cans as heat riser tubes?

[mram50]

I was wondering since cans are so easy to come by are they OK to use for riser tubes in a pinch for making a short term stove? I have a LOT of sheet metal being i sell scrap metal for a living, but i also have a few gazillion coffee cans hanging about. I have one large sheet from the top of an old stove with the porcelain coating (darn tough to cut with tin snips, but not impossible) but the hardest part is bending it to make tubes. I have a big U shape now, but...y'know. Are can's OK to mig weld together till they eventually burn up?

[Donkey]

I've built the little tin can camp stove rockets.. They work great, can't say how long they last. Longer than the camping trip.

Try it and see what ya get.

[hpmer]

They don't last very long. I built one using a stove pipe "T" with a juice can for the riser to boil down maple sap into syrup over the course of three weekends. The stove lasted about 30 hours before the metal crumbled into powder (including the stove pipe). Probably would have lasted a bit longer but I poked it thinking it was just ash build up on the inside of the can, but it was actually the perlite insulation that was coming through. Poking it hastened the process and now I'm going to rebuild it.

I'll probably try face bricks as the guts of the system to see if that improves its longevity.

So, long answer short: yes it does work, but only for a while.

[hpmer]

Donkey,

After considering how I might rebuild the thing with brick, it occured to me that I could just use another stove pipe "T" as a mold around which I wrap some sort of cob mixture. That way when the metal pipe flakes away I'd still be left with the masonary structure.

Does this sound plausible? If so, since I've never worked with cob before, do you have a suggestion for an appropriate mixture that could withstand the burn tunnel/heat riser issues?

[Donkey]

You can do that..

It would be almost impossible for me to give you a mix appropriate for your soil(s) without seeing them first.Seeing as how you plan to put it right where the heat is, your mix should have no straw, just sand and clay rich soil.
Yer gonna have to make some tests. Starting with pure, clay rich soil, make test pucks or bricks of similar size. Progressively add sand, making pucks with different mixes, labeled well, till you KNOW you've got too much in. Set 'em out to dry. The ones that crack have too little sand, the crumbly ones too much.. Somewhere in between is yer best mix. For fire cob you want stuff that is quite sandy without being too crumbly, sometimes it's a bit of a compromise, depends on your soil(s)..
You should test fire your best candidate to see how it handles heat before you try to build something permanent..

[hpmer]

How about refractory cement? Would that be a simpler option?

[Donkey]

Refractory will work fine. It's more expensive but there's something to be said for the ease of just-add-water...

[hpmer]

O.k., thanks. So where does one find it, a Home Depot type store? And what exactly is it called that I will be looking for, refractory cement or is it known by some other name in the trade?

[Donkey]

Err... "Refractory cement" might work. You'll probably need to go to a pottery place and get the stuff used for making kilns. I doubt Home Depot will have it.

There is a home-made refractory mix, layed out on the Backyard Metal Casting site..

[hpmer]

Perfect. Thanks.

[peterberg]

About refractories:
Portland cement contains a lot of silicide, sand and gravel normally used for filler of concrete do as well. Exact numbers I don't have at hand, but silicide crystals change shape at a temperature of about 760 F. And, more importantly, expand during that process. That's why a concrete building can be destroyed by fire.

Refractory cement on the other hand, consist of aluminum cement. The filling in refractory castable is made of ground porcelain or firebrick, depending on the temperatures it can withstand. The kind I am using is designed for maximum temperatures of 2190 Fahrenheit. It is a relatively light material with a specific weight of 1.9 instead of the 2.5 to 3.0 for normal concrete. As such, it is a medium insulating concrete. In the Netherlands it is only obtainable in quantities of no less than 1.1 US tons.

To obtain the best results, the wet material has to be vibrated to get rid of most of the enclosed air. A vibrating needle is not suitable, the hole left when taking out the needle rapidly fills up with cement water without the filling. A vibrating table is much better, that's what I am using for this purpose. Within 8 hours the concrete is getting solid and quite hot as well.

Aluminum cement has a very rapidly binding process. So much so, that it can reach 80% of maximum strength in 24 hours instead of several weeks. On the other hand, there is the heat curing process during which the temperature rise has to be relatively slow. Otherwise the enclosed water can form steam inside the material with a weaker concrete as the undesirable result.

In short, making refractories with sand and portland cement is of no use at all. Clay and maybe other ingredients could help to some extent. At best you will end up with a material that has a limited temperature resistance. Certainly not the 1830 F which is easily reachable in whatever ceramic firebox.

[hpmer]

So you are suggeting the link given above is far less than ideal.

Then how about some sort of mixture used for kilns? I'm assuming it is mostly firebrick? Would that work, and if so, have you a reasonable recipe?

[peterberg]

It's advisable to avoid portland cement and use aluminum cement instead. Sand doesn't belong in refractories either. Fireclay is alright, perlite or vermiculite is usable at best. A possible ingredient nobody mentioned here is expanded clay pellets. Sometimes known as Liapor, Hydroton, Leca and names like that. This aggregate is produced in high-temp kilns at 2700 F.
Grinding half of the filling to smaller particles combined with aluminum cement would make a very workable mixture.

Much better is the ready-made mixture of refractory concrete, of course. Artists sometimes use it for casting statues, because of the rapid strength development. Professional heater masons do make castings in almost every stove. Checking whether one of them is willing to sell you a bag of, say, 50 pounds would sound logical to me.

[johnf]

peterburg: great info on refractories. I'd seen info on the nature of refractories before but nice to see it concisely laid out as we here need it.

I would like to experiment with making things like this.
What it seems we need it something durable and high-temp for the lining of the burn tunnel/vertical combustion chimney (whether you happen to be using the "J" or the "L" sort). A thin lining for durability, but then filling in the body volume outside this liner with much lighter insulative mixture like perlite/clay, and then something more durable but standard concrete/cob like for the outer body.

It would be nice to make something as simple as the mud stoves you see with the banana stem mold parts, but which don't fall apart as quickly as the ant-hill dirt/sawdust mix.
Maybe possible to make the entire body and maybe even the heat-sink mass bench of such mixes, without stove pipe or fire-brick? Just a metal top of the vertical burn tunnel, like the Justa stove.