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posted 9 years ago

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This is my inner form for my RMH combustion chamber and riser. I will be casting with a dense/packed mix of sodium silicate and sand. Is 4" of casting enough to insulate the riser to get complete combustion? It's a 6" system.

I built this out of scrap wood because it's going to be burned out when casting is done.

Any comments about design are definitely welcomed. The horizontal run is slanted to create an instant draft when getting it started.

Thanks in advance!
Brant

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Glenn Herbert

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posted 9 years ago

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Why just sodium silicate and sand? Sand may not be very conductive, but it is not an especially good insulator, and with a binder, the tiny air spaces that make sand insulative at all will be reduced or eliminated. You need some actual insulating component to the mix, like perlite. Also, I haven't investigated the fired strength or stability of sodium silicate plus sand, but unless it is a particularly stable sand it is not likely to hold up well to repeated heating and cooling without shrinkage and cracking.

You need a heat-resistant material for the inner core of the combustion zone, like refractory cememt or a fireclay mixture, with a good insulating layer around it. 4" of an actual insulator would be plenty, 2" might be enough.

Glenn Herbert

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posted 9 years ago

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Also, the combustion zone is recommended to maintain a consistent cross section. Making the burn tunnel expand significantly as you show may be bad for combustion. If you want to slope the burn tunnei in an effort to get instant draft, slope the whole thing, top and bottom, equally.

Brant Foster

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posted 9 years ago

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Sodium silicate and sand is frequently used in foundries as crucibles to melt cast iron and aluminum for casting. Holds up very well to extreme heat.

Once the brick dome is placed over my core, some of the heat should be reflecting back towards the stack. Can't hurt the lesser insulating sand and sodium silicate.

One thing I've noticed all over you tube is people using perlite tend to crush it when packing a mold. It loses its effectiveness when packed.

Just finished my exhaust port and the outer box to the mold. Going to finish the stack extension tonight after dinner.

I have another idea for the combustion chamber that I can do later. I'll add a "speed bump" out of half round sodium silicate along the bottom. It'll maintain the same cross sectional area while allowing the fast ignition.

Thanks for the input!

Glenn Herbert

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Location: Upstate NY, zone 5

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posted 9 years ago

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If that is a tried-and-true strong heat-resistant mix, fine. But you only want enough thickness of that to be durable. Outside of the strong liner, you want some truly insulating material. A "brick dome", whatever you mean by that, is not going to do the job you need. How are you thinking of the dome configuration, and what is its purpose? Is it in place of the typical 55 gallon barrel in an RMH? The riser needs insulation to prevent the space around it from being as hot as it is inside; this is one of the "pumps" that drives the draft. And unless the space around the riser is also freaky hot, you would be losing heat in the riser space where final combustion is supposed to happen.

By the way, perlite can be packed firmly and still be a very effectve insulator. Of course it can be crushed and rendered ineffective, but that will not happen with proper care. I would be interested in seeing one of the videos where you believe perlite is being crushed, as that would be a good one to point to and say "don't do this!"
Vermiculite in clay can be crushed pretty easily, and may be more readily rendered ineffective.

Brant Foster

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posted 9 years ago

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What if... I substitute perlite or vermiculite for the sand and poured the form without packing the mold. Maybe only using a concrete vibratory to settle it in gently. Just aggregate and sodium silicate binder? It would be much lighter and more insulated. As is, my forms will hold just over 10 cubic feet of sand and will weigh over 300 lbs.

Due to the higher expansion of perlite when heated, won't this cause cracking with the expansion and contraction over time?

Glenn Herbert

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Location: Upstate NY, zone 5

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posted 9 years ago

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Perlite and sodium silicate alone might work, but I have not heard of anybody doing that and reporting on the results. I would experiment with a small sample of it and see how it holds up to heat. I would also research homemade refractory cement, to see what other components are commonly included. For low-abrasion areas like the heat riser, a mixture of perlite with just enough fireclay to stick it together works well, and you don't have to crush it to get it packed. You do want it packed or vibrated enough to eliminate voids. Casting the riser inside a metal outer form like ductwork will give a durable wrapper that will allow easy handling.

Making the casting 4" thick is overkill, and may exacerbate differential heat stresses. I think you would be better with a casting 1 1/2" to 2" thick of a relatively dense and sturdy mix, surrounded by 2-4" of very light insulating material like perlite-clay. If your crucible mix is as strong as I think, you could probably make it 1 1/4" thick, with the riser cast separately for ease of moving and reducing the likelihood of uncontrolled cracking. You might need to fire the feed tube and burn tunnel assembly in a kiln to get it all to full strength, as parts of the feed area will never get significantly hot from normal operation.

allen lumley

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posted 9 years ago

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Brant Foster wrote:This is my inner form for my RMH combustion chamber and riser. I will be casting with a dense/packed mix of
sodium silicate and sand. Is 4" of casting enough to insulate the riser to get complete combustion? It's a 6" system.

I built this out of scrap wood because it's going to be burned out when casting is done.
Any comments about design are definitely welcomed. The horizontal run is slanted to create an instant draft when getting it started.

Thanks in advance!
Brant[/quote

Glen Herbert said :

The riser needs insulation to prevent the space around it from being as hot as it is inside; this is one of the "pumps" that drives the draft.
And unless the space around the riser is also freaky hot, you would be losing heat in the riser space where final combustion is supposed to happen.

- - - I just wanted to expand on the last part of Glens comment !

A lot of people in the past have used just Fire clay and Perlite/Vermiculite and they have had mixed results with durability* adding Sand to that mix
will reduce the Insulating qualities but will add strength- producing a lightweight fire brick like material with much greater durability.
Certainly adding Sodium Silicate or the less corrosive Potassium Silicate will add strength.

With the Durability of the inside wall of the Heat Riser taken care of. We still need further protection to assure that there will be a minimum amount of
Heat Energy Transference Through the inner wall of the Heat Riser !

Failure to isolate these Two streams from each other can result in a Balancing or Equilibrium of the temperatures of the Two Streams -Causing
The "Pumping Action'' which promotes the 30'+ Horizontal flow of our hot exhaust gasses through the Thermal Mass to STALL !

This will only be made worse ( I Think ) by your use of a Brick Dome which will retard the Transfer of Heat Energy Through itself - The rapid cooling
of the Hot Exhaust Gasses by Radiation out through the ductile steel of the 55 gal drum is part of that '' Heat Pump !

The Need for insulation to surround the formed Heat Riser can be as simple as a 2'' layer of Mineral wool (rulex )wrapped around the outside of the
Heat Riser !

This is definitely the old-school and cheapest way to go !

Hope this helps and is timely, for the good of the Craft ! Big AL

* There is much discussion about the use of materials like light insulating fire brick in the Feed Tube, Burn Tunnel and Heat Riser, which promotes
rapid heating, and the idea of using a Heavier, Denser, "Kiln Brick with its increased durability and ability to both refract most of the Heat Energy
back into the Combustion Zone while stabilizing the higher temps and resisting Heat Energy Transference !

This usually boils down to what you can afford and what you can get your hands on ! A.L.

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