cantenary arch

Hey,
I'm a newbie so I'm going to try to restrain myself on comments till I've read "The Book" (I've ordered the hardcopy and I'm still waiting for it to come in) so it might take a while.

One of the stoves I'm looking to replace with a RMH is in a place where I want to keep its footprint as small as possible. So, I'm looking to go up with the thermal mass rather than out with a bench type structure. It's in my workshop, which is already crowded, and I really don't need anything that will encourage me to sit down in there. I'm thinking of stacking bells for efficient heat capture, and it seems like the best shape for a bell made out of any kind of masonry material would be a cantenary arch, or maybe a roman arch shaped chamber cast inside a rectangular shaped box to make them stackable. See http://www.house-design-coffee.com/catenary-arch.html and at the bottom of the page there's a link to more on roman arches.

My wife is a potter and the cantenary arch is a standard in kiln design. Search doesn't bring up anything on cantenary or roman arches in your forum so I'm wondering if if it's been tried for heater mass. I don't feel the need to reinvent any wheels.

Hi Tom.

Will it be fired everyday? If not, i'd say, use a metal bell. Because you will need fast heat. Rather than heat storage.

Tom, I'm not an expert on this subject, but have followed a lot of the discussions with some interest for a while.

I'm not sure quite what you mean by stacking bells. Are you wanting to put a bell directly on top of the furnace as opposed to running a short length of pipe into your vertical mass?

There may be some drawbacks and issues, but I suppose the mass could be made vertical. Unless you find one to copy, I would definitely put it in the "experimental" column though since it would be a pretty significant change. To slow down the escape of heat, you might be able to use a coil chimney up through the vertical mass, but the last time I checked, they don't generally sell stovepipe that coils. Nevertheless, there might be something that would work. Certainly elbows and short straight sections, but that would be a lot of expense for a bunch of elbows.

It's a big metal building and although I've done my best to tighten it up and insulate it it still doesn't hold heat well. The stove I have there now I keep burning constantly while I'm in the building, stoked up and damped down, it takes the edge off the chill a bit but mostly it's something warm I can back up to. Before I learned of rocket stoves I was thinking of putting in an old fashioned masonry mass heater. Radiant heat, yes, try to heat all the air in the building, forget it. With as small as the firebox is on the rocket stove I think I do need heat storage.

I am aware of the DVD's. I can't afford to buy them till I've recovered from Christmas a whole lot more. I figure on reading the book, saving up my nickles and dimes and scrounging materials, then renting them when I'm ready to get going on it. I won't make a final plan till I've watched them,but like the book I want to have them handy to refer to when I have to change the plan in the middle of the project. Meanwhile I'll gather what wisdom I can from these forums.

So batch rocket and big massonry heater mixture is for you.

http://batchrocket.hostoi.com/html/foto.html

But remember, you need big if your building is drafty.

Yes, I have gone through the masonry stove book and at least read the pictures. So, I was looking at using a batch rocket for the higher efficiently burn then exhausting it through a masonry mass to catch the heat and then radiate it off over a longer time frame.

Your tubing through a cob mass depends on the surface area of the tubing to extract the heat... whether it's straight or coiled tubing. A bell operates on a whole different principle. See http://www.stove.ru/index.php?lng=1&rs=16

Bells laid up from brick give a rectangular cross section to the chamber and brick ducting gets complicated. I'm thinking that a roman arch shaped chamber molded into a rectangular block shape puts the most heat up where the masonry is the thickest. Mold the blocks to be stackable, run sheet metal ducting vertically between them, and keep adding blocks till the exhaust temperature gets down to where you want it. Then pass the mold on to the next guy. By throwing this out there I was hoping to get a "been there, done that, I'll put you on the list to use the mold" response.

I've scanned a quick sketch that might help. Lets see if I've figured out how to attach it.

Neat concept. I've never seen anything like it, but suspect the concept has promise for some applications. I think you could reduce the footprint slightly running the stovepipe straight up through the top of the domes.

I just realized you were talking about making a masonry heater, and not necessarily a rocket mas heater. I don't understand masonry stoves that well, but it appears to me there are really two very different types of woodburners although some of the concepts may overlap.

