fire bricks for bell

[hammer]

What temperatures does the top of the bell protentional reach, I am trying to work out what fire bricks i need for the internal wall of the bell wall above the riser , there are some hard firebricks max 1000 degrees some 1200 degrees and some 1500 degrees could anyone advise please, the heater is 180 mm and the roof will be 400 to 500mm above the riser.

[fiedia]

Some measurements from my 5" batchbox (HR1 : HR bottom; HR3 : HR top; F1 : firebox).

[fiedia]

Jul 19, 2022 19:35:35 GMT -8 hammer said:

[I wanted the heat to be consistant from top to bottom

there is a temperature gradient along the bell height. This post gives some ideas :donkey32.proboards.com/thread/3821/results-chaining-batch-box-bells

[hammer]

Thanks once again Fiedia, I have found someone selling some old heavy white fire bricks, for the upper part of the bell, that I will pick up at the weekend.

[Forsythe]

Jul 12, 2022 3:58:53 GMT -8 hammer said:

Hi, can anyone tell me if it is a good idea to use insulating fire bricks to make the upper wall and roof of the bell, Peter used the insulation blanket as part of the upper part of the bell. I have extra bricks and wanted to utilise them.

Hey Hammer — Just to toss out some other ideas that may offer some inspiration / creative problem solving…

In kiln building, flat top kilns have a very similar problem to flat top heater bells — the heat is the most intense at the roof, and so that’s where expansion and contraction is the greatest. (And it’s why all traditional pottery kilns had arch roofs to distribute the load and expand/contract without falling in on themselves.)

In some masonry heaters, the solution for this is to lay one or two solid-cast refractory slabs across the top, with ceramic fiber gasketing to allow the roof to lift and shift — up to a couple inches — and therefore avoid cracking and plying open an exhaust leak.

However, In wide-diameter bells (as with large kilns,) this becomes increasing impractical as the size of the bell increases. The slab thickness necessary to structurally support a span of several feet becomes immensely costly — and immensely heavy— so much so that you risk crushing the supporting walls under the combined strain from weight + heat… unless the walls are at least a couple brick wythes thick… which further exponentially increases the cost and overall weight of the build, while also impeding thermal transfer through all that added mass of beefy heater walls. (This appears to be one of the rationalizations for why the only *currently* ASTM code-approved North American masonry heaters are of the overly large, massively heavy, outrageously expensive design.)

One way around this is to use lighter-weight insulating refractory materials for the bell roof… which, depending on your design goals, may also help keep a more-even heat gradient between floor and ceiling — a dense refractory roof can transfer too much heat upward toward the ceiling (as Peter pointed out) and make the air in the upper part of the room the hottest, while the floor remains too cool.

The first idea is to drill holes through insulating firebricks and string them along threaded rods, with connecting wires attaching the threaded rods to support steel laid cross-wise. This may be a good option if you already have a lot of extra insulating brick on hand.






It’s very similar to Hof’s use of threaded rod and firebrick units for his lintel construction here:
donkey32.proboards.com/post/37099/thread

Pictures below cribbed from Hof's thread for quick reference:





…Although I think I would recommend drilling holes, rather than carving out an open channel and relying on the strength of furnace cement backfill to support the hanging bricks’ weight.



Another method is to accordion-fold ceramic fiber blanket and hang it on a similar girder-grid pattern — this could even allow you to put threaded rods through the fiber at 90° angles from each other, and all those metal components are shielded from heat by the fiber hanging from them. (The first couple images below show kiln wall construction this way, but it would apply similarly for roof construction, albeit with sturdier horizontal beams on the cold-face [outside] of the roof, from which the threaded rods holding the ceramic fiber are hung… as with the first example using brick.)






There are some companies that sell accordion folded ceramic fiber modules, but they’re probably overkill for a heater bell, and you may have to buy in bulk to get them. Nevertheless, seeing how they’re constructed gives an idea for how you could fold and hang them, DIY. 

