Author Topic: Earthstone 110 or FGM?  (Read 16246 times)

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Offline shuboyje

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Re: Earthstone 110 or FGM?
« Reply #50 on: September 30, 2012, 10:41:32 AM »
The challenge with using those diagrams is that they don't really accurately reflect the thermodynamics.  The air is obviously a continuum of temperatures, not quantum levels, and also the oven should be full of convection currents stirring the air, not to mention what happens when you move wood around or put a pizza in and out.

I don't disagree that a lower door might be important, but you would have to look at a lot more variables to scientifically come to that conclusion.  Maybe I will publish a paper in Pizza Science Magazine if I can get the Reep door to work.

This is actually not true.  In a properly built oven the air is very stratified, you can actually visibly see it via smoke, and you can read it slightly with an IR thermometer and you move down from the peak of the dome.  It's all about density and the pressure differences it causes.  Inside the oven you have a huge pressure difference between the ambient outside air coming in to feed the fire and the hot gases coming off the fire.  These lead to a huge pressure differential between the two gases and a VERY stratified environment.
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Offline TXCraig1

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Re: Earthstone 110 or FGM?
« Reply #51 on: September 30, 2012, 02:14:43 PM »
Now, a high door, like a high ceiling, can definitely be worked around.  If you throw enough wood at it, it will get plenty hot.  

And you will have terribly unbalanced heat that will burn the edges of your pies.

I'm still unclear how a couple extra inches of hot air at the ceiling is going to make a meaningful difference?

The temperature of the dome itself and the height of the dome are what really matter, right? The power radiated from the dome is a 4th order function of the temperature of the dome - for example, a 1200F dome would theoretically radiate more than 2X as much power as a 900F dome. Likewise, increasing the dome height decreases the view factor thus reducing the radiated power that strikes the pie. Compounding this, a high dome is harder to heat for the same reason. What role does the extra hot air play?
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Offline shuboyje

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Re: Earthstone 110 or FGM?
« Reply #52 on: September 30, 2012, 03:17:00 PM »
Have you ever cooked pizza in your oven without live fire Craig?  I understand the reasoning, and it is sound, but doesn't match what I have experienced in multiple ovens, I know we touched on this from a different angle in another thread.  Radiation off of the bricks doesn't have the power to cook the top of a pizza at neapolitan bake times, it's all about the convection.  Change the dynamics of the door and you change that convection.

Regardless of all the technical and theoretical stuff, the 950B works, and cooks a great Neapolitan pie from everything we have seen.  Added to that Antoine has offered a great solution for the case where the door height is an issue.  I think we are really talking about a moot point.
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Offline TXCraig1

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Re: Earthstone 110 or FGM?
« Reply #53 on: September 30, 2012, 04:07:13 PM »
Have you ever cooked pizza in your oven without live fire Craig?  I understand the reasoning, and it is sound, but doesn't match what I have experienced in multiple ovens, I know we touched on this from a different angle in another thread.  Radiation off of the bricks doesn't have the power to cook the top of a pizza at neapolitan bake times, it's all about the convection.  Change the dynamics of the door and you change that convection.

Regardless of all the technical and theoretical stuff, the 950B works, and cooks a great Neapolitan pie from everything we have seen.  Added to that Antoine has offered a great solution for the case where the door height is an issue.  I think we are really talking about a moot point.

That's where I was trying to get to - we know the 950B works. I think it is the low dome overriding other factors. 

I have cooked a pizza without live fire, and I don't like it. I don't however understand how does that relates to the issue at hand? The flames put off a lot of radiant energy, right? That's the difference between flames and no flames.
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Online Tscarborough

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Re: Earthstone 110 or FGM?
« Reply #54 on: September 30, 2012, 04:16:27 PM »
I didn't read the whole thread, but Scott's diagrams are not correct.  There is no inversion layer trapping superheated air against the dome, the convection is visible in the oven in use and it sweeps the dome.  Any superheated air is below that in the center of the dome where a rolling cloud of smoke is visible if enough logs are added to create smoke.

