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Before pulling the trigger, in need of criticism - Forno Bravo Forum: The Wood-Fired Oven Community

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Before pulling the trigger, in need of criticism

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  • Before pulling the trigger, in need of criticism

    Hello all,
    I am a newbie in this forum and after reading a lot of content here for the past month i decided to make a WFO on my balcony.
    Some background first i live in Greece in an floor/aparment at the top floor of a 3 storie building. I have a good location for the oven that can hold the weight and the smoke will have a clear path away from buildings. My balcony is quite big but my wife's tolerance is not so i have secured a 40x55 inch space for the oven.
    I am going to use a metal base that size to put the oven on (with heavy duty wheels so i can move it few feet back and forth ico).
    My idea so far is to build an oval kindoff pompei oven with cooking space dimensions 36 inch deep x 26 inch wide. The ceiling will be at 13 inches . The door opening will be 16 inches wide and 8 inches tall.
    The thermal mass of the firebrick is going to be 5 inch (the firebricks i found are 9x5x2 '').
    Outside the firebrick i am going to use a Medium cement castable insulation concrete reinforced with chickenwire. i expect to use 2-3 inch of this since it is very high quality (i have tested it on my gas bbq -bulit a pizzahacker kind of dome- and with 800F degrees inside the outside was cold). Below you can see a rough sketch i made.
    The chimney is going to be 8x10 built with the refractory insulator and the gases will be routed over the front top of the oven and will exit from the top of the dome vertically with stainless steel flue.
    The metal base will be covered with a metal plate with 5 inch high sides in order to make the cement casting easy. I am going to use 3' of ytong blocks and on top i am going to use the insulating concrete. Above that, fireclay slurry and the 2 inch thick firebricks.
    Please if you see something wrong with my plan tell me before i start.
    Thank you.
    https://docs.google.com/file/d/0B3EI...it?usp=sharing
    Last edited by giol; 03-20-2013, 08:43 AM.

  • #2
    Re: Before pulling the trigger, in need of criticism

    Sounds like a really interesting project. I hope you can share some pictures when the time comes, yours is a unique design and build site.

    If it doesn't complicate your design in any way, you might want to consider rotating your cooking surface by 90 degrees (making it 36" wide x 26" deep). I only suggest this because when you are cooking pizza, roasting or smoking meats you will need to have a small fire (or smoldering ashes) burning inside the oven. It might be a little easier to manage the fire if it is on the side vs in the back of the oven. A fire on the side also reduces the chances of pushing your pizza into the ashes with your peel since you aren't actually pushing the food toward the fire. I know there are some people who prefer to place their fire in the back so in the end it is really just a matter of choice and small one at that.

    Good Luck
    AT

    PS - Is that really an insulating cement (as in it does not absorb heat) or is it a refractory cement that would add thermal mass, like adding another layer of firebrick?

    Comment


    • #3
      Re: Before pulling the trigger, in need of criticism

      I thought of making it wider than deeper but i suspect that the draft will be severely comprimised and i would have stale smoke on the sides. But you got me thinking that maybe i am wrong.
      The cement is used for insulation of high temp furnaces up to 2000F . It does have some chammote in the mix which adds thermal mass (i suppose). It is lightweight -less than 1 gram per cm3. And the Thermal Conductivity at 600C is 0.25.

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      • #4
        Re: Before pulling the trigger, in need of criticism

        I don't see why your draft would be compromised either way. It sounds like your vent will be plenty big enough in area - as long as you can extend you chimney up about 1 m you should be fine (you might even be able to get away with less if you wanted to). You are venting to open air above? I.E. you don't have a roof over head - sorry but I had to ask.

        I read somewhere that the thermal conductivity of a 4:1 mix of Vermiculite and portand cement is ~ 0.16 which is about the same as the cement you are planning to use. If you find that cement to be cost prohibitive (I've not seen anything like that here), mixing vermiculite or perlite with cement is a proven method here (you can even go with a leaner mix if you are not too concerned about compressive strength - but it can get crumbly if you don't get it right).

