I built an MDF top-hat to fit on top of a 60-gal plastic drum, with a 6" inlet and a 5" outlet that fits a commercial DC blower (about 1.5 HP). A pretty straightforward design with the only innovation being a lining of 1/8" UHMW to minimize wear on something like 1/8" hardboard.
Up until recently, everything was working good.
But last week I was milling 110 bd-ft of cherry, and noticed a large quantity of chips ending up in the DC bag when usually I'd only get a smattering of dust. It appears that the Byrd heads on my jointer and planer are creating much thinner, fluffier chips than the old HSS knives did. The effect is that the chips actually seem to be collecting on top of the baffle, rather than all falling through the drop slot, and thus getting sucked up through the blower and into the bag and filter assembly. This starts with the drum almost empty, and gets progressively worse as it starts to fill. I even thought about actually cutting the baffle out of the bottom of the top-hat, but wisely restrained myself.
So, I'm trying to figure out what to do.
The outlet extends into the separation chamber about 2" (the chamber is about 6 1/2" tall), and the drop-slot is about 1 1/4" if I recall correctly. I'm wondering if shortening the outlet tube, or widening the drop-slot might improve the performance.
Thanks.
Excellent question.
I recall reading a post from someone at (I think) Sawmillcreek.com where he said he was experiencing the same problem with a conventional cyclone (an Oneida, or Clearvue, IIRC). He even contacted the manufacturer and they informed him they weren't surprised.
Apparently, the chips coming from Byrd heads are like feathers. :)
Can you increase the cut depth? Perhaps if your cut depth were increased, the weight to size ratio would change enough (favoring weight) to make that chips fall?
Just a thought.
Like feathers - yeah, that's an excellent way to describe them!
Increasing the depth of cut might slightly change the thickness of the chips, but would probably be offset by a slower feed rate so I think I'd be back to the starting point.
Very interesting that a similar problem has been observed with cyclones! (and here I was thinking that I might have to invest in a cyclone to solve the problem!!)
I'm trying to visualize what's happening inside the separator, and I'm thinking that because the chips are lighter they aren't being forced against the sides as strongly by centrifugal force, and are getting caught by the baffle, and that maybe increasing the size of the drop slot would improve the situation. Then again, it could be that they're simply remaining airborne long enough to get caught by the baffle at the end of the drop slot, instead of falling into the drum, in which case changing the width of the slot would have little effect.
This might be a case where a shallower separation chamber would work better. Since your outlet pipe is only 5", nothing would be lost by reducing your inlet pipe from 6" to 5" ahead of the separator. Then the separator could be built shallower since it only needs to accomodate a 5" inlet pipe. The result should be higher velocity in the separator and therefore greater centrifigal force.
Based on my experience with a rectangular inlet, I would also recommend that over a round inlet because the waste stream is introduced along the outside wall. You would probably have to do what I did, i.e. have a sheet metal fabricator build a welded transition piece for you, but it will cost a whole lot less than a cyclone separator. You might also have to open up your drop slot a little more.
Now, the bad news is that is possible that none of these suggestions will help, and you won't know until you've gone to the all the work of making the changes!!
Now here's a crazy thought ;D. Could you put a leaf shredder/blower after the planer to shred the "feathers" before they go into the DC line?
Quote from: galerdude on January 05, 2012, 03:51:03 PM
Now here's a crazy thought ;D. Could you put a leaf shredder/blower after the planer to shred the "feathers" before they go into the DC line?
Hah! Gettin' closer to a Rube Goldberg setup ever' day...
Quote from: retired2 on January 05, 2012, 03:49:09 PMNow, the bad news is that is possible that none of these suggestions will help, and you won't know until you've gone to the all the work of making the changes!!
Aye! There's the rub!!
