Combustion of gases

Hi to all you interesting folks,

Is there a way to determine if my 8 inch RMH is experiencing the high temp reburn (2000 plus deg F)? Some times it burns rather quietly esp if i have 2 larger pieces of wood in the feed tube -3" to 4" diameter logs. A full tube of smaller pieces burns with more flame and mild torch sound.

I expected to see higher barrel temps, a very vigorous burn, produces only max of 600 deg F on the top of barrel located directly above the heat riser. As it burns down it soon cools to 250 or 300 deg on top of the barrel, other wise it seems to be functioning great. It is not producing smoke from the chimney except for a few minutes when I add more wood. Is that proof of gases reburning or just the result of an efficient burn ?

Heat riser is 29 inches high from the ceiling of burn chamber. Made of 1 1/4 inch thick dense fire brick with 3/4 inch dry perlite packed around outside of brick, contained by 12 inch stove pipe.

Is there a test I can do? To be sure?

Thanks, Lester

29" above the burn tunnel doesn't sound like a lot of height. What are the dimensions of the feed tube and burn tunnel? (Typical measurement styles are along the outside edges - feed top to floor, burn tunnel end to end, riser floor to top; or along the centerlines of each element, from intersection to intersection/end.) An approximately 36" max. riser is unlikely to be as much as 3 times the feed tube height, while the recommended proportions are 1:2:4 feed:burn:riser. This would give the rocket less power and explain your gentle sound.

If you are getting really effective mixing in the burn tunnel & riser, there may not be enough fuel left to reburn. So the only important question is whether you are getting full combustion. If you can manage to get a whiff of the exit gases (very quickly at first) you may be able to get a sense of this. It has been said that a properly burning RMH will just smell faintly of woodsiness.

What is the rest of your system like? Lengths, number of elbows, chimney type, height & interior/exterior?

Lester Hurst : Within 15 min or even much sooner your firebrick forming the walls of your combustion chamber should glow red hot in a dark room !

A conventional smart Phone camera visibly 'leans toward' and 'photographs' the normally invisible radiated infrared Energy ! A Clean burning / high

temperature Rocket's core should show both signs !


Within several minutes of establishing a 'red hot combustion zone' a lump of charcoal introduced into the burn tunnel should immediately burst into flame.

When you have a single chunk of wood its combustion can only occur on it exterior surface, the same hunk of wood split into 4 pieces has that much

larger area undergoing surface combustion -and the immediate release of wood gases ! Also the spaces between individual chunks of wood act like

reverse-flow Chimneys pre warming the needed combustion air and raising the initial surface combustion temperatures !


Your description of Your Rockets burn and temperatures sounds normal to me ! Especially if the gap between the top of the Heat Riser and the underside

of the barrel is 2'' or bigger ! Remember that the pyrometer can only measure surface temps and not the temperature of the gases flowing past that point !


Hope this is timely and helps ! For the Good of the Craft ! Big AL

Thank you very much for your helpful responses.

Yes I can see the fire brick glowing at the inside beginning of the burn tunnel, I cannot see or photograph the exterior as that is covered with sand /clay mix already. A whiff of the exit gas if very mild smelling and does not burn your nose.

Dimensions are as follows : feed tube floor to top =16". Burn tunnel end to end = 27". Cross section is 9" high by 5.5" wide =49.5 sq in, this is the same thru out the 8 in. system except for the external chimney which is 42 sq in. Heat riser floor to top is 38.5". Top of riser to top of barrel it's about 2.5". Mass tubing is 8" round, with 5 elbows to chimney entrance,of course one final additional turn to begin journey up the chimney, and 25'of straight. External Chimney is 20' high, ending 2' above roof peak.

Learning more all the time, I see 2 issues that may improve performance, I do not have my burn tunnel insulated (just the 2.5" dense fire brick ) cover with mass, and I can increase the height of the heat riser.

Also I would like to add an additional loop of pipe and mass if possible with out hindering performance. This would add 4 more elbows and 20 additional feet of straight pipe. Bringing the total straight pipe to 45' of straight and 9 elbows, plus the 20 ft of external chimney. Thoughts on this ??

Thanks again both Glenn and Allen !
-Lester

So you have J-tube proportions of about 1:1.7:2.4. This is far less than the recommended differential, just enough to give the proper chimney effect under good conditions. You should probably make the riser at least 48" tall, to give a 1:3 proportion which was the recommendation in early years. 1:4 gives a stronger, more foolproof pump and allows for some sub-optimal conditions somewhere else in the system. The slightly shorter than 1:2 ratio for burn tunnel length should not be a problem.

You currently have about 50' equivalent of duct (25' + 5 elbows@5'), which is the maximum length for an 8" system in ordinary circumstances. Lengthening it would put you in risky territory, especially with a sub-optimal heat riser.
*In the pictures, I only see three elbows and four lengths of horizontal duct (about 22-23' total); how do you get 25' and five elbows?You made a pure 8" diameter opening under the barrel and immediately a 90degree bend to horizontal. You would get better flow here if you dug out the base of the barrel area to make a wide funnel leading directly to the horizontal duct, reducing entry transition drag and eliminating a 90 degree elbow.

I counted the 5 ft vertical exhaust pipe and the elbow at either end of it, imediately behind the barrel.

Only the horizontal ducting counts for length; the vertical duct counts as part of the chimney height (which benefits draft and can sometimes add a little bit to the allowable horizontal run). But the two elbows of the jog over to the chimney count for length, so you are still pretty near the maximum length. I wouldn't add more unless you tested the whole current system and found that it had lots of draft under marginal conditions, like a cold start, warmish weather, or gusty winds.