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Dimensions - Forno Bravo Forum: The Wood-Fired Oven Community



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  • Dimensions

    How critical is it that the entry height ratio be 65% of the dome height? I am at a stage where I can make some adjsutments before...the project is in cement. I have a
    32" diameter, 17" width, 12" entry height. I planned on a 16" high dome. That puts the ratio at 75%. I could always reduce opening height by puttimg 1-2" of mortar on the underside of the arch.(already installed- to avoid ripping it out)

    Your comments would be appreciated.



  • #2
    Re: Dimensions

    In my opinion, the 63% ratio is critical. This ratio allows the burning hot gasses to circulate in the dome long enough to give up their heat before exiting the opening. A higher ratio (75%) means that you will have to burn more wood - maybe a lot more wood.


    • #3
      Re: Dimensions

      I agree with Neil, 63% is considered ideal; but anything between 60-65% is acceptable. 75% is really pushing it, it may act more like a fireplace than an oven with that large an opening.



      • #4
        Re: Dimensions

        I wasn't so much skeptical about the ratio as I was ignorant, but I must say that that nugget of knowledge alone made my oven work. I would hazard a guess that it just about the most important design specification of the entire project.


        • #5
          Re: Dimensions

          Thanks for the clarification.


          • #6
            Re: Dimensions

            Ever consider adding a second arch of 2" bricks under the existing arch? Sure, it's a lot of work, but the result would render an effective fix and appear to part of the original design.


            • #7
              Re: Dimensions

              Thanks, good idea. I was thinking of just that- but cutting brick into crescent moon shapes and cementing them in to have a flat surface vs an arch. It won't be that visible anyway.


              • #8
                Re: Dimensions

                You just need to lower the high part of the arch down a couple of inches not the whole width of the arch of course. Hunt around for a scrap of sheet metal or bar or angle iron the right size. If you do it at the inside of the arch it will soon get black with soot and won't be visible.
                Last edited by Neil2; 05-04-2010, 01:57 PM.


                • #9
                  Re: Dimensions

                  How does the shape of the oven opening (arch top versus straight across) affect performance/wood consumption? It seems that if it matters at the very point in the centre of the opening it would matter all the way across, right? Or is this just one of those questions that is unanswerable because no one has two exactly the same ovens that differ only with this variable? (Or I suppose an advanced thermodynamics simulator.)

                  Originally posted by Tscarborough View Post
                  I wasn't so much skeptical about the ratio as I was ignorant, but I must say that that nugget of knowledge alone made my oven work. I would hazard a guess that it just about the most important design specification of the entire project.
                  My Oven Thread: http://www.fornobravo.com/forum/f8/i...rio-12842.html

                  My Picasa Photo Album: http://picasaweb.google.ca/davidort/...eat=directlink

                  My Food Blog that includes posts about the oven build: http://www.foodwithlegs.com


                  • #10
                    Re: Dimensions

                    I would think that there are 2 factors at play:

                    1. The overall square inches of the opening.

                    As an example the rule of thumb for fireplace opening to flue size is 10%. I used this to figure my oven flue size: door opening=325SqIn, Flue=36SqIn.

                    2. The differential between the dome height and the door height.

                    So far as I can tell by watching the smoke during startup, the differential creates a roll of smoke that assists with the draw, similar to the way a smoke shelf helps a fireplace. I would also assume that it allows more heat to be trapped in the oven after initial warmup.


                    • #11
                      Re: Dimensions

                      I was wondering about the exact same thing the other day, arch vs straight across, relating to the height %. Straight across seems to allow more smoke and hot gas to exit the oven at a given height.

                      Let's say, theoretically, you have a triangular inner entry. Where exactly do you measure the entry height? I'm sure there is a formula to figure this out, taking the total dome volume and ceiling height into account.
                      Last edited by fxpose; 05-06-2010, 09:15 AM.

                      My 34" WFO build

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


                      • #12
                        Re: Dimensions

                        V Notch Weir Discharge Calculator and Equations


                        • #13
                          Re: Dimensions

                          Considering that initial fire hot air/smoke fills the dome from the top down, the first opportunity for it to escape is the apex of the entry arch. A curved arch would allow hot air/smoke to exit from the top-center of the arch first and eventually spread the exit profile across the width of the arch until equilibrium is reached. As George said, a horizontal entry would allow the hot air/smoke to exit across the arch span evenly. A flatter curved arch would allow a wider exit path than a narrower curved arch.
                          Based on the comparison below, given identical opening dimensions, it would appear the most efficient design for dome heat-up and retention is the hemispherical arch.
                          Attached Files


                          • #14
                            Re: Dimensions

                            A curved arch will be much stronger than a flat top. Also I think that the flue should be oversize because the smoke problem at start up needs as much draw as you can provide. The entry into the flue also needs some volume. Ovens that don't provide this are notoriously smokey at start up.
                            Kindled with zeal and fired with passion.