The most important point would be to have the ALL supporting steel rods and ties tucked away within the insulating fiber. That way the fiber bears only the heat—not the weight; and the metal structure bears only the weight—not the heat.








Here, also, are a couple videos showing one professional-type process for accordion-folding such modules, which can be replicated in a home workshop by a single person:

youtu.be/3MLSDYO26Ww

youtu.be/rZ0wuOD1NSA

And, of course, once you have the structure of the ceramic fiber roof assembled, you’d absolutely want to rigidize that fiber with colloidal silica or similar air-setting rigidizer — to make sure the fibers adhere to one another and don’t blow loose on the inside with hot flue gas impingement flowing up from the riser, and don’t come loose on the outside, posing inhalation risk to the people in the room. A bucket of Sairset would do the trick — and Sairset wouldn’t need to be fired in order to cure it hard (unlike fumed silica’s need for hot-firing to cure and harden.) Sodium silicate waterglass may also be sufficient here, since it’s not directly in the flame path of the combustion zones.


...Anyhow, those are just some additional ideas that might offer a jumping-off point to create a roof solution. Can't wait to see this heater come together for you however you decide to build it!

[hammer]

wow Forsythe, thanks so much for this, a lot of good ideas to digest and think about, you know your onions! I am taking my time building it A; because I am using mostly recycled and surplus materials B; I live in a rural location and C; The more I read the more it morphs and hopefully improves. I tend to overthink things, so it goes at a snails pace!! I think this info will continue that trend 😂, but it will be worth it to try and make it as best that i can. I will let you know what I do.

[hammer]

Thanks again to everyone and to this great forum, I have been away from the computer, but have been slowly building the heater, in the end I decided to insulate the roof of the bell with a mixture of the suggestions by Forsythe, I made the bell round because of the space restrictions of building it in a corner next to a window, so the inside roof of the bell was made similar to the concertina method in Forsythes discription in the previous post, I had small pieces of blanket which I folded in half to 6inch pieces and packed them into the rim of an old bicycle wheel, and pushed some steel rods through the blanket so the rods sat on top of the rim and could not fall through. I then rested that on top of the inside fire bricks (after spraying it with water glass) and then covered it with some refractory insulation board that i salvaged, then to make sure I put thin slate tiles and sealed it all. The roof of the bell does not change tempreture at all no heat gets through. The insulation bricks I used to make the firebox seem to work well, I used hard firebricks for the angled 45 degree bottom to avoid damaging when loading. I have included some pictures here flic.kr/ps/3ZJmWH

[Forsythe]

hammer niiiiiiiiiiiiice! You sure did an excellent job making an aesthetically pleasing heater build — that’s not an easy thing to accomplish when using mostly salvaged materials. This is beautiful!

[hammer]

Thanks Forsythe, just dont zoom in too much! I am a long way from finishing I am going to tile most of the lower bell and paving slabs. I have had a couple of dramas lighting it and have tinkered around with the secondary air tube. When I light it the majority of the flame seems to go through the port straight away but the flame that hits the rear roof of the firebox the smoke seem to want come back toward the front and if I leave the door open it comes out, if I close the door to the sweet spot where it is open only slightly its not so bad, if I close completely you can hear the rocket sound start to die out and so I open it and the smoke pours out for a short time. I darn't leave it closed as I smoked the house out the first couple of times when I done that (I can laugh at it now... my smoke alarm worked though!) The door cannot be closed for quite a while after lighting even though it looks and sounds good. It is a 7 inch batch with insulated brick hexagonal riser with no extra insulation on the outside of the riser.
I stuck as close as i could to the measurments that Peter gives and what works for everyone.
128 base
width should be 256 mine is 240 =16mm smaller
height should be 384 mine is 410 =26mm higher
min length is 512 mine is 540 =within the parameters 
port 281x 64 as per calculation
I cannot think of anything else that could affect it, my flue pipe is inside the first 700mm of the bell but is well insulated but just single skin outside the bell and is 7mtrs long and at the peak of the burn I can keep my hand on it for 5 or more seconds.
Is there is something that I have missed? once it is really going it is fantastic the bricks are still releasing heat in the morning 17 or so hours later.
Just as a warning for anyone wanting to make the bell rounded like mine, there is one hell of a lot of cutting and head scratching, I needed a project to take my mind off of recently losing my wife and this certainly helped, it took a lot of time to get to this stage of the build, but rewarding. 