Rocket stoves/rocket mass heaters seem pretty fussy when it comes to seemingly insignificant design changes. I'm thinking that looking at this as a masonry heater instead of a rocket stove would be more productive.

Hopefully someone will respond who has done something similar.

I think you could reduce the footprint slightly running the stovepipe straight up through the top of the domes.

If he did this, it would no longer be a bell. It would just be a funny looking flue system. The idea behind taking the exhaust from the bottom of each bell is that the coolest exhaust moves to the next bell and the warmest gases stay until they cool off by heating the inside of the dome.

I just wrote a post on the three issues which frequently confuse on another thread, Rocket Mass Heaters vs. Scandinavian / Slavic Masonry Heaters. Particularly, it would be useful if the combustion system is distinguished from what happens to the exhaust after it is created.

Tom's idea of a mold is interesting. What material would you use for the casting?

Cindy Mathieu wrote:

I think you could reduce the footprint slightly running the stovepipe straight up through the top of the domes.

If he did this, it would no longer be a bell. It would just be a funny looking flue system. The idea behind taking the exhaust from the bottom of each bell is that the coolest exhaust moves to the next bell and the warmest gases stay until they cool off by heating the inside of the dome.

I just wrote a post on the three issues which frequently confuse on another thread, Rocket Mass Heaters vs. Scandinavian / Slavic Masonry Heaters. Particularly, it would be useful if the combustion system is distinguished from what happens to the exhaust after it is created.

Tom's idea of a mold is interesting. What material would you use for the casting?

Cindy, i've tried this, kindof, i've made the vertical flue pipe out of 35kg gas bottles, with a 4 inch exit, on my second version of a rocket stove. And, since the gases expand in the chamber created, they slow down, giving it more time to exchange heat with the surounding material. It's not as eficient as a bell, but has the advantage of keeping the draft stronger.

Tom, have you ever seen rectangular home or tractor fuel tanks?

I've been given this. Which is similar.

http://donkey32.proboards.com/thread/1017/pure-genius

The plan is to use it as a bell, covered with dry stacked bricks. That's the simplest way imho.

Hth.

Max.

Cindy, you are right, but what I am picturing is a pipe through the top that extends to the bottom of the bell thereby pulling air from the bottom of the bell. I'm not saying for sure it would work, but that is what I was thinking.

The masonry mass is a heat exchanger. The stove is a heat generator. The traditional masonry mass heater had a firebox built in, usually under the mass of the heat exchanger, and maybe there was an oven, water heater, or other bells and whistles built in too.

As I said up front, I don't know enough to talk about rocket stoves yet. But, most of what I've heard about rocket stoves assumes you're going to run it's exhaust through a long bench shaped heat exchanger before it goes out the flue. I know I want a different shape for my heat exchanger so I brought that up.

The stove, the heat exchanger, and the flue/chimney are all parts of a system. they have to be sized to work together. I'm looking for a heat exchanger that I can start building while I'm getting up to speed on how to build rocket stoves.

I have a 6" flue coming off my existing stove. I figure I can plan on making a 6" rocket stove and size the heat exchanger for a 6" input. Then if I don't get around to making the rocket stove this summer I can hook the heat exchanger to my existing stove to get me through next winter. If the rocket stove runs hotter than my existing stove and I need more heat exchanger for it I can add more bells on top (and, maybe a whistle or two). There is that option for adjusting the sizing, which I think will be easier than add or subtracting bench. I've got 14 ft. before the flue goes out the wall so I've got a lot of room to go up.

But, if I do go up 14 ft I'm putting an awful lot of load on that bottom bell. That's why I was looking at going to the roman arch as being the best structural shape to bear the load. And, it's why "what material would I use for the casting?" becomes such a great question. I was going to get around to asking that one myself. Any suggestions?

Tom, myself, i would KISS it.

Something like Peterberg workshop heater,



Which is a batch rocket. Surounded by mass. Would it be cob or bricks or cobbles or anything.

