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Large vs. Small  RSS feed

 
Richard Maltese
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Location: Greensboro, North Carolina
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Now that I've become a little familiar with the concepts I'd like to try and build a stove. But here's the thing, it seems like a waste to build a full scale one as a beginner.
I remember reading somewhere that the smaller the stove the higher the max temperature must be to drive the combustion. Is that true?
And is there a (simple) method for calculating these numbers?
 
Glenn Herbert
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I suppose you could say that a smaller RMH needs a higher temperature to drive the draft, but the actual situation is that a smaller unit will have a lower maximum temperature than a larger one of the same construction. It is considered an advanced project to build an RMH smaller than 6" diameter; 6" is forgiving enough that it will work if you get the construction close to right. 6" is also large enough to put out real heat and be generally useful. If you do your test builds using fireclay as mortar (not refractory cement or furnace cement) on the bricks, it will come apart easily and the bricks can be reused in your final build.
 
Trev Ortex
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I've been wondering the same thing. I remember reading somewhere (on the Donkey32 Proboards rocket stove forum) that they only really work down to 3 inch, but I cant find where I read it now and there doesn't seem to be any other mention of it. I was thinking of building a scale model 2 inch batchbox rocket out of clay to see how it behaved. Sure would make experimenting a lot easier.
 
Satamax Antone
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3 inchers are not working, despite what some "claim"

4 can, with real care, and a real drafty chimney. I like my range retrofit, which is About 5 inches (120mm) batch.
 
Glenn Herbert
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Very small rocket STOVES can work well down to maybe 2"; these are for quick efficient cooking, and do not route their exhaust through a mass. rocket mass heaters, as Max says, need to be bigger to work correctly.
 
Glenn Herbert
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It is totally possible to build a RMH out of just cob, with a sacrificial wooden inner form. Its durability is unknown to me at this time, but you can experiment this way without spending any real money.
 
Satamax Antone
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Yep Glenn, daft pompous me, i didn't think about the cooking stoves! Mind you, i'm not a fan in heating aplications, due to the boundary layer slowing things down. And having dual purpose rockets is a good thing above may be the 40 parallel
 
Trev Ortex
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I'm guessing this is the result of the volume of the pipe verses the internal surface area? I know from Hydro-Electrics that below a certain size pipe the frictional losses quickly add up to 100% and nothing can be gained.
 
Satamax Antone
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Richard Maltese
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Thanks, for the discussion there's some great info here. I'll start with a 4 inch and see how that goes, maybe I'll try it with cob as you suggest Glenn.
 
Glenn Herbert
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If you do build a 4" system for testing, make sure you include a good tall chimney to help with the draft.

Take note of this other thread started by someone who decided to go small: http://www.permies.com/t/50568/rocket-stoves/Exhaust-design
 
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