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Post by Calaf on Feb 6, 2012 21:37:45 GMT
Looking at Rich’s steel drum build with the storage heater blocks I’ve decided to redesign the oven floor. My original design used 6-8 of the heater blocks as the back wall but as I have 16 to use I will deploy them as thermal mass in the floor. However, I’m not too keen on food being in contact with them so I will use fresh quarry tiles on top. This reduces my firebrick count by a further 13, although the saving will be more than lost by the additional cost of the quarry tiles. I'm also thinking it should also be advantageous for heat-up times. The thermal mass of a 76mm firebrick oven floor would be 64,326cc. A 55mm storage heater block floor, plus 10mm quarry tiles, would be <43,856cc. I shall hope this is enough to use the oven for bread and meat, noting there will be another 136,694cc in the vault and rear wall. Instead of 15mm ceramic fibre I have also decided to employ 25mm Calcium Silicate board under the blocks and incorporate a CalSil skirt around the hearth. This will be particularly important to isolate the heat in the cooking floor from the landing, nearside of the aperture arch. The cooking floor will be 90cm off the ground, about 20cm below the recommended elbow height. I think this will be fine, I still have some bend in my knees.
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Post by Calaf on Feb 6, 2012 21:38:31 GMT
Reinforced the perlite raft with a buttress wedge of 2:1:1:1 perlite, vermiculite, sand, portland to help prevent crumbling of the edges while working on the oven brickwork. Slapped a bit of rustic render on the base blockwork. Almost pretty now. I think I will give my trusty rusty cement mixer a bit of a rest for now until the enclosure phase.
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Post by Calaf on Feb 6, 2012 21:40:58 GMT
Looks very nice. I like the smaller foot print. The footprint on mine is rather larger than I envisioned. Bill Calaf........never mind pretty.....it's looking really good. Love the diagrams and the stage pics.......bring it on.... Terry Hi Bill, One mag 4 quake and mine will be a pile of rubble. Whereas yours will probably still be standing after WW3.
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Post by Calaf on Feb 6, 2012 21:41:48 GMT
I have ordered the following
70 Firebricks - 76 x 230 x 115mm 2 Calcium Silicate Boards - 1000 x 610 x 25mm 1 Ceramic Fiber Blanket - 4.88m x 38mm @128kg density Loosefill Rockwool - 10Kg
Total inc delivery -- £235
I'm taking a punt on the Rockwool as the density is unstated.
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Post by Calaf on Feb 6, 2012 21:44:04 GMT
A friend watching this thread and considering constructing his own oven emailed me with a question. Why use a fancy Calcium Silicate board for insulation when an aerated concrete building block probably would provide the same level insulation for less money?
So I’ve tried to give my rationale based on my understanding of what we want to achieve here. It may need correcting from experts but here goes.
We aren’t trying to protect the base from the oven heat, rather contain that heat within the oven. To illustrate, even a sufficiently thick wall of copper (a good conductor) could provide the same level of “insulation” (R-value) as CalSil but would achieve that insulation by dissipating the heat throughout its mass, thereby sucking the heat from the oven. This is the key functional distinction between insulation value (measured over thickness) and thermal resistance (measured over a surface).
Here are the typical thermal conductivity (inverse of thermal resistance) values in metric for several insulators.
• Perfect vacuum 0.0 • perlite 0.03 • vermiculite 0.06 • ceramic fibre 0.05 • calcium silicate 0.05 • aerated cement block 0.15
The lower the value of the insulator the more heat will be diffusely reflected back into the oven. So, when combining insulation layers, it’s better to put the material with the lowest conductivity adjacent to the thermal mass.
In reality I doubt that in domestic use a CalSil board is significantly better than celcon blocks as so many other factors will affect performance. But I thought for £40 I get the peace of mind that I’m using a rated refractory material.
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Post by Calaf on Feb 6, 2012 21:47:43 GMT
The insulation values quoted for common-or-garden materials are presumably those that apply at normal atmospheric temperatures. Insulating material in a WFO has to withstand some pretty elevated temperatures. I've seen it suggested that concrete based on Portland cement loses 50% of its strength at 400°C, so over a few years of thermal cycling, you might conceivably get some breaking down of a celcon block's structure. Far safer to use refractory materials. Mark I think Mark's point is valid but not necessarily relevant to a domestic occasional use wfo. Personally, I don't think there are sufficient dynamic mechanical stresses in the base in a typical domestic wfo situation which probably only gets used 20-50 times a year to make me worried. As in most wfo design considerations it is one of the trade-offs, increased (unknown) risk vs cost. I would definitely go for the refractory materials on a commercial build. TonyB Hi Mark, yes, I think my source (a friend teaching in the Department of Materials Science at a local U) said 20'C, and the conductivity increases with temperature for all materials but at similar rates. So, for example, Vermiculite doesn't get better than Perlite at 250'C, which raises the question why Vermiculite is de-rigueur for loosefill in the enclosure. I suspect it might be Vermiculite affords better convection resistence for hot gases squirting through mortar cracks. :roll: I'm using neither in the enclosure and have opted for cheaper Rockwool as I don't see it's critical beyond the ceramic fiber blanket.
