Help required with mini rocket stove

Hi all, I'm new to the forum and looking for some help please.

I'm making a rocket stove for my live in van, which is primarily used for heat, although I probably will use for boiling kettle etc.

I'm having issues with my design, and I'm pretty sure it's either the flue size and/or the heat riser. Although it seems to draw well enough, the flames do start to move up the wood. This in turn means it burns down really quickly. I'm also getting lots of smoke, which does disappear but only when the wood has burned down sufficiently. As soon as I refill it, the smoke starts again. I've been chopping the pieces quite thin too, between 10-20mm. The flue is 50mm tube.



It's primarily constructed from 100mm box section, it's not clear from the images but the heat riser is 75mm box section. I was aware before starting, that the height of the riser should be more than the feed chamber. The bottom section used to be 100mm box section too, but I reduced it to 50mm high to try and solve the problem. I thought the burn chamber may have been to deep.





I've alternated where I have the air inlet, either at the base of the feed tube, or from the opening of the feed tube as pictured.




Does anyone have any pointers?

Hey all, I wondered why my recent post about a steel heater didn't get any replies. Well I've been reading more on the subject, and I've read it's not good to use steel for a riser!!

Well this has put a spanner in the works, I've been welding and cutting, and re-welding and re-cutting! All trying to make an efficient stove.

I really want to make a mini rocket stove for my live in van.

My plan was to use 3" box section for the J tube, and insulate the riser with perlite.


Now this stove doesn't have to be mega hot, I'd be happy with 3-400 degrees C.

Is it still a bad idea to use steel? I'd look for an alternative for the riser, but I'm not sure how anything of a cast nature would handle movement of being in a van.

Any help appreciated.

Scott

How do you plan to vent the rocket stove?

Hi Scott,

there are several reasons why your current heater configuration doesn´t work properly:

1) the recommended ratio of the lengths of feed tube, burn tunnel and heat riser are 1:2:4, So your heat riser is way on the sort side.
2) the area around the heat riser where the gases must come down again seems very narrow, normaly the minmum gap should be 2 inches / 5 cms all around. Not to forget a big enough top gap between riser and ceiling for the gases to turn direction.
3) the use of metal to contain the burning fuel has serveral disadvatages:
It conducts heat away from the fire, thus cooling the burn and limiting its efficiency.
Due to this the feed tube will also heat up and serve as a competing chimney causing smokeback.
With an unsinsualted heat riser the metal wil also conduct heat between the inner (upwards) and outer (downwards) exhaust gas path, so the heat riser gets less hot and produces less updraft and the surrounding chamber gets hotter and produces less downdraft.
If you manage to get hot and clean rocket combustion by insulating the burn area form the outside, metal will spall and disintegrate over time.

However metal is fine as a casing around an insulated burn tunnel and heat riser made of refractory materials,

I understand well that it´s tempting to construct a rocket stove with the materials you have at hand, but if you want to heat with very little wood and no smokeback and a clean exhaust you´d have to redesign your stove qite a bit.
First thing would be that you construct your feed, burn tunnel and riser of firebrick splits. There´s a mock up for a small fire brick core and heat riser that I found very nice in the first few minues of this video  
This fire brick core should be wrapped either in cermic insulative fiber board or put into a bigger metal tube that is backfilled with a mixture of 1 part fireclay and 3 parts Perlite, both serving as insulation to keep the combustion super hot and clean.
Also such a " soft insulation" will protect the firebrick core from shocks while driving with your van.
And the radiating casing around all this should be big enough to not restrict the downflow of the burn gases.

I hope this is helpful.

All the best,
Ralf

J-tube systems of 4"/100mm or less seldom work well, as you run into issues of scale. The cross section and the power of the stove vary as the square of the diameter, so a 4" stove has not 2/3 but less than half the potential power of a 6" system; and the radiating surface pulling heat out only varies linearly, so is proportionately worse in a small system.

