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Flue dimensions for 37" Pompeii Oven

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  • Flue dimensions for 37" Pompeii Oven

    I wanted to know before I finalise the design of the oven opening arch, whether a rectangular flue is an issue, it seems that a circular 6" flue pipe is suitable up to 36" and that would have a surface area of 182 cm squared. The alternative would be the rectangular flue which I think works out at 301cm squared so unless I've forgotten my geometry calculations from school the flow of air is at least 50% better if I go down this route - assuming that the height of the flue is the same for a fair comparison of course. Secondly, any strong feelings about suitable chimney height in a relatively sheltered corner of the garden, I was thinking just 400mm, but could afford and build up to 600mm if it would make a huge difference?

    Here was my original post which got lost on my main build page as a topic:

    1. For the size of oven (37inch diameter) is there any problem with selecting the following middle sized option of these cavity liners (by my calculations it has a larger surface area than a 6 inch circular flue) and would work well. *(see photo) they are 215mm x 140mm https://www.selcobw.com/products/bui...er-215-x-140mm and would be easier to connect to my intended rectangular exit through the oven entrance archway than a 6 inch diameter circular alternative. The 215mm x 215mm flue seems far to big for the purpose.

    2. What height should I go to for the chimney, I've read a few thoughts and was planning on having three of these, connected with home brew mortar which would take the height up to 600mm from the point of connection with the arch, could/should I just opt for two and 400mm of height in the chimney?

  • #2
    I can't answer your query with absolute certainty, but logically there should be no problem with the area of the flue.
    Two things I do know, with certainty, the taller the flue the more the draw, although realistically you only the the smoke to go above peoples heads, and more importantly, you will have to insulate those flue liners or they will crack.

    Comment


    • #3
      I don't know where you are building your oven but if it is close to a structure or if you are going to build a roof there are most likely local codes you should follow. I did not need a permit to build my oven or roof but followed local code for height above structure to cover us in case there was ever a fire.
      My build thread
      http://www.fornobravo.com/community/...h-corner-build

      Comment


      • #4
        Originally posted by hughjamton View Post
        I can't answer your query with absolute certainty, but logically there should be no problem with the area of the flue.
        Two things I do know, with certainty, the taller the flue the more the draw, although realistically you only the the smoke to go above peoples heads, and more importantly, you will have to insulate those flue liners or they will crack.
        Guessing I can use either insulation blanket (used for the dome insulation too) or vermiculite fed in to the gap between the bricks I intend to run up alongside the flue with a very small gap between brickwork and the flue? Would I need the insulation if I have the brickwork running up alongside the flue?

        Comment


        • #5
          You might enjoy (or be terrified...) watching this video of thermal shock on a chimney tile. Insulating your rectangular chimney tile is a good idea as is keeping your flue clean. The insulation simply helps moderate the thermal shock in the flue tile material when you start up your oven. Generally, with a good smoke collection chamber (think of an upside down funnel that leads into your chimney) and normal firing of the oven, you won't see the extreme thermal shock generated in the video. Your flue will expand and contract slightly with firing/cool down, so leaving a little cushion between it and your chimney bricks is a good idea IMHO...

          https://www.youtube.com/watch?v=bxOkwQyY-2w
          Mike Stansbury - The Traveling Loafer
          Roseburg, Oregon

          FB Forum: The Dragonfly Den build thread
          Available only if you're logged in = FB Photo Albums-Select media tab on profile
          Blog: http://thetravelingloafer.blogspot.com/

          Comment


          • #6
            Originally posted by SableSprings View Post
            You might enjoy (or be terrified...) watching this video of thermal shock on a chimney tile. Insulating your rectangular chimney tile is a good idea as is keeping your flue clean. The insulation simply helps moderate the thermal shock in the flue tile material when you start up your oven. Generally, with a good smoke collection chamber (think of an upside down funnel that leads into your chimney) and normal firing of the oven, you won't see the extreme thermal shock generated in the video. Your flue will expand and contract slightly with firing/cool down, so leaving a little cushion between it and your chimney bricks is a good idea IMHO...

            https://www.youtube.com/watch?v=bxOkwQyY-2w
            Thanks - and wow, that's very stark warning about getting this right!

