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Post by Deleted on Dec 22, 2014 2:09:15 GMT -8
Hi,
I have been reading these forums for some time and have built several small steel rocket stoves.
I'm now wanting to move onto a nice Rocket mass heater for a small courtyard.
After reading through many threads and posts, i think a 6in batch box system will be the most appropriate system, but i am a little confused as to what the "most ideal" material will be for the batch box and riser.
I have access to a range of refractory products, from refractory plastics, insulating fire bricks and castables including silicon carbide
So here is my question, if you could have your pick, what would choose to build the batch box and riser from and why?
Time.
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Post by pinhead on Dec 22, 2014 6:35:33 GMT -8
I would build the firebox out of insulating fire brick and the heat riser in the same way that clkindred made his " trashcan forge." EDIT: You asked why. I like firebrick because you don't have to worry about it crumbling apart; the seams between the bricks prevent undue stresses from building within the bricks themselves. And for the heat riser, I like the excellent insulation properties of perlite and the temperature resilience of sodium silicate and alumina. |
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Post by peterberg on Dec 22, 2014 7:19:34 GMT -8
When I have the choice, building my first batch box in a court yard, not inside the living space of the house. I would use hard firebrick for the firebox and insulating firebrick for the riser. See if it's possible to cut the riser bricks so you could build an octagon riser. This way you don't need insulation material around the riser and it is close to the ideal shape. The real deal is a rigidized ceramic fibre riser but those are expensive and at most places hard to come by.
Refractory castable could be used for the firebox, but using bricks you are able to try things out. Lots of fun!
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Post by matthewwalker on Dec 22, 2014 8:22:24 GMT -8
I think it's going to be different for everyone, there is no "best" here. For me it is insulated castable refractory for the firebox and formed ceramic fiber for the riser. Because it's fast, easy, durable, and I can make any shape I want simply.
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Post by satamax on Dec 22, 2014 14:46:39 GMT -8
I quite like the way an insulating firebrick firebox reacts. Mind you, Peter says it's not necessary. For the heat riser, insulating firebricks, chimney flue (my case), cast, insulation fiber, and whatnot. Stick with tried and true solutions. With plenty of insulation.
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Post by Deleted on Dec 22, 2014 17:16:22 GMT -8
When I have the choice, building my first batch box in a court yard, not inside the living space of the house. I would use hard firebrick for the firebox and insulating firebrick for the riser Is there a reason you differentiate between outdoor and indoor construction? There are a number of hot face insulating castables that would be able to be formed into the ideal shape not just close. For that matter there are also a number of long life formed ceramics from which the firebox and riser could be made from. Why choose insulated firebrick over a cast insulating refractory riser or ceramic? Also, Why do you not feel a insulative combustion chamber is needed? surely a higher temperature chamber is desirable, even if only to cause additional heating of the secondary air supply? Finally, Can a riser be over insulated? Time. |
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Post by patamos on Dec 22, 2014 20:06:49 GMT -8
I have gotten into the practice of casting high heat duty (1300c) refractory for heat risers using 6"+ ABS or PVC plastic tubes. The inner can be removed shortly after the cast has been made so as to accelerate drying. the outer i remove shortly after. Alternately you can use disposable sona tubes. I have been casting as thin as 3/4" but feel disinclined to go any thinner on the chance that they will destabilize too much when they crack. Using clay and perlite as the insulative surround can help to keep things in place (and thereby problem free). If you use soft ceramic blanket or loose perlite or vermiculite or the like (contained by a larger diameter surround), then the risk of movement of the heat riser after it cracks is increased.
Casting an ocatgonal shape is easier to form up with wood.
Matt's preference for formed ceramic fibre heat riser is more of a cutting edge approach. Generally the more insulative the heat riser is the better it will perform at boosting the combustion temperatures - particularly in the early stages of the burn. If your RMH does not have a start up by pass flue this early rise in temperatures is of greater benefit/importance. That said, there are many ways to go about it. Bricks, insulated or not, and store bought castable mixed done properly are 'tried and true' and work just fine. The heavier they are the slower the warm up time. But if your system is well designed overall, this is of little concern.
So far as i know, you cannot have too much insulation. Bt it gets to a point of impracticality as the return on the time/cost/space involved diminishes the more you add. 4-6" of clay-perlite is all most people add around the heat riser. One full wrap around of 1" ceramic blanket is also common.