I am running a similar setup. I have a 6" inlet, 5" outlet top hat that is connected to a 2 HP Griz. One of the first tests I ran after building the baffle was to plane a piece of wide cherry though a Griz 15" planer with a Byrd cutter head.
http://youtu.be/SswUX_keN1M (http://youtu.be/SswUX_keN1M)
I do not have the bypass that you are experiencing. My collection drum is 30 gallons, so the tighter circle may be keeping the rotation speed of the collected chips higher. The DC was upgraded with a Wynn filter to maximize CFM. Do you possibly have a clogged filter or filter bag?
Also, I re-routed my inlet pipe so that I had a straight shot into the tophat. The last 90 is a few feet from the inlet.
(http://i552.photobucket.com/albums/jj359/mannsj/DSCF0031.jpg)
The final thing I would look for is leaks in the system. It seems that a loss of moving air will greatly impact the rotation and thereby the scrubbing.
Steve
Quote from: Bulldog8 on January 06, 2012, 04:02:14 AM
The final thing I would look for is leaks in the system. It seems that a loss of moving air will greatly impact the rotation and thereby the scrubbing.
Steve
That is really a great point. As your chips become simultaneous lighter and larger, any air leaks will be highlighted.
Here's a picture of my rig. Based on the lack of dust at joints and stuff, I think it's pretty leak-free. Just before I had this marathon milling session where I observed this problem I thoroughly cleaned the filter, so it's definitely not clogged.
(http://semiww.org/forum/download/file.php?id=1908&mode=view)
Your impeller moves air counter-clockwise as viewed from above and your separator moves air/dust clockwise. Therefore the two air streams are counter rotating against each other. (my setup also moves air in the same direction as yours does) The difference in our two setups is that I used an extension pipe between the top hat and the blower instead of directly mounting it on top. Could this be why I get a different result with the planer?
A different dust collection website says that you should always have the impeller move in the same direction as the separator. This didn't work for me as I would have had to include a sharp 90 right at the entrance of the tophat to keep both streams moving in the same direction. I chose to counter rotate the air streams and hope for the best. It has worked out well for me.
Maybe you should consider an air straightener system like Retired2 uses.
Here is an excerpt from a Cincinnati Fan Engineering Manual that I have posted once or twice before. I put a great deal of stock in what they have to say on the subject of air rotation entering a blower, and they say that rotation in either direction is bad.
Duct Inlet Spin
"A major cause of reduced fan performance is an inlet duct connection that produces a spin or pre-rotation of the air entering the fan inlet. Inlet spin in the same direction of the fan wheel will reduce air volume and pressure ratings. Inlet spin in the opposite direction of the fan rotation will substantially increase the motor horsepower requirements. An ideal inlet condition is one which allows the air to enter the fan axially and evenly without spin in either direction."
Now what Cincinnati does not say is which of the two rotations is the lesser of the two evils. But they are quite clear about the ideal, and that is where the air straightener comes in. From the test data I published an air straightener clearly makes a difference in a close-coupled configuration.
Regarding leaks. "Based on the lack of dust at joints and stuff, I think it's pretty leak-free." I wouldn't agree with that conclusion. A leak on the vacuum side of a DC system is not going to leave any dust as evidence. It will get sucked into the system. And a leak on the pressure side won't hurt the performance other than to put dust in the air before it gets to the filter. The best way to check of leaks is a smoke test. Start by checking the seal at the waste drum.
So does anyone have photos of an air straightener? Just curious how one is configured/works.. Thanks.
Quote from: elfmanp on January 07, 2012, 10:51:20 AM
So does anyone have photos of an air straightener? Just curious how one is configured/works.. Thanks.
Read this thread.
http://www.jpthien.com/smf/index.php?topic=563.0
Well! That's quite a thread.
Just a few thoughts:
First - I lined my top-hat with 1/8" UHMW. It doesn't allow you to see what's going on inside, and right now, I wish it was plexiglass, but it was extremely easy to work with.
Second - I have a very thick baffle, and could make some improvement there. I believe from what I've read around here at one time or another that I could make it somewhat thinner, but also taper the edges to improve performance.
Third - the "straightener" outlet would be an easy fix, but I think I'm also hearing that making the outlet pipe longer will also improve performance, so doing both might help.
