Hello All....great forum, been stalking for a few days...
So I am going to embark on my first RMH. Your feedback would be much appreciated.
So, I have a bunch of questions and few odd design criteria’s, but that’s for later............ for now, I wanted some input on the J tube length ratios.
I am doing some drawings before I commit to any mortar. I need some help or guidance with the feed, burn, and heat tunnels sizes collectively known (I think) as the “J” tube. Is the proper ratio 1:2:4?
FYI, I am building my RMH on the smaller side. I’ll explain when my drawings are more prepared. So, onward..
Please see attached, Is that correct? The Heat riser seems tall?
A feed of 8”
Equals a burn tunnel of 16”
And a heat riser of 32”
In my case all measured on the “long” side of the tunnel.
If your sketch is near to scale, it appears that you are thinking of building a 4" system. This is decidedly smaller than the recommended sizes (6" or 8" diameter), and experienced people have found that this smaller size is very tricky to make work well. As you get smaller, you get more surface area to volume, and thus more heat loss with less fuel/flame to make up for it. The surface laminar flow effects which reduce effectiveness begin to outweigh the internal turbulence which makes for good efficient mixing.
Measuring along the outer faces of the elements will give you the most extreme differences in lengths, which will give the most powerful effects for a given overall size (riser height). But if you have a 6" system, the 8:16:32" dimensions will leave only 4" for the top length of the burn tunnel - hardly enough space for the riser insulation and barrel to clear the edge of the feed tube.
posted 5 years ago
Thanks for the input.
So, is the 1:2:4 correct? And as your pointing out, it matters how you apply that ratio.
I understand the ratios are nonlinear. So, how is that ratio supposed to be applied and laid out for construction? Inside edges, center line, etc?
On the cross section, yes I am going to maybe build a smaller unit and see how she runs, but your points on surface area to volume are understood, and perhaps I will just stick with the 6 inch size, not sure.
Location: Upstate NY, zone 5
posted 5 years ago
There is no "official" standard on where the dimensions are taken from; I like the "centerline" method of measuring, but it depends on your situation. I have heard it said that the outside method is easier if you are playing with real bricks to come up with your layout, while centerline is easier if you are working on paper and especially if you are planning on casting your core where you can make dimensions precisely what you want. Bricks force you to certain modular dimensions, some of which are easier than others to fit to the theoretical dimensions. The proportions are not ironclad - minor differences can be tolerated, with the proviso that the burn tunnel should be no larger in cross section than any other part, and the feed tube should not be appreciably larger than the burn tunnel. A taller heat riser in proportiion will give a stronger rocket (the original standard for lengths was 1:2:3; 1:2:4 is more reliable.)
Given your example, the 16" = D, and the 32" = E. Were I using these measurements, I would use the 16" = D as you show in your diagram, but I would only measure my vertical fire riser from the roof of the burn chamber, so it would be shorter than 32" the way I'm measuring my system. I would suggest adding height to your vertical fire riser so that the new height, measured from the roof of the burn chamber is a min. of 32-inches (so that, E >= 2D).
With regard to the feed, just make it as small as practical. Mine is two fire bricks tall because that's how the brick stacking works out nicely.
With regard to your first build, I'm just dry stacking the fire brick. It is not perfect but it works and works well enough for you to begin taking some test measurements, and get a feel for what you are doing. Importantly, it is simple to change. I would not use mortar until the final build.
I found a local brick yard and discovered they sold fire brick (2500*F) for about half the price as the big box stores, and a 2500*F brick instead of a 2200*F brick. I have 50 of these which I am using to build my test J rocket stove. I expect I will pick up another 20 or 30 before I start my final build, but 50 is sufficient for now.
That's what I'm doing, so that's what I suggest. But as the more experienced builders have pointed out, there is some wiggle room here, and the smaller you get the trickier the build.
I think the most important thing you could do is get 30 to 60 fire brick and a wheel barrow of sand, and start some dry stacked builds, and see what you think.
I am assuming you have bought the "book" ("Rocket Mass Heaters" 3rd Ed.) and read it a couple times. If not, I would recommend doing so.
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