Offline Reep

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Re: Earthstone 110 or FGM?
« Reply #55 on: September 30, 2012, 10:01:58 PM »
Well, to partially close out this thread, I am scheduled to call Antoine tomorrow and discuss the options.  Either way it looks like I'm going with a 1500B (22-short) high (12.8" dome).  Thank you all for your valuable input.  It has been very helpful.

Rich

P.S. I start a new thread with pictures and progress of the construction.

scott123

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Re: Earthstone 110 or FGM?
« Reply #56 on: September 30, 2012, 11:21:33 PM »
I didn't read the whole thread, but Scott's diagrams are not correct.  There is no inversion layer trapping superheated air against the dome, the convection is visible in the oven in use and it sweeps the dome.  Any superheated air is below that in the center of the dome where a rolling cloud of smoke is visible if enough logs are added to create smoke.

Tom, the exhaust layer isn't trapping heat, it's the top of the dome that's trapping the heat as it rises. This really isn't all that complicated.  Hot air rises.  Cooler exhaust air sinks.  The rolling layer of smoke we've all seen at the door level is cooler then the air above it.

scott123

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Re: Earthstone 110 or FGM?
« Reply #57 on: September 30, 2012, 11:45:09 PM »
I'm still unclear how a couple extra inches of hot air at the ceiling is going to make a meaningful difference?

The temperature of the dome itself and the height of the dome are what really matter, right? The power radiated from the dome is a 4th order function of the temperature of the dome - for example, a 1200F dome would theoretically radiate more than 2X as much power as a 900F dome. Likewise, increasing the dome height decreases the view factor thus reducing the radiated power that strikes the pie. Compounding this, a high dome is harder to heat for the same reason. What role does the extra hot air play?

The extra hot air makes for a hotter dome. It's kind of like when you pre-heat a home oven using the bake function and then turn the broiler on. The convective air rising from the bake element below pre-heats the ceiling of the oven, so that when you turn the broiler on, the top of the oven AND the broiler are radiating heat down.  Just like the active fire/red hot coals in a WFO are doing the lions share of the IR, the broiler element is doing the bulk of the work, but the air heated ceiling plays a part. In a WFO, where the ceiling has significant thermal mass, this role is even greater.

In a perfectly cylindrical 42" oven, a 2" lower door translates into 264 square inches of additional brick surface area contacting the hot air and 2772 cubic inches (19 cubic feet) of additional hot air volume.  That triples the volume of the 1" head space created by the original door.  And that's with a perfect cylinder.  With a rounded dome, that 1" of head space with the original door could be 1/5 to 1/6 of the head space with the 2" lower door. 5 times more volume translates into 5 times the space for superheated air.
« Last Edit: September 30, 2012, 11:50:42 PM by scott123 »

Offline Reep

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Re: Earthstone 110 or FGM?
« Reply #58 on: October 01, 2012, 12:06:23 AM »
The extra hot air makes for a hotter dome. It's kind of like when you pre-heat a home oven using the bake function and then turn the broiler on. The convective air rising from the bake element below pre-heats the ceiling of the oven, so that when you turn the broiler on, the top of the oven AND the broiler are radiating heat down.  Just like the active fire/red hot coals in a WFO are doing the lions share of the IR, the broiler element is doing the bulk of the work, but the air heated ceiling plays a part. In a WFO, where the ceiling has significant thermal mass, this role is even greater.

In a perfectly cylindrical 42" oven, a 2" lower door translates into 264 square inches of additional brick surface area contacting the hot air and 2772 cubic inches (19 cubic feet) of additional hot air volume.  That triples the volume of the 1" head space created by the original door.  And that's with a perfect cylinder.  With a rounded dome, that 1" of head space with the original door could be 1/5 to 1/6 of the head space with the 2" lower door. 5 times more volume translates into 5 times the space for superheated air.

If that is true, and the primary factor, then should it take 5-6x as long for an FGM oven to heat up, and 5-6x the amount of fuel to keep it hot?  This isn't the case according to those who have used both door heights.  I think there are many more factors involved than just the hot air trapped above the door.

