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two ?s - door shape and chimney opening - Forno Bravo Forum: The Wood-Fired Oven Community

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two ?s - door shape and chimney opening

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  • two ?s - door shape and chimney opening

    Hi everyone,

    I have noticed 2 variations on door shapes. complete arch or arch top with a brick or partial brick vertically as the sides. Is there a performance difference? Height is always mentioned as critical but what about width and shape/ overall area of the opening?

    2nd question is the chimney opening size. Is there an ideal ratio of size opening/area to final chimney pipe size? I am doing a 42" oven, 8" chimney pipe.

    thanks for any insight.

  • #2
    Re: two ?s - door shape and chimney opening

    Height is the only critical dimension. (Should be 63% of inside vault height.)

    The rest is a mater of aesthetics and personal preference.

    As for chimney or vent size there is a wide range of opinions. If you just want to keep the fumes from your face when cooking and don't mind smoke coming out of the opening on firing, a 6 inch vent on your 42 incher would do. If you positively want to keep all smoke at all time from the opening you will need something much bigger - even then smoke will come out the front sometime. I have seen lots of wood fired ovens in Sicily and Croatia that had no vents at all.

    Comment


    • #3
      Re: two ?s - door shape and chimney opening

      Thanks for the info Neil. That makes my life easier but I admit I am very suprised about the shape of the opening being considered mostly inconsiquential. This is based on my knowledge of combustion engines as a hobby and what is done to port or otherwise modify them to improve airflow. Volume, shapes and several other factors weigh heavily when considering how to modify an engines characteristics. While there is a lot that is not common between an engine and oven, there is also a great deal that is. We are still dealing with intake/exhaust of gasses and temperature and pressure differentials that drive the exchanges.

      thanks again, onward with the project we go!

      Comment


      • #4
        Re: two ?s - door shape and chimney opening

        I think the opening shape does make a difference, but have no basis on which to define it. For the vent size, it appears that the fireplace rule of thumb of 10 percent of the opening size is about right.

        Comment


        • #5
          Re: two ?s - door shape and chimney opening

          You're describing segmental arches (not a full semicircle) and semicircular arches. The classic Roman arch is a semicircular arch.

          The primary reason people use segmental arches (I posit) is that the opening, being more rectangular, offers easier access to the interior, i.e., it's easier for the cook to work with. The argument in its favor certainly has nothing to do with efficiency of heating an oven.

          One reason to consider a semicircular arch is that it is simultaneously stronger and more resistant to "slop" in the manufacturing process. What do I mean? It's stronger because the load is better distributed down to the feet. Segmental arches require strong wide buttresses to avoid buckling outwards. There is a conception that the true strongest arch is a catenary, but this is not actually correct. The strongest unloaded free-standing arch is a catenary (like the St. Louis arch), but an arch loaded from above with an evenly distributed force (like one with a building standing on top of it) is usually strongest in a shape approximating a semicircle (so the Romans got it right after all, since all their arches support heavy infrastructure above).

          This is easy to demonstrate. Hang a cord on a wall and it will conform to a catenary. Now, start hanging weights along the length of the cord in an evenly distributed fashion. The cord will alter shape and become nearly semicircular. Voila!

          I also said a semicircular arch is more resistant to imprecision. What did I mean? For a given opening width, a semicircular arch is, by definition, of a tighter radius of curvature than a segmental arch, which means that when constructed from bricks of a fixed starting size, those bricks will be more severely wedge-shaped. A more severe wedge is less likely to fall through a gap from above for a given degree of slop in the design (by slop I mean imprecision in the shape of the bricks or the width of the gap between the bricks)...so a semicircular design ought to be more forgiving to imprecision in the construction without catastrophic failure (stones literally falling out).

          HOWEVER...
          ...while I find all of this fascinating from an engineering point of view, we aren't designing ten story buildings here; we're designing ovens. There are virtually no reports on FB of anyone's arch ever failing, regardless of design or precision of construction.

          The thermal arguments are also interesting (Tscar commented on this a bit) but are difficult to measure precisely, so I will leave that issue off the table in this post.

          So, build it according to your personal aesthetic guidelines, meaning, build it the way you want it to look, and don't worry about any other aspect of it (structural, thermal, etc.) because it really doesn't matter. Just make the oven look the way you want it to look and it will do a perfectly good job of cooking food for you in return.

          In my opinion (and I'm not very qualified to have one), the biggest areas of design for we amateurs to focus on in our Pompeii ovens (because they are aspects of the design which if done improperly can truly impede our ovens' performance) are insulation and door-height-to-dome-height ratio.

          ...

          If you read this post all the way to here, I'll send you $100.

          ...

          If you read this post all the way to here, the previous promise was false.

          Cheers!
          Last edited by kebwi; 06-09-2010, 01:46 PM.

