Top hat performance fall-off

[retired2]

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.   

[SteveSawyer]

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.

[retired2]

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.     

[SteveSawyer]

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.

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.

[retired2]

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.

[RonS]

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

[rumwrks]

Steve,

I had almost exactly the same problem.  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

[SteveSawyer]

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.

[rumwrks]

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.

[SteveSawyer]

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...

[kwoodworker]

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?