Also, remember that you can only count the dome space in the vicinity of the door, not the back of the oven.  Otherwise you could argue that a 1-inch wide and 16-inch high door on a 16-inch dome oven would not retain any heat, which isn't the case either.
« Last Edit: October 01, 2012, 12:10:47 AM by Reep »


scott123

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Re: Earthstone 110 or FGM?
« Reply #59 on: October 01, 2012, 12:15:39 AM »
If that is true, and the primary factor, then should it take 5-6x as long for an FGM oven to heat up, and 5-6x the amount of fuel to keep it hot?  This isn't the case according to those who have used both door heights.  I think there are many more factors involved than just the hot air trapped above the door.

I didn't say that hot air volume was a primary factor in pre-heating and high temperature maintenance.  It is a contributing factor.

scott123

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Re: Earthstone 110 or FGM?
« Reply #60 on: October 01, 2012, 12:43:41 AM »
Also, remember that you can only count the dome space in the vicinity of the door, not the back of the oven.

While the front of the dome gets hotter than the rear, the whole dome is heated by the superheated air.  If the whole dome is heated, the dome in the back gets counted.

Fabricate an insert, and pre-heat the oven with and without it. If the insert doesn't trim off at least 1/4 of the pre-heat time (and use at least 1/4 less wood to reach the same temps), I'll eat my hat.
« Last Edit: October 01, 2012, 01:07:16 AM by scott123 »

Offline TXCraig1

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Re: Earthstone 110 or FGM?
« Reply #61 on: October 01, 2012, 09:55:40 AM »
Tom, the exhaust layer isn't trapping heat, it's the top of the dome that's trapping the heat as it rises. This really isn't all that complicated.  Hot air rises.  Cooler exhaust air sinks.  The rolling layer of smoke we've all seen at the door level is cooler then the air above it.

Are you sure about that? My recollection is that the smoke after adding a log to a hot oven is at the upper level of the oven. Isn't that why there is a layer? The cooler, denser air below traps the particulate matter in the upper part of the oven?
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Offline TXCraig1

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Re: Earthstone 110 or FGM?
« Reply #62 on: October 01, 2012, 11:24:58 AM »
In a perfectly cylindrical 42" oven, a 2" lower door translates into 264 square inches of additional brick surface area contacting the hot air

And 264in2 of additional brick surface area and who knows how much additional mass that must be heated.


Quote
and 2772 cubic inches (19 cubic feet) of additional hot air volume.  That triples the volume of the 1" head space created by the original door.  

Triple sounds impressive, but consider the numbers. (N.B. 2,772 in3 = 1.6ft3)

Let’s say the superheated air is 1500F/1089K

2,772in3 = 0.0454m3

At 1089K, the density of air is .3243kg/m3, so you have 0.0147kg of air in the extra 2”

At 1089K, the specific enthalpy of air is 1,149kJ/kg, so the energy of the extra 2” of air is 16.9kJ or 0.017MJ

There is about 10.4MJ/kg recoverable energy in oak firewood, so the energy in the air in the extra 2” is about the equivalent of 0.0016kg or 0.06oz of oak.

Here is what that looks like (paper clip for scale):
« Last Edit: October 01, 2012, 11:40:34 AM by TXCraig1 »
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Offline Chicago Bob

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Re: Earthstone 110 or FGM?
« Reply #63 on: October 01, 2012, 11:42:59 AM »

Fabricate an insert, and pre-heat the oven with and without it. If the insert doesn't trim off at least 1/4 of the pre-heat time (and use at least 1/4 less wood to reach the same temps), I'll eat my hat.
Mmmm.... :chef:
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Offline Reep

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Re: Earthstone 110 or FGM?
« Reply #64 on: October 01, 2012, 11:46:19 AM »
Craig is giving me nightmares of may P-chem courses in graduate school.

It will be a while, but I will have a way to test the oven under both conditions.  I'll weigh out the same amount of wood, burn it under the same conditions and at the same time point after lighting I will take a temp measurement of the dome and floor and compare them.  It will be a while though, so Scott better not salt his hat yet.

Offline TXCraig1

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Re: Earthstone 110 or FGM?
« Reply #65 on: October 01, 2012, 12:05:39 PM »
It will be a while, but I will have a way to test the oven under both conditions.  I'll weigh out the same amount of wood, burn it under the same conditions and at the same time point after lighting I will take a temp measurement of the dome and floor and compare them.  It will be a while though, so Scott better not salt his hat yet.