Whaddya think?
Quote from: SteveSawyer on January 07, 2012, 11:58:27 PM
Well! That's quite a thread.
Just a few thoughts:
Third - the "straightener" outlet would be an easy fix, but I think I'm also hearing that making the outlet pipe longer will also improve performance, so doing both might help.
Whaddya think?
I don't think there is a lot of experience on this forum with the issue of spinning entry air, or the design and use of air straighteners. In fact, I may be the first to utilize one with a Thien separator. Maybe others have followed my lead, but I am not aware of any one else.
So, I think it is safe to say that no one on this forum, myself included, can tell you definitively how many feet of pipe (or ells) negate the benefit of an air straightener on the exit side of a close-coupled top hat separator. But I would bet a ton of money that less than 6' of straight pipe does very little to stop the performance robbing spin.
Ok - so I'm on my own, huh? ;D
I may make my outlet pipe just a tad longer if for no other reason than to give myself something more substantial into which to install the "straightener" elements. right now that tube (as you can see from the picture) is only about 6" long at most.
Since I'm only running about 1 1/2 HP (you may recognize the blower and filter assembly as being salvaged from a Harbor Freight DC unit), I'd like to really minimize any restriction, and may play with the idea of making some kind of "egg crate" insert if it's not too hard to fabricate.
Quote from: SteveSawyer on January 08, 2012, 12:56:42 PM
Ok - so I'm on my own, huh? ;D
I may make my outlet pipe just a tad longer if for no other reason than to give myself something more substantial into which to install the "straightener" elements. right now that tube (as you can see from the picture) is only about 6" long at most.
Since I'm only running about 1 1/2 HP (you may recognize the blower and filter assembly as being salvaged from a Harbor Freight DC unit), I'd like to really minimize any restriction, and may play with the idea of making some kind of "egg crate" insert if it's not too hard to fabricate.
Yes, you are pretty much on your own. When I finally concluded an air straightener would improve my set up, I was too burned out to build and test other variations. However, I believe a solution that might work just as well as my "straws", or your proposed "egg crate" would be four simple vanes installed axially in the pipe. I think this would be easier to fabricate, it would be sturdier, and it would stop the air rotation.
Quote from: retired2 on January 08, 2012, 04:16:08 PMfour simple vanes installed axially in the pipe. I think this would be easier to fabricate, it would be sturdier, and it would stop the air rotation.
I think you're probably right - I always try to make things more complicated than they need to be!
However, I'm trying to analyze what is happening. We have a material (Byrd head-produced shavings) that are more prone to remaining airborne than what I was previously putting through my separator - chips produced by HSS knives. This is causing this material to get caught in the air-flow into the output pipe. What I'm visualizing is a "cloud" of shavings circulating around the separator rather rather than a "sheet" of chips caught in the slower-moving laminar air-flow around the outside of the separator, and thus ending up on top of the baffle rather than falling into the collection drum.
One of the other things that I've observed is that the chips in the collection drum always form a lopsided inverted cone which is consistent with them falling from around the sides of the drum below the drop-slot. What I noticed about the Byrd-head-produced flakes is that this cone is much deeper, having a much steeper slope from the outside of the drum toward the apex of the "cone" at the bottom. I think this attests to the tendency of these chips to resist the pull of gravity, and therefore to remain airborne a little longer.
So, it seems that the trick is to get the flakes to hug the outside of the separator, which the increased velocity from the air-flow "straightener" may help to achieve, but I'm trying to think what else I could do to get enough differential in the air velocity between that outer layer of air above the drop-slot to get the wood flakes to drop. I'm not sure what effect the thickness of the baffle has on all this, other than the "thinner is good" rule-of-thumb, but perhaps someone could help me determine if thinning my baffle might also help the cause.
Trying to separate feathers with a device that is based on centrifigal force may be a bit of a lost cause. A thinner baffle is desirable, but I doubt it will have much effect on this problem.