            Comment


            • #7
              Hi Whackers, I am actually considering the same material for the chimney in my build. I found 3 pieces of clay flue tile, approx 8x17 ( or 203mmx431mm in your terms.) While this was a larger opening than I was intending, the pieces came from my father and I thought it might be fun to incorporate some of his old yard flare into the build.

              Does anyone know if an 7" x 16" (ID) chimney opening would be "too" big for a 36" oven? There's lots of mind boggling info on chimneys, but I'm having trouble nailing down the best size window to be in. I suspect the main disadvantage would be thermal loss through a larger chimney?

              Comment


              • #8
                I don't have the extensive dynamics of flue flow () in my background, but thermal loss would not be the problem with a larger chimney diameter. Since chimneys work best as they heat up a bit (increasing the draw), the problem I see with a large diameter flue is that it may not pull all the smoke up...you'd probably have a significant amount of smoke coming out the front of the oven as is going up the chimney. I suspect that if you used a piece of that larger flue to funnel smoke into a 6" to 8" pipe (normal size for a 36" to 39" oven) it would work really well. The smaller pipe producing the suction required to pull that smoke up into the intended path. Your transition to the smaller pipe would present some interesting problems, but I suspect it would be doable. I agree that it would be great to incorporate something of your Dad's in the build. You could also just stack them to the side of your oven and use them as a stand for your peel & other tools.
                Last edited by SableSprings; 09-07-2020, 10:53 PM.
                Mike Stansbury - The Traveling Loafer
                Roseburg, Oregon

                FB Forum: The Dragonfly Den build thread
                Available only if you're logged in = FB Photo Albums-Select media tab on profile
                Blog: http://thetravelingloafer.blogspot.com/

                Comment


                • #9
                  Originally posted by Whackers View Post
                  I wanted to know before I finalise the design of the oven opening arch, whether a rectangular flue is an issue, it seems that a circular 6" flue pipe is suitable up to 36" and that would have a surface area of 182 cm squared. The alternative would be the rectangular flue which I think works out at 301cm squared so unless I've forgotten my geometry calculations from school the flow of air is at least 50% better if I go down this route - assuming that the height of the flue is the same for a fair comparison of course. Secondly, any strong feelings about suitable chimney height in a relatively sheltered corner of the garden, I was thinking just 400mm, but could afford and build up to 600mm if it would make a huge difference?

                  Here was my original post which got lost on my main build page as a topic:

                  1. For the size of oven (37inch diameter) is there any problem with selecting the following middle sized option of these cavity liners (by my calculations it has a larger surface area than a 6 inch circular flue) and would work well. *(see photo) they are 215mm x 140mm https://www.selcobw.com/products/bui...er-215-x-140mm and would be easier to connect to my intended rectangular exit through the oven entrance archway than a 6 inch diameter circular alternative. The 215mm x 215mm flue seems far to big for the purpose.

                  2. What height should I go to for the chimney, I've read a few thoughts and was planning on having three of these, connected with home brew mortar which would take the height up to 600mm from the point of connection with the arch, could/should I just opt for two and 400mm of height in the chimney?
                  I too considered those liners from Selco but moved away from the idea because they are cavity liners not flue liners so they are not intended for high temperature use. They are terracotta so they might work but it may be worth testing them to see how they withstand high temps.

                  Comment


                  • #10
                    Hi, I'm a bricklayer and wouldn't use the clay flue for a pizza oven. I've done alot of chimney rebuilds and believe me they break down. Spend the money and use a proper insulated chimney pipe, adapter and cap. You know the old saying " good isn't cheap and cheap isn't good" Cheers

                    Comment


                    • #11
                      Thanks, for the comments, since you are a pro who deals with this material all the time this pretty much reinforces the video that David S links to here and there on how clay chimney liners cannot handle the thermal shock of high temps.
                      Russell
                      Google Photo Album [https://photos.google.com/share/AF1Q...JneXVXc3hVNHd3/]

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