7" (18cm), my first RMH starting to appear

[dvawolk]

I decided to build a RMH with schamott flue with 18cm diameter (that is 7.1") for heat riser. Based on 18cm flue dimension i calculated other vales based on Ianto's book.

I quickly built the heating part of RMH. The schamott is 6cm thick (2,4") and the schamott flue has 1,5cm (0,6") wall thickness. I sketched it in google software to show other dimensions.

When i tossed together the bricks and flue i then "sealed" holes and gaps with clay.

I insulated the whole thing, flue included, with 4cm (1.6") of rock wool board. The only thing that was not insulated was the floor, which was 10cm concrete. But the floor of the burning chamber was 3cm thick schamott on that concrete(see sketch).

Now, gathered dimensions:
Flue -inner diameter 18cm (7,1")
feed chamber and burning tunnel 13,3 x 18cm ( 5,2"x 7.1").
C.S.A. of feed chamber and burning tunel is slightly smaller compared to flue diameter. (5,9% less).
The height of flue, just the round part, is 86cm (33,8")
Some other dimensions are found on sketches.

Now after burning the thing for a few hours i can still smell and feel in the eyes that not everything has burned.
A few days ago i did a small rocket burner with hight temp. insulation board (looks like AAC but heat resistant). There was no bad smell.

Is there still too thin insulation? Is the problem in concrete floor with only 3cm schamott on top?

On top of a flue i put a 5cm (2") distancer and added a barrel lid on top of that. There i measured (IR) temperature on the surface: 310°C /590F. I also measured temperature with that fully insulated small rocket burner (also on the lid). There it was over 500°C/932F.

When i blew into the heat riser, that flame apeared just after the blew. I would say that this "second" flame should always be there if the thing would work normally.

What can be a problem?
Still damp schamott after 3 hours of burning?
Not enough insulated?
Concrete floor?
Are dimensions ok? Are they ideal for 18cm flue?

The thing had a nice draft all the time but IMO it just didn't get out as much as it could!
Here are the sketches:





, Klemen

[dvawolk]

Sorry, next time smaller images :-|

[Donkey]

when in doubt, insulate more, including the floor. That concrete is definitely NOT helping you.
Yeah, when the thing dries out completely, likely as not, it'll run better.
The machine really should be the same cross sectional area (csa) all the way through. Sure, if anything is smaller, its safest if it's smaller at the feed/burn tunnel, but it's best to maintain csa throughout.
On that note (as per your images), placing a round heat riser on a square burn tunnel can create a pinch-point at the junction. You've gotta cheat the bricks open under the heat riser as much as you can (and still hold the thing up), then ya gotta fill in around the junction of the two with your stickum/filler mix..

I do that a lot, square base w/round heat riser.. I like the flow of the build, makes for a quick installation, you save on brick and they end up nicely lightweight/insulated.
Unfortunately, I think evidence has been coming in that indicates that keeping the SHAPE of the flow the same (or similar) can make a difference. The jury's still out on this, but it seems that drastic shape changes might slow things up, which makes a certain amount of sense. If nothing else, it would tend to minimize the pinch-point issues that come up trying to fit different shapes together.

[Donkey]

It would be easier to diagnose your issues if you post images of the build.
Remember to keep them small.

[dvawolk]

So.
Yesterday i made a new, better insulated rocket burner.
Here are the new dimensions:

Now, gathered dimensions:
Flue -inner diameter 18cm (7,1")
feed chamber and burning tunnel 14 x 18cm ( 5,5"x 7.1").
C.S.A. of feed chamber and burning tunel is now almost identical to C.S.A. of flue
The height of flue, just the round part, is 86cm (33,8")
I have also shortened the burning chamber, so the overall lenght of lower part is 50cm (19.7")

The floor below the burning chamber is now:
10 cm (4") concrete
5 cm (2") AAC
Below you see the floor of burning chamber. In the front it is only AAC. This is where the feeding area comes down. Away from that it is a step that was insulated with 2cm of rockwool and 3cm of that product similar to AAC but that can stand fire temperatures.


I filled every gap with clay. Again walls were maid of 6cm schamott and insulated with 5cm (2") of AAC and some rockwool. The chimney flue is insulated with 5cm (2") of rockwool for the below 66cm (26"). The top 20cm (8") of a flue was not insulated.


The draft was good from the begining, i use hardwood, beech that was drying for a fat year time. I can still hear moisture escaping when burning. And on the top of the burning wood (above the feeding area i can even see the sap escaping from the heat below).
I was again for the hunting of that hotter flame that apears even on the top of the chimney. Finally i was able to reach that state, the fire flames (secondary flames?!) were just reaching over the chimney flue. There i had some iron mesh (seen on previous photo) with 5x5mm thicnes of iron. It started to glow red (visible in the daylight). On top of that i again put a barrel lid the way it is shown on the below photo. This photo is from previous trying, so ignore the lower part (now it is insulated better).

On this lid i again measured the temperature of 450 °C /840F. I guess that temperature on the iron mesh was about 550-600°C/840F-1100F.

When reaching this state all three parts of schamott chimney flue audibly cracket from top to bottom :-(. Also when acheaving thist part of temperature, all the blackens from heat riser burned - it is clean now).
Here is a short video of burning (i think the draft was good all the time)


And now my questions:
Should this kind of burner easily achieve the temperatures to get to the secondary flame? Is the problem only partly dried wood or something else? I really had a feeling of being just there near the secondary flame most of the time. But it appeared only twice for a few minutes. I am talking about this kind of flame:
see at 1:30 minutes:



I was baking pancakes on top and i saw water droplets forming on the pan (so the wood obviously was not totaly dry). But the pan wasn't blackened below - no carbon gases anymore?
I thought chimney schamotte flue was a good idea? But now it cracked :-)

Also those scamotte bricks for burning chamber - i would say they are quicker of removing heat compared to red bricks? So the bricks would also be a good idea.

I would say that this option:
donkey32.proboards.com/index.cgi?action=downloadattachmentpage&board=cooking&thread=508&post=4551
would be even a much better idea. What do you think?

Well i was thinking that this secondary flame was something that should be expected in many rocket stove burners but now i more of think that it is not that easy to come to there, at least with schamott and stuff, compared to metal construction.
What gasses are in this secondary flame, anyway? I would say that at the lower part of a heat riser there is enough heat (probably over 700°C /1300F) to burn all the carbon away. (thatswhy the pancake can has not blackenned?).

Is there ANY place in this part of the rocket burner (up to the end of a chimney) that using materials like schamott is better than high temperature insulative boards?

If i can use the following materials:
schamott, red bricks, clay, high temp insulative board, rock wool, AAC. What would be the best combination of using them to make most usefull burner?

Oh, i was also baptized with slightly curled hair and eyebrows because i was constantly looking into the flue from the top (it appears to me that there is also some other types of drafts going on around rocket stoves, ahaha :-). It is really nice to see so lively flame :-)

Cheers, Klemen

[dvawolk]

I ended this testing original RMH burner. I have read adwentures with a horizontal feed and i found it to be more useful for me. First test was today!