ray ejom

Glenn Herbert wrote:Unless your scrap wood is all straight and smooth, fuel can easily hang up on the edge of a slanted feed tube. So lumber yes, sticks no. A J-tube has less of an issue with this.

To get good combustion with less smoke (desirable regardless of efficiency concerns), you want the flame path long enough that big flames don't reach the top of the riser. A J-tube has a horizontal burn tunnel which increases the flame path for a given riser top height over a K-style. It also has more sharp changes in direction which increase beneficial turbulence.

For J-tube dimensions, I like a 1:1.5:3 ratio measured along the outside edge for feed tube, burn tunnel and riser. You could probably make the feed a bit taller and burn tunnel a bit shorter for cookstove use.

As a long steady burning unit, I like the L-tube style. It can accommodate long fuel completely within the burn tunnel so no danger of it hanging up or falling out, and no smokeback risk. I have used about 30" length and rise.

I would not make the air inlet, if that is somehow separate from wood feed, smaller than the combustion zone to try to get a jet of air. You probably want air restricted some at the entry so there is not a huge amount of excess air which will just cool the fire.

It's all timber, not branches and sticks.

Why an L style over a K style? Is K style not favourable just because of smokeback? Would having a lid covering the feed tube solve that?

thomas rubino wrote:Hi Ray;
Welcome to Permies!
I'll try to answer your questions for you.
1) No, a J-tube design is better, less prone to burning backward up the feed tube, than a K-design.
2) 6x6" feed tube 16" tall, a 6x6" burn tunnel with a 12" roof, and a 6x6" riser no less than 36" tall.
3) All internal sizing needs to remain the same dia.
4)Steel can be used but will quickly degrade, clay bricks work much better and firebricks are the brick of choice if you can get them.
5 & 6)  We have a poster here at Permies that specializes in building Rocket cookers, his name is Fox James.
Here is a link to one of his posts  https://permies.com/t/226771/messing-vermiculite
If you click on his name it will take you to his profile and then click on the number of posts under his name, there you will find a list of all his posts.
Fox experiments and builds some outstanding cooking stoves, I am sure you will learn much by reading his posts.

Hi thanks!

I've also read that a J style is better, but K style seems to be more popular. Why is that?

I have 6" and 4" steel box section lying around so it will cost me nothing to build. It won't see daily use, so I have no worries about it degrading.

I've had a quick look at Fox James. It appears that he has used building materials other than steel box section. I don't know if that's relevant? I'll give it a more thorough look later.

Hi all, I need some advice on constructing a rocket stove for cooking. This would be for high heat and fast cooking, so fuel efficiency is not a concern. I have a lot of scrap wood to get through.

1. I would like to build one that is self feeding, is a K type the best solution?

2. What are the ideal dimensions or ratio of dimensions?

3. Is making the air inlet narrower than the burn tube to increase the air velocity a good idea, or should it be constructed out of the same size material?

4. I have 150mm/ 6" box section steel lying around, I thought it would be a good idea to use this.

5. Are there any design considerations worth making?

6. Anything else I should know before attempting this?

Many thanks for your help.