Tuesday, April 3, 2012

Brick Oven Specs #1

The materials from which a wood-fired oven is constructed vary greatly.  Brick oven materials are more specific.
The thermal requirements of a brick oven point to quality materials.  The oven is constructed of firebrick with a balanced heat-coefficient.  The equation to the below says it all...
Q = heat flow in input or lost heat flow , J/s = W
h = heat transfer coefficient, W/(m2K)
A = heat transfer surface area, m2
\Delta T = difference in temperature between the solid surface and surrounding fluid area, K
Right.   The long and short of it is: you want a material that absorbs the heat of the fire sufficiently well to radiate that heat for long periods of time so that the oven environment remains an even temperature. In this equation, the surrounding air is considered the 'fluid'.  If the difference between the temperature of the brick and the internal temperature of the oven are close to even, an ideal material has been used.
Porous bricks do not absorb heat well.  Very dense bricks absorb heat too quickly. Some materials transfer heat to the exterior of the oven. Others just heat up on the interior surface.  That said, you can make pizza in the most rudimentary oven because the 'live fire' is doing half the work. The real work of the brick oven begins after the fire is out.  The residual heat on my ovens last for about 30 hours. The hours you take to heat the brick of the oven is rewarded by 30 hours of cooking temperatures (these temps decline slowly giving you a baking environment suitable for a dozen types of foods)

Standard duty fire brick is well matched to brick oven construction.  They are inexpensive enough to justify using them over the more fragile and less efficient red brick one might pick up for free.  They are also exponentially (big mathematical word again)  more durable than cob.

For residential ovens and ovens not used on a daily basis, a single thickness floor is adequate but for professionals, a double layer of floor bricks is recommended.  Modular fire brick in the U.S. is 9" x 4.5" x 2.25" which makes laying them in herringbone or any other symmetrical pattern straightforward.  Some masonry yards supply fire brick that are not modular. Don't use them.
I've noticed that in other parts of the world fire bricks are commonly available in much bigger dimensions. But then I don't live there.
Arch bricks and fire brick tiles are also available but they are pricey.  An entire brick oven can be beautifully built using the one standard modular brick size.

The domes of my ovens are 4.5" thick. This gives me two dome segments per brick.  Standard duty fire brick cuts easily with diamond blades.

The next posts will address mortar, proportional dimensions of mouth, throat and door openings and heights of chimneys.  In the meantime keep this equation in mind (V= volume):

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