          Website: http://keithwiley.com
          WFO Webpage: http://keithwiley.com/brickPizzaOven.shtml
          Thread: http://www.fornobravo.com/forum/f21/...ttle-7878.html

          Comment


          • #6
            Re: two ?s - door shape and chimney opening

            Originally posted by kebwi View Post
            ...

            If you read this post all the way to here, I'll send you $100.

            ...

            If you read this post all the way to here, the previous promise was false.

            Cheers!
            Great Post....snore...............100 $ !!!! Oh....snore....


            Just kidding. Thanks for the great info
            Sharpei Diem.....Seize the wrinkle dog

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            • #7
              Re: two ?s - door shape and chimney opening

              There are virtually no reports on FB of anyone's arch ever failing, regardless of design or precision of construction.
              It is, however, a notorious crack location. It's very common to get a crack on one side of the keystone or the other. Mine certainly cracked there. I'd give serious thought to a semicircular opening if I did another oven.
              There is a conception that the true strongest arch is a catenary, but this is not actually correct. The strongest unloaded free-standing arch is a catenary (like the St. Louis arch), but an arch loaded from above with an evenly distributed force (like one with a building standing on top of it) is usually strongest in a shape approximating a semicircle (so the Romans got it right after all, since all their arches support heavy infrastructure above).

              This is easy to demonstrate. Hang a cord on a wall and it will conform to a catenary. Now, start hanging weights along the length of the cord in an evenly distributed fashion. The cord will alter shape and become nearly semicircular. Voila!
              We've discussed the catenary arch ad infinitum, and no one has ever mentioned this. Love your little demonstration. I'm gonna try that out.
              My geodesic oven project: part 1, part 2

              Comment


              • #8
                Re: two ?s - door shape and chimney opening

                Originally posted by dmun View Post
                It is, however, a notorious crack location. It's very common to get a crack on one side of the keystone or the other. Mine certainly cracked there. I'd give serious thought to a semicircular opening if I did another oven.
                Not that a semicircular arch reduces the risk of the crack in the first place. Mine cracked on one side of the keystone on the outer arch and on both sides of an off-center keystone on an inner arch (that stone actually wobbles about 1/16th in). But they absolutely cannot fall through, partially in thanks to the tight-radius arch.

                Originally posted by dmun View Post
                We've discussed the catenary arch ad infinitum, and no one has ever mentioned this. Love your little demonstration. I'm gonna try that out.
                Auroville Earth Institute, training courses, workshops on Vaults, Arches, Domes(VAD), stabilized rammed earth walls, compressed earth blocks, vaulted structures, compressed stabilised earth blocks, rammed earth. has an image which illustrates this example.

                Website: http://keithwiley.com
                WFO Webpage: http://keithwiley.com/brickPizzaOven.shtml
                Thread: http://www.fornobravo.com/forum/f21/...ttle-7878.html

                Comment


                • #9
                  Re: two ?s - door shape and chimney opening

                  Good discussion. I definitely fall into the camp of "strength is not an overbearing concern in opening design". For my own oven I am building a steel entranceway (ala Naples ovens I have seen) and will be assembling an arch in brick on a steel support/frame and have a steel face plate covering this area. The questions came from my need to cut the steel this weekend for the opening, face plate, and chimney and it struck me that I did not 'know' beyond height ratio of opening to oven interior height what concerns I should have.


                  I found this recent image and info posted very insteresting for visualizing the air flow in an oven.

                  PhotoPlog - Dome and Floor
                  Last edited by dhs; 06-10-2010, 07:35 AM.

                  Comment


                  • #10
                    Re: two ?s - door shape and chimney opening

                    Originally posted by dhs View Post
                    For my own oven I am building a steel entranceway (ala Naples ovens I have seen) and will be assembling an arch in brick on a steel support/frame and have a steel face plate covering this area.
                    Yeah, a steel entry will be really cool.
                    I originally wanted a steel face/arch for the inner entry and looked at a few wood stove door kits to see if I can adapt any. I did not find one.

                    George
                    George

                    My 34" WFO build

                    Weber 22-OTG / Ugly Drum Smoker / 34" WFO

                    Comment


                    • #11
                      Re: two ?s - door shape and chimney opening

                      Keith,

                      Thanks for pointing out the loaded catenary properties. It's interesting to note that if the inverted catenary arch is loaded only where the approximate vent and chimney reside (the middle 50%), the effect to the arch is to pull the sides into a more vertical position.

                      Depending on the mass of any given vent transition/chimney, I agree that the entry arches with truly vertical sides shouldn't have a stability concern. However, builds are shown with additional entryway bricks that effectively 'increase the width of the pier.' This additional mass in the entryway would presumably wick heat from the oven, and since it is not insulated on the inside, lose that stored heat over time even though the door is on.

                      According to the Auroville Earth Institute data 'Catenary arches are always the most stable so their thickness can be reduced'. Maybe this is another reason to consider a catenary (hemispherical) entryway.
                      Last edited by GianniFocaccia; 06-10-2010, 09:54 AM.

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