Please clarify turbulence question

[guy48065]

I'm a newbie to baffle concepts but definitely not new to home shop DC's.  My 2-stage 4" pvc system pre-dates the internet and works fine, despite all the grave warnings about dust explosions that were tossed about in the magazines back then :) 
Although I get very little fines in my bag (less than 5%), and none on the walls of the closet it's located in, I'd still like to try to improve performance.  I'm baffled (ha!) by posts I've read about the desired effect in the drum.  Some posts talk about wanting a dead drum where chips just fall down while other posts talk about spinning the chips out in a vortex.

Which is the intended effect of a Thien baffle?

[phil (admin)]

The goal is to keep the stuff spinning BELOW the baffle spinning as slowly as possible.  It will spin, but slower is better.

Am I answering your question?

If not let me know and I'll try to focus-in.

[guy48065]

Answer is good--but pretty subjective.  I have a 45 gal. white translucent barrel like you've previously described.  With a backlight I can see the dust swirl.  Presently with no baffle the stream appears to corkscrew down 1 full turn in the drum before hitting the bottom.  I probably can't see the finest flour this way so it may be spinning around endlessly in there but in a test I performed Tuesday night only 1.4 out of 28 lbs of dust got into the bag (no baffle).  That works out to 5% bypass--is that good or bad per everyone's experience?  This test seemed pretty extreme as I normally don't suck up 28 pounds in 10 minutes, and never get more than a handful in the bag per drum-full, and nothing on the walls around the DC.

[alan m]

phils baffle isnt really there to stop bypass it is there to stop scrubbing when the drum is nearly full.
when the level is up the air spining around will lift the settled dust back into the air and allow it to be sucked up through the outlet

[guy48065]

So at, say, less than 1/2 full the baffle has no benefit?  Good to know so I won't waste a bunch of time testing it on 1/3 drum of dust :)

[alan m]

im sure there are other benifits to the baffle with less dust in the drum and probably a few minusses.
but having the baffle there does keep the air above it moving quickly and thus improving seperation.

[phil (admin)]

I think 5% bypass is quite a bit.  I should be keeping better track of it, but I suspect I was getting a pound for every four to five empties of my drum and that was when I was using a drum sander (which generates a ton of fines) a lot more.

But there are a ton of variables here.

Like, how much fine dust was in the mix, how many micron's your bag filters down to, etc.

The baffle does help even when your drum is empty, BTW.  It just helps a ton more as it fills.

[guy48065]

I've gone this far with the testing so I do plan to try the baffle but I need to adapt it a little differently to my drum.

Lid with 12D x 12H shroud removed:


My lid is 24" but the drum is 22" so I made the lipped ring so I could get a consistently good seal.  Just suspending a baffle under the intake scoop will make it a PITA to lift & replace the lid so I will make an integrated ring-sleeve-baffle hanging down into the drum.  Kind of like an internal "tophat".  Alternately I could make a normal tophat and not lose any drum capacity.  I think I have enough height above the DC motor for this and still have adequate lift space for the rope & pulleys.

Are there any other "enhancements" that I should consider for this type of unit?  I've seen mention of bellmouths, woks and chimneys on the impeller intake--would that make any difference used with the baffle?  Maybe some sort of guard or vane so the impeller doesn't "see" the intake scoop?

[alan m]

i would go with the top hat style
. its a pain to remove the suspended  baffle if you are restricted to the height you can lift. i also think that you can build a top hat more accuratly  because you dont have to allow everything to drop into position

[phil (admin)]

I've seen baffles attached to a leg which extends to the bottom of the drum.  So the baffle isn't connected to blower and lid at all.  I've seen people use 1" square stock, and even PVC.  You have to improvise a couple of additional tabs on the baffle to keep it centered.  And some way to keep the baffle from spinning in relation to the inlet.

The downside to the leg method is, if you remove the baffle to check the contents of the drum, you probably have to empty the drum before you will get the leg back in (because the sawdust will fill the spot the leg took as soon as you pull up on it).