RSH dimensions...again

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RSH dimensions...again

Postby Rhett » Wed Mar 12, 2014 12:39 am

Ok seems like this topic is beat to death but I do have a question on dimensions. I have a 25 gallon propane tank that I wanted to use for the shroud (or whatever you call it) for a 6" stove. It's about 40" tall with a 14" inner diameter. Looks like most of you guys are using that kind of dimension for the outer layer of a cast riser not the outer shroud. I was thinking of casting my riser from a 6" sonotube or stove pipe nested in a 12" sonotube. That's 3" wall thickness on that riser but that only leaves 2" between the riser and the shroud. Is this doable? I like the aesthetics of these dimensions better than a typical 55 gal barrel stove but is there a good way to get the exhaust out of that tank? Thanks for any advice.
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Re: RSH dimensions...again

Postby matt walker » Wed Mar 12, 2014 2:22 am

Hey, welcome Rhett. That's a tight fit, but I think it could work given a proper chimney, maybe. I think it would be worth trying out.
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Re: RSH dimensions...again

Postby matt walker » Wed Mar 12, 2014 2:24 am

Sorry, I missed the part about exhaust. Well, what will the rest of the system look like?
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Re: RSH dimensions...again

Postby Rhett » Thu Mar 13, 2014 12:20 am

Well, for one thing I was not planning on having a mass component, so this would be a straight up stove. I originally had the idea in the attached sketchup model. The propane tank bell was to sit over the 12 inch riser and a 6 inch exhaust port, but that is when I thought (for some reason!) that my tank was 18 inches. it isn't, it's 14 inches (so the scale in the image below is wrong.) At 14 inches, I cannot put a 6 inch exhaust port under there with a 12 inch riser. I am beginning to think it wont work.

Can I also ask about turbulence? I see a lot of people trying to make their stoves turbulent. This doesn't make sense to me. If we are trying to get a more complete burn shouldn't we insulate the riser more or use a taller riser? Purposefully introducing turbulence seems like asking for trouble. Wouldn't smooth laminar flow better ensure exhaust goes the right direction and the rocket effect is optimal? Do aerospace rockets and jets try to make their exhaust turbulent? Just seems odd to me.

Anyway, enjoy the site and thanks for any feedback.

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Re: RSH dimensions...again

Postby matt walker » Thu Mar 13, 2014 1:01 am

The way you have it drawn there is great, that would be my preference. I understand that won't work with your current dimensions though. Frankly, you can be pretty cavalier with the system if you aren't going to have any mass. You will have a much warmer exhaust then you would if you had mass, so you can get away with some fudging dimensions if you put a bit of a stack on it and use that to pull. I think you might be able to do a basic side connection if it's going right into a vertical stack. It should pull through the pinch. Maybe.

The idea of turbulence is to help mixing of the oxygen and fuel gasses, so it's only useful in exactly the right spots. Everywhere else, low friction is desired, just as you suspected. In a normal system I don't think turbulence in the riser is a good thing, although I've not seen it in many heating J tubes. It does seem to help in cooking L stoves where the fuel is right at the bottom of the riser, so there is no horizontal component to the burn. I think a misunderstanding of the difference in the two stove shapes has probably led to well intentioned riser turbulence inducers. Maybe. :D

Or, maybe you could do your design with a small intermediary box that encompasses both riser base and exhaust hole, and reduces to bell size on top.
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Re: RSH dimensions...again

Postby Rhett » Thu Mar 13, 2014 2:15 am

you know I was thinking about taking some thin sheet metal and building a box over the whole thing. If I did that I could just use the propane tank as the outer shell on my riser and move to an 8" system. Or maybe just use a 55 barrel and be done with it. I don't know.

I had this idea of using two 6" inch elbows taped together to make the feed tube and burn chamber as in the attached image. I could just cast around these rather than building a form. Is this too short for a 6" system? I like your hardiboard form, but for whatever reason I prefer the idea of a completely tubular J-tube - seems like flow would be smoother. I could connect them the way they are meant to be connected (male to female rather than male to male the way I have them in the picture) but I would loose a couple of inches that way. Not sure if that matters.

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Re: RSH dimensions...again

Postby matt walker » Thu Mar 13, 2014 4:29 pm

I've seen that method proposed quite a few times Rhett, and I can't say that I recall ever hearing any feedback from anyone who has cast a burn unit that way. I think it would work. It would go through a rough moment or two when the metal stuff was burning out and curling into the burn chamber, but that's normal for my riser casting method as well and is no big deal. It might be a hair short on the horizontal, but that's not necessarily a bad thing. I think it would be fine.
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