We are building a 6 inch rmh using Erica and Ernie's plans. We are using 1/2 firebricks for the heat riser, and are proposing using a 15 inch barrel packed with perlite to insulate the heat riser. That would give us 3 1/2 inch clearance inside the 55 gallon drum. Is that sufficient for good air flow? What would be the ideal diameter for the heat riser plus insulation inside a 22 inch 55 gal. drum? Thanks.
According to Erica and Ernie's book, the downdraft space should be at least 4X the cross sectional area of the main plumbing. In your case it is a 6" round pipe which is about 30 square inches of cross sectional area and your 3.5 inch gap gives roughly 205 square inches so you are between 6X and 7X. Should work great.
BTW if you want to calculate this yourself, the formula for area of a circle is pi * r^2 ( i.e. pi * radius squared). This calculation gives 30 si for the 6 in circle. Then you have a 7.5in (15 in Dia ) inner diameter which gives 177 si and for the outer diameter you have 11in (22 in Dia) which gives 380 si. We subtract the inner diameter area from the outer diameter area and divide by the system diameter area. This gives (380 - 177)/30 = 203/30 = 6.77.