I'll continue to run that off a small shop vac and a Dust Deputy. But, if I build the toe kick thing, it might mean that I have a better place to put a cabinet to house the shopvac/DD and enclose it to reduce the noise.

I gotta stop visiting this site. Too many new projects, which I need like a hole in the head.

[Jim O'Dell]

Like Rod said, my understanding is that there is more wall surface area, and therefore more drag on the air flow, causing a reduction of air speed/flow, and therefore efficiency. The more out of square the "pipe" is, the more wall space. Jim.

[Van Huskey]

I think you have your answer but have you ever seen rectangular duct work expand/contract with just the low pressure/vacuum placed on it by an HVAC system depending on which side of the fan it iis on? Round pipe is inherently much stronger, thats why you don't see much square or rectangular tubing on bicycles.