If you really want a very small system, try a batch box, which does scale down directly while still working well. Read all about them at batchrocket.eu. I built a working mockup of a 4" batch box firebox and riser which was only 9" wide x 21" long x 29" high outside firebrick dimensions, without cutting any bricks. (22cm x 53cm x 72cm) It burned strongly and was so clean that I comfortably breathed the exhaust (from 4' above) for several full deep breaths. If you have a tile or masonry saw to cut bricks, you could make a smaller one.

Thanks for the replies guys.

Gilbert, do you mean vent the exhaust? It would have a flue up through the ceiling.


I've actually moved on from the one pictured, as like you say it's out of proportion. My new design would've been 3" box section from feed to flue. With an insulated riser, and sufficient gap around the riser.

Until you lovely people ruined my plans .

The main issue I have is space, I can't afford a wide stove.

I'll do some more research on the batch rockets, they look interesting. However, I think even those would be too bulky.


Does anyone know of a 3" csa rocket heater that's worked? I could start from scratch with my design, and use refractory cement for burn tunnel and riser. But if it's useless on a small scale, there's no point.

Thanks again everyone, you've saved me a lot of time a money. I was going to go out and buy new steel today!

Regards,
Scott

Scott,
Your dimensions are off, 1:2:4 should be measured along the centerline of each part, the feed tube should be vertical, no low air inlet, preferably the same cross section area throughout the system, the whole thing should be insulated around bar the top part of the feed, don't shove fuel in the burn tunnel at all. Please, build a straightforward J-tube, insulate the hell out of it and try again. The steel wouldn't survive this long, but it's just to demonstrate what's possible.

And I am really puzzled by the placement of the chimney pipe, it looks like it is connected to the base of the heat riser? That's completely the wrong spot, no wonder why it smokes. The exhaust shouldn't be connected to the core itself but low in a heat extracting drum or something and the same diameter as system size. Please read as least the first and second page of this thread, that'll explains a lot.

scott dimelow wrote:Does anyone know of a 3" csa rocket heater that's worked? I could start from scratch with my design, and use refractory cement for burn tunnel and riser. But if it's useless on a small scale, there's no point.

I would say it's pretty useless, even a 4" system wouldn't play ball every the time without the bells and whistles mentioned on Donkey's forum. That one is probably the smallest footprint that can be realised although it isn't a mass heater.

Thanks Peter, I'd actually move on from this design so it wasn't related to these pics. And the flue doesn't come out of the riser, the riser is within the outer box section.

I'm gutted to hear a small rocket stove won't work, I just love them.

I'll have to look at designs for a normal wood stove, although I hate the inefficiency of a steel box jammed with wood.

If a rocket stove isn't practical on such a small scale, what is?  What kind of system would be efficient for cooking and occasionally heating with wood?

By the way, I think it's marvelous that you can not only build your own stove, but you are also willing to build a second one.  Some people might get frustrated when faced with so many challenges.  Keep up the good work.

Thank you for the encouragement

I'm finding it really hard to fight the urge to just try a smaller design. I'm going to make a sketchup plan, using the correct ratios, and vermiculite bricks.

Even if I have to spend a bit more money on another build, I'll enjoy doing it, and maybe find a design that works?

Scott

So guys, I've been busy in sketchup!

I hear what you're saying about smaller rocket stoves not working, but I think I'll have to try it anyway. Even if it's not 100% efficient I can still use it, and it has to be better than a micro wood stove.

This time I'll use vermiculite blocks for the hot stuff, and construct a steel 'barrel' around it all. The feed tube will also be cased externally in sheet steel.

So if you wouldn't mind, would you look over my ratios to see if I've worked them out correctly?





I measured everything from the centre line this time, so the feed tube is 63mm the burn tube is 126mm and the riser is more than 4x the feed tube. That's just where a full length block ended up.

