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What are the consequences of building your dome too high? - Forno Bravo Forum: The Wood-Fired Oven Community

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What are the consequences of building your dome too high?

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  • What are the consequences of building your dome too high?

    I built my dome without the use of any forms - just a piece of plywood that I used as a guide all the way up. I wasn't aggressive enough at following the curve from the beginning and by the time I got halfway up the dome, I knew I'd be too high. I ended up 3" too high (needed 18" and finished at 21"). Does this just make my dome less efficient or am I looking at seeing other problems as well?

  • #2
    Re: What are the consequences of building your dome too high?

    Your oven is neither less efficient nor problematic as a result of the finished height of the dome. Essentially, it is what it is.

    A recent study of dome radiation by mklingles suggested that the heat pattern from a 21"h dome compared to an 18"h dome was virtually identical.

    http://www.fornobravo.com/forum/f9/m...try-15292.html

    I think the most interesting thing I've learned is that the intensity is a bit more then double at the edges of the oven. Also in a 42" oven there's a good 20" middle section with nearly uniform radiation, but beyond that the intensity starts to increase quickly. Also here you clearly see that 18" vs 21" height didn't make much difference.
    Also, don't forget the vast history of beehive ovens that worked their magic for many, many years.

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    • #3
      Re: What are the consequences of building your dome too high?

      Cool post...makes me feel better!

      Comment


      • #4
        Re: What are the consequences of building your dome too high?

        Your opening should still be 63% of your finished dome height.

        For your dome height the opening should be 13 1/4 inches. If it is less than that you will get more incomplete combustion, more smoke and greatly reduced efficiency.

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        • #5
          Re: What are the consequences of building your dome too high?

          Well thanks a lot Neill for bursting my bubble! Oh, well - you live and you learn. Hopefully it will be ok because my arch height is definitely 12".

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          • #6
            Re: What are the consequences of building your dome too high?

            Santino, throw a 3cm layer of Soapstone on the floor, voila!

            Chris

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            • #7
              Re: What are the consequences of building your dome too high?

              Originally posted by SCChris View Post
              Santino, throw a 3cm layer of Soapstone on the floor, voila!

              Chris
              Wouldn't that just make the inner arch opening smaller and make the ratio worse?

              Chip
              Chip

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              • #8
                Re: What are the consequences of building your dome too high?

                Hmmm, I guess I didn't think that one out quite right.. Now if you could add the SS to the roof of the oven that would do it.

                Chris

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                • #9
                  Re: What are the consequences of building your dome too high?

                  As I understand the difference it is between Higher being better for bread or Lower being better for Pizza. If that is the case in your oven then you could as Chris said put in a layer that would
                  take up the extra space but since you are making Pizza you would only use enough to cook Pizza on. You could then if you are getting smoke like Neil2 said use a small fan to add the extra air needed to get complete combustion. It would be best if it were a controllable exhaust fan as you would be drawing the air in as fast as you are exhausting it. Pushing air might be more of a problem than you have now. Think about a forge or a horseshoer working on a horseshoe they push air in faster than would be normally used . I think it might be called a blast furnace. By adjusting the air flow you should be able to overcome the small opening. I also think I remember seeing a post that said somewhere between 60% and 65% was fine 60% would be 12.6" might make you feel better.
                  Last edited by Hank10746; 08-16-2011, 08:28 PM.

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                  • #10
                    Re: What are the consequences of building your dome too high?

                    Ok so I may be branded a blasfemer, but has anyone seen any combustion science relating to entry height / dome height ratios relate to combustion?

                    I know that the 60% to 65% is the sacred mantra here, but I haven’t seen any evidence.

                    Is this ratio just our "urban legend"?

                    I’m sure that air / exhaust flows can be modeled. I remember seeing a model a couple of years ago of this on this site that showed the heating of the oven over time.

                    It would be interesting to see the science and how various door height ratios, door widths and opening shapes relate to combustion.. BTW my opening is at the standard 60-65%.

                    Chris
                    Last edited by SCChris; 08-17-2011, 08:13 AM.

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                    • #11
                      Re: What are the consequences of building your dome too high?

                      has anyone seen any science relating to entry height and how it relates to dome height?
                      The closest thing I've seen here is PizzaBob's heat flow analysis which illustrates the flow of air in through the bottom of the door, swirl into the dome and exit out the top of the door and up the flue. An elegant graphic with color-coded arrows showing flow direction, but damn if I can find it. It does not compare variances in door dimensions, tho.

                      John

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                      • #12
                        Re: What are the consequences of building your dome too high?

                        John,

                        That's the one that I remember as well.

                        I know that if I constrict the entry opening and focus the air flow onto the fire that the combustion is more vigorous. I’m sure that if there were a vortex around the fire swirling around the interior of the oven this is a very different heating than is a fire at the back of the oven exhausting out the front and top of the doorway in a less energetic way. I could envision that in the second fire example a dead zone, inversion might be created at the top of an oven, or at least to a greater degree than a high energy fire.

                        Anyway questioning door to oven height ratios is food for thought..

                        Chris
                        Last edited by SCChris; 08-17-2011, 08:22 AM.

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                        • #13
                          Re: What are the consequences of building your dome too high?

                          Originally posted by SCChris View Post
                          Ok so I may be branded a blasfemer, but has anyone seen any combustion science relating to entry height / dome height ratios relate to combustion?

                          I know that the 60% to 65% is the sacred mantra here, but I haven’t seen any evidence.

                          Is this ratio just our "urban legend"?

                          I’m sure that air / exhaust flows can be modeled. I remember seeing a model a couple of years ago of this on this site that showed the heating of the oven over time.

                          It would be interesting to see the science and how various door height ratios, door widths and opening shapes relate to combustion.. BTW my opening is at the standard 60-65%.

                          Chris
                          I don't know for sure but it probably comes from experience of building them over the years or comparing those that were built over the centuries and the one's in use today in Italy. Just thinking about it I would think it would have to do more with the size of the dome and the size of the flue and then the size of the door would be determined by that.

                          Comment


                          • #14
                            Re: What are the consequences of building your dome too high?

                            Hank,

                            I agree that there must be some basis of efficiency that sets the rough parameters of design. If it didn't work well then you surely wouldn't build another one like it.. I wonder also about the reasoning of the ovens that are seen in ancient sites.. These ovens were not being used to cook pizza, at least not modern pizza. The cooking done in these ovens was more diverse and more ongoing heating than many of our residential ovens.

                            Were the door openings based on allowing a worker to enter to maintain these ovens? If so was this a minimum sized opening mostly for the convenience of maintenance?

                            I’m sure that there is science behind these ratios. I just wonder to what degree.

                            Chris
                            Last edited by SCChris; 08-19-2011, 12:59 PM.

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                            • #15
                              Re: What are the consequences of building your dome too high?

                              The 63% ratio came out of a study on the bread ovens of Quebec. In some way(I'm not sure what the test was) they found the most efficient ovens, determined their opening ratio, and found 63% to he ideal. I think it is covered in "the bread ovens of Quebec" which is availiable to read online.

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