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quick ? - Rocket stove flue.  RSS feed

 
                            
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I am interested in making a rocket stove for cooking purposes.

I'm interested in maximuzing heat at the top of the barrel (where cook pot will sit) and also minimum smoke out the end.

I had the tought of piping the exaust, or part of the exaust, back through the burn chamber.  My idea was that this would super heat the smoke (woodgas?) and increase the combusion - thereby adding more heat to the area below my cook pot, and also cleaning up the exhaust.

Part of me is thinking this won't work - but I can't put my finger on why it isn't a good idea.

maybe the added heat to the exaust pipe isn't meanful in terms of cleaning up the smoke - and is just robbing energy from the stove?  thanks for input!
 
Kirk Mobert
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Location: Point Arena, Ca
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Seems to me that running the exhaust (or part of it) back through the stove wouldn't work. The main reason (right off the top 'o my head) is that the exhaust has no (or not enough) oxygen in it and will take up space that oxygenated air would otherwise use. This solution would likely cause more problems than it solves.
A well built rocket stove really doesn't need it's exhaust cleaned up.. That work should be done satisfactorily by the stove in the first go and usually is.

My little six inch rocket stove with the pot-belly stove top gets cherry red and when its running there ain't no smoke coming out the chimney. Its plenty hot enough to cook on (maybe too hot), plenty clean and is about as simple as it gets.
So far, in the 10 years (or so) that I've been fooling with these stoves, the thing that seems to effect heat to the TOP of the barrel the most is a large(ish) volume below the barrel, downstream from the heat riser.
Also, the barrel itself isn't strictly necessary. The stove is using some of it's heat energy to pump exhaust back down, you save energy for other uses if you don't do that. The barrel, more than ANYTHING else is simply a cheap, simple (to do) convenience for heating people and spaces.
 
                            
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Thanks Donkey, I appreciate the input.

I wonder what effect, if any, you'd have if you pulled exhaust (with a fan) back from top of the stove into the very bottom of burn chamber.  This hot gas would then have the chance to be re-burned and re-mixed with oxygen...  maybe even have a small venturie to pull in some cold O2 rich air from under unit.

Would that result in evenless "smoke" output?  And reburning would theoretically also increase heat output per volume of wood.

* I know there are added complexities to the fan idea, but I'm just thinking through all options at the moment...

And Donkey - ten years of fooling with these stoves is awesome - would you mind sharing a picture, or maybe some dimentions & plans?  I'd love to learn from your experiences!
 
Kirk Mobert
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One of the beautiful thing about these stoves is that when they're built properly, there's no smoke left to burn. It does the job right the first time and that's all she wrote. No need to do anything more.

After all these years, it never ceases to amaze me, how when folks approach something like rocket stoves, they IMMEDIATELY look at it and think to themselves (something like) "That's just too simple to work, maybe if I add complexity, it'll work better.."
I know I'm TOTALLY guilty of that thinking.. It's taken a long time for me to be otherwise.
So, my best wisdom (after 10 years) is to keep a simple thing simple and reduce complex things to simplicity.
 
                            
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I'll resist the temptation to complicate - and break - it!
 
Kirk Mobert
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The No. 1 place to look when the stove isn't working properly is at the bottom of the barrel, where the exhaust re-gathers into a pipe again.. That space needs to be generous  and sometimes it's deceiving.

The No. 2 place is at the gap above the heat riser, between IT and the top of the barrel. Some stoves often run best when that gap is a bit higher than "regulation".

If you have a long bench or are heating volumes of water or have other big heat draws and the exhaust cools quite a lot, it will slow the draft considerably and increase likelihood of condensation. In some cases you can step down in pipe (or channel) size to maintain proper through-flow, improve draw and maintain higher relative internal pipe temperature.

I personally think that the common working model for why rocket stoves perform so well is incomplete. Current theory is based entirely on the buoyancy model, stack effect. There is certainly a lot of stack effect happening in a well built stove and it would be foolish to entirely ignore the needs of your chimney, however there are some quirks and features about rocket stoves that lead me to believe that there's something else going on as well.
Heat riser temperatures get up around somewhere between 1500 and 2500 degrees Fahrenheit. Air expands when you heat it, and 70 or 80 degree air will want to expand to several times it's original volume by the time it hits heat riser temperatures..
I'm of the opinion that resistance to expansion by the inside of the stove should cause a bit of positive pressure, pushing from behind (so to speak) AS stack effect ALSO pulls from the front (if you will).. Expanding gasses should squirt forward under a bit of pressure, without the help of a chimney.
A possible way to test this idea would be to build a lightweight metal rocket stove with some kind of wood-keeper so that the thing can be turned upside down while firing. You'd get the thing running upright like normal, then turn it over. It should keep right on working to some degree if my idea is correct..
 
                            
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Donkey -  would you be willing to share any of your design sketches, or measurements with me?  I'm specifically interested in the rocket stove cooking, not the bench warmer. 

Thanks in advance for your consideration,
Scott
 
Kirk Mobert
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Scott911 wrote:
Donkey -  would you be willing to share any of your design sketches, or measurements with me?  I'm specifically interested in the rocket stove cooking, not the bench warmer. 

Thanks in advance for your consideration,
Scott


For cooking, Aprovecho's done the most work. Their site even has a little design program on it.. I'd go there.
 
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