Lightweight stove for a small mountain hut

[pigbuttons]

WOW!!!! That looks amazing. Nice crisp edges and very consistent appearance of the cast with no grainy spot or streaks. The tiny bubble holes are of no consequence. I know you'd like it smooth as glass but they are probably only on the side facing up as cast. The bottom side should be smooth, though that won't matter in the long run either.

[bombadyl]

I finally removed the inner foam blocks to see the final result.

It looks pretty good. There are tiny bubble holes on the inside as well but I am not too worried. As expected the external bottom face of the firebox is not flat because of the concrete pushing the wall where there was no duct tape, but it will be mounted on a stone floor which is not flat either so I will need to compensate with some mortar in between.

Good news is : the walls and bottom board are ready to be used for the 2nd mould. The foam blocks can be reused as well, but the 2nd mould is a mirror image so I will reuse only the symmetrical ones.

Next step is to cure it !

I have read the instructions for the product I use and they recommend the following:

- rise temperature by 50°C per hour until you reach 110°C, then keep at 110°C during 24 hours !

- then rise slowly without interruption by 10°C per hour until you reach 1400°C => meaning rising temperature continuously during 6 days ! ha ha ha

Clearly this is not achievable without an industrial oven !

I was thinking:

- small open fire without the top of the firebox

- small fire with top of the firebox but without door and riser

- same with riser

- increase wood quantity, etc...

This is clearly very far from the instructions but... any other ideas ?

[martyn]

Look fantastic especially for a first try!

You mentioned sitting the stove on a stone floor ... what about insulation?

[bombadyl]

Thanks it is encouraging !
I was not really planning on insulating the firebox, only the 5min riser would be insulated with ceramic wool inside and outside. And maybe also bonding some wool on the outside faces of the cast.
From your comment I can feel it is probably a mistake to have the bottom face directly in contact with the floor...
I could add some wool also underneath but I was counting on fresh mortar to level the firebox during installation and compensate for both floor and cast not being flat.
Should I use clay or sand instead?

[martyn]

You will need insulation of some sort, you need to keep the heat inside the core.
If you just sat it on the floor, the floor will just wick away the heat but even with 50mm of high tech insulation the outside can reach untouchable temperatures.
Also cracking occurs  due to temperature differentials, insulation will avoid spot heating and distribute the heat more evenly.
Rocket stoves of the kind you want, rely on very high temperatures inside the core and require good insulation.
The cheapest is probably vermiculite and cement mix but ideally you would want 50 mm of ceramic fibre board under the fire with an air gap below!
A 100mm Vcret slab would be ok but it takes an age to set and dry.
Hopefully this link will take you to a good diagram showing insulation and temperatures... www.dragonheaters.com/blog/installing-jtube-rocket-heaters-safely/

[pigbuttons]

Again, martyn is giving you great advice about insulation. At the very least you should have 50mm of ceramic fiber, either board ( you could contour it to get your core to sit flat on the floor ), or ceramic fiber blanket/pad, or super wool, of 50mm. You could put a sand base under the insulation to make it a little easier to level. I would avoid clay as it can be unpredictable as far as settling and holding moisture unless it is properly dried.

That casting looks really good. Yea, the curing thing is crazy complicated but understand they are thinking industrial kiln or boiler installations that will run at 1700C (3000f) continuously for months at a time. Your plan should do just fine, just make sure to burn small fires until all of the steam is out before you start to stress test it with a full fire.

You are doing great for a first try.

[bombadyl]

Thank you both so much for the great advice !

And thank you Martin for the link which gives a good idea of the phenomena...

I was clearly underestimating the temperatures on outer faces of the batchbox.

I have been looking at insulating materials I can easily get my hands on.

Still did not get any luck with ceramic fiber boards but I found some Calcium Silicate boards which seems to have good enough properties as long as they are not in direct contact with flames:

- Max temp around 1000°C

- Lightweight : 0.22 density

- Good mechanical properties : used to build stove supports and housings.

- Low thermal conductivity : 0.1 W/(m.K) @ 600°C

- good thermal stability

There are several manufacturers such as Promat and Skamol. Any experience with this material ?

