Portable Oven Designs

[Wiley]

Re: Portable Oven Designs

cvdukes,
Actually I got the dome as part of a trade from our local scrap metal guy. He has a contract to cut up old propane spheres and tanks for recycle. He had the half dome and another full sphere one time when I was in dropping stuff off and they looked cool and we worked out a trade. The half dome is only 40" in diameter the other is much larger closer to 48".

As for thermal expansion I was thinking along the lines of a product our local quarry sells called "1/4 minus" for covering the dome itself. It's basically finely crushed basalt, they use it for top dressing on walkways and drives. It packs down quite well and I figured I could mix a bit of Fundu ciment as binder and basically "dry pack" the thermal mass layer. Perhaps creating deliberate score lines for "controlled cracking". I read where it is recommended to cover the bricks in conventional brick ovens with aluminum foil to allow movement. I was thinking doing this over the steel hemisphere. That it gets bigger and smaller shouldn't be a problem if that movement doesn't effect the weather resistant outer skin. I was even thinking at one point of simply cutting the larger sphere in two and using it over the other sphere with 3" of basalt pack and 1" of Frax. From what you'all are indicating that may have enough thermal mass for pizza and perhaps some bread too? Perhaps I could save on the cost of Frax and simply fill the extra space with dry vermiculite poured in thru a hole in the top?

I came across a company called "Pivot" located in Australia that makes ovens that are basically two hemispheres (one inside the other with a couple inches spacing) and which they fill that space with sand. Seems to me that without insulation it would cool down fast and when hot be pretty dangerous as in "Don't lean on that!!!"

Thanks one and all for your input. I'm not totally locked into using the hemisphere this way. I was even thinking one could use it as a "plug" in the manner some boats are built. Cast the refractory dome in sections over the top of the steel hemisphere and then remove and reassemble the cast pieces. Then there would be the chance of spalling, right? anybody cast their dome?

thanks,
Wiley

[Wiley]

Re: Portable Oven Designs

cvdukes,
Thanks for the links on your postings on ferro cement construction. What threw me was the term ferro concrete. I'm familiar with ferro cement at least in regards to boat construction. It was a popular method in the 70s and 80s for DIY backyard boat construction. A good medium for freeform consruction.

I'm curous, you mention glass filaments in the mortar mix. Around here they have redimix with filaments in the concrete but I was under the impression that they were some sort of high tech plastic. Glass would make more sense considering the temperatures involved. Where do you get glass filaments for addition to concrete?
Thanks,
Wiley

[dmun]

Re: Portable Oven Designs

I have a 44" dia. 1/4 thick sphere that I initialy was going to use as an oven until I happened upon this site.
Still open to the steel sphere if anyone has any good ideas.

I've long thought about building an oven upside down, so you could easily and properly finish the interior. The problem is that is the weight and logistics of turning the oven over when it's done. The thick steel hemisphere makes this easy, you just roll it over. Of course you'd want to set it in a sandbox while you were working so it wouldn't roll about while you were working. You could even tilt it to get a better angle at the place you were working on.

The simple answer to the expansion problem is that after you've coated the interior with castable refractory, you lay down squares or strips of insulating blanket, to give it an expansion layer, as you build your dome up from the peak.

[cvdukes]

Re: Portable Oven Designs

Craig,

any reason not to consider some rebar in the ferro mix (is there a negative, beyond weight)?

...and how do you avoid voids when laying it up? You're basically packing mortar into a mash of wires right?

and again, how thick were your dome and liner layers?

Thanks

I've never included rebar just because its harder to work with and hasn't been needed for anything I've built. Of course if I were building a really huge structure (say a 55 foot long ketch that I saw built down in Charleston harbor)... I would definitely want the rebar to give the shape to the structure because that large of frame built with just reinforcing wire (or my usual cheap hogwire) would warp before the cement mix was applied. If you have a 1/2 thick peice of rebar stuck in the middle of your chicken wire mesh layers, then you'll end up having to use several more layers. Even with just a relatively thin peice of reinforcing wire or hog wire in the middle of the mesh layers, you end up with a packet of wire mesh about 3/4 inch thick. By the
time the mortar is packed in, you're getting up around 1 1/4 inch thick. Putting rebar in would take you to 2 inches or thicker.

The hard part of ferro-concrete comes in that you basically end up packing the mortar in by hand. Filling voids just depends on backing up other side of the mesh so you can pack against. Easy enough if the structure is small enough to get your hands on both sides of it (or if you can get someone on the other side like stuffing my son up in the monster chimena). 3 layers of 1 inch chicken wire is about the minimum that you can easily pack... and its easier with more layers becasue there is more for the mortar to adhere to.

I'll address your last question on thicknesses, etc. back on my original thread on the pizza oven as I see you had a couple others there.

[cvdukes]

Re: Portable Oven Designs

Wiley,
After seeing your and Mine's posts on the steel half-domes, I was wondering where ya'll are doing your shopping...thanks for clearing that up.