Wow, that open a whole new door. Do you have more pictures or a diagram to what's happening inside of that?

http://donkey32.proboards.com/thread/951/shop-rocket-heater?page=1#page=2

http://donkey32.proboards.com/thread/511/adventures-horizontal-feed?page=23#scrollTo=8038

http://donkey32.proboards.com/thread/849/green-machine-inch-tube-help?page=2#page=1

Tom Strode wrote:Wow, that open a whole new door. Do you have more pictures or a diagram to what's happening inside of that?

The links provided by Max should help a lot to understand how it's done. Here's a diagram of the principal layout, the thing is dead simple. The three 55 gallon barrels do form an open cylinder together, when it's at full bore the riser is acting like a flame thrower and the noise the thing is emitting is best described as aggressive.

Bah humbug. I took to long to writing my reply and when I tried to post it the computer ate it and gave me an error message.

I'll rewrite it... when I get more time.

Thank you. This is a fast and nasty addition I can put on my existing stove to get more heat out of it. I'll still want to put in a rocket stove with more mass to stretch the heating time out. I want to try the roman arch idea and if I have one of these already in place I can run the rocket stove to it for a while first to get a benchmark to compare the other to. Being able to put it off is good, this stove isn't my highest priority.

I have a Kentucky wood gobbler on the other end of the building that uses four times the wood that this stove does. It's a water heater, and yeah, I know, squish boom. But, I'm committed to hot water heat. The under-floor heating elements are literally cast in concrete. Plumbing, pumps, and manifold for downstairs and radiators, blowers and ductwork to get heat to the loft. It's taken three years to set it up and work the bugs out, and it works well... but it takes me ten cords of wood to get through the winter. I bartered some backhoe work for this old furnace so I don't have a big investment in it. It's paid for itself by giving me the time to work on the rest of the system. But, it's about three generations behind on being designed for efficiency so it's ready for retirement. And, I'm ready to tackle the learning curve of rocket stoves. There's the furnace, the stove in back, and I have a south slope that's just begging for a walipini (http://www.bensoninstitute.org/Publication/Manuals/Walipini.pdf) and, that could use a heater. With three of them to build that justifies some effort.

I'll read the book and then we can talk about it.

This link talks about a system similar to what you have been discussing. Unfortunately, it is very short on specifics. http://0costliving.blogspot.com/p/rocket-stove.html

Specifics isn't all it's short on, he leaves out the heat riser from the rocket stove. The riser is an air pump driven by heat differential which gets the draft going strong enough to get the "rocket effect" going. That fast hot burn is what makes it so efficient. He describes the riser as being part of the burn chamber. The riser can act as a secondary burn chamber but that's not what it's there for. The masonry mass heaters like the Russians use don't have that. The bell is a design for a specific type of heat exchanger, which was used by few, if any, of the older Russian masonry mass heaters. This thread has been about connecting a rocket stove to a bell type heat exchanger instead of the usual tube through a cob mass. I think it's the same thing he's talking about. Where he says, "these passages could be replaced with bell chambers thus allowing a simpler, more flexible and more easily constructed design" is where I have to differ. A tube with cob packed around it is about as simple and easy to construct as you can get. We were talking here about the flexibility part of being able to have a smaller footprint by stacking bells. Near as I can tell that isn't what he is doing.

http://batchrocket.hostoi.com/html/foto.html

http://technologieforum.forumatic.com/viewtopic.php?f=19&t=27

http://donkey32.proboards.com/thread/848/18cm-inch-double-batch-system?page=2#page=1

Okay, It's that time of year again and I'm getting ready to go. I've made a heat exchanger for my inside stove based on Peter Burgs drawing.

I had some old dead pressure tanks that I'd gotten from a well driller and made up into tubing (16" dia.) for another project (which didn't pan out). I've welded ends on an am using them for the outside drum. 6 inch stovepipe comes out of the stove and goes up 4 feet. I put a piece of 10 inch duct over that, extending below the elbow, and packed it with a perlite/clay slip mix.

There is 16 foot of stove pipe going up inside the building that should radiate a lot of heat too. A door on the tee at the bottom of the chimney gives access for getting the draft started.

I've started it and it does draft. I'm running a tiny fire in it to cook the moisture out of the perlite/clay mix and I'll be doing that for days before I crank it up to try it out.

Thanks for the help guys, I'll keep you posted as I go.

