fire bricks for bell

[hammer]

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. I was thinking maybe it would take longer to heat these but but release the heat slowly at a later stage, would this be correct. Thanks for reading.

[martyn]

Most rocket stoves are individual builds, custom built to suit the owner, so there may be a reason to use insulated bricks in a bell but generally speaking this is not the case and solid mass bricks will be the normal choice.
Insulating brick will not hold any heat for more that a few minutes.
I dont know why insulating brick would be used for a roof structure unless more heat is required further dow the line maybe?

[hammer]

Thanks Martyn,I did not realise they did not hold heat. Could you tell me just so i get the idea, why the insulating blanket is used in top part of the bell.

[martyn]

The only reason I can think of is because the builder or owner would not want the top of the bell to get hot, perhaps because it is to close to a combustible surface?
Generally speaking the only place in a rocket stove where insulating material is used would be the fire box, combustion chamber or afterburner.
Those are areas where maximum heat is required but normally a bell or other form of heat storage will be built with dense high mass material like fire brick or refractory cement.
Peters site explains it all …. batchrocket.eu/en/workings

[hammer]

thanks again Martyn, I have read as much as my little brain can the site you linked and have started my build. I get most of it but the bit that has confused me is the part where the upper part of the bell has a narrow brick on edge then insulating blanket and then another brick on edge I have seen it a few times one of them was Peters Mallorca build on the Batchrocket eu site, thats the thing I cannot get my head around.

[martyn]

Ok well I guess Peter is the only one who can answer your question, he is active on this site.
Perhaps it is a system that protects the steel and allows for expansion of the fire brick roof.
Personally I would avoid using ceramic fibre mat for health reasons however if you are building a bell with a low roof over the riser then that area will get seriously hot, so that might be another reason.

[hammer]

Thanks Martyn, much appreciated

[peterberg]

hammer, in general, the reason for building the top half or so of the bell like this is expansion. The space above the riser gets seriously hot, a bell with a wall built with just a single brick on flat will always crack badly above riser level. It doesn't matter what material you are using, even dense firebrick will expand and contract, tending to walk apart over time.

In order to avoid this situation one could build a bell out of two skins. An inner skin that could be allowed to expand and an outer skin to keep the bell airtight. In order to avoid cracking of the outer skin, both skins should be completely separated from each other. One way to do this is using a material like superwool between the skins. Since the temperature differences between outer and inner skin are quite high, the insulation properties of this wool won't be an issue. However, a single skin bell is required for a situation where lower weight and/or higher warming up speed would be required.
In short, a lighter bell with quicker response could be built with only the top end of the bell's wall in double skin. At the same time it also provides a way to close the bell's top without having the top closing bricks in sight. The Brussels build is another example of this method, using moderate sized slabs this time.

For the Mallorca build there weren't any larger refractory slabs available. Therefore I came up with the possibility of using steel T-bars and firebricks for closing the top. Done in such a way that the steel would nowhere touched the bricks and could expand freely without doing any damage. This method is used quite extensively since then, also by professional builders. Regarding quite some insulation on top of that bricks: people aren't living on top of the heater, the radiation from the top is largely heating the ceiling. And it provided a means of making sure all crevices would be permanently closed. The bricks on top of the insulation made sure nothing of the ceramic wool was exposed to the living space.

I hope all this sounds sensible.

[hammer]

Thanks Peter, yes that all does make a lot of sense and a good explanation. Your knowledge sharing is a generous thing to do. I am part way through my build I am yet to build the batch box, I managed pick up some insulated refractory bricks, 2 different types one type 230x180x75 is fairly hard and the other type 230x110x75 is very light and delicate, I will make the fire box from the tougher ones but would the softer ones be better for the riser or would the harder ones be better for that as well? Maybe I should use them on the roof of the bell as well?
Kind regards

[fiedia]

Hello hammer,

before installing your soft firebricks (preferably in the heat riser), you should have a look at this post : donkey32.proboards.com/thread/3909/firebricks-refractories-fail-silica-flux

It strongly advises to coat your soft firebricks with zircon coating.

[hammer]

Hello again Fiedia, wow that was a lot of information!!! Very useful, I would have been dissapointed to have read that after i built it, so thankyou very much, I have ordered a pint of the 100 HT.

[fiedia]

Hello hammer,

Regarding firebricks for the fire box, I would rather use the hard ones because :

1) the soft bricks will be easily damaged when loading wood and retrieving ashes

2) I do not see an advantage insulating the firebox. It may enhance thermal runaway and the fire box is an interesting heat source at the bottom of your bell (if you want uniform temperature profile along your bell).

The firebox gets very hot. You need two skins like the top of your bell.

 

[fuegos]

" I do not see an advantage insulating the firebox. It may enhance thermal runaway and the fire box is an interesting heat source at the bottom of your bell (if you want uniform temperature profile along your bell)"
The point you make about wear on the soft IFBs is valid but the idea of an insulated firebox is that it quickly gets up to temperature ,overall a cleaner burn.My riseless core oven build has dense firebricks in the core but im planning another build with IFBs. A more efficient burn means less firewood & for this next oven build in a community space , firing all year & daily for 6 or 7 months a year this could be significant.

[martyn]

For the fire box construction, It would appear that both approaches work depending on your perspective ie… hard bricks hold heat and are abrasion resistant but take a fair bit of time to get hot and they remain very hot making re loading tricky unless you wear gauntlets .

Soft brick provides a much quicker start up and makes reloading safer but not very abrasion resistant!

Other points are … soft bricks are very easy to work with and can be cut with a hand saw, hard bricks are very difficult to cut neatly by hand and ideally require a wet cutting saw.

Some options might be coating the soft bricks with a refractory paint or lining a hard brick firebox with replaceable vermiculite components..

Most of Peters designs use relatively thin, cast refractory, backed by insulating material = a good compromise perhaps?

The ‘very high temperature’ insulating bricks that I have used are in fact quite hard when super heated but are expensive and not as insulating as standard soft fire brick.

Finally there seems to be more and more refractory coating products coming on the market, most I have not tried but, thin, hard coating, soft bricks seems to be the ideal way forward ?

[hammer]

Jul 18, 2022 23:47:15 GMT -8 fiedia said:

Hello hammer ,

Regarding firebricks for the fire box, I would rather use the hard ones because :

1) the soft bricks will be easily damaged when loading wood and retrieving ashes

2) I do not see an advantage insulating the firebox. It may enhance thermal runaway and the fire box is an interesting heat source at the bottom of your bell (if you want uniform temperature profile along your bell).

The firebox gets very hot. You need two skins like the top of your bell.

 

Hi Fiedia, Thanks for that. The insulating bricks for the fire box are fairly hard, in my mind I wanted the heat to be consistant from top to bottom and allow the coolest air to exit some of which will pass under the firebox which is 90mm off the floor.