[peterberg]

Perhaps your heater isn't thoroughly dry yet? Even when it looks like that doesn't mean it actually is. In case the thing has been in use for over a month, some other reason might be in play. You said you could hold your hand on the bare stovepipe for 5 secs or more when the thing is going full bore.

In a completely dry heater that would indicate a chimney temperature that's too low. So when starting up there's insufficient draft. in case you are doing top-down burns, try to establish a small fire in front of the port first, load more when that's going strong, wait a couple of minutes more and add the rest of the load.

In case this won't help: maybe the bell size is a bit overspecced OR there's an unwanted restriction somewhere. Since the exhaust pipe is inside the bell, the easiest (hopefully?) remedy would be to take the insulation off and/or lift the pipe 10 or 20 cm.

[Forsythe]

Could it potentially be a threshold that's not tall enough ...or a threshold that's not far enough back away from the air inlet in the door, causing the smoke to pool at the firebox roof? ...resulting from the incoming primary air not getting swept forcefully enough up and over the fuel — and along the firebox roof— to coax those gasses toward the port?

[peterberg]

Oct 3, 2022 3:37:22 GMT -8 Forsythe said:

Could it potentially be a threshold that's not tall enough ...or a threshold that's not far enough back away from the air inlet in the door, causing the smoke to pool at the firebox roof? ...resulting from the incoming primary air not getting swept forcefully enough up and over the fuel — and along the firebox roof— to coax those gasses toward the port?

Don't think so, the underpressure in the port should do it. It (the port) acts as a draft amplifier, which means the draft need to be there in the first place in order to be amplified.
The threshold has nothing to do with it, it's convenient to keep the ashes in. And it directs part of the incoming air upwards, handy for to keep the glass clean.

[hammer]

Oct 3, 2022 1:12:10 GMT -8 peterberg said:

Perhaps your heater isn't thoroughly dry yet? Even when it looks like that doesn't mean it actually is. In case the thing has been in use for over a month, some other reason might be in play. You said you could hold your hand on the bare stovepipe for 5 secs or more when the thing is going full bore.

In a completely dry heater that would indicate a chimney temperature that's too low. So when starting up there's insufficient draft. in case you are doing top-down burns, try to establish a small fire in front of the port first, load more when that's going strong, wait a couple of minutes more and add the rest of the load.

In case this won't help: maybe the bell size is a bit overspecced OR there's an unwanted restriction somewhere. Since the exhaust pipe is inside the bell, the easiest (hopefully?) remedy would be to take the insulation off and/or lift the pipe 10 or 20 cm.

It could possibly be still wet, it certainly was wet the first couple of times, when the door was open steam would come off the iron and drips appear on the floor I have lit it about 12 times, its spring here now. The riser was made from very light insulation bricks that I dipped in water before smearing with refractory glue/mortar and then coated with zircon, they felt damp for ages while i was building the rest of the bell.
The bottom of the flue is about 150mm off the floor at the moment,
I will take what you have said on board and let you know how i get on, thanks for your time.

[josephcrawley]

Looking at the photos, nice build by the way, it looks like you don't have a cold start bypass in the system. I would recommend adding one if your smokey starts continue.

[hammer]

Oct 3, 2022 13:49:37 GMT -8 josephcrawley said:

Looking at the photos, nice build by the way, it looks like you don't have a cold start bypass in the system. I would recommend adding one if your smokey starts continue.

Thanks josephcrawley can you give me a link or brief explanation of a cold start bypass, thanks