For the burn chamber durability is a priority, so denser brick is a good idea. Better to insulate around the chamber than in it, unless you are taking Matt's approach involving glue rich mixes (eg sodium silicate)... If you want to stretch the bounds of a brick burn chamber you can use high heat duty splits (1.5" x 4'5" x 9")
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Post by Deleted on Dec 23, 2014 0:10:53 GMT -8
Ok Thank you for the Reply. To get the ball rolling, i think i will take your advice and build a firebrick box and riser, and put it on a dolly so i can work out where the best place in the courtyard is to build the fixed "ideal version". I'm planing to build a metal bell that extends back over the firebox. Would anyone be willing to look at the attached file and tell me if it would work? I'm after radiant heat but a compact system. Is this a viable design? Also, with the P channel, is a door with primary air hole, better than an open box? Time. |
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Post by peterberg on Dec 23, 2014 4:21:02 GMT -8
Is there a reason you differentiate between outdoor and indoor construction? When the result isn't what you expect and you need to redo things. When outside it's hardly a critical application as opposed to the living room. Why choose insulated firebrick over a cast insulating refractory riser or ceramic? Insulating fire brick is much more insulative (the type I've seen) than insulative refractory. And it's easier to change and re-use after tearing down. A refractory cast riser need insulation anyway. A ceramic riser in the shape of a tube won't survive without cracks and you need insulation again. So, insulative firebrck is easy to use, you can shape it with a hacksaw. A cast riser cannot be reworked, and you need to make a mold. Also, Why do you not feel a insulative combustion chamber is needed? surely a higher temperature chamber is desirable, even if only to cause additional heating of the secondary air supply? The firebox has to be abrasive resistant, insulating brick won't deliver that. And sure, you can insulate the firebox around its perimeter, it will help. Finally, Can a riser be over insulated? No, I'll second Patamos in this. |
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Post by peterberg on Dec 23, 2014 4:27:07 GMT -8
Would anyone be willing to look at the attached file and tell me if it would work? Yes, you need more space above the riser I would say. I'm after radiant heat but a compact system. Is this a viable design? This design is probably too compact, maybe it would be better to add another barrel or a bench. Or scale down the core to a 5" system for instance. Also, with the P channel, is a door with primary air hole, better than an open box? Yes, definetely. |
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Post by pinhead on Dec 23, 2014 6:50:22 GMT -8
Ok Thank you for the Reply. I'm after radiant heat but a compact system. Is this a viable design? Also, with the P channel, is a door with primary air hole, better than an open box? Time. Yes, it's a great design, though with a 6" batch box you'll be bordering on red-hot metal every time you fire it up; you don't have nearly enough radiating surface area. I'd double the height at a minimum or, like Peter said, reduce the size of the batch box. |
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Post by Deleted on Dec 23, 2014 15:33:38 GMT -8
OK Thanks,
I might redraw it for a 4in system then. I think the physical size as it is drawn in the previously attached file is the largest I can go and still comfortably be able to wheel it about.
What is your recommendation for clearance from the top of the riser to the bell?
What SQM surface area would you recommend for a radiant metal bell on a 4in system?
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Post by peterberg on Dec 24, 2014 2:40:24 GMT -8
Recommended minimum top gap in a 6" system: 1'. Ideal top gap: the same as the riser length, i.e. 8 to 10 times the base measurement. in a 4" system that would be 23" to 28.8". A maximum top gap is never tried out yet so perhaps there isn't one in practise. I don't know about a SQM abbreviation, what does it mean? In case you mean the same as maximum internal surface area (ISA) that would be 44.4% of a 6" system. That last one is 6 m2 or 64.6 sq ft, so in your situation it would be 28.7 sq ft. And you need an adequate chimney stack of course. Beware though, this isn't tried out yet so here is your chance to contribute to development in true international fashion.  |
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Post by satamax on Dec 24, 2014 9:38:33 GMT -8
A maximum top gap is never tried out yet so perhaps there isn't one in practise. Well, Peter, that's not the first time you say that. But we can determin the maximum top gap by the maximum ISA. Even if the barrel isn't a true bell, it's maximum size, for a 6 incher is still 6m² or thereabouts. Let's say 58cm diameter for the barrel, so (60000cm² - 2462cm²) / (58 x PI) = 314cm barrel height. Minus 110cm for the heat riser; that's aproximately 204cm top gap  Or something along thoses lines! |
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Post by peterberg on Dec 24, 2014 14:21:50 GMT -8
Or something along thoses lines! I'd agree, but steel barrels are shedding heat faster than brick bells. So for steel, the maximum ISA is 5 m2. With that in mind, the equasion would be (50000cm2 - 2462cm2) / (58 x PI) = 260cm barrel height. Incidentally, that is very close to a tower of three barrels on top of each other. Minus 110cm for the riser, that would mean 150cm top gap. The largest top gap in that tower I've tried is about 1.3 barrel high, that would be 110.5cm. Close, but not yet there I would say. But wait! The first ever casted batch box which is still in the Netherlands in a former pig's stall, is also in a three barrel tower. That one has the firebox installed about halfway in the first barrel, the riser is a mere 40cm above the top of the firebox. Inside, the top gap should be very, very close to this theoretical maximum. We had it running as a bare steel tower for one night only but it did work alright. The next morning it was clad in cob, except the top barrel. After that, the exhaust temperature happened to be much higher but the cob shell stayed hot longer after the fire went down. Happy Christmas to you all, hope your shoulder blade will get well soon Satamax! |
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