burn tunnel length question.

i am cross posting this from the other forum. hope that is ok.
as i want to use the heater in my yurt i like the idea of having the feed outside as done by fishermans Daugher a few years ago. as i want to be able to cook on the top of the drum i don't want it too high. i can bury the burn chamber to some extent but was wondering about the ratios. if i keep the burn tunnel the same CSA as the 6inch riser but use a 8 inch riser would that allow me to have a longer burn tunnel with out increasing the height of the riser too much?

Watching this for an answer...

bob golding wrote:i am cross posting this from the other forum. hope that is ok.
as i want to use the heater in my yurt i like the idea of having the feed outside as done by fishermans Daugher a few years ago. as i want to be able to cook on the top of the drum i don't want it too high. i can bury the burn chamber to some extent but was wondering about the ratios. if i keep the burn tunnel the same CSA as the 6inch riser but use a 8 inch riser would that allow me to have a longer burn tunnel with out increasing the height of the riser too much?

Not much experience to draw from, but since nobody else has answered I'll put my 2c in.

I think the CSA doesn't make much difference as long as it's not restrictive. A column of mercury under a vacuum will read the same atmospheric pressure no matter the diameter, and I think the same principle applies. Vertical distance and temperature will determine how strong the draw is.

I've heard of other people wanting to have a longer combustion chamber but haven't seen the results or their experiences. I don't believe a longer combustion chamber will negatively affect draw so long has you have a insulated tall heat riser. I see one potential problem of ash build-up choking the system as the ash will tend to get pulled to the back of the combustion chamber and start to block the heat riser/combustion chamber intersection. If you clean the combustion chamber regularly this probably won't be a problem.

I also believe your barrel temperatures will be colder than the standard design as the barrel is much farther from the combustion chamber and the heat of combustion will be absorbed in the larger J-tube mass. Your proposed system will probably take longer to heat-up and start-up.

Your logic of increasing the cross-sectional area to compensate for the longer burn tunnel will decrease pressure loss. Yet, if you increase the the cross sectional area of your heat riser you will want to keep the same cross sectional area throughout the rest of the entire system.