Rocket in Mobile Home... new design considerations

[jgreen]

Hey group. This is my first post, though I've been reading the board for a month or two now. My first rocket project is to be a mass heater with the flue traveling through a cob bench. I've read the rocket mass heaters book several times and have built several mockup's of the unit he shows designs for (the 8" 'standard' rocket dimensions). At this point, I have most of my materials gathered, including having scavenged 50" of metal flue pipe. However it is 7" flue pipe and as such, not suitable for the dimensions of the 8" stove in the book. I am hoping to get some advice or ideas about how to proceed with the design of the combustion unit to fit 7" flue pipe. From what I understand the diameters of the heat riser (D) and the horizontal burn tube (C) should be relatively close to each other with (D) being no smaller than (C). Then, the actual flue diameter should be no smaller than (C) or (D) but can be quite a bit larger. So I have two major hang-ups in going forward.

First, we are wanting to run the flue pipe through a 12' long bench (in a right angle shape) and loop it around and go back through the bench and then take the flue out through the roof. That ends up being atleast 25' of flue in the bench with several 90 degree turns and one 180 degree turn. Then count on atleast 15' (or more) to get out of the house and up past the peak of the roof. So it seems to me that I'll need a pretty hot rocket to a) get enough draw to push the gases through such a length of pipe, with so many turns and b) keep the combustion gases hot enough to avoid them cooling and condensing before they leave the flue outside. So I'm wondering what I can do with a 7" (38.5 inches sq.) maximum cross-sectional area, to make things hot enough. I was planning on using a steel tube for the heat riser (but have not found one yet) and was wanting to avoid welding two metal drums together to get a taller heat riser. Of course, I was planning to insulate everything in and around the combustion unit as best as I can. But I don't have enough experience with different rocket stoves to know what can be expected, or at least what has been done and what should be possible.

My other confusion is about building the combustion unit and the actual dimensions of the bricks I'm working with. I'm using firebricks with the same dimensions as the bricks in Ianto's design, and his design calls for a square or nearly square shaped burn tunnel and heat riser (7x7" or 6x8"). The bricks seem to lend themselves to those two shapes (especially 7x7). But to reduce the cross-sectional area from 48" in sq. (as in a 6x8" burn tunnel) to less than 38.5" in sq. and use these same bricks requires some creativity. The easy answer is that a 6x6" square gives 36" in sq. and that is probably close enough but I haven't been able to wrap my head around how to make bricks that are 8.5x4.5x2.5" do that. The closest I've gotten just fooling around was an arrangement 4.5" tall by 8" wide (giving me 36" sq.) but that seems like an odd shape and I was hoping someone here with more experience could speak to how important the geometry of this part of the stove is. Even the 4.5x8" arrangement had some practical issues with brick laying but I was not wanting to pursue it further without getting some different perspectives on what direction to go in.

If anyone is wondering... it is truly going into a mobile home that friends of mine live in. We've already beefed up the flooring and foundation supports and have tackled most of the issues of how to arrange it to be safe and have adequate clearances from combustible surfaces. From what I've gathered, its the first time anyone I've spoken to has heard of a rocket stove in a mobile home. So I'm excited to be doing something potentially new and creative. I'm also really appreciative of this forum and having a place to share information and support experimenting with rocket stoves. I'm hoping that my project will add to the collective rocket wisdom here and keep me warm at the same time

-Jay in Merlin, OR

[Donkey]

There is no particular geometry requirement.. I like to make the channels deeper than wide. It seems that a slightly deeper channel right next to the feed tube is a little easier to keep clean.. Just a thought, not a rule. Personal preference, really.

Just place the same stack of bricks slightly closer together to narrow the channel. You will either need to cut the bricks in the front/back faces to fit or let them fly long where they are pushed together.. I advise that you fly where it's buried and cut where it shows for a neater look.

To make for a longer heat riser, you could just build the cob base up a little higher.
I know of a couple of six inch systems about as long as what you plan, so I figure that a seven incher should do fine.
If you are at all worried about cold start problems, you should place a primer box below the chimney. Your arrangement is perfect for it as having the pipe double back to somewhere close to the feed will insure that you actually use it.

[Donkey]

And..
Welcome to the board!

[jgreen]

Hey ya'll. Thanks for the ideas and the welcome donkey. I hadn't thought of cutting the brick to make it fit the new length on the ends. Thats a good idea. So I built a mockup of the brick part of the unit with the right interior dimensions for a 7" pipe. Now I have another question. I was originally planning on using a 7" steel pipe for the heat riser rather than building it with bricks. This is now dependent on if I can find a suitable piece of pipe. I'm not really having much luck with local scrap steel places (ie. they don't sell steel at all), but my question is if I do want to use a 7" round pipe, how do I make the transition from a 5.25" x 7" rectangular shape to the 7" circle. I was imagining using clay/sand mortar to sculpt a shape that gradually changes into a circle without reducing the total area. But I'm wondering... since this is one of the hottest parts of the system, is this amount of clay/sand mortar going to be a sound support in a place subject to this much heat and expansion? How have other people tackled this?

I included links to a couple of pictures to help show what I mean:

[Donkey]

I've thought a bit about this.
Seems to me one of the easiest ways to deal with it is to weld (or have welded) on some toggles a couple inches back from the edge of the pipe. Cheat the bricks open slightly and slide the pipe inside till it rests on the toggles. Tighten the bricks up around the pipe as much as possible and stuff the cracks and corners with a high temperature cob filler mix. A quick and easy way to hold back cob in the corners (so it won't fall when wet) is to jam in some balled up newspaper to fill the voids and cob over it. The paper will burn out later.
You don't need a smooth transition. In fact, a jarring transition will produce more turbulence mixing the fuel/air better.
Just don't create a constriction, be sure to seal everything up well and yer pretty much fine.