Plans for a 70 cm/28 inch hombrew oven

[spoton]

Hi,

Have been reading this forum for some time, and time to put the theory into practice.

Planning to build a 70 cm ID oven using the homebrew mix.

A few questions that I got so far:

1. Size of the base. Want to make it compact on the sides, so I guess to leave room for around 8 cm of wall thickness + some perlcrete insulation on each side I shouldn't go below 110 cm width.
Lenghtwise I want to leave room for the flue gallery + some open space, so around 120cm?

2. I that see some are people casting their flue gallery separately from the sphere. Are there any pros/cons of doing that?

3. Materials:
- Fireclay is an undefined word here. But I can get kaolin/china-clay. Having done a search I see that others has used it with success. But open for any comments.
- As for reinforcement I see that the main concrete supplier here has both SS-needles and basalt fibers in stock. They claim the latter has better refractory properties. They also look a bit finer and maybe easier to use than SS-needles. Has anyone tried it?

Thanks in advance for any advice

[david s]

1. Draw a plan, starting with internal diameter. 8cm is very thick for a small over. The usual inner dome cast thickness is 5cm. Any thinner starts to reduce strength too much.
Plan depth of flue gallery (depth needs to be big enough to accomodate flue pipe fitment and gather smoke), but shallow makes working the oven easier.

2. Casting the flue gallery separately (but in situ), cast domes first then cast gallery in front of it, allows some relief from a difference in thermal expansion between the hotter dome and the cooler gallery. Look at the build threads on cast domes in the Other Oven Types Section.
Creating the door rebate in a combined, one piece dome/gallery is difficult.

3. Kaolin or China Clay is more refractory, but expensive and not required for our service temperatures. Bricklayers Clay from building or concrete suppliers is a better choice.
Basalt fibres (at least the ones I have) comprise a range of length and thickness and the finer fibres do not disperse well. They tend to clump, as all really fine fibres do, which means much more extended mixing. I prefer AR fibres which have a consistent 0.5 x 19mm size and disperse far more readily with similar strength and cost characteristics.
SS needles make placement more difficult are expensive and small quantities difficult to obtain.
I also use nano reinforcing products but also expensive and hard to obtain small quantities. Do not use steel rebar because it’s too thick to withstand the thermal conductivity differences. Chicken wire is another alternative, but hard to apply over a compound curve. It can be done with smaller overlapping pieces, but must be embedded in the middle of the casting which is way harder than random mixed fibres.

It is well worth the trouble of deep planning and research before forging ahead.

[spoton]

Thanks for the good feedback David.

Have started on the foundation and drafted some plans:

So far sourced SS Needles, PP-fibers, sand, portland cement & clay which I think will be Kaolin (Found a quite cheap supplier selling 5 & 1 kg bags).
Only thing I'm struggling with is hydraulic lime. I have read others using hydrated lime though, so might go for that.

I think I am quite happy with the calculations I have done for the dome. Might add a ceramic blanket if I can get that for a decent price.

Regarding the flue gallery I'm not quite done yet, but so far I'm thinking casting it separately, attaching it to the outher edges of the dome using perlcrete or something similar. Thinking 15cm flue, but will need to read a bit more deeply as I read you mentioned using caution because of expansion of the metal.

Uploading my draft so far. Considering my paint skills I don't blame anyone for not understanding it

[UtahBeehiver]

The dimensions are a little hard to read due to the photo resolution. The Pcrete seems a little light in thickness, recommend a minimum 5 cm thickness of 8 - 10 to 1 ratio. Pcrete has a K value (insulation value) about 50% of ceramic blanket. NickJC from the UK did a really nice cast oven so you might want to look at his build thread. Attached is a table show Pcrete K values. Note the different recommended ratios for the floor vs the dome insulation.

[spoton]

Thanks. Probably unclear from the pic, but I'm thinking a 10 cm pcrete layer.

I'll try to reupload

[UtahBeehiver]

Pictures are better and everything looks good. 8-10 ratio on dome, 5-1 ration on floor. Need to protect the weep holes from getting filled up during the pcrete floor pour

[david s]

[QUOTE=spoton;n466752]Thanks. Probably unclear from the pic, but I'm thinking a 10 cm pcrete layer.

I've found a 35mm layer suits me better. At 100mm the mix takes way longer to dry out. Before the new generation of blanket (exonerated as a carcinogen) became the standard and its price became affordable, I used to use a 10:1 mix for maximum insulation efficiency, but found its very high proportion of water made the drying very slow. (see attached test on drying of a vermicrete slab). My solution was to apply the vermicrete in layers of 35mm thick and allow a week of drying after applying each layer. With the cost of the safer blanket now coming down and its widespread adoption, I now apply a 10:1 vermicrete layer 35mm thick over the blanket. A 10:1 mix results in around the same insulation value as blanket (more cement drastically reduces insulation), 35mm being sufficient to both cover blanket lumps, restore a perfect form as well as providing a sufficiently strong enough substrate for a render/stucco application. A thicker or richer layer is fine, but will have the aforementioned drawbacks.

Vermicrete insulating slab PDF.pdf

[spoton]

[QUOTE=david s;n466767]

Thanks. Probably unclear from the pic, but I'm thinking a 10 cm pcrete layer.

I've found a 35mm layer suits me better. At 100mm the mix takes way longer to dry out. Before the new generation of blanket (exonerated as a carcinogen) became the standard and its price became affordable, I used to use a 10:1 mix for maximum insulation efficiency, but found its very high proportion of water made the drying very slow. (see attached test on drying of a vermicrete slab). My solution was to apply the vermicrete in layers of 35mm thick and allow a week of drying after applying each layer. With the cost of the safer blanket now coming down and its widespread adoption, I now apply a 10:1 vermicrete layer 35mm thick over the blanket. A 10:1 mix results in around the same insulation value as blanket (more cement drastically reduces insulation), 35mm being sufficient to both cover blanket lumps, restore a perfect form as well as providing a sufficiently strong enough substrate for a render/stucco application. A thicker or richer layer is fine, but will have the aforementioned drawbacks.

[ATTACH]n466768[/ATTACH]

Interesting. Will have a look for what I can find of insulating blankets. Soon ready to cast the top slab now. Would you also recommend a similar thickness for the insulating layer there?

[david s]

Blanket is unsuitable as an insulating layer under the oven floor due to the oven weight which will compress the blanket thereby reducing its insulation value. Use a minimum of a layer of calcium silicate insulating board which comes in either 40 or 50 mm thickness. Two layers is far better, three perhaps overkill as there is a rule of diminished returns and the stuff is quite expensive. An alternative is to cast in situ (at least a 10cm thick) vermicrete slab, but use a 5:1 ratio as any leaner mix is not really strong enough and any richer drastically reduces its insulation value. The downside is that while it's far cheaper than the cal sil board it requires extended drying. (see attached slab drying experiment)

Vermicrete insulating slab PDF.pdf

[spoton]

Since the fall is here soon with all its rainy days and I'm only done with creating the top slab, I reckon I need to put up some kind of roof (at least temporary) to be able to finish the rest of the project without it raining away while curing. That got me thinking about the flue/chimney. It would be tempting to go for a rather short chimney (50cm) so that I might avoid having to have it trough the roof. Will that be too short? I read here that for a single walled pipe 18" is the min recommended distance to combustible materials. Any advice on that?