I don't know about this basalt you talk about (not a southern thing) but shucks, if I had one half dome 40 inches and one that was 48, I would sure try something with 'em, 'cause it don't sound like you got much to lose.

I would take the 48 incher and lay it with the open bottom side facing up (the sandbox idea seems good to me). Then take a piece of styrofoam about 4 inch thick and cut it out to shape of archway the width of a door into this thing. Glue the foam inside the bigger dome with the arch pointing down. Then slip the smaller one in, also with the open bottom side facing up. Figure out to make two bottoms even with each other...Maybe drill a few holes around the circumference of the smaller one so you can stick a few stiff wires to suspend in place. Pour a regular cheap 5000 psi concrete into the open space (use a finishing sander on the outside dome to vibrate it in place)...give it a fw days to set up good then cut out the metal of both domes where the foam arch is. Buy a case of beer and get your friends to help roll and lift it into place.
Insulate on the outside of the outside dome. If you wanted to cut down on the thermal mass a little bit, figure out how to insulate the inside of the bigger dome before you put them together... you want to keep the thermal mass next to the inside dome.

Just posted more info on the fibeglass concrete at http://www.fornobravo.com/forum/7/ex...html#post28234

[david s]

Re: Portable Oven Designs

For my mobile oven I cast the dome in one piece which eliminates brick joints and weakness where cracking is likely to occur. I was trying to keep the weight down and made the outershell ferrocement shell 10mm thick. If you want ferrocement stronger you use more mesh rather than more cement. I've also been experimenting with fibre and would chuck some of that in too if I were doing it again. I think the big problem with a mobile oven is the transportation. Road corrugations and speed bumps are not kind to dense,dry brittle material. Oven holding up ok so far.I kept the weight of mine down to 160 Kgs.

[david s]

Re: Portable Oven Designs

Wiley,
The fibre to use for reinforcing is fibreglass, but it has to be AR (alkaline resistant) type. The other fibre you refer to is probably the polypropolene fibre, which is quite short and fine and does not provide a lot of strength. It was originally used to fireproof concrete. When the temp hits 165 C the stuff melts and leaves tiny holes that steam pressure can escape through, leaving a fire damaged concrete wall or floor intact. Castable refractories use it in their products for the same purpose. I think we can all benefit from using it too to help when curing our ovens although the outer shell is unlikely to get that hot. Better to use it in the oven walls and the AR fibre in the outer shell. I believe this to be a vital improvement utilizing modern materials.

[Groves]

Re: Portable Oven Designs

Wow, how did you keep it to 160kg?

[david s]

Re: Portable Oven Designs

The oven interior is 540mm diam and I made the inner dome thinner (50 mm) but compensated by increasing insulation. Also made the outer shell as thin as possible (10mm) and the supporting slab used Hebel (airated reinforced concrete). The flue is a sliding fit, into an outer s. steel sleeve, for transportation.

[Robert Collins]

Re: Portable Oven Designs

Ive started buildilg out a duel axel trailer to put a brick oven on. i have built the steal frame out of 1/4 inch angle. like the one pictured in this web site. I was going pore my
5' x6' X2and a half inchslab and set my refractory bricks in the cement.then build up the oven with brick just like in the FB plans. then Wrap the dome in ceramic blancket and coverit with chucken wire then tucco, See or experanced any problems with this?

[dcobble]

Lighter weight cooking floor

Can anyone suggest a lighter weight cooking surface than 8lb bricks? Ferrocement is probably not foodsafe right?

[Robert Collins]

Re: Portable Oven Designs

O k. I've built my double axel trailer out now im going to pour my 4 inch structural slab that is 50"x60". and I think Ive decided on refractory cement renforced with fiberglass and some rebar. and while it is still wet set my fire brick into the slab 1/4 th inch.Ive also concidered cutting 1/4th inch groves into the bottom of the brick for more adhearance. any thoughts?

[Wiley]

Re: Portable Oven Designs

Robert,
I like the idea of making a physical join between the bricks and the support by means of the saw cuts. The only downside I can see is replacing a brick might prove problematic should that need arise.

When you say refractory cement are you referring to a heat sink type material (calcium aluminate based) or some sort of castable insulating material? If the former, then what is your plan for insulating beneath the firebricks and slab?

Bests,
Wiley

[dmun]

Re: Portable Oven Designs

Yeah, don't forget insulation, it's not possible to retrofit your under-floor-insulation after the dome is built.

[david s]

Re: Portable Oven Designs

A castable refractory is not as strong as normal concrete. You should make your structural slab from regular reinforced concrete, then on top of that the insulation layer, then the firebrick floor.If the structural slab is separated in this way it will not be affected by any excessive heat so it doesn't need to be refractory. To save weight you might consider using Hebel slabs (aerated concrete) which will act as both structural and an insulator. It can be purchased with steel reinforcing already cast into it, but you will still need a steel frame with bracing underneath.