Thanks for the follow-up. I've done a lot of reading since posting (Fred Olsen's The Kiln Book, Nils Lou's The Art of Firing , Mel Jacobsen's 21st Century Kilns, and even Kiln Construction by Joe Finch), and have also plugged in numbers from different kilns in the books and those I have access to locally. Unfortunately, there are no consistent ratios here. I love the idea of the double venturi as described by Nils Lou, but after running the numbers on his MTF kiln (that he uses as an example of a double venturi), I found the area increases after the entrance to the exit flue as follows: 31.5"² @ exit flue >35"² @ base of chimney >41.5"² @ damper >81"²@ chimney >31.64"²@ the reduction at top of chimney/base of stack. The second venturi at the top of the chimney/base of the stack is really clear, but the exit flue into the chimney increases in area, which would decrease pressure and velocity, not demonstrating a venturi effect.
Here are my numbers. 4 MR-100 burners firing into 3" diameter intake ports give me an intake area of 28.28" sq. I'm designing the exit flue to match that and will be 4.5" tall x 6.28" wide (28.28"sq). From here, Fred Olsen suggested a 25% reduction at the back of the exit flue, which would be consistent with a double venturi design. I'm creating a spot in the back of the exit flue/base of the chimney to insert brick and decrease the flow area. A 25% reduction is about 4.71" wide by 4.5" tall (21.2"sq), opening to my chimney base which will be 9x4.5" wide (40.5"sq). I can decrease again at the top of the chimney heading into the base of the stack to create a second venturi, but I'll figure that out once I get an exit flue and chimney size I feel good about.
Two examples from research.
A similarly designed downdraft at FireArts fires with 6 MR-750's, and has a combined intake size of 29.46"sq. The exit flue entrance is 42.19, opening to 64.12"sq, and opening again to 76"sq going through the damper. It then opens again to 108"sq above the damper. Not sure on the size of the decrease at the base of the stack. That means the exit flue is 43% larger than the intake, opening an additional 74% through the exit flue, and even more still through the damper and chimney. Next, we have Vince Pitella's Downdraft Soda Kiln. 3 MR-100's fire through 60"sq of intake. That feeds through a flue 9x7.5" (67.5"sq) and into a chimney 9x9" (81"sq).
So, despite lots of research, I'm not all that clearer about the science. Thoughts?