Clarification on Dust Collector Design

[Bill Adamsen]

A one person shop - typically one machine operating at a time - I'm looking at re-installing a dust collection system following move to a new shop. The collector is an old but functioning Torit cyclone with 3hp 3-phase Baldor running at 3450rpm, 8 inch intake and 600 square foot of cartridge media (Wynn 15 merv). I have a Magnehelic attached to the exhaust plenum basically monitoring filter condition. Everything is disassembled at the moment and I'm at that stage of ordering some new pipe and then reassembling, and hoping not to make too many mistakes.

I created a spreadsheet (attached below) of the velocity issues created for the one person shop with using a range of pipe from 8" to 4". I was (sort of) shocked at the performance issues created by mixing 4" through 8" ducts. This is assuming the collector can produce 1200 to 1400 cfm at least with no friction. 4000fpm velocity in a 4" branch fed to an 8" trunk drops the velocity to 1000fpm ... enough velocity drop to "settle" dust. Keeping the velocity at 3500fpm in the 8" main trunk would require the 4" line to pull 14,000fpm of air. Neither scenario really works. Are my calculations correct? Based on this analysis, in trying to maintain trunk line velocity (3500fpm) and branch lines (4000fpm) it becomes pretty clear that the options for a single person shop include:

• keep the trunk diameters closer to the branch line diameters which means smaller than 8" (which increases static friction)
• keep branch diameter closer in size to the trunk, or not too much smaller than the trunk
• plan on keeping several gates open simultaneously (reduces efficiency at operating machine) to keep the flow of the trunk at the optimum 3500fpm

What is the best practice to maintain that 3500 fpm in the main trunk?

System Air Velocity
Read a great post started by Mathieu B. on measuring and balancing a system. I think I need to do this right now before any purchasing or assembly - the theoretical is only good to a point. I really don't know what CFM the system can optimally produce, and there is nothing like a real world test to find out. So, what is the best practice today (cheap) for measuring cfm?

Attachment 281795

[Jim Andrew]

Do you know what size impeller your system has? And what is the RPM of the motor?

[Bill Adamsen]

Do you know what size impeller your system has? And what is the RPM of the motor?

I knew this question would arise, but I'm not really sure I understand the relevance ... at least to the question asked. Can impeller/rpm/hp provide a CFM without looking at any sources of static resistance from viscous drag or efficiency of moving the air in the cyclone?

Impeller is 14"x3.25", RPM was stated at 3450, hp at 3, though the impeller is not a high efficiency impeller based on what I read at Bill Pentz' site. The reason I'm looking at the anenometer is to derive (free of duct resistance but including exhaust and filter resistance) the cfm. Then adding known drag the anenometer should provide a reading to validate impeller capability.

But none of this answers the question about air velocity. Cyclone aside (which I can always replace) the ducts will either work or not based on fpm. To achieve fpm will require overcoming drag, and the proper sizing of the pipe. If I need 1220 cfm of suction to pull 3500fpm in the 8" trunk, and then there is a 4" upstream on a branch, the principle of conservation of mass tells me that 4" pipe will see 14,000fpm. This would seem to favor alternative designs (I'm trying to calculate the Reynolds number to see if 14kfpm would induce turbulent flow).

[Ole Anderson]

We all have systems with varying sizes of inlets and they work well in general. Mine has lots of 4" inlets with flex and a 7"main run. Yes the velocity will drop in the main and you might get some settling, but as soon as you open a second gate, or use a different machine with a large inlet, the flow and velocity goes up in the main run, it clears itself, and all is good with the world again. There have been discussions regarding the fact that ducts will self clean, meaning that when they start to clog, duct area decreases and velocity goes up and the dust starts to move again. Consider that you might be overthinking the issue.

