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Thread: optimum efficiency for flues in brick ovens

  1. #1
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    optimum efficiency for flues in brick ovens

    Hello all,

    I came across your forum and was wondering if you could help me understand or guide myself in the right direction for some questions perhaps basic in your field and knowledge but I am having trouble understanding.

    I am building a brick oven with a flue, one of those pizza ovens. My oven is a pure semicircle with a diameter of 128cm and a dome height of 64cm. My 2 main issues are about the dome entrance size and flue size.

    1) Dome Entrance: I'm sure there is a exact formula to the science behind it all im sure and Im guessing that the archway the dome entrance should fit into a percentage of the whole volume of an oven to work at its optimum)

    However we have managed to get the hight of the arch to an overall size due history and peoples experiences.

    In relation to the heigh of the arch generally most people who build brick ovens say that it's approximately 63% = +/- 5% to the height of the top of the inner dome. If it’s too high their will be too much heat loss, too low and not enough air flows through not heating it sufficiently. O.K. so how did they come up with figure? After all the research I haven’t got a clue! A valid reason was that of all the oven measured including the ones still standing today after 2000 years it seems that it’s around the 63%. Good enough for me but still it bugged me as I had a itch to find why? I found out that it could be a empirical ratio. An empirical formula of a mathematical equation that predicts observed results, but is derived from experiment or conjecture and directly from first principles. perhaps a empirical ratio that is ultimately related to the properties of pizza and brick? There is no fundamental heat transfer law that governs this. It is simply an empirical ratio, boring as that may be. Could it have to do with the “Venturi effect” as cold air is drawn in and hot air expelled from the flue?

    Perhaps It could be the “stoichiometric mixture" formula since it's a Air-Fuel ratio much like a car. There is a optimal efficiency to the combustion of the air/fuel mixture. So if the stoichiometric formula is correct and I am using the normal chemical equation for wood CxHyOz + O2 ------> CO2 + H20 ) which I don't understand as I pulled off the net. Could we work out the optimum size of the entry arch and would this equate to the 63% that has been averagely measured over 2000 years of pizza ovens (some which still stand, like in Sicily Italy)?

    If you were feeding your oven with a regulated amount of gas or oil then the stoichimetric ratio might apply, but if you have a pile of wood of varying composition, size and surface area, then the ratio is not applicable. and could perhaps be moot.

    The only number that I could come up with was of the fibonacci level of 61.8% of natural flow and expansion. Does it somehow naturally effect the growth of heat at its maximum capacity without the loss of heat (so its maximum efficacy so to speak)?


    2) Flue Design: What would be the optimum flue size for maximum efficiency, Would it have to do with a percentage of the oven size? A lot of people are saying a minimum of 8inches but I'd like to find out the formula to this or how they come to that figure apart from the bigger the more draw,

    Any help and guidance would be greatly appreciated and credit and reference will be stated in a book that I will publish on building ovens.

    Thanks in advance

  2. #2
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    Hi, you seem to me to be over-complicating a simple situation. I don't mean to talk down to you, but it is just a fireplace. If I were doing this project, and I would love to have a wood fired pizza oven, I would make the flue and the opening around the same area.

    I would make the opening as small as is practical, maybe 14" for a 12" pizza plate. In the flue I would have a damper that can be regulated with a screw device for precise adjustments that stay put. It may not be a 2000 year old approach, but my truck has air conditioning in it so I see nothing wrong with going along with modern technology.

    Am I missing something?

    Make mine a meat-lovers with anchovies please.

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    Thanks for your reply, I guess I was trying to find out the exact measurements for the optimum efficiency of an oven with the entrance and flue design. Yes, perhaps I have overcomplicated the question but as I like to learn as I build it would be nice to find out what the calculations are rather than to slap it together and not know any principals behind it.

  4. #4
    Lead Engineer RWOLFEJR's Avatar
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    When the first of that type of oven was made I really doubt that the efficiency of the ratio of opening to oven size was any concern. They were looking for a structure that they could build and would hold up with what they had available. Today we can pretend that it was specifically engineered for preparing the ultimate pizza... but it was simply a container to hold the heat in, that could be built to last out of what they had plenty of ... free rocks. Then you have the opening... I'm pretty much certain that the opening was designed solely for its purpose, and with no "rule." The builder asked himself... "Self... How do I make an opening in this thing that will endure (arch) and how big will the things be that I need to put in it?" They would've been throwing crocks of beans or stew in there as well. I'd bet heavy that the cooks using those ovens back then probably played around with choking off the flue some (dampers). And they might also have covered part of the opening with a wet hide, or stack some wood needing dried at the front of the opening, to keep heat losses down. They surely fiddled with it...

    Imagine if some cook from way back would've had a non-stick pizza pan... they'd have been the talk of the town. Look... they'd say... Frank isn't putting his pizza right on those dirty rocks. He has a magic plate that nothing sticks to... oooooh.... aaaaahhh....

    That isn't to say that the furnaces and ovens of today aren't highly engineered... Just don't think the efficiency idea was a big factor when this type of oven came to be. Capture this heat to cook my food was all that was on their mind.

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    I applaud the idea of learning and understanding principles, please do not stop that approach. But, I suspect Bob is correct in that the arch was pretty-much, the only self-supporting structure of it's time. In the days before concrete self-supporting was the only option. Mud was probably used to fill in the gaps but it added little to no structural content. Probably how they discovered firing clay with some specific muds.

    The things of the distant past still pretty much hold to the old 1-2-Root(3) principles of what looks good and is in proportion.

