#11  
Old 08-17-2011, 08:00 AM
GianniFocaccia's Avatar
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Default Re: What are the consequences of building your dome too high?

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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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Old 08-17-2011, 08:08 AM
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Default 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 at 08:22 AM.
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Old 08-17-2011, 08:26 AM
Peasant
 
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Default Re: What are the consequences of building your dome too high?

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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 havenít seen any evidence.

Is this ratio just our "urban legend"?

Iím 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.
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Old 08-17-2011, 08:49 AM
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Default 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 at 12:59 PM.
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Old 08-17-2011, 09:54 AM
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Default 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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Old 08-17-2011, 10:00 AM
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Default 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.
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Old 08-17-2011, 10:44 AM
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Default 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

Last edited by SCChris; 08-17-2011 at 10:51 AM.
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Old 08-17-2011, 01:55 PM
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Default 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.
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Old 08-17-2011, 02:40 PM
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Default Re: What are the consequences of building your dome too high?

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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.
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  #20  
Old 08-17-2011, 06:11 PM
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Thumbs up 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.
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