Hello all.
For the last year I have been studying the book by Mr Evans, and just about every
thread and you tube
video available. The proportions of the
feed, burn tunnel, and riser are not a problem - I have got my head around those.
However, at risk of appearing incredibly stupid and despite the research, I am unable to find, or have missed, an
answer to this:
What ratio is the riser height (and therefore all other dimensions) to the cross sectional area of the burner tunnel?
Or:
Does the burning flame/gas/smoke length remain the same whatever volume (CSA of burn tunnel) of
wood is burned?
For example:
Ianto says the riser
should be twice the height of the burn tunnel length, but anything between 25 and 50 inches will work. But, a 4 inch burner tunnel will have a cross section of 16 sq in, a 6 inch = 36 sq in, an 8 inch = 64 sq in, a 10 inch =100 sq in. This would suggest to me that for complete combustion a 10 inch J tube would need to be considerably bigger in all dimensions than a 6 inch. Approximately 3 times as big? Probably not, but surely bigger.
If you take the Erica and Ernie plan for a 6 inch as being tried and tested dimension, the burn tunnel is 21 inch the riser 47 inch. Would it follow that a 4 inch (roughly half the CSA) work best with a burn tunnel of 10 inch and a riser of 25 inch, an 8 inch at say 36 inch and 84 inch?
Clearly this cannot be the case because I cannot find any
RMH with those extremes of dimension.
Would it be safe to assume a state of complete combustion has not occurred if flame, rather than just hot exhaust products, are coming out of the top of the riser? Therefore the optimal dimension of the riser would be when the flame tip is level with the exit from the riser? This, to my simple mind, would mean the air to fuel ratio, and burn tunnel/riser dimensions, are correct because no secondary oxygen is being supplied (without the barrel fitted) to the flame exiting the riser and a state of complete combustion has occurred within the J tube. Am I right in believing this is the aim?
If so, this poses more questions. Does the resistance in the exhaust and barrel of the completed
RMH hold back the burning gas and smoke inside the riser long
enough to achieve a complete burn? Does this resistance increase when an incomplete burn (if it is) tries to draw oxygen back through the exhaust system to achieve a complete burn? Is this why some RMH systems fail with smoke back?
Or is none of the above relevant?
Thanks, in advance, for any comments. Please be kind I'm in Wales, UK, and this is my first post!