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Post by Calaf on Feb 6, 2012 21:48:56 GMT
Received my Firebricks and insulation materials. Was somewhat peeved to find one of the CalSil boards is 23mm thick, the other 25mm. Thinking now I should have stuck with a single ceramic fibre board. I hope this does not cause any issues for construction. The surface of the CalSil is a little friable. I assume it’ll hold up 600Kg of masonry if it is commonly used under ovens. It’s also rather dessicant leaving your hands very dry after handling. The Rockwool was on special offer but still would have been cheaper at Wickes. The ceramic fibre blankie looks good but ob the side of the box there is a warning, "Restricted to professional users". How silly.
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Post by Calaf on Feb 6, 2012 21:50:37 GMT
There's plenty of that nonsense around. The container of strong brick acid I bought at Wickes says "Trade only" on it. I think the forum account counts as "Trade" but I'm sure they'd sell the stuff to anyone anyway. I hired a concrete breaker yesterday - first time I've ever used one - and I noticed that the guy had ticked the box on the paperwork that said "Operator is experienced". I wonder how he knew that, 'cos we never discussed it, and also what the implications of not ticking it would be?
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Post by Calaf on Feb 6, 2012 21:51:14 GMT
Here is the final design for the hearth. The thermal mass of the oven floor will be contained in a CalSil skirt to help reduce thermal leeching, particularly into the landing.
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Post by Calaf on Feb 6, 2012 21:52:14 GMT
Here is a dry lay of the storage heater blocks on the CalSil and measuring up the cuts.
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Post by Calaf on Feb 6, 2012 21:52:45 GMT
When I saw the price of the quarry tiles I thought they must be substitute porcelain but no, they are real. Just shockingly poorly cut so will need straightening with the grinder. About 1/3 look like this 42 x 150mm red clay quarry tiles - £25 – B&Q 2 bags of fireclay and 2 tubs of ready-mix refractory cement - £30 – leftover from the build of a fellow forum member who lives nearby.
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Post by Calaf on Feb 6, 2012 21:54:32 GMT
They don't look right to me Calaf, the bevelled egde seems to disappear with the edge. Normally they're pretty spot on. If it's not too far to go, i'd take the duff ones back, have all the boxes open on the floor of BnQ and hand pick the good ones! ;D Good idea. To be honest, that was the worst one, but I'll pick out the 21 worst, take one box back and check I get gooduns this time. But at 59p each I wasn't expecting posh.
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Post by Calaf on Feb 6, 2012 21:56:23 GMT
Calaf I've used these quarry tiles before (on a building project) and They were exactly the same then. As they were used on a porch entrance I was able to use a flexible tiling grout to joint them. As they are to be used for your finished hearth, then instead of butting them up tight you might have to leave a small joint/gap and fill the void with a mix of fire-clay and fine sand. Last year I posted on the forum that I had a couple of hundred "clay pavers" for sale or barter. Someone from the forum turned up last year and filled their boot with clay pavers and gave me a case of "bow" in payment...both parties were content. Shame you don't live closer to my area as I still have about a hundred of these pavers which would be much better than "new" quarry tiles .....it would still cost you a "four can" of bow. Terry I think I already owe you more than 4 bows.
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Post by Calaf on Feb 6, 2012 21:57:39 GMT
The Calcium Silicate board has been cut to size (easy with a stanley knife) and laid out. I attempted to spread a mortar bed of 2:1:1:1 Sand, Perlite, Vermiculite, Portland on the sloppy side to secure the boards to the Perlicrete raft but they will not adhere, exacerbated by the fact that they curve a bit. The weight of the heater blocks should keep it down. Only the firebricks are to be mortared onto the board with fireclay cement as I think mortaring down the heater blocks is unnecessary so they are laid dry. They are not going anywhere. Here you can see the storage heater blocks laid out and the skirt which will thermally isolate the oven floor from the landing. The skirt is secured merely with decking screws at this point. Setting aside some CalSil for door insulation I didn't have sufficient to complete the skirt at the rear but I think this is the part of the structure that will receive the least thermal stress so it's not an issue. The CF blanket will suffice.The three 55mm firebricks at the front are the base of the aperture arch. It occurred to me that the profiled side of the storage heater bricks will be of some benefit so I’ve arranged the blocks to create 4 air pockets facing the CalSil board to augment insulation a bit. I had to resist the temptation to embed thermocouples in the hearth. I am such a nerd.
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Post by Calaf on Feb 6, 2012 21:59:06 GMT
Calaf I love the special touches you are making to your individual bespoke Wood-fired oven. Normally 4" of either vermiculie/perlite concrete or 4" thermolite blocks are used, but your addition of the cal-sil over the perlite will stop ANY dissipation of heat from your cooking hearth. Great build....looking forward to the rest. Terry I think by "special touches" you mean over-engineered vanities. But it is engaging and enjoyable and makes the oven more mine.
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