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    yes, I totally understand the reasons for the arch, that is a given. It's like saying you need a window to see out from. However the question remains unanswered to the area size of the entrance and flue for optimum efficiency. Sorry if i sound brash, I just find that in todays understanding of thermal conductivity and the likes there would be a equation or formula. Surely in the manufacture of industrial kilns and wood stoves. Its just that I have scoured the engineering sites and have yet to find a answer. But It could be that there is no real equation and it just is more of an aesthetic situation if you want a large opening or small opening. Any other thoughts?
    P.S
    And thanks so much for your replies as well.

  7. #7
    Lead Engineer RWOLFEJR's Avatar
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    The word efficiency is what trips me up on this topic. Efficiency and brick pizza oven is an oxymoron. A modern day efficient fuel fired oven isn't left open. It will only have an exit. You pump air with your fuel to make your burn. So if efficiency is what you're after... you need to build a door for the thing and pump air in your fire box... more or less depending on your fuel and the desired temperature.

  8. #8
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    Brash or not, modern or old, the size of an oven has zero bearing on the method or efficiency to heat whatever it is inside.

    Ovens are made to encase what needs to be heated. Heat, and there are certainly some calculations for efficiency and BTU comparisons of fuels, but that is an entirely separate issue.

    Clinging to your hopeful-theory then, if a 4-cft oven required 10 burning sticks with a 1-sft area opening and flue, to reach 400F, then a 16-cft oven would need 40-sticks and a 4-sft opening and flue, and that just ain't so. The ten sticks will get the bigger oven just as hot, regardless of opening and flue areas, but take a while longer to do so.

    Best idea for you and this seemingly pointless pursuit is to go to some Native American Reservations and talk to some of the old Indians there. Ask to measure some of their bread baking Adobe ovens and scale it up or down to suit your patio or pizza pan. There are several Reservations near here and the old Hopi and Navajo people are an amazing source of information.

    I have several Hopi cookbooks for preparing and eating the local wild foods, meat and veg and they are simply awesome. Study their Farming technology, they have ratios of land-area to plant-type to natural watering and it makes our Farming methods look clumsy and ill-considered. I don't believe a single Hopi Male has ever used a tape measure to build an Adobe oven.

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    Thanks for your replies. Although nothing is pointless if it serves a goal, and if in the end the goal was not as expected then it was the experience and interaction that drives us. Everyone, everything evermore has a purpose.

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    Project Engineer CCR5600Design's Avatar
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    Quote Originally Posted by hahnlove View Post
    Thanks for your replies. Although nothing is pointless if it serves a goal, and if in the end the goal was not as expected then it was the experience and interaction that drives us. Everyone, everything evermore has a purpose.
    And if that purpose includes hot pizza and cold Diet Bud, then it's all good.

    Ron

  11. #11
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    Ron, I think you have a typo there.

    "includes hot pizza and cold Diet Bud,"

    should that not be...
    "includes hot pizza and cold Diet of Bud,"

    Are you serious? Dog-pee yellow club Soda?

  12. #12
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    "kettlepizza"

  13. #13
    Project Engineer CCR5600Design's Avatar
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    Quote Originally Posted by PinkertonD View Post
    "kettlepizza"

    He shoots, HE SCORES!!!!

    Where's the cooler?



    Ron

  14. #14
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    Quote Originally Posted by CCR5600Design View Post
    Where's the cooler?
    In transit, I got a couple as they are on special at the moment...

  15. #15
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    Hahnlove,

    The questions you ask regarding optimum flue dimensions are not only valid, but practical as well, since there are several different materials that are currently in vogue when constructing a flue for a wood-fired oven (WFO). It is true, to operate, a brick oven does not require a flue at all, however a flue does provide chef's relief from smoke and oven-exhaust during heat-up and operation. It is not true that a Pompeii-style brick oven is just a fireplace, and the numbers used to calculate the optimum flue size for a fireplace is indeed different than a WFO.

    The considerations in WFO flue design are not complicated, but involve fluid dynamics. The objective is to derive a minimum flue pressure level to match your WFO's internal volume and it's output. Note that part of the WFO's enduring (and magical) performance is not borne from 'burning sticks', but from the stored heat captured within the firebricks. In conjunction with conductive heat (hot floor), convective heat (breathing oven) the release of this reserved heat as radiation is what makes WFO dishes (including pizza) incomparable.

    There are a number of considerations that will allow you to 'tune' your oven so that it combusts fuel (breathes efficiently) in concert with the aforementioned radiation and conduction. These include door shape, height and width as well as door height-to-ceiling ratio, which you are already aware of.

    You want to go to http://www.fornobravo.com. There is a wonderful worldwide community with a wealth of knowledge to share, not just guesses or silly anecdotes. The beauty of the Forno Bravo Forum is that it is frequented by builders who can share real-life experience from their own and observed ovens. Their knowledge extends beyond the oven to all types of construction, thermal dynamics, heat-transfer and general refractory properties and, of course wonderful recipes. The best part is that if any of these gentlemen feel they are not qualified to advise you on a subject, they will tell you so.

    Good luck,

    Gianni
    Last edited by Gianni; 04-03-2012 at 02:00 PM.

  16. #16
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    Hi Gianni,

    Thanks so very much in pointing me in the right direction. Yes indeed I would like to work out the optimum formula for the fluid dynamics you mentioned in relation to my oven. Off I go down the yellow brick road to find the answers.

    Thanks again

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