[David Kumm]

Torit uses the same size cyclone for the 3 and 5 hp motors. I believe the impeller is a straight blade radial. Not as efficient at low pressure but better able to deliver cfm at higher pressure. Blade configuration is relevant to your question. The 8" main should likely be reduced down to 7" as the 8" inlet is designed for the 5 hp 15" blower. The straight radial handles the additional resistance of the smaller pipe and increase velocity. The impeller will actually pull more cfm through a 4" port than a higher efficiency curved blade. If I were setting up your system I would run a 6" blast gate a few feet from the machine and then wye into two 4" lines. one to the machine and the second to a hose to clean off machines or with a second gate to adjust flow. You will quickly figure out if you need both open, one open , or a combination of the two and then leave it alone. With a straight blade impeller you need to be careful to not overamp the motor so a wide open 8" port would probably need a bigger motor than 3 hp. It also helps if you can wye off the main horizontally rather than vertically. Then any chips that stay in the main won't drop down into another vertical. The main will clear out when another gate is opened. I think you will be surprised to find that your impeller will pull chips from a 4" port up a 6" vertical and through a 7" port without a second gate but to be safe, add the second. The straight blade is the key here. It is the type that benefits from a vfd to vary the speed and is similar to what Oneida uses on their smart collector. Dave

[Bill Adamsen]

David:

All great ideas. I'd been considering decreasing the diameter to 7" for the main and using 6" (or 5") for the branches fits with the velocity figures. The horizontal y idea is fab, though it will add 45° of bends to each drop though the benefit outweighs the cost. And yes I am using a VFD which is programmed for soft start ... hadn't thought about varying speed but will experiment with that. Thanks!

[David Kumm]

Bill, look for a reasonable muffler as those impellers are loud. When you hook up the system, set the vfd to monitor amps, and then start opening gates. When the amp draw stops increasing see how close you are to FLA. If over you know you have to be careful if running several gates and dial the speed back. When running just one machine you might find you can speed it up and get closer to FLA. The Torit impeller, if like mine, is heavy steel with a taper lock bushing which is a better design than a double set screw. Still, I stay at 66 hz as my max. Could go higher but it gets scary sounding. I have an extra 5 hp motor and 15" blower for a Torit. If you put a 5 hp motor on yours it would be much like the Oneida Smart without the auto speed adjustment. you will find a sweet spot that handles your needs most of the time and only occasionally will you need to adjust speed. Works pretty well for just filtering the shop at a lower speed too. Dave

[Jim Neeley]

Bill,

When you look at commercial installations they often have large mains with smaller feeders and still smaller end connections. This is a good design if you normally have multiple (or all) ports open at one time. That way no single small port bottlenecks the system while leaving most of the SP drop for the end run.

Optimizing for a 1-man shop, you're really best off using a single sized piping system (generally 6" for 3hp to 5hp DCs) to as close to the tool as possible. In a dream world you modify your tool's DC connection to accept a 6" connection. That's good where it works but sometimes that's not possible. In that case you may need to either split the 6" load to two 4" lines to a pair of tools (and a single 6" blast gate) to keep the velocities up in the mains or install a 6x4x4 wye with one 4" connected to a tool and the other to a blast gate that you open partially to ensure sufficient air flow in your mains.

IF you don't like "wasting" the suction, most tools would benefit by having you add secondary collection to that second 4" port using something like the flex set in the picture I've attached.

Woodcraft, Rockler and others offer such beasts in 4".

Just my $0.02.. YMMV.

Jim in Alaska

[David Kumm]

You have some tradeoffs in the 6" all the way through design too. A 7" main at 3500 fpm carries 950 cfm vs 700 for a 6". The 6" with a couple of wyes and ells can add up to 1" of SP. Doesn't sound like much but at higher pressure 9-10" you can lose 100-200 cfm with that inch. I'd either open a second port part way or risk slowing the horizontal velocity down to 3000 fpm to give myself the additional cfm all the time. Overall you will get more cfm at the machine port, large or small if you can reduce the SP through pipe and fittings. Filters with less resistance, a better hood design, less flex all help. Curved blade impellers benefit the most - within reason. If I were using the 6" all the way my first choice would be the straight blade 14" or a 15-16" BI. The 14" straight will still outperform but might need 5 hp to do that vs 3 hp for the curved impeller. Dave

