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Post by philippeelskens on Feb 17, 2016 12:11:15 GMT -8
Ok, the concept is basically a white oven (hot air circulates in a gap AROUND oven, not IN the oven), sitting on top of the riser of a P Batch Box (130mm diameter; 5").
The inside of the oven is half a sphere with radius 30 cm (12").
I'm trying to figure out, and would love to hear your input:
-How thick should I make the inside wall (cob), and the bottom plate (cob)?
-How high should the bottom plate be located above the riser? I've found 5 cm (2") in other discussions, so this seems pretty good.
-How thick should the gap be between the inside wall of the oven (cob) and the outside wall (perlite clay). I've also read 5 cm (2") in another thread.
-How thick should the insulated outer wall be (perlite clay). This is obviously something that I can adapt more easily by putting more insulation on top...
Thanks!
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Post by patamos on Feb 20, 2016 10:14:17 GMT -8
Much depends upon your baking intentions. How often will you be wanting 3+ hour slow roasts? How often a pizza? Large gathering community kitchen or private small group?
Thicker mass generally = longer flywheel once heated but also longer warm up time til functional.
I have built a few a 6" L-feed with 2.5' and 3' diameter oven domes. One had with 1.5" inner mass, then 3" clay perlite. and worked very well. Convection heating early on, then more mass accumulation as time went on.
Another i built with 4" of mass in the dome and it didn't fare as well for the first 1/2 hour.
5" batch will likely produce more KwH than a constantly fed 6" L. So maybe the wall thickness ought to be in between the above mentioned.
Also, are you planning a top exit flue or a lower side exit (with longer external flue). The latter will make a better bell for harvesting heat…
my 2 bits
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Post by philippeelskens on Feb 23, 2016 18:21:29 GMT -8
Much depends upon your baking intentions. How often will you be wanting 3+ hour slow roasts? How often a pizza? Large gathering community kitchen or private small group? Thicker mass generally = longer flywheel once heated but also longer warm up time til functional. I have built a few a 6" L-feed with 2.5' and 3' diameter oven domes. One had with 1.5" inner mass, then 3" clay perlite. and worked very well. Convection heating early on, then more mass accumulation as time went on. Another i built with 4" of mass in the dome and it didn't fare as well for the first 1/2 hour. 5" batch will likely produce more KwH than a constantly fed 6" L. So maybe the wall thickness ought to be in between the above mentioned. Also, are you planning a top exit flue or a lower side exit (with longer external flue). The latter will make a better bell for harvesting heat… my 2 bits Thanks for the numbers Patamos! Most of the time, we will be using the oven to cook for a very small group. Using the oven relatively fast is more important than having it radiate for half a day. A typical use would be to put in a pizza, and when we take it out put in some potatoes/tomatoes for the day afterwards an make some granola. So a relatively thin wall seems in place (1.5" cob if I follow your suggestion, or rather 2-2.5" since it is 5" batch instead of 6" L). Outside wall 3" perlite clay, and I will not finish the outside until I am sure enough of the oven's performance that no additional insulation is necessary. Some additional questions: -Do you make the bottom mass as thin as the wall? I've never worked with cob before, so I'm curious as to how thin you can make it so that it is still strong... -Did you have an air gap in between the 2 layers, and if yes, how thick? Did this gap also run under the bottom mass? Oh, almost forget: a sketch of my oven to illustrate. The riser (left bottom: red) is located under the air gap (transparent) of the oven. Hot air circulates in the gap, over the oven, and exits through the exhaust (right top: blue), located at the same height as the riser but on the opposite side. After that, the exhaust will run through a bench to use rest heat.    |
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Post by patamos on Feb 23, 2016 21:22:45 GMT -8
Ahh, the sketch gives me a better understanding of what you mean by white oven. I have not yet built one with that thin an air gap surrounding the dome. My first gut feeling is that you may have issues with getting the gasses to flow evenly around the dome. Low entry and exit ports are always good. On opposite sides if possible. For air gap under the ledge of the bottom plate i'd say at least 1/5". ANd watch out for 'pinch points' - where the gasses are changing direction they want more room to turn… For ideas about cob check out Donkey's posts in the reference section on cob and fire cob have fun  |
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