What are the consequences of building your dome too high?

[shuboyje]

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

As for my opinion on the topic, I think the 63% ratio is way over emphasized. If you are building sure build to it, why the heck not, but otherwise wait until there is an issue to worry. The Quebec ovens didn't have flues for instance. We also have multiple door shapes. A 63% hemisphere is not the same as a rectangle for instance, but nobody mentions or considers this.

[SCChris]

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

Shuboyje,

I agree that there are so many variables and door shape is just one of them.. A rectangular Alan Scott oven is a very different animal than a hemisphere FB style oven. And I definatly agree you build it to the 60-65% if you're building and if you didn't and have real problems address these problems and move on..

Santino, Fire it up and see where your oven goes.

Chris

[david s]

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

In my experience altering the door height makes little difference. A friend of mine wanted an oven with a higher door, so he could fit in larger roasts (my oven is small) So I cast an extra ring around the base of the dome then placed the cast dome on top of this. This altered the dome height ratio from the original 64% to 69% I was worried it would not perform well, but there is no perceptible difference in the way it operates compared to the original.

[GianniFocaccia]

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

In my experience altering the door height makes little difference

I agree. Since oven exhaust leaves the oven top-down, as long as the top-of-the-door to the top-of-the-dome ratio remains constant, the distance from the dome ceiling to the floor will not influence exhaust to a giant degree.

Door shape may make a bigger difference in how an oven retains its heat, but I would think regardless of its door geometry, every oven fills it's dome with the hottest gas at the top until a balance is reached. Like David says, I'd guess anything +/- 5% of the golden 63% works.

[Lburou]

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

We're just going to have to wait for some fires to see how well it does as built.

Santino, In your shoes, I'd be looking for a local pompeii oven to compare to your oven's performance.

My dome was two inches higher than half the diameter of the base. The door is 63% of the dome height (about 1.5 inches higher than without the extra two inches of dome height). Works fine so far as I can tell: 750 F in 40 minutes with mid sized fire. 1,000 F in 75 minutes.

[shuboyje]

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

When it comes to efficiency the lower and smaller the door opening the better in my opinion so long as it is big enough to let in enough air. That really is the balance we are trying to achieve, a door just big enough to load food and just big enough for enough airflow.

Stack effect drives natural draft. Stack effect creates a certain amount of pressure. That pressure is the driving force behind air velocity. Air velocity combined with cross sectional area gives you the air volume. A hemisphere has less cross sectional area then a rectangle at the same height, therefore a lower rectangle is capable of the same amount of air flow at the same velocity given the same pressure as a higher hemisphere. Now if the smae amount of air is coming into the oven at the same speed and the same amount of air is leaving the oven at the same speed, shouldn't the oven function nearly identical minus turbulence differences which should be minor? To me this proves door shape does play a role, albeit an overlooked one.

I hope that makes atleast a little bit of sense, it does to me but airflow is a big part of what I do for a living and I have a pretty strong understanding of it.

I agree. Since oven exhaust leaves the oven top-down, as long as the top-of-the-door to the top-of-the-dome ratio remains constant, the distance from the dome ceiling to the floor will not influence exhaust to a giant degree.

Door shape may make a bigger difference in how an oven retains its heat, but I would think regardless of its door geometry, every oven fills it's dome with the hottest gas at the top until a balance is reached. Like David says, I'd guess anything +/- 5% of the golden 63% works.

[Santino]

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

[QUOTE=Lburou;119205]Santino, In your shoes, I'd be looking for a local pompeii oven to compare to your oven's performance. QUOTE]

Well if there is anyone in the Charlotte, NC that would be willing to let me take a look at their oven in action, I'd be all for it. If anyone is nearby that would be up for it, let me know.

[Lburou]

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

Santino, In your shoes, I'd be looking for a local pompeii oven to compare to your oven's performance.

Well if there is anyone in the Charlotte, NC that would be willing to let me take a look at their oven in action, I'd be all for it. If anyone is nearby that would be up for it, let me know.

Do a search (showing results as 'posts') for 'Charlotte' here on fornobravo.com and you will find over thirty posts that mention Charlotte amidst several threads. That will be a place to start, it is likely there is another pompeii oven owner happy to share.

Good Luck

[SCChris]

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

Shuboyje, I took a look and didn't find the reference within the book.

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.

It's not really important other than it would be nice to have some research somewhere that was the basis of the rule..

I've run into reference to the book a few times and had found it on Amazon for sale, but was delighted that I could download it.. Interesting read.

