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Post by krakensmack on Sept 9, 2014 3:17:37 GMT -8
Hi everyone! I'm some random guy from Poland, usually heating my home with bituminous coal using top-lit updraft furnace. My experience with rocket stoves is limited to several experiments with firebrick constructions in the backyard, which were quite promising. Now I needed some small wood-burning water heater, so I've managed to build rocket stove like this: It was built from scrap metal, using copper pipe coil as a heat exchanger to avoid welding. I admit I didn't pay much attention to the dimensions, only trying to preserve general rules as far as it was possible with metal elements I got. The problem is: in this thing there's no rocket, nor stove. It's a lazy smoking machine. Whatever I put into it, it burns very slowly, even worse than in open air, exhausting tremendous amounts of smoke. It cannot create internal draft and heat up properly. I tried to connect it to the 10m tall chimney, but it didn't help at all. I came to the conclusion that the most probable reason is the coil which is dramatically reducing drop channel diameter - from 12cm to about 7cm. Or maybe do you see any other obvious mistake I made? If coil is the problem, how to avoid it? Would making drop channel from a pipe of much larger diameter help at all? I suppose that the best solution would be external water jacket, but unfortunately I don't have enough welding skills to make it. Any help would be appreciated.
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Post by DCish on Sept 9, 2014 4:02:34 GMT -8
1) Have you read Ianto Evans' and Leslie Jackson's book on the topic? It's a very good starting place to answer the questions your project raises. (http://www.rocketstoves.com/)
2) A primary principle of the rocket stove is separating out the combustion from the heat utilization. Wherever there is flame, there must be insulation. This is essential to creating a hot, clean burn. After the burn is done, then you start harvesting the heat.
3) Another concept is the idea of creating some draft by having an insulated heat riser followed by rapid heat harvest as the gasses fall. Your water tubes would seem to be heat harvesters, but do you have an insulated riser to ensure maximum temperature differential? And even with a well insulated burn area and riser followed by optimal rapid gas cooling, a chimney at the end is still advised for reliable operation.
4) Water likes to escape, and flashing to steam creates very dangerous situations. Donkey has created a safe and effective water heater consisting basically of a small pot of water with copper tubing immersed in it. The pot of water is subjected to direct heat and is open to the atmosphere, no chance of explosion. The immersed copper coil is never exposed to direct heat, and thus can never absorb enough energy to flash to steam and explode. Very effective and safe.
5) Another principle is keeping a stable cross-sectional area (CSA) throughout critical areas. A popular variation discussed on this forum incorporates larger open areas (bells) for heat harvest, but constrictions will strongly affect performance. As you observe, your coil is likely restricting gas flow at a critical point. A small amount of reduction in CSA after gasses have cooled significantly can be functional, but I don't think that exception would apply to your stove
6) On the use of metal as a core construction material, cue Satamax in 3... 2... 1...
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Post by lincsoldbird on Sept 9, 2014 14:02:43 GMT -8
I will second Satamax metal is doomed!!!
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Post by satamax on Sept 9, 2014 20:55:10 GMT -8
I will second Satamax metal is doomed!!! you said it first.
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