Top hat performance fall-off

[SteveSawyer]

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.

[phil (admin)]

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.

[SteveSawyer]

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.

[retired2]

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!!

[galerdude]

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?

[SteveSawyer]

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...

[SteveSawyer]

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!!

Aye! There's the rub!!

[Bulldog8]

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

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.


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

[phil (admin)]

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.

[SteveSawyer]

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.

[Bulldog8]

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.

[retired2]

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. 

[elfmanp]

So does anyone have photos of an air straightener?  Just curious how one is configured/works..  Thanks.

[retired2]

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

[SteveSawyer]

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?