[Bill Adamsen]

David and Jim:

Thanks for the feedback. I did a few calculations for 2 X 4" and 2 X 5" just to see what the impact might be of having the 7" main drop to 6" on the riser, and then split to either double 5" or double 4". The results are interesting and I updated the "scratch sheet" to reflect those. The short of it is that a 7" main (3500fpm at 935cfm) would pull that same cfm at 4764fpm through a 6" drop, which would then pull 5359fpm through two fully open 4" ducts ... or 3430fpm through two fully open 5" ducts. Ostensibly a bit faster if modulated on one, or if there was a 4" and 5" (Spiral for instance makes reducing Ys as well as a tapered reducing lateral - from 6" to 5" and 4"). Of course this is all calculated without static pressure. And to David's point, that is likely less the larger the pipe and the closer the reduction is to the end. But these "real world" scenarios and calculations have really helped me think about the layout and drops ... and confirmed some of my speculation based on taking FFD some 40 years ago.

Now I'll start to add in the static resistance from any one run of air to see what it does to the required cfm. But it feels like I'm moving in the right direction. Thanks!

[Michael W. Clark]

Hi Bill, you've gotten some good advice and you are on the right track.

Since you are a 1-person, 1-machine shope, the simplist approach is to use all the same duct diameter. I prefer a design velocity of 4000 FPM+, but that is for industrial and I don't want to get called back for plugged ductwork. I would go all 6", then split the 6" into two 4" where you have to or a 5" and 3" (such as for a TS). You may get by with the 7" main if you are willing to reduce the velocity.

If you try to run the higher velocities in the 4", the hood losses will get you. Its not just the duct and fittings loss. You have to get the air in the hood and accelerate it up to conveying speed. If you are running 4000 FPM, this would be about 2"wg of SP if you have a plain end duct. The loss is different for other hood configurations and velocities. Your duct and fittings losses add to this as you work your way back to the collector.

Do you have a performance curve for your Torit? Donaldson/Torit is generally good about providing design data required.

Mike

[Bill Adamsen]

Update:

System is mostly in and it works great! I ran 7" for the main trunk and then 6" for branch and vertical, 5" or 6" typically for tools unless they had smaller ports. Some, like the Centauro Bandsaw I enlarged.

On startup, the cyclone's power was not very strong. It pulled all the shavings out of the planer and up the drop to the cyclone ... but I was pretty freaked out. What could have happened? It's powered by 3-phase so I wondered ... could I have wired it to run in reverse? I climbed up to the top of the unit and looked at the motor on wind-down ... indeed, it was turning in reverse. I can't believe it worked at all in reverse but it did. Reversed the leads and the CFM came right back to what I expected. I've posted a few photos below. There is also a thread elsewhere where I installed a wireless remote to turn on/off from anywhere in the shop ... that works great!

Several interesting learnings. Tight couplings (gates for instance into male ends) can be eased by heating the pipe. Worked like a charm for 5" gates. Also, european 410 mm fittings perfectly fit the inside of Fernco 4"clay to pipe. See the last photo. Lastly, duct cement is tenacious, think before gluing!

DSC_1551.jpgDSC_1554.jpgDSC_1556.jpg

[glenn bradley]

Looks great Bill. Thanks for the tip on the clay to pipe sleeve.

[Jim Andrew]

I re installed my cyclone and piping this winter, took the opportunity to open up a couple too small openings, replaced my filter with a larger one, and sealed a leak between the blower and cyclone, now my system is greatly improved. Chipped a piece of plywood, which went into the shroud on my Hammer TS, and so looked in the barrel for the chip, and was amazed at the quantity of sanding dust in the drum. The filter does not seem to be collecting much dust so far.

[Michael W. Clark]

Nice looking shop Bill! (And DC System) If you have any more pictures, I'm sure it won't hurt anyone's feelings.