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I just snagged a 1.5hp 700CFM Oneida cyclone dust collector on Craig's list and now need to run some ducting.

Does anyone know if the 6" steel HVAC ducting at Lowe's/Home Despot etc. can handle the vacuum or does one have to go with the thicker or spiral stuff that the DC manufacturers are selling?

I imagine that at only 1.5hp and 700CFM I should be safe. The stuff at the Big-Marts is pretty cheap.

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I've had a 2000cfm unit for years with such ducting with no troubles. I do, however, keep one line free of blast gates whatsoever so the system never builds excessive vacuum.

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Doug

Don't let fear or common sense stop you from trying to build something

I use that stuff too. It'll be fine. Use as little spiral ducting as possible- every foot of it cuts your airflow big time, and with 700cfm you can't afford to lose much.
I found that if you accidentally start the collector with all the blast gates closed, it doesn't generate very much vacuum, but the motor spins up to a higher speed; you can hear it, and it's probably not good for things. If it's already running and you close the last gate though, you might get a crinkle, especially on a long straight run of ducting. Not the end of the world.

Not being a tard here my friend , but ive been selling and installing systems for years and have never seen any math to back up this claim . Granted he doesnt need spiral pipe , however it is superior to the thinner wall materials simply because of the quality of the built connectors and elbows .

What you WILL NEED to do is get the thin foil duct tape and wrap your elbow joints and connections it will reduce the leaks and improve your draw. ESP. the elbows . Those adjustable el's well come apart on u and leak air badly .

my 2 cents worth

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I imagine that Mike was thinking of spiral-wound hose, which is indeed horrible for air flow. Spiral metal duct is only slightly worse than straight galvanized for friction. For 100' of straight 6" duct, 4000 fpm, here are the losses in inches of water column (iwc):
Spiral: 4.22 iwc (http://www.spiralmfg.com/downloads/engineering_data.pdf)
Standard: 4.1 iwc (http://www.engineeringtoolbox.com/ductw ... _1122.html)
Pentz says flex hose is 3x-9x worse than duct!

What does make a difference, as Jim mentioned, is the quality of the elbows, branches, transitions, and inlets. Spiralmfg has data on die-formed vs 5-segment elbows (4" dia, 90 deg, long-radius (r/d = 1.5), at 4000 fpm):
Die-formed loss = 0.105 iwc
5-segment loss = 0.37 iwc
A typical short-radius elbow (r/d = 0.75) is more than 2x worse (http://www.webpages.uidaho.edu/me414_51 ... /elbow.pdf)
So a big-box 4" elbow (3 segment short-radius) would probably have a loss of about 1.0 iwc, vs 0.1 iwc for a good one.

For a dust collector, closing off all inlets won't hurt it since those motors have their own cooling air supply. The motor speeds up because the impeller can't pump anything, which takes less HP and amps, so the motor actually runs cooler. BUT, some vacuum cleaners use the pumped air to cool the motor (called "flow-through"); those will indeed burn up if closed-off too long.

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David Malicky

That would be absolutely correct .


You must be a Spiral Mfg Dealer or Rep . Thats whose products I sell . Excellent stuff , but for the small guy its hard to justify . I have approx 500 sq ft in my shop , a 2hp Jet and 10 machines hooked up and the cost for me was about $1,000.00 to set up my shop . You might want to keep in mind that using 45 degree elbows and angling all your lines toward the collector will help your system be much more efficent .

Note : The schematic here http://www.spiralmfg.com/prod.htm

BTW ... Dust collection is the ONLY product I sell where I can Tell people " If it SUX Thats A GOOD Thing !

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My system uses snap together ducting from lowes. Been up and running full speed ahead for three years with no issues what so ever. The key to great suction is to seal all the seams and use blast gates so that the only path open is the one to the tool you are using. That way it doesn't matter how much total CFM you need for all your tools. It only matters how much you need for your longest run.

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I run the stuff, too. 13+ years, no issue. As said, tape all joints....

6" will be too large, though, and you'll lose a lot of air velocity with such a small dust collector and a too-large duct. I'd go 4", or at most, 5" straight metal ducting. Do some research; overly large ducts are great for volume but absolutely kill velocity(IE: air speed), and you need to balance volume with velocity if you want a reasonably effective system. When in doubt, go one size smaller than you -think- you need, in fact.

Good advice from Jim, of course. A 45 has half the loss of a 90 and the diagonal run is shorter. I do like Spiral Mfg's extensive test data.

Optimal duct diameter depends on the exact specs of that Oneida, the ducting SP loss, use and maintenance, and design priorities.

Is that 700 cfm an absolute max flow at 0 iwc with clean filters, or a rated flow at either X iwc or seasoned filters? With 1.5 HP, it's likely more towards the former. If Oneida publishes the 'fan curve' for that cyclone (or the max static pressure and max CFM, then drawing an approx curve), the actual cfm can be estimated through various ducting options.

High duct velocity is important to keep the dust from settling in the duct and then restricting/plugging it. 4000 fpm is typically recommended for wood dust. Factories need at least that. Home shops produce small amounts of dust so a little slower velocity is ok if the ducts are inspected periodically. Lower duct velocity gives: less pressure loss, more CFM, less noise, but more risk of dust settling. With a small blower in a home shop, I'd aim for about 3500 fpm to keep the CFM up, and put a take-apart joint in each run for periodic inspection/clean-out.

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David Malicky

For a real-world example, mine is a large(ish) industrial 4 bag system that I got at an auction. The motor is a 3 or 5hp(pulls 20 amps at 220v) and has a 8" intake. After the initial run of 8" ducting from impeller to the ceiling, I neck-down to 5" straight metal ducting from there, for the rest of my layout.

I had a good bit of 7" ducting that came with the unit, but it felt "sluggish" and a fair amount of dust and chips could be seen building-up on the bottom of the ductwork after just a short while. When it came time to permanently install my system, I read Pentz's book, and decided on the 5" ducting. After 11 years here, I have never had to clean out the ducts, they stay very clean. And even with as many as 3 ports wide open, there's very little detectable velocity drop-off at the furthest downstream port. I do try to run with only one open, but it still shows that the system isn't being starved by the small-ish ducting. Basically, I couldn't be happier!

If you don't have a cyclone(I don't), I do recommend a dropoff "trap" ahead of the impeller, to prevent large pieces of wood or otherwise from getting into the impeller. This trap can be as simple as a 6x6x6 tee with a blast gate on the bottom, added a few feet ahead of the impeller. What happens is that the air velocity will drop a bit in this section, and the larger, heavier pieces will hopefully fall out of suspension and drop to the blast gate. After each use, simply open the gate to extract the heavy items. I have one of these ahead, and also -behind- the 90° elbow at the impeller. This last one takes the heaviest, fastest moving objects that were still moving too fast to fall out at the first trap. Works! I haven't had a single heavy piece reach the impeller since installing these traps, where before it was a common occurrence(as witnessed by the many dents in the 1/8" thick steel impeller's housing!!).

Also, someone mentioned that running with all the blast gates closed makes the motor run faster. This is a good thing. These motors have their own cooling fans and don't use the impeller's fan for cooling. The motor runs faster because there is -less- load on it. In other words, the motor isn't working as hard. We should actually be closing all blast gates before starting our DCs, but who bothers doing that....

Thanks guys. I might go for the 5" stuff for my long parallel run. The run to the collector will be slightly downhill so I can probably go 6" there. I also found another cyclone on CraigsList that comes with about 40' of 5" ducting and solenoid blast gates for about what I paid for just my DC so.....opportunity to mix and match.

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