I tried attaching three photos yesterday but it only took two. I'll add one here. Or try to, anyway.

Keeping a small fire going is a problem. I'm sure that the moisture I was trying to cook out was a large part of the problem too. Water expands as it vaporizes and since it was expanding into the drum that tended to shut the draft back down. It was condensing in the chimney and dripping out the tee. I should have known to put a pan under it before I started.

It took hours before the chimney started feeling warm, the insulation with all that water in it was soaking up the heat as I was trying to get it going. With the chimney staying cool the riser/drum was creating all the draft. Smoke would back up into the room if I opened the door to check the fire. When I turned the vent fan on to cool the bldg. (it was an 80 degree day) it reversed the draft. I had to keep the door open next to the stove for it to work at all. The longer it went the better it did work, and I can see where it will roar when I get the moisture out and build a real fire in it.

No boom, no squish, it's doing fine.

Traditional woodstoves depend on a hot chimney to create draft, and sucking most of the heat out of the flue gases for efficiency robs the chimney of much of its draft.
Rocket mass heaters create draft by the heat riser/barrel radiator combination driven by a very hot fire. A small fire may not have enough push to overcome system losses, especially on a warm day where the chimney is hardly warmer than the outside air. With your woodstove not burning efficiently (hot) you may not even have as much push as a rocket core idling along would give. The perlite/clay should have enough porosity to allow higher heat without spalling even at early stages of curing. You might try a good fire for a relativly short time and see how it drafts, then let it cool down if you are concerned about heating the insulation layer too much too soon.

Having just seen the beginning of this thread, it is late to be commenting on the stacking of small arched bells (others have done it already), but one tall bell would do the same job as the multiplicity of short ones and be much simpler. You would want to study bell to firebox/system size ratios so as not to make a bell much too big for your heat input.

That slip took a lot more water than I thought it would, around three gallons. Having that much water trapped between two pieces of metal, with the only opening that strip at the top, it just seemed wise to bring it up slow. It was a lot easier to start this morning, and to keep it running. I was starting it then letting it die and cool a bit when I first started it yesterday. I've been gradually running it longer and hotter and it does seem to have the porosity to take it. I prefer to learn that sort of thing the easy way.

The idea behind stacking the small bells was to lessen the footprint. I have limited space. What I'm doing takes less space, is a lot easier to build, and should work well enough.

A small footprint does not mean you need to use short bells. One tall bell with the same footprint may give as much or more heat absorption inner surface as a stack of short ones with much less complexity.

I was thinking that with the hot fast fire we're looking for that the heat would blow across without having time to separate out. Now that I think about it I see more possibilities. We need to keep the same cross sectional area on the pipes but could fan it out to a short wide entrance and exit to the bell, maybe put a short barrier in the middle to give it more flow length to separate out over. There would be limits, but, yes, I may have been to hasty. Still, this was a quick cheap fix using stuff I had on hand, and it's done (that's a biggie). If someone else tries it I would like to hear how it works.

The tall bell you have is working exactly as it should: the heat is rising to rhe top (assisted by the vertical entry) and only the coolest gas is exiting at the bottom, after it gives up its heat. A standard bell works the same way, where the gas entering it is much hotter than the rest so it rises instead of flowing straight across to the exit. In a narrow bell you would want a baffle or arrangement to keep the gas from short-circuiting, as you mentioned. The bell does not depend on a specific flow arrangement for heat transfer.

Tom Strode : Without belaboring the point, you are better set up to get the heat unto the ceiling AND out than holding it in your comfort zone.

you really need to fork on getting your teat energy trapped at the ceiling back down to you. if you figured total cubic feet your room seems
3Xs bigger than the working space and all of it over head ! good luck, for the crafts ! Big AL

You are only seeing a small part of the building. There's that tall garage door in that wall and I wanted head room to put in a chain hoist for unloading, but the building is only open all the way up for 10 ft in from that wall. Our living space is up in the loft next to, and open to, that space you see. The heat actually does go to where we want it pretty well. The pottery shop in the front of the building has underfloor heat fed from a furnace (another modification project). That stove was never intended to heat the whole building. There will be fans for circulation.`