Glenn Herbert wrote:Gap C between riser and pot needs to be at least 5 or 6 cm and gap D around the pot probably 2 or 3 cm; and you need a big transition area from the top of gap D to the chimney pipe - ideally a thick donut of airspace that the flue gases can flow easily around and reach the chimney. All of these are to avoid choking the draft. 8" is a pretty big system and I think as big as you want to get.
Depending on the consistency of the beer material you might have issues with overheating and scorching, so keep that in mind as you test and be prepared to add a diffuser plate so the 2000F flue gases don't hit the pot directly. (Water doesn't care of course as it can't scorch )