Silly DC Experiment

[Rob Will]

In an effort to test a new planer I temporarily hooked up one section of 8" spiral pipe to an 8" radial blade blower. I pointed the outlet of the blower at a wagon that I have with a sloped roof covering the top. The gable ends are open on the wagon's roof. The fan was blowing toward an opening on the side.

Here's the funny part. The blower did a fine job of blowing the shavings into the wagon.......and then the shavings blew out of every opening. The wagon actually caught perhaps 10% of the shavings. It was like pointing a fire hose at a bucket!

So gentlemen (& ladies), how do I slow down the air velocity at the end of an open run such as this?

[Bob Dodge]

Hi Rob,

I'm not quite clear on your set-up. Is that 8" pipe, between the planer and the blower (sucking)? Or is it at the outlet, aimed at the wagon?

If it's between the planer and blower, what size is your blower outlet? Can you fasten a pipe there?

Also, is that blower's fan-wheel only 8" in diameter, or did you mean that the blower inlet accepts an 8" pipe?

The way to reduce your outlet velocity, will be to increase the size of the outlet. Simply add a large wye at the outlet, and the newly created outlet will be twice the original size. That'll cut the velocity by half, through each branch.

If that's not enough, you can add a pair of wyes to the first one. (4 outlets/1/4 the speed.) Lot's of ways to go there, including building a plenum with multiple outlets.

Good luck, Bob

[Rob Will]

Bob, the 8" duct is between the planer and the blower. The blower is 7.5 hp, 8" inlet with a 5 x 7" outlet. Wheel diameter is 13.5". It is a pull-through design. I had the blower mounted on a stand at the correct height to blow directly into the wagon......but that made a big mess. There was no pipe or plenum on the output.

Thanks,
Rob

[Jim O'Dell]

What about a piece of plywood to knock the projectiles down? Kind of like a shop vac where the incoming hits a plastic wall where it drops and the air escapes out the exhaust. Of course you may have so much air velocity that it just blows the shavings around the wagon and out. But still might be worth the experiment. Jim.

[Jamie Buxton]

how do I slow down the air velocity at the end of an open run such as this?

Like Bob says, increasing the duct diameter reduces the air velocity. You could imagine shooting the sawdust into a gymnasium. That's a very big duct diameter. The sawdust would mostly land on the floor. A more compact way is to use a cyclone. Cyclones are darn close to magic. They reduce the velocity of the air enough that the sawdust falls to the bottom, and do it in a very small footprint.

[Rob Will]

I'm trying to come up with the best shape for a plenum to drop chips into a wagon. Should it use a centrifugal design like an open-bottom cyclone or should it be a gradual increase in cross section to slow the air and let the chips......fall where they may? This place is full of round metal tanks, culverts, 55 gal drums etc. any ideas?

The details of the problem: If my system is flowing 1600 cfm and the size of the fan outlet is 6" x 7" then the exit velocity is just under 5500 fpm. The chips are very light and are flying everywhere with this fan pointed directly at the wagon.

Thanks,
Rob

[Barry O'Mahony]

You need to enclose the wagon with filter cloth.

[Bob Dodge]

Hi Rob,

Well, I have a clearer idea, so thanks for that info.

There are all sorts of tricks to dealing with this problem. You can tackle it at the front end of the blower, or the back-end, or both. Obviously, there's way too much capacity here. It wouldn't surprise me a bit if you're pulling in the order of 2500 CFM or more with that 8" pipe, especially with an unrestricted outlet. Even a decent 3hp single-stage with a 13.2" fan-wheel and a 7" inlet, pulls 1400+ CFM at 8.5" static.

I think you'll have to make a decision here, as to whether or not you want to use this blower in the first place. We can choke it down dramatically to reduce airflow, but you'll still be running a 7 1/2 hp motor to do the job of a 2hp-3hp blower, depending on your duct run.

A combination of solutions can be applied to this problem. You can choose "permanent", or "temporary". There are just so many ways to go, I'm not quite sure where to start, so I'll just list a few, and we can refine this a little further as we go along.

1. You could add a cyclone separator, with a clean-air return to shop, BUT, you'd need a rotary air-lock, or a hopper. With the hopper, you'd simply install it above your wagon, and manually operate the waste chute from time to time.