I was thinking:

- 2 x 30mm thick boards between the batchbox and the floor (with small air gap because the floor is not flat)

- 1 x 30mm on the sides and at the back.

 

[bombadyl]

Oh and one more question : how do you assemble the 2 halves of the batchbox together ?
I did not understand whether it should be a hard joint with refractory cement (same mix ?) , or a soft join with some sort of refractory caulk/sealant to allow thermal expansion and avoid cracking.

[lsch]

Hello,
I follow with interest the evolution of your project and I see that you are progressing well.
I add my grain of salt.
in my opinion, the insulation is only necessary under the box since it is included in the bell.
a too high temperature in the stall leads to an accelerated degassing of the wood and an overload of the unburned (your reference model is the 3 peter barrels and it is not insulated I think).
For the assembly, I will go on the side of the batch block so angles that enclose the three pieces. The ash will fill the gaps.
It will allow you to easily fix your door moreover...

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[pigbuttons]

I've never tried to join two large pieces together like you are doing. Mine always have a steel frame to keep everything straight and well supported. But I do know that the same mix that you used to cast your box is very good at adhering to itself, so that would be my advice. I would tend to agree with Isch on not needing side wall insulation if your stove is completely inside the barrel.

[bombadyl]

Ok, no insulation on the sides, only at the bottom.

The 2nd half is now ready ! It looks even better than the 1st one thanks to the lessons learnt (more duct tape at the bottom of the mould, pre position some of the upper foam blocks and silicon joint everywhere, side reinforcement of the walls to keep them rigid enough).

This 2nd half therefore has a flat bottom side. But as the 1st one does not, they do not fit very well together on a flat surface.

So I put them upside down and added some refractory mortar to level them. I added some fillers (white rectangles on pictures) to keep 2 location without the extra thickness for a potential metallic frame.

Now it sits perfectly flat and looks good to go to the next phase : slowly heating !

Actually I was wondering whether I could keep the steel clamp you see on pictures in the final installation... It only costs 4€ and if I remove the wood handle, it is pretty thick steel. Would the pressure be an issue when heating ? More than a steel frame that would also expand more than the concrete ?

[pigbuttons]

What an awesome outcome, especially as a first pass. It looks very professional.

I wouldn't tempt fate by using the clamp because it is a very small pressure point and the rest of the casting is free to flex. I know there is supposed to be only tiny expansion of the refractory but I'd be more concerned with the pressure on such a small area while the curing process is occurring. Point loads are seldom a good idea except in destructive processes like drilling, cutting, breaking. A thin sheet metal box would be sufficient and would allow for holding everything in place without creating a pressure point. Even a wooden box would work as a start and if it began to smoke you'd know about how hot the outside of casting is getting. Once the halves are joined with refractory or silicone whichever, then a thin steel band top and bottom would probably do.

[lsch]

Hello,
I agree with the answer of pigbuttons (which corresponds with what I had already proposed to you at the beginning).
try to find a 2 in 1 solution that assembles your 3 pieces and allows you to fix the door.
for example 4 strapped longitudinal angles and maybe 4 others to form a frame at the front weld or bolt together and on the first ones.
on the other hand, I would not glue them together so that they can "live" without too many constraints.


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[martyn]

The two half's need to room to move independently, you will be surprised just how much they will expand and contract.

I dont know of any flexible glues that work beyond 300c, some of the non flexible fire cements will seal joints but will not allow for any movement at all.

You have to remember that steel will expand at a different rate than cement and any form of rigid fixing will encourage cracking.

Wire can be used to bind the half's together as it will offer some movement or a steel frame over insulation as the insulation will absorb the expansion.

Perhaps ask isch for more details of how he used a steel frame that allows expansion without insulation?

[lsch]

Hello,
I have seen the martyn's answer and so am I.
I have no experience of assembling castings (I assemble dry bricks) but after reading the forum, the different solutions used and the feedback, I think that it is necessary to keep the pieces in place but not to tighten them (like the bricks).
the new "uzume" models are made on the same principle as yours so maybe take inspiration from their methods.....

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