You've already defined the objective, i.e. keep the waste against the outside walls and the velocity high. The air straightener may help a little, but again I'm not sure if it will make a significant difference. I think your best hope (and you know what they say about opinions) may rest with a rectangular inlet (taller than wide). If you've read the thread on my build you know I am an advocate of the rectangular inlet. I'm also quick to admit that unlike some of the other enhancements I employed, there are no measurements I could take to prove the benefits of the rectangular shape.
Good luck.
If the only problem you are having is at your jointer/planer, you could run some experiments by building a smaller diameter tophat, at the jointer/planer output, to increase velocity and also play with larger baffel slots to see what works best. If you find that it works you could just leave it there. It shouldn't impact the other branches of your DC system.
Ron
Steve,
I had almost exactly the same problem. http://www.jpthien.com/smf/index.php?topic=574.msg3213#msg3213 (http://www.jpthien.com/smf/index.php?topic=574.msg3213#msg3213)
Things that I found helped (I'm currently separating pretty well at all sizes, but did take a wee bit of a hit on the small stuff):
- POLISH ALL THE EDGES EVERYWHERE. The wood feathers really hang up on anything and once one does you're hosed as the rest build up behind it in a hurry. I believe that this is really the #1 thing to try first. I know this from having spent several hours throwing handfuls of shavings into my separator and then taking it all apart to see what got stuck where. With your build this may involve adding some sort of surfacing to make it super smooth.
- bevel the edges of the slot to make it thinner. A thinner baffle does work better, although I believe that there is a lower limit somewhere (I had started with a piece if 1/32 sheet metal over a piece of wood (various thicknesses from 3/4" to 1/4" depending on where I was in the troubleshooting process), and that seemed to be to thin - although see item #1 as its very possible that the two issues were aggravating each other, and I never tried thinner than ~1/4" with the wider slot so .. yeah mostly speculation.
- Open up the slot. I ended up at 2" which is pretty wide. But that was - in the end - what solved my problem. I did it in several steps of < 1/8" or so until it started working :o Took time but was trying to save as much fine dust separation as I could.
- Drop the inlet size to 5". The advantage of the 6" inlet was minimal for me and I moved to 5" - this didn't totally fix the problem (I had done this before opening the slot) and - I believe (without strong evidence) that the size of the inlet needs to be less that some percentage of the diameter of the separator (what those numbers are - dunno!! - but I have a relatively small separator and it did change the behavior for the good some for me...), I'd probably do this last as you appear to have a fair bit of piping invested
Thanks for the suggestions. Most of the inside of the separator is already extremely smooth. I faced the inside with 1/8" UHMW instead of hardboard. It was secured using a combination of double-sided fiberglass tape (works extremely well with UHMW) and countersunk flat-head bolts. I even ran a patch of packing tape over the heads of the bolts to ensure even the slots in the screw heads wouldn't introduce turbulence. The top and bottom are "virgin" MDF which is also very smooth.
However I have a REALLY THICK baffle - two layers of 1/2" MDF - and the edges are very crisp. I don't recall the drop-slot size, but I think it's 1 1/4". So obviously based on your experience there are two things I can do immediately - thin that baffle (maybe by sharply beveling the edges??), and increasing the size of the drop-slot.
Well - did anything help? The peanut gallery is curious :D
On the smoothing comment, mostly I found it critical around the edges of the baffle into the drop slot. I didn't so much have a turbulence problem as a small "catch" problem where one strand would be temporarily snagged and then it was all downhill from there.
Sorry if I'm keeping the Peanut Gallery in suspense, but I'm in the middle of a project that is going to keep me pretty busy for the next couple of months, and I just lost a weekend making a base for my mortiser so I can move on to the next phase of the project. I won't get around to making a couple of modifications to my DC separator until this project is at least into the finishing stages.
But stay tuned...
Quote from: galerdude on January 05, 2012, 03:51:03 PM
Now here's a crazy thought ;D. Could you put a leaf shredder/blower after the planer to shred the "feathers" before they go into the DC line?