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        • #5
          Re: Before pulling the trigger, in need of criticism

          Originally posted by giol View Post
          I thought of making it wider than deeper but i suspect that the draft will be severely comprimised and i would have stale smoke on the sides. But you got me thinking that maybe i am wrong.
          The cement is used for insulation of high temp furnaces up to 2000F . It does have some chammote in the mix which adds thermal mass (i suppose). It is lightweight -less than 1 gram per cm3. And the Thermal Conductivity at 600C is 0.25.
          I assume the 0.25 is in metric? The 0.77 for Vermicrete is in Imperial units. That converts to 0.036 Metric. As you can see what you have here is not an insulator, it would be considered thermal mass in the US. A lot of materials considered insulators in Europe are not actually insulators, and are not considered to be in the new world countries.

          Comment


          • #6
            Re: Before pulling the trigger, in need of criticism

            Ive used the so called insulating refractory concrete and if you can find an alternative insulating material for similar price I would grab the alternative.
            Last edited by brickie in oz; 03-21-2013, 01:30 AM.
            The English language was invented by people who couldnt spell.

            My Build.

            Books.

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            • #7
              Re: Before pulling the trigger, in need of criticism

              There is no roof above.
              The concrete does contain vermiculite and the TC is in w/m/k which coverts to 0.14 btu/hr/ft/sqft. I can get an even better insulator if needed. I was thinking that the AAC block below the concrete will finish the job since it is also insulating.
              Last edited by giol; 03-21-2013, 06:41 AM.

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              • #8
                Re: Before pulling the trigger, in need of criticism

                I just bumped on wfo shape i have never seen before
                How do you think this oven works with two different levels?
                Maybe i can go for something like this?

                Comment


                • #9
                  Re: Before pulling the trigger, in need of criticism

                  Originally posted by giol View Post
                  AAC block below the concrete will finish the job since it is also insulating.
                  Insulation below the concrete is all wrong, it needs to go on top of the concrete unless you own a forest of firewood.
                  The English language was invented by people who couldnt spell.

                  My Build.

                  Books.

                  Comment


                  • #10
                    Re: Before pulling the trigger, in need of criticism

                    The concrete i am going to use is castble insulating of very high quality. The AAC will be additional insulation below a layer of 5cm. The company engineer that suggested the concrete told me that 5 cm would be more than enough but i thought "you can never have enough".
                    What do you think about going for a wider than deeper wfo?

                    Comment


                    • #11
                      Re: Before pulling the trigger, in need of criticism

                      If it has a thermal conductivity of 0.25 in metric it is not an insulator. They can call it what ever they want but the numbers don't lie.

                      Comment


                      • #12
                        Re: Before pulling the trigger, in need of criticism

                        Yes, sometimes you have to be careful of engineers suggesting things. They often operate under a different set of criteria when they get slightly out of their field. Insulating castable is a castable with insulating properties, it is not, for this application, considered to be insulation.

                        As for the wider than deep, my gut says no unless the ratio is very small. The dual level oven may work, but the levels should probably be reversed.

                        Comment


                        • #13
                          Re: Before pulling the trigger, in need of criticism

                          It is a calcium-silicate cement with vermiculite as main ingredient, so it should be just a bit a better than vermicrete. It is used as insulating lining for Kiln and in the ceramic industry.

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                          • #14
                            Re: Before pulling the trigger, in need of criticism

                            Originally posted by giol View Post
                            It is used as insulating lining for Kiln and in the ceramic industry.
                            Why would they put an insulator at the hot face of a kiln?
                            Sort of defeats the whole purpose of a kiln I would imagine?
                            Last edited by brickie in oz; 03-22-2013, 11:08 PM.
                            The English language was invented by people who couldnt spell.

                            My Build.

                            Books.

                            Comment


                            • #15
                              Re: Before pulling the trigger, in need of criticism

                              It is used as lining outside the refractory lining. In imperial units this cement coverts from 0.19 w/m/k to 0.10 btu/hr/ft/F. What is a good number in imperial units to use as a guide?
                              A ceramic FB in the US (http://www.ktrefractories.com/Cerami...FiberBoard.htm) has 0.15w/m/k.
                              Last edited by giol; 03-22-2013, 11:39 PM.

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