Author Topic: Conduction factor calculation  (Read 1240 times)

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Offline pizzaneer

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Conduction factor calculation
« on: July 28, 2012, 07:48:37 AM »
After looking at many, many pictures of WFO pizza compared to pizzas made in LBE's, home ovens, etc, one thought really came to mind.

WFO's usually have a hearth made from firebrick.
LBE's usually have a hearth made from cordierite (kiln shelf).

Cordierite is much more conductive than firebrick, due to its metal content.  Therefore, if cooking the same dough at the same TEMP on cordierite vs firebrick, the pie cooked on cordierite will char much faster.  So you have two variables in play to produce cook time.
 
What I would like to see is a table that will give us equivalent (by conduction factor per minute of cooking) temps on firebrick, cordierite, fibrament, etc.  For instance, 850 on fb = 770 on cordierite, etc. 
I'd rather eat one good meal a day than 3 squares of garbage.


Offline TXCraig1

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Re: Conduction factor calculation
« Reply #1 on: July 28, 2012, 08:06:20 AM »
Your chart would probably need a third dimension as the material thickness would also be a variable.

CL

Pizza is not bread.

Offline pizzaneer

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Re: Conduction factor calculation
« Reply #2 on: July 28, 2012, 08:09:53 AM »
Call it a factor, and perhaps the chart would become a spreadsheet like the Dough Calculator.  It would be most useful for people trying to assemble their setups or trying to duplicate other people's results using different hearth material.
I'd rather eat one good meal a day than 3 squares of garbage.

Offline TXCraig1

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Re: Conduction factor calculation
« Reply #3 on: July 28, 2012, 08:19:48 AM »
Have you seen this thread?

http://www.pizzamaking.com/forum/index.php/topic,16663.0.html

It would be interesting to expand on this work incorporating the question you're asking here. Scott touched on it, but we never really looked into it.

CL
Pizza is not bread.

Offline pizzaneer

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Re: Conduction factor calculation
« Reply #4 on: July 28, 2012, 10:12:41 AM »
Craig thanks for the link.  Scott's comment is actually very close to what I have in mind.

"I don't know how deep you want to get into this, but instead of just looking at thickness, material and temp, and how those relate to bake times, you could drill down on the material and get density and specific heat capacity, as well as specific heat capacity per volume."

It should be more than possible to come up with a conduction factor that could be applied to temperature to result in a specific bake time.  For example, that means saying that you want B bake time, equivalent to a WFO running at T temp.  The equation ideally would work like this:  T * cf(th) = B, where cf is a known product per material of conductivity and th is thickness.   You solve for T.

Insulation, convection and radiance is not really my concern at this point.  Just the bottom bake of the crust.  Eventually, with much more work, they could be approximated, but they could never have the specifity of going by a MDS per material.
I'd rather eat one good meal a day than 3 squares of garbage.

Offline shuboyje

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Re: Conduction factor calculation
« Reply #5 on: July 28, 2012, 11:45:25 AM »
I've been playing with some new Finite Element Analysis software that could probably do this study pretty easily.  I'll play around with it a bit when I get a chance.
-Jeff

Offline pizzaneer

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Re: Conduction factor calculation
« Reply #6 on: July 28, 2012, 12:07:46 PM »
Doing a search on thermal data of cordierite vs fibrament led me OF COURSE right back to this forum, one of red_november's posts:  http://www.pizzamaking.com/forum/index.php/topic,5645.msg47871.html#msg47871


FibraMent
Density: 1.762 g/cm3
Thermal Conductivity: 0.69183088225 W/m-K
Maximum Temperature: 538 C
Strength
Flexure: 10.58345 MPa
Compressive: 73.29127 MPa

Cordierite
Density: 2.6 g/cm3
Thermal Conductivity: 3.0 W/m-K
Maximum Temperature: 1315 C
Strength
Flexure: 117 MPa
Compressive: 350 MPa
I'd rather eat one good meal a day than 3 squares of garbage.

Online scott123

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Re: Conduction factor calculation
« Reply #7 on: July 29, 2012, 12:59:31 AM »
While November's figures are very useful, in the context of that thread, for showing how, in most applications, cordierite is far superior to Fibrament, the conductivity quoted for cordierite doesn't tell the whole story. Cordierite varies greatly from manufacturer to manufacturer.  It can be extruded or it can be pressed. The formula is rarely the same. The ingredients vary, the density varies, and, most, importantly, it can vary in conductivity. From the research that I've done, 3 is towards the upper end of the conductivity spectrum, with some cordierite clocking in as low as 1.5.  Density can tell part of the story (higher density usually translates into greater conductivity), but the formula can impact conductivity as well.  The greater the alumina, the greater the conductivity.

Firebrick suffers from similar variations, although the parameters are a bit tighter.  Here's an example of a range of firebricks based on alumina content:

http://www.traditionaloven.com/articles/84/firebricks-heavy-dense-fire-clay-bricks

As you can see, the conductive ranges from .98 to 1.13.  The article references '18% alumina' bricks but doesn't go into technical specs for those, so .98 is most likely not the lowest parameter. I wouldn't be at all surprised if someone was making non insulating firebricks with conductivity as low as .75.

Specific brands of retail cordierite stones probably (hopefully) have static specifications that are easily graphed/analyzed, but, since we don't recommend retail cordierite stones (only kiln shelves), I'm not sure how useful that information would be.

If cordierite and firebricks were single entities with consistent formulas/properties, then this kind of analysis would be useful.  The way it is now, though, is that there are too many variables.


 

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