I tried a spiral top hat

[AndyF]

I'd been thinking about a spiral top hat for a while.  At some point a while back, Phil mentioned having a "lead-in" section of the duct that curves to put the material on the outside of the duct before it enters the chamber.

I combined those ideas.

The spiral is an Archimedes spiral.  In this design, every 90deg, the radius increases by 1/2in.  My brother had been playing with Ruby script plugins for Sketchup to make spirals; so, I fiddled with the settings (initial radius and growth rate) to get a spiral I liked.  The spiral is for 450deg.  The last 90deg (or first 90 deg depending on how you look at it), represent the lead-in curve section of the duct.  That kept the width of the rectangular duct cross section consistent. 

I sized the top hat for an 18in diameter 10gal tub.  I wanted a tub less than 20gal so if I can get a bag in the tub (without it sucking up into the top hat), the weight of the waste would be in the <20lb range.  Sawdust density: http://www.engineeringtoolbox.com/density-materials-d_1652.html

I'm getting some by pass but I think it's because the outlet is too close the the shortest radius.  If I were to do this again, I'd make the initial radius larger by about 1 to 1-1/2in (this would be radius from the center of the outlet duct to the wall at the closest point).

As you can tell from the pictures, my plexi bending skills leave a lot to be desired.  I used metal duct tape to seal both sides of the wall when ever I had a break....

Cutting the spiral with a jig saw was the hardest part for me.  Once I had one spiral mounting guide for the plexi, I screwed it to another spiral mounting guide and used a flush trim bit to even out any differences.  (I wish I had access to a CNC for this.)

I followed the general top hat assembly techniques of several others here.


The top view picture of the top hat shows the path of the plexi following the spiral.


I think the initial 90deg curve in the duct is a really good idea based on watching the material fly around the outer wall of the top hat.  If you build a spiral, make it a little larger that what I made.  Using a spiral may be more trouble than it's worth unless you have access to a CNC (or better jig saw skills).

I've attached a Sketchup drawing and the Ruby script plugin for Sketchup along with a few pictures.

Hope this helps somebody.
Andy

[BradD]

Let me start by saying I don't know nothin' about what I'm talking about. I took Fluid Mechanics about 40 years ago, and I've forgotten more than I ever knew. The math went totally over my head.

Now that we have that straight, it looks to me like you need to do some work on the inlet, if that's it on the lower left of your template. I say this because if air is flowing up from the bottom, it almost immediately gets deflected about 45*. If either of the last two "ifs" are incorrect, then somebody tell me so I don't embarrass myself any further.

You could continue the outer spiral around to where you could inlet your air more or less at a tangent to the spiral.

Also, there's a post around here where someone got good results with a spiral-in only on the last little bit of the wall, just before the intake.

I'll look and if I find it, I'll edit in a link.

Brad

Edits:
This is the post I was thinking of:
THE CASE FOR A "TWEAKED" RECTANGULAR INLET Reply #68 in 5", Rectangular Inlet, Bellmouth Outlet with Air Straightener, Top Hat Separator: http://www.jpthien.com/smf/index.php?topic=563.60. The entire thread is a "must read."


Others of interest:
Short discussion on scroll (spiral) design: http://www.jpthien.com/smf/index.php?topic=1055.0
"Vortex trip" Reply #5 in http://www.jpthien.com/smf/index.php?topic=1055.0

[BernardNaish]

I am sorry to say do not understand this design or the design principle. I think Phil suggested an inlet duct that curved just before entering the separator. retired2 put a tweak onto the downstream side of his inlet pipe to aid smooth flow as the swirl comes around to hit the inlet stream. Elsewhere I think I have seen the idea that the outlet pipe be offset from the geometric centre of the chamber. Is this a mix of these ideas? What is the objective of this design? Yes I know it is separation of wood waste from the airstream but how would your design achieve this? As I see it you are making a shorter and shorter path that leads the air stream closer and closer to the outlet pipe. I think this allows the stream to slip across to the outlet pipe before it can drop its load. I suspect that this is likely to cause the by-pass you are getting. There is also the question of the the airflow. Does this slow it down or increase/maintain it?

I have tried sealing with duct tape as you have and found it very ineffective so you probably have a leak that will not help.

Plexiglass (Perspex) is not the ideal material for making curved surfaces because it is so hard it is liable to shatter even if drilled when laid flat. Stress it, by bending it, then drilling is a sure way of getting a crack. Polycarbonate sheet, such as Lexan, is ideal as it bends readily, is not so hard that it holds stress so will not shatter when drilled and screwed in place if you wanted to do that. Polycarbonate seals easily with a silicone sealant and you can avoid mess by masking up with tape.

Polycarbonate is also very scratch and impact resistant, that is why it was used as bullet proof windows, so will withstand years of circulating abrasive sanding particles and silica laden wood debris. It is so tough that 3mm (1/8th") or even 2mm (3/32") thick sheet can be used and it will cost you less than Perspex. It is readily available on eBay here in England. I have no commercial ot technical connection with any firm that makes or sells polycarbonate.

[AndyF]

The spiral or scroll design is not new.  It was presented a while back:
  http://www.jpthien.com/smf/index.php?topic=492

Extending the spiral around on itself seemed to also address Phil's idea of a curved inlet.

Part of the fun of doing this is seeing if things work or not.

[phil (admin)]

Very interesting, has promise.

[AndyF]

I had a chance to test it more today and the bypass is due to overloading.  What I thought was bypass was dust still in the bag filter and the ring from before making the separator.  I took it off and let the impeller discharge to open air and nothing came out visually unless I overloaded the duct.  I did the typical pour a pile of dust on the driveway and suck it up while watching the outlet.

The dust stays suspended along the walls for several revolutions, I'm guessing that's because of the increased velocities from the spiral.  It does eventually drop into the can.

So, I'll be using this since it's working well.

I might try a version that is simpler to construct like the industrial versions bill70j posted awhile back.  It has 180deg of arc at one radius and 180deg of arc at a larger radius. 

If you want to try an Archimedes, might look at how Bill Pentz made a spiral template:
  http://billpentz.com/woodworking/Cyclone/blower.cfm#spiral_configuration