The cross section throughout will be 75x75mm = 5625 mm2
Gap between top of riser and hot plate - 30mm which gives a csa of 10500 mm2
The area around the riser is 12936 mm2
The gap between riser and 'barrel' is 25mm all around



Your help is greatly appreciated.

Scott

This thread got my head spinning

Scott, if you have room for a footprint of approx. 30 x 50 cms I have got an idea:
Your heat extraction bell can be a capped standing steel tube (with removable top lid /cooking plate in case of inspection) of 30 cms diameter, about 1 meter high or more.
Cut an opening low on the side to fit in a rectangular box-like tube of steel plates which protrudes towards the inside of the big tube as well as towards the outside.
The box should be just big enough to contain a small (4 inch /10 cm) batch box (maybe cast of refractory concrete with thin walls) wrapped or embedded in refractory (and shock absorbing) insulation.
Heat riser could be made of 5 cms /2 inch refractory fiber blanket fitted inside a 20 cms / 8 inch diameter steel tube which is fixed near the rear inner wall of the big tube (viewed from the opening of the fire box).
This way you will further shorten the required length you need for the batch box by "shoving" it deeper into the big tube.
Then you only need a door at the front of your batch box "insert" and that´s it.
You´d only have to watch out that there´s enough space to the left and right of your inserted box so that the gases can pass it unhindered on their way down to the exhaust opening.

This would be basically a shrunk version of Peter van den Berg´s workshop heater made of three full-sized barrels stacked on top of each other.
It can be found here.

Ralf

Hi Ralf, thanks for your reply. Unfortunately I just don't have the space for anything that big.

The design I've come up with is basically the size I have to play with, I have quite a narrow gap to play with.

Hi Scott,

I will leave the evaluation if your new design will work to someone with more practical experience under his/her belt.

My feeling is that the top gap would work better with 50 mms /2 inch (it´s not only the cross-section needed but also room for the direction change of the gases).
And if you could somehow construct a round or at least octagon heat riser in that square tube it would leave a little more room for the gases to stream down on all four corners.
It would not harm the upward draft through the riser as long as it´s got 75 mms inner diameter

Just my 5 cents.

Good luck with your build !

Thanks Ralf, from what I've gleaned from various sources, the area above the riser should be 1.5-2x the csa of the riser. That's how I came up with the 30mm gap.

Zero told me that if the gap is too big, then not enough turbulence is created.

Good idea with the octagonal shape, I'd have to figure out how to do that without a sheet metal bender.

If space is so tight around the stove, so that you can't make anything bigger, will the hot stove set anything on fire?

Interesting thread, thanks for posting your experiments!

That's the reason I can't go too big Gilbert, I've allowed a sensible gap around it. I will then line the surrounding area with aluminium, with an air gap.

Dimensions looking correct now, although I would like to make the riser a bit higher. The smaller the system size the more unfavorable the aerodynamics are. The recommended top gap is a minimum value, not an absolute one. I built 4" systems with a top gap of 600 mm without adverse effects according to my Testo analyser so in practise there isn't a maximum. Don't forget you need a proper vertical chimney stack, venting horizontally through the wall won't work in most cases. Beware for your knockles when adding fuel, don't touch the metal bell housing. And, I fully expect the thing to heat up like... a rocket?

Thanks Peter, that's good to hear.

I did think about making the riser taller, which I probably will do actually.

I'll get some fire bricks and get building

The J-tube suffers from effects of scale, but the batch box doesn't. You could probably make a 3" batch box system (the dimensions are given in this table), and get your heating and cooking without taking too much space.

Thanks Glenn, that's the reason the new design uses vermiculite bricks for the feed, tunnel and riser.


What are peoples thoughts on the thickness of the barrel? I was thinking either 2 or 3mm. The hotplate will be 6mm.

The barrel only needs to be thick enough to stand up to any knocks it will get. After the combustion zone, there is not enough oxygen for scaling or corrosion to be a problem. A 6mm hotplate sounds like a good idea, as buckling or bowing there is not helpful.

