castable refractory questions

[dave]

I am putting together ideas for a 5" system and want to cast the entire thing, but am unsure on the refractory. Can you just use high temp castable refractory? Does it need a steel liner on the burn side of the tunnel and riser, or will exposed refractory be OK?

I found this "recipe" for a crucible furnace, does it sound ok, or any other suggestions?

www.backyardmetalcasting.com/refractories.html

Here is the recipe:
The refractory mix is composed of Portland cement (1.5 parts), silica sand (2 parts), perlite (1.5 parts) and fireclay (2 parts). The first three components are mixed together thoroughly.

Or, should I just use refractory cement mixed with perlite to lighten/insulate it?

[dave]

Another question, if this system is entirely cast, is there a minimum thickness? I planned to use 5 gallon steel pails like paint thinner or something comes in for the riser. This would be about 2.5" wall thickness with a 5" round riser. For the tunnel, I planned to make it 5" wide x a little under 4" high rectangle. This is close to the 19 cu/in to match the 5" round riser.

Also, it sounds like this cement is gooey and won't pour, so I will have to plan my assembly of this to fill the outside steel form?

[Donkey]

Seems to me that if it's good enough for a metal casting furnace, it's good enough for a wood stove. Either of your ideas might work just fine. I would be looking for a more insulative mix, even though it will be fragile around the wood box area. Perhaps you could cast the wood feed out of the heavier, heartier stuff and the burn tunnel and heat riser using the insulative mix.


I don't really know about minimum thickness in your castings. peterberg's opinion would be valuable here.

I've found that the intuition can get it roughly right more often than not. If you don't get a decent answer from someone, just make your best guess and go for it.

[peterberg]

Minimum thickness would be 1 1/4 ", this will heat up very quickly.

My opinion about Portland cement in refractory: it won't hold. Normal cement and sand contains silicium. This is changing christal structure at about 1000 F and expands at the same time.
I've been using proper refractory castable which contains aluminum cement and ground porcelain. Specification: 2190 Fahrenheit and it's still intact. Instead of elephant grey it's color has been changed to vaguely pink. Tapping it, it do sound fragile.

[carllittle]

It appears great minds.... Yesterday I cast a burn tunnel and the beginning of a heat riser using the refractory mix Dave mentions. My idea was to have one piece that would serve as the burn tunnel and the base that would hold a stainless 8" heat riser. My plan is to build Peterburg's initial rocket/bell heater. I have a section of double walled stainless stove pipe left over from when I built my house that seems perfect to build a heat riser. Here is a link to a couple of pictures.
picasaweb.google.com/Carl.Little.law/Desktop?feat=directlink
I'm not sure of this cast will work, especially after Peterburg's warning about Portland in refractory. ( I wish I would have seen it before i did this.) If I were to do it again, I wouldn't. I think the few extra bucks for firebrick would be worth it. This thing weighs probably 200 lbs and I greatly underestimated the amount of fireclay that I would need. Therefore, the top of the firewall is just at 1 1/4 inches, but I imagine it will need more insulation. The reason that it is the shape and size it is, is to fit just inside the brick tower that will act as the bell. It will also sit on concrete fireplace foundation, so I wanted plenty of refractory between the bottom and the concrete.

[canyon]

Wow carlittle that is huge looking! Perhaps you can shell out the money for some proper refractory cement and plaster your burn tunnel with and inch or so to take the high temp shock (or precast some that slide in)? Might even need an expansion joint or two? I don't know, just ideas.

[dave]

Well after researching the weight and cost (incl shipping) of refractory cement, I'm wondering if it is worth it considering I want this to be somewhat moveable, plus the expense.

If I used heavier walled steel tubing for the burn tunnel and riser, with an outer sheet steel shell, could I just fill it up with perlite? I understand that it greatly expands when heated above a certain temp, but does it stay in that form? Perhaps build the thing, fill it with perlite with open tops, fire it, then cover the tops with steel again?

[peterberg]

As far as I am aware of, perlite doesn't expand during heating up.

Silicon (Si) does, and because of that it isn't suited for refractory materials.

[johnf]

I am greatly interested in hearing how this works out.
I've seen refractory recipes, but they all seem to require a high temperature kiln firing.
Sculpting the entire innards of the stove seems like a good thing, but all the mud stoves I've seen are fragile and vulnerable to water and cracking. (the banana stem form and such)
Could something like a two-part cast work: the inner lining out of more durable cement mix, but not thick.
Surround that with the mostly perlite insulative mix.
And then finish with a coating of cement to make the outside durable.

I'll go off and do a lot more reading on such things.

(first add)

probably too much info
Heat loss from stoves:
Thermal properties of insulative bricks
www.bioenergylists.org/stovesdoc/Andreatta/Heatloss.htm

(another add)
I'm getting mixed results, digging around.
There's this cast rocket stove : 1 part cement, 5 parts vermiculite, non-fired whole stove body:
Rocket stove en vermex (vermiculite expansee)
www.youtube.com/watch?v=28FtcxsKkzQ

and this, with 1000 degree fired bricks of sawdust/clay, and a poured fill of vermiculite/cement.
How to build a Rocket Stove
www.youtube.com/watch?v=aLh7FVla6RU