Satamax Antone wrote:http://donkey32.proboards.com/thread/1406/calculating-ring-circumference-projection-gap
Ok I figured out, using the minimum 1.5 suggested at the post you linked to, my 8" diameter system would need a 3" gap at the top of the barrel. I had only planned for 2 " so I will have to adjust the height of the barrel for that.
As for my "sunken" collection ring, it is nominally a 1-1/2" gap, 2/3s of the way around the barrel, at the top, which is what you can see in the picture, but the cavity widens to 3-1/2" at the bottom, which is 6" below the top of the collection ring. And then the two sides of the collection channel dump into the central chamber which is 6-1/2" wide and 7-1/2" tall, before transitioning to an 8" diameter stovepipe and the first clean out.
This sketch is a side view of the situation (cast in concrete...)
Also the channels widen a bit at the top from 1-1/2" to 2-1/2" as the approach the central chamber.