Opinion needed on separator design

[George Courson]

OK, I had another thread where I was asking about distances needed to make a dust separator. Didn't get any help on the math.
So, I went to CAD and used the measurements of the intake (7-3/4") of my blower. I was able to get a cardboard drum that has just enough room inside so that if I can get a tub mounted perfectly, the airflow into the separator is just to the side of the clean-air-exhaust that is 7-3/4".

348969 (Drawn without the carriage bolts from the top to the bottom plate.)

The inside of the barrel is 17-1/2" tall and 17-1/4" side to side.
If I cut the bottom off and mount it to the cardboard barrel I found today and mount a round surface so that it would be at the location of the original bottom with carriage bolts to the top, it leaves an unobstructed drop for the sawdust.

How big of a gap should I leave between the bottom plate and the sides of the barrel?
How far down into or away from the bottom plate should the exhaust tube be, to allow it to draw the air out, but pick up the least amount of the fine dust?

I was also thinking about putting a Helix band against the wall of the barrel, to help force the particles down. Should I run the Helix all the way down the side?

I know that I would need to use a thine section of tin on the side where the air enters the separator, to keep it from destroying the side, so I thought I'd be able to also use a section to form the Helix, but would it make a difference?

Thanks

[Andy Giddings]

George, you might get more response from a specialist website forum such as Phil Thein's separator design - http://www.jpthien.com/smf/index.php?PHPSESSID=1m3k38ojss4as01t4cftsa3se1&board=1.0

[Jim Andrew]

Looks like you are designing a cyclone, minus the cone. My cyclone is 18" diameter, and the tube goes down close to the bottom of the cone. Think you can find diy instructions on Bill Pentz website on how to build your own cyclone.

[Dan Schrecongost]

George-

Andy is right - check out Phil's website and look at the top hat designs.
Briefly:
1. the gap is typically 1 1/8". Note that the gap should not cover 360°; rather the gap should exist for only 2/3 of the drum circumference or 240°. This helps to de-couple the air mass below the bottom with that above. This decoupling encourages debris in the bottom to stay there.
2. The standard answer is the distance between the bottom plate and the end of the exhaust tube should be at least 1/2 the diameter of the exhaust tube. As long as the end of the exhaust tube is below the lowest portion of the inlet tube you should be okay.
Regarding the Helix along the side wall, this is unnecessary. You want dust to fall into the barrel, not air. Since the barrel is sealed, if you direct the airflow down with a helix, it has nowhere to go but back up into the separator, and it will probably carry dust with it. Dust particles will hug the separator wall and fall in their own good time. The airflow is more agile and will exit through the exhaust tube - unless you have leaks in your barrel or between the barrel and separator. Air leaks are bad and will hinder dust separation.
Good luck.
-Dan

[Alan Schaffter]

If you will look at Bill Pentz's website you will also see that the intake extends to the center creating what is called a "neutral vane" which reduces turbulence. Bill's design, a version of which has been adopted by Oneida, takes the neutral vane one step farther by using a spiral inlet ramp- a helix. By the way, the information on Bill's site will only allow you to make an approximation of the helix- unless you bend and stretch the material, you physically (and mathematically) can't make a helix from a flat sheet. Without a cone or other means to get the dust away from the central discharge tube, turbulence will keep all but the heaviest of dust airborne in the cyclone and it will be sucked out the discharge tube.