I plan to use fire brick to construct my entire J tube. I was wondering about the round vs square riser, inside as well as outside of said riser. I am using the fire brick for ease of construction, and durability, but have some question on efficient combustion.
1 Should I forego on the fire brick riser altogether, and use a cast tunnel made from vermiculite/fire mortar?
2 Does a square fire brick riser work best if the outside of it is "rounded" with vermiculite/ fire mortar?
3 Does a square fire brick mortar work best if the -inside- of it is also rounded " " / " " ?
This video from the dynamic duo of RMH's shows a square j-tube with the barrel exterior. It
seems they would know best with having made nearly a thousand of these. I think the round
exterior helps with the vortex of the flame to create a super-efficient burn.
When in doubt some say you should read the instructions and they even made it easy with a DVD set.
Paul goes into the idea of it here for those that may not know of this method of squeezing all the heat
out of a stick of wood.
A round interior is more efficient for airflow (as in it can be smaller in cross section than a square one for the same capacity), but the ease of construction of a square firebrick riser may balance that. If you have an 8" diameter system, a square riser would be recommended to be about 8" square (or the same size as the burn tunnel). Ernie and Erica's standard 8" design has a 7" x 7 1/2" feed tube, burn tunnel and riser.
A rounded interior would have thin fragile spots at the flats and I would not expect it to be durable; you would essentially be replicating all the effort of casting a round riser.
A rounded exterior to the riser would ease airflow to some degree, but it is mainly a practical construction matter, in my opinion. The corners don't need as much insulation, as the interior at that point is not quite as hot, and there is more thickness of brick on the diagonal of the corner. A wire mesh wrapper for loose insulation will naturally give a rounded form and concentrate the insulation at the centers of the riser faces where the interior heat is highest. The turbulence reduction would matter most in the case of a barrel that approached the minimum size for the riser bulk. As long as the corners have 2" or so of airspace I don't think the shape really matters.