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Will this Flue be effective? - Forno Bravo Forum: The Wood-Fired Oven Community

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Will this Flue be effective?

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  • Will this Flue be effective?

    I have just started building the molds for my 42" cast oven, I have had a sleepless weekend trying to figure out how to get an 8" flue in such a small space whilst still allowing a 3" of concrete everywhere, Eureka I awoke this morning with what I thought was a good idea, What do you think of the model I have made? I'd just like to point out that I am going to take the shoulders of the rectangle 45 degrees into the round part allowing for a larger aperture.
    Attached Files
    Last edited by sonomacast; 02-14-2011, 02:53 PM.

  • #2
    Re: Will this Flue be effective?

    If you can give me some dimensions I'll model the airflow in my CFD software for you.

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    • #3
      Re: Will this Flue be effective?

      The Oven floor is 1050mm in diameter (41”)
      The dome height is 472mm (18.5”)
      The doorway is a semicircle 558mm (22”) wide 297mm (11.6”) High (225.2"square)
      The final Flue is 8” in Diameter (50” square)
      The rectangle Flue entrance is approx 75mm x400mm (3”x16”)

      I shall extend the shoulders of the rectangle 45 degrees to meet the circular flue.
      Thank you
      Last edited by sonomacast; 02-14-2011, 03:44 PM.

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      • #4
        Re: Will this Flue be effective?

        Here's an overly simplified model of the vent running a pretty realistic volume of standard air. It's not a model of a real world situation in operation in an oven, so the actual numbers mean little, but air is air and oven exhaust being drawn through by natural convection will experience the same effects. As you can see in the model the vent isn't really doing you much good as the air is not able to make that sharp of a turn at that velocity in that short of a distance.

        I'll try to model the sloped sides you mention, but they will be very difficult if at all possible due to the limitations of the software I use.
        Attached Files

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        • #5
          Re: Will this Flue be effective?

          Thank you very much, interesting, So how would this one fare Modena Assembly: Attach the Vent

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          • #6
            Re: Will this Flue be effective?

            Your design is similar to mine (but MUCH more refined). The portion of the flue that transitions into the dome will cause you problems with heat loss. FB says the flue opening should not be in the dome area but build into the arch (opening). You can see my mistake below is similar to your design. I was able to add some fire brick (on the inside) to correct my heat loss issue. You may not be so lucky.
            Attached Files

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            • #7
              Re: Will this Flue be effective?

              The Flue is attached to the first arch (Porch/entrance), First there is the porch, then the doorway and then finally the oven itself It just looks weird in the photo as I havn't built the dome form up with foam/sand yet, If you look carefully on the right of the photo you can see the end of the ribs I cut to use as a guide for the dome.
              My whole problem was getting an 8" flue (50" squared) into such a tight space whilst allowing 3" concrete all around, This seemed the best way. Today I modified the flue by bringing the far ends of the rectangle up into the round flue at 45 degrees. Hopefully maintaining as close to the 50" square area I wish for..... Getting there slowly
              Attached Files
              Last edited by sonomacast; 02-15-2011, 12:32 PM.

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              • #8
                Re: Will this Flue be effective?

                It seems from todays studies smoke channel shape is very important for better gas flow.
                Round channels provide the best gas flow, square second best and rectangular channels offer the poorest gas flow The further a rectangle retreats from a square the greater resistance to gas flow.

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                • #9
                  Re: Will this Flue be effective?

                  Flue size. The US underwriters Laboratories state that it is best to use a flue as large as you need and no larger;
                  So if you were burning 12 lbs/hour of wood you would be generating 97,000 btu's and therefore need a 6" Flue (My 41" oven would burn less than this)
                  16.5 lbs/hr=7" according to The US underwriters Laboratories.
                  21.6 lbs/hr=8"
                  27.3 lbs/hr=9" and so on

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                  • #10
                    Re: Will this Flue be effective?

                    Round channels provide the best gas flow, square second best and rectangular channels offer the poorest gas flow The further a rectangle retreats from a square the greater resistance to gas flow.
                    I'm sure this is correct in theory, but in the US, almost all masonry flue liners are square or rectangular. I've used both without difficulty.
                    Flue size. The US underwriters Laboratories state that it is best to use a flue as large as you need and no larger;
                    True for inside fireplaces and appliances. The concern is that your oversize flue will suck the heated air out of the house. Less a concern for a freestanding oven.
                    My geodesic oven project: part 1, part 2

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                    • #11
                      Re: Will this Flue be effective?

                      Yes, we do not even sell any round flue, the point is how to figure the cross sectional area.

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                      • #12
                        Re: Will this Flue be effective?

                        Although the Forno Bravo design looked like a good one from the get go and I assume it was designed by an engineer I modeled it anyway for the heck of it. I put it into the exact same scenario as the other vent which means again the numbers are all arbitrary but the general characteristics should hold true as should comparisons. As expected it passed with flying colors in my book. The airflow looks fairly smooth and uniform.
                        Attached Files

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                        • #13
                          Re: Will this Flue be effective?

                          Originally posted by Tscarborough View Post
                          Yes, we do not even sell any round flue, the point is how to figure the cross sectional area.
                          There's more to it then cross section. Airflow through a duct is subject to friction loses. In the HVAC industry this is measured in inches of water column per 100 feet. Friction is proportional to the surface area. Round ducting is most efficient because it encloses a given volume with the smallest surface area. Square duct will give you less surface area then a rectangle and the further out of ratio you go the higher the surface area gets. It sounds petty but it can actually make a pretty big difference, even different materials can make a large difference due to different textures.

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                          • #14
                            Re: Will this Flue be effective?

                            That is true, and I still have the reference books for frictional losses through various materials and cross sections with fluids and gases, but we are dealing with 3-4' of flue in this application.

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                            • #15
                              Re: Will this Flue be effective?

                              I'll let those with more smarts than me argue the vent situation, I just want to see more pics of this build.
                              My oven (for now):
                              http://www.fornobravo.com/forum/f43/...ven-14269.html

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