Thanks again Glenn, I think I'll go with 2mm to keep the weight down.

Hi guys and gals, I just thought I'd share my progress thus far.

I got the bulk of the work done yesterday, I'm now just waiting around for the vermiculite bricks to arrive.

It looks big in the pic, but it's quite a compact unit. I had to keep checking my measurements because it seemed too small lol.

Hello folks. Well I'm really excited tonight! I've had my stove burning, the bricks arrived so I got it all assembled.

I lit a small fire lighter in the feed tube, and within a couple of seconds it was burning horizontally. So I loaded it up with wood, and away it chooched! Burt clean from the start which amazed me, even with a petroleum based fire lighter.

I got it loaded up properly and it was rocketing nicely, it was at around 340 centigrade on the hot plate. Which is more than enough for my needs. I reduced the wood down to a couple of sticks, and it held a temp around 150. Which is most likely what I'll keep it at in most cases.

Here's a video, you can just about hear the rocket sound over the crackling.



I'll do a longer test burn over the weekend.


As this is for my live in van, the only concern I have is the bricks moving around. I'm tempted to make the core a single, or 2 piece unit. What do you think? Any recommendations on a good castable mix?

Regs,
Scott

Here's the finished stove, with new shoes and a couple of coats of paint

Can't wait to install it.




Thanks for all your input folks.

Do you think you could possibly expand UP to compensate for the lack of total space? I don't know the clearance you're working with inside the van, but two thirty gallon drums stacked ontop of eachother might provide enough power to drive a 3 inch batch box.

[All armchair theory here, I've yet to construct my own Rocket Heater.]

EDIT: that's what I get for running my metaphorical mouth [aka fingers] before finishing reading the thread. Glad to see you got your stove running the way you want it.

I think it's much better now And it's a cool little unit.   However I agree with Kurt.   It could be taller.   Especially the feed tube, I noticed a tiny bit of smoke-back on your video.   Good work though in your revised design.  

Are "vermiculite bricks" the same as firebricks?   Good job on the van window.   Is the black cat yours?

It can't be taller as I'm short on space, it'd be too imposing in the van.

It's not actually smoking back in the clip, it draws down nicely 😁

Vermiculite bricks would be insulating firebricks, not the hard common variety.

Thanks for the clarification Glenn, those are what I used in my riser and Venturi.  

I saw a bus conversion once and the guy used a 5" transport truck exhaust stack for his rocket stove chimney.   I gotta admit it looks more natural and awesome on a vehicle.   Do you have a final exhaust stack plan?  

Riser needs to be same or smaller cross sectional area as feed tube. It'll work better if the bottom of the feed tube is the front of the riser.
See: http://www.instove.org/sites/default/files/20Lstove_tech_bulletin_v1.3.pdf

Scott, take a look online at what is working small in Japan - might be to your needs - See: http://rocketstove.tokyo/lineup/rockeko/  and  
http://rocketstove.tokyo/lineup/wanco/

Robert, a rocket stove for cooking is not the same as a rocket mass heater. I don't know for sure if comparative studies have been done on the efficiency or combustion completeness of an angled feed straight to the base of the riser versus the standard J-tube, but part of the design of the J-tube is to have sharp changes in direction of the flames for turbulent mixing. You would not get this with the "V" configuration, nor would you probably get as much dwell time of gases in the hot zone for combustion to finish.

Also, the angled feed will let any stick that is not perfectly smooth snag on feed edges and hang up; the vertical feed is less prone to this, and allows the fuel to be positioned so air is drawn between sticks for maximum oxygen access rather than above the sticks which may allow coals to be buried out of the airstream.

Rocket stove combustion is rocket stove combustion, after that it's heat transfer.

Combustion efficiency depends on the details of the core. A typical rocket stove (outdoor type) is going to be distinctly less efficient than a well-designed RMH core.