Thermal Expansion Question - Dome vs Hearth

[Kemo]

I have a question concerning thermal expansion of firebrick at different zones of the oven. Specifically, where the dome meets the hearth. Is there a structural benefit to building the dome on TOP of the hearth as opposed to AROUND the hearth?

My gut tells me that the dome would expand a bit faster than the hearth based on the assumption that heat rises. With that in mind, it makes more sense, IMHO, to build the dome around the hearth rather than on top of it, keeping the dome completely separate from the hearth.

Any thoughts on this? I was hoping for some of the Engineers on the board to chime in with an independent analysis of this theory

[asudavew]

Re: Thermal Expansion Question - Dome vs Hearth

By hearth, do you mean the brick floor?

And I'm interested in the answer too.

[wlively]

Re: Thermal Expansion Question - Dome vs Hearth

Not an engineer but if I interpolated the correct data, the thermal expansion of firebrick is a constant curve up to 0.5% at a bit over 600deg C, a higher temp than our bricks will absorb. That translates to 0.0225 inches with a 4.5 inch brick. If you take that across a 42 dia then that would be 0.21 inches total or 0.105 inches per side.

I followed the original directions with the dome built on top of the floor. With ovens our size and temps, I don't think it matters very much.

Data found here; Frank M. Gentry: The Technology of Low Temperature Carbonization ~ Chap. 7: Operation, Design & Materials of Construction

[Kemo]

Re: Thermal Expansion Question - Dome vs Hearth

By hearth, do you mean the brick floor?

And I'm interested in the answer too.

Dave,

yeppers...i was wondering if the floor expanded horizontally along the tabletop at a different rate than the dome.

in the end, my real question should have been, "is it better to build the dome on top of the hearth or on the side?"

I was just wondering if one method creates less cracks over another.

[asudavew]

Re: Thermal Expansion Question - Dome vs Hearth

Dave,

yeppers...i was wondering if the floor expanded horizontally along the tabletop at a different rate than the dome.

in the end, my real question should have been, "is it better to build the dome on top of the hearth or on the side?"

I was just wondering if one method creates less cracks over another.

From what I can gather its seems about half the people build on the floor and the other build around the floor.

Ken is building around the floor and using spacers to create an expansion joint, and Hendo is doing the same thing.

I guess we need to ask them what they know.

[wlively]

Re: Thermal Expansion Question - Dome vs Hearth

I should have added, that there is definitely a difference in expansion rates between the dome and the floor. My thermocouples indicate that while the dome is around 600deg (315 deg C)(heat soaked, center of mass reading) the floor is around 300F (148 deg C) deg at the same instant. These reading were taken about 5 hrs after firing started, in which time I had already reached white dome (2 hrs), cooked 6-7 pizza's, and no longer feeding the fire.

So unless you are doing alot of very long firings it is unlikely that you will get the center of mass of the floor over 400deg F which means your amount of floor expansion is going to be very very small indeed. This also brings to mind that dry setting or loose setting the floor has a built in advantage. The spaces betwen the floor bricks can act like expansion joints and with max expansion possible probably around 0.25%, more than adequate to do the job. Where was this question a year ago, when I was designing the floor!

I guess a good question to ask is who has built their domes, ON the floor, and have at least 3 yrs of regular use? How is the dome holding up? I would think, well. Not trying to make that the standard, just asking since that would be the method most effected by expansion.

[james]

Re: Thermal Expansion Question - Dome vs Hearth

Nice thread. I think this gets to the important points. The floor and dome expand and contract at different rates, so that if you build the floor around the dome, you need to leave space. All of the FB ovens (Casa, Premio, Ristorante, Artigiano, and Modena) are made this way, as are virtually all of the well-made Italian ovens. The dome wraps around the floor. There are a few lower-end French made models that put the dome on the floor.

The benefit of putting the dome directly on the insulated hearth, around the oven, is that it is more thermally efficient. The heat of the cooking floor that is move sideways runs into the dome, where is can be stored (at least that's how it has been explained to me, and it makes intuitive sense).

For Pompeii builders, putting the oven dome around the cooking floor adds a layer of complexity, in terms of cutting the floor pieces and working around the round floor.

I think it makes sense to describe both methods, and let builders choose for themselves.

James

[Kemo]

Re: Thermal Expansion Question - Dome vs Hearth

so, if the dome expands at a rate faster than the hearth, then would it be necessary to use spacers? It seems to me that the firebrick floor would never get hot enough to "out run" the dome walls.