Shuboyje, you indicted that you have some basis in airflow.. If it's not prying what do you do or did you do for a living?

Chris

[Aegis]

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

Here is an OLD post that gives me hope that my oven will be fine. My dome height is 20.2 inches. and arched door is 12.5 inches high. My curing fires seem to be fine so far, although they are small. Draft up the flue is great as only some small wisps of smoke leave the front door, all else is up the flue and out!

Here are dome shape and opening size calculations I have made that might be helpful. I don't think it rocket science other than not making your opening so small you cannot get your favorite peel or pan into it (which I did on my first oven because I was trying to optimize it for bread), and so large that you loose heat too fast. If you stay roughly within these guidelines you should be safe:

Oven Sizes
Interior D Interior H Opening W Opening H Volume Ratio
Casa90 35.4 16.5 18.5 9.8 0.31
Casa100 39.3 15.3 18.9 11.8 0.37
Casa110 43.2 15.3 18.9 11.8 0.34


Premio90 35.4 19.7 18.9 12.2 0.33
Premio100 39.3 19.7 18.9 12.6 0.31
Premio110 43.2 19.7 18.9 12.6 0.28

To get an idea of curve difference, you could look at the prefab data sheets on our site. There are graphics that show the curvature difference between the two style. In fact, our producer calls their low style (our Casa) "Volta Bassa" or low vault.

James

Hope this helps
John

[Hank10746]

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

Hi
I would say this came about the same way a lot of other things did. Communities of people built what they thought was right each one in the community doing according to his or her opinion. Then after a while after seeing the differences and what seemed to work best they Incorporated the best ideas they found from each in the ongoing search for perfection. Perfection, not being able to do all jobs perfectly but being able to do the jobs it was intended for. So I am sure there were trade offs when needing to heat and cook in the same oven. Today we have forced air heat and Micro waves steam ovens inferred heat lamps and a myriad of other ways to cook or heat. So today because of Computers the Internet we can boil down to 1 or 2 inches being the difference between a bread oven and a pizza oven. But in most cases I would bet that even the worst of both would do a better job of either than what we are used to. The only real problem I see with a home oven is to make sure the flue and door opening are of the size to make sure smoke travels up the flue and not out the door. I would almost bet there is a program that let's you plug in the size and dimensions and what the intended needs were and it would give you a heat profile needed with all wall sizes flue and door openings needed to be the most efficient possible. I think Forno Bravo has come close enough with instructions for us to build a oven all of us would be happy to have either kit or brick built. Personally I want the Pizza oven because I believe that if insulated right in the cool down process it will get down enough to be able to cook bread perfectly. At least enough for my family.

[Neil2]

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

Heuristic methods have determined that 63% is the optimum ratio. This has been discussed at length before. Door width or shape makes little difference.

More or less than that ratio will still work but will not be the optimum.

[Hank10746]

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

Just in case
"Heuristics stand for strategies using readily accessible information to control problem-solving processes in man and machine. An analysis of the nature and the power of typical heuristic methods, primarily those used in artificial intelligence and operations research, to solve problems in areas such as reasoning, design, scheduling, planning, signal interpretation, symbolic computation, and combinatorial optimization. It is intended for advanced undergraduate or graduate students in artificial intelligence and for researchers in engineering, mathematics, and operations research."

Another way of saying." I would almost bet there is a program that let's you plug in the size and dimensions and what the intended needs were and it would give you a heat profile needed with all wall sizes flue and door openings needed to be the most efficient possible."

[Lburou]

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

IF there is such a program, it would be proprietary and very close hold information.....We aren't likely to see it until someone 'in the know' discloses it to the puublic.

[Hank10746]

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

Short search not sure this is DIY type program or business but further searching might find one.

Refractory Design : heat transfer calculation, insulation layer, furnace refractory lining


The whole thing I tried to say is this is a supposed to be fun not something having to to be within .0001 of a inch to work. I am sure that for perfect combustion you would need surfaces that were smooth so the flame would hug the wall's with no disturbance. There would be a wall thickness that would give you a perfect temp for the least amount of wood fired. Even if the door opening was to big you should be able to fabricate a door that would work for heat up then use another door for holding the temp in. Again if I was a betting man I would think you could be off by more than 2 inches on all of the design and you would be happy with the product produced. I don't remember off hand but I think this started with a oven with a rise of about 2 inches higher and a door opening about 1.5 inches to big. Other than more wood I don't think he has anything to worry about. He will get nothing but complements for the food produced.
Again I did not use Heuristic methods to back up what I thought?