2. You could place the blower next to your planer, and use the blower as an exhauster, choking it down with a smaller diameter outlet. That outlet restriction, will create a pressure drop at the intake, and you'll flow less air, but still at high outlet velocity. You'd have to dramatically increase the diameter of that discharge, close to the wagon. Far from ideal.

If you choose this route, I'd recommend no more than a 5" pipe at the blower outlet. As you approach the wagon, add a series of enlargers, to increase that pipe diameter to 8", then ad an 8x8x8 pants wye, with the legs set horizontally. That'll simply divide the velocity in half through each leg. Then, you can add two more 8x8x8 wyes. One on each leg. Your total airflow from that 1st 8"wye, will have an outlet velocity reduced by a factor of 4. Not pretty, but better than the present set-up.

Since you're dealing with larger planer waste here, you could add some burlap "socks" to the end of each pipe. Just an open-ended tube hanging above the wagon. You want the ends of those wide enough, so they don't back-up. Maybe reinforce the opening with a stiff wire. That should diffuse the exhaust-blast somewhat.

3. You could seriously reduce the inlet to restrict airflow, limit volume.

4. You could use those drums you mentioned, to make a double-barrelled cyclone, mounted on top of your shop-built hopper. Cut the bottoms of the drums out, mount them side by side to holes on top of the plenum/hopper. Run the planer pipe into ports cut into the sides of those drums, near the top, and add an air ramp inside the drums, to improve flow(downward spiral). Have your blower pulling or pushing air to those "cyclones". You won't need a cone. Vent the "cyclones" as usual, through the top, with an 8" pipe as the uptake tube inside each cyclone. Cap the drums with MDF "lids". If you're going with pull-through design, run an 8" pipe from the blower inlet, to an 8x8x8 wye, and an 8" pipe to each "cyclone" outlet.

Filter the return air appropriately.

With this arrangement, you could still run an 8" pipe from your planer if necessary, to just before each "cyclone", then split that 8" pipe to two 7"'s. A rectanglur entry-pipe, is far easier to work with.

Let me know if any of these solutions are of interest to you, and we can refine the ideas.

Bob

[Rob Will]

Do you think it is a mistake to use the radial blade blower? For a permanent install, I could indeed go to a backward incline non-overloading blower. The problem with the blower that I currently have (according to the Grainger catalog) is that the blower overloads a 5 hp motor at low static pressures....hence the big motor. The 24" planer seems to need the cfm to stay clear.

Rob

[Rob Will]

I wonder how many square feet of cloth would be required to vent 2500 cfm? What material would you use?

Rob

[Steve Clardy]

I've got the same problem. Chips hit the trailer bed and bounce out.

[Rob Will]

Steve, this is very similar to my setup except I have a roof on the wagon. I do remember that a local sawmill had a chip blower directed into an open top rail car. At the end of the pipe they had some sort of a round diffuser, perhaps 3' in diameter. I never looked to see how it was built. Hence the question: should chips enter such a diffuser tangentially for a bit of cyclone effect?

Rob

[Jim Becker]

Rob...the bottom line is that you need to slow the flow down just before the bin in order to tame the output. Cyclones do that via friction and there are probably a bunch of ways to do the same with some creativity...

[Steve Clardy]

Well. I finally just pulled the trailer out and let er blow.
Now, with the pipe outlet about ten foot from the ground, it still has just dirt, rocks directly under the blower. [May have a hole in the ground one of these days.]
When time permits, I'm going to put side boards on the trailer, maybe with a heavy tarp on top, and run the outlet into the top of tarp. Then I won't have to shovel everything off the ground into a trailer to get it to the garden area.
This 3500 cfm blower, with the fan I put on it tries to suck some of my 4" hose together in the shop, so I have to have at least two gates open most of the time.

I've seen a diffuser type thing at a mill where I sometimes get some off size lumber. I'll have to look at it a little closer next time I'm down there.

[Rob Will]

Rob...the bottom line is that you need to slow the flow down just before the bin in order to tame the output. Cyclones do that via friction and there are probably a bunch of ways to do the same with some creativity...

OK, lets say that a certain hog farmer turned woodworker had some cone bottom feed tanks mounted on angle iron legs. If the tanks are about 6 feet in diameter, would the air velocity be sufficient to do any centrifugal separation? Would said tank need to be mounted to a sealed dust container or would it be open top and bottom?

Rob