If this all makes sense, then I will be building my dome AROUND the hearth with no spacers.

Thanks for all the replies.

[wlively]

Re: Thermal Expansion Question - Dome vs Hearth

Yes the dome expands at a faster rate, but it expands in every direction. You also have to remember that the temperature across the dome or floor will not be uniform. There will be both "hot" and "cold" spots while the fire is heating everything up, so expansion will not be uniform across either. So, if you are building the dome around the floor it would probably be a good idea to leave the floor bricks loose and leave at least a very small gap between the floor and the dome. Given the values in the previously mentioned graph, I would think a gap of 1/16-1/8 inch would be more than sufficient insurance. I guess you could probably get a 1/16" gap with just the variation in the bricks, you probably don't need spacers, you will have to see.

But we have got to remember, this basic oven design tenants are centuries old, before computers, engineers, ect. Domes built on top, domes built around, they have both been done and work.

[Carl]

Re: Thermal Expansion Question - Dome vs Hearth

My build's going slowly right now, but I plan to build my dome around the floor with a space between the edge of the floor and the wall of the dome. I'm concerned about expansion, but not due to the firebricks expanding. There's a story about a barrel oven on the Yahoo brick oven list:

Each bake day at Wild Flour would (and still does) begin with a batch of sticky
buns which were so sticky apparently that the melted sugar overflowed the
shallow bakers pans and seeped into the cracks between the hearth bricks.
During the next firing the sugar would carbonize and expand eventually pushing
out the oven walls from the bottom

The full post is here: Yahoo! Groups - credit due to the original poster.

This oven had to be rebuilt - so either don't spill sugar on your hearth, or leave a gap! I'm a messy cook at the best of times, so I'm going to leave a gap.

[Unofornaio]

Re: Thermal Expansion Question - Dome vs Hearth

The post this came from on that site was in regards to arches failing in vault ovens because of design or construction. This supposed info was never verified by Alan (who posts to that site now and then) and quite honestly I don't believe it.
The root cause of this matter was that the early design of Alan's when expanded (for commercial sized ovens) did not anticipate the downward force the larger arch spans exerted on the side walls and as a result some failed. No others made sticky buns by the way

Even if this "sweet theory" was true the domes weight in a Pompeii oven is distributed evenly and this expansion scenario would not bring it down. In a barrel oven the side walls are under strain exerted from holding up the arch and the load of cladding and insulation on top of it creating an outward pressure on the sidewalls. Repeated expansion and contraction in an area under force (as in the side walls) will eventually fail. This early design proved that.

In regards to the gap for the Pompeii if you are going to do it on the outside in my opinion there is really NO question, you should leave a gap. Not because the oven is going to fall down but because having something expand in a confined space (confined by the dome bricks) is just asking for it, it needs to go somewhere.

As to the on or around choice I believe the impact difference is so insignificant its not worth the added trouble to put them on the outside.
I really think its just a personal design choice.

[Dutchoven]

Re: Thermal Expansion Question - Dome vs Hearth

Having built an oven that was loosely based on an AS I also discount the sweet theory. Many of his ovens are built with the walls on top of the floor bricks and they are usually not mortared down...at least from what I have seen. The reason those ovens fail is for just the reason Uno mentioned. Some of these larger ovens are reinforced with a harness created using allthread and angle iron in the top one third of the wall height. Another solution is to load the hauches of the oven by making the lower courses wider than that of the arch and not springing the dome quite so aggresively.
The difference of on or around is IMHO the same as Uno, personal design preference.
Best
Dutch

[Carl]

Re: Thermal Expansion Question - Dome vs Hearth

Thanks for the feedback on the story above - being a newbie I had no idea how realistic it was. I've got a lot of respect for Alan, and I wasn't aware that he hadn't verified the story . His book has been a big help to me so far, and was the inspiration that started me off building an oven.

[Kemo]

Re: Thermal Expansion Question - Dome vs Hearth

I've been mulling this over for a few days. Thanks for all the feedback guys. In the end, I have concluded that it doesnt matter if you build 'on top' or 'around' the hearth. Since the hearth will never expand faster than the dome, they are basically independent of one another. As long as they are not mortared together, there should be no interaction. The choice to build one way or another seems a matter of style. For me, my decision will be based on cost and labor

[Kemo]

Re: Thermal Expansion Question - Dome vs Hearth

sorry Carl, I meant labour