Remember the most important of the "same conditions" are the moisture content of the wood, the ambient air temperature, and the initial heat content of the oven which, of course, is at least partially a function of the ambient air temperature. If there has been a big swing in ambient temperature (particularly if there is also wind) before the test, the starting temperature of the oven might not be a reliable indicator of heat content. Small differences in moisture content of the wood will make a big difference in the test.
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Offline Reep

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Re: Earthstone 110 or FGM?
« Reply #66 on: October 01, 2012, 12:27:28 PM »
Remember the most important of the "same conditions" are the moisture content of the wood, the ambient air temperature, and the initial heat content of the oven which, of course, is at least partially a function of the ambient air temperature. If there has been a big swing in ambient temperature (particularly if there is also wind) before the test, the starting temperature of the oven might not be a reliable indicator of heat content. Small differences in moisture content of the wood will make a big difference in the test.

Yeah, I can use the same lot of wood, and in SoCal we have a lot of very mild days, so consistency isn't a problem.  I can even take starting surface temperatures to make sure it's the same.  I have a Ph.D. in science so I think I can eliminate most variables and get a good result.  I will wait until the oven is well broken in too so we don't get any variation from that.

Now, off to order my oven.


Offline TXCraig1

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Re: Earthstone 110 or FGM?
« Reply #67 on: October 01, 2012, 12:56:01 PM »
Yeah, I can use the same lot of wood, and in SoCal we have a lot of very mild days, so consistency isn't a problem.  I can even take starting surface temperatures to make sure it's the same.  I have a Ph.D. in science so I think I can eliminate most variables and get a good result.  I will wait until the oven is well broken in too so we don't get any variation from that.

Now, off to order my oven.

I look forward to the results.
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Offline JConk007

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Re: Earthstone 110 or FGM?
« Reply #68 on: October 01, 2012, 08:14:57 PM »
Holy Moly way too complicated Lost me on the first X2?  I prefer results
The first 2 pictures  are from ???? oven and second 2 are from ????
same Batch of wood !
options are Mobile , Earthstone , and Four Grand Mere   ???
John
« Last Edit: October 01, 2012, 08:18:40 PM by JConk007 »
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Offline Chicago Bob

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Re: Earthstone 110 or FGM?
« Reply #69 on: October 01, 2012, 08:25:30 PM »
Gorgeous pies John...you deserve an icy cold Margareta...top shelf.  ;)
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Offline JConk007

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Re: Earthstone 110 or FGM?
« Reply #70 on: October 01, 2012, 08:54:46 PM »
its  about the different ovens and all the hoopla on heat loss, floor temp gases....  :o
Boggles my mind you cant go wrong with either as I said before. Build 1 . go to italy for a month and train ? come back its done and have some fun !
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Offline Reep

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Re: Earthstone 110 or FGM?
« Reply #71 on: October 01, 2012, 10:33:40 PM »
its  about the different ovens and all the hoopla on heat loss, floor temp gases....  :o
Boggles my mind you cant go wrong with either as I said before. Build 1 . go to italy for a month and train ? come back its done and have some fun !

Yeah.  Your first hand experience with all these was very valuable.  I still want to play with the door opening out of curiosity, but I know I will love my oven. 

Okay, so I like the bottom pie the best.  Which oven?

Offline JConk007

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Re: Earthstone 110 or FGM?
« Reply #72 on: October 01, 2012, 10:42:35 PM »
The Bottom Pie was from the  Four Grand Mere Party I did for RickM now a member here andcame over last night. You can confirm it in my FWF thread toward end  before bacholorette party same pics are there  ;)
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Offline Reep

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Re: Earthstone 110 or FGM?
« Reply #73 on: October 02, 2012, 12:27:21 AM »
The Bottom Pie was from the  Four Grand Mere Party I did for RickM now a member here andcame over last night. You can confirm it in my FWF thread toward end  before bacholorette party same pics are there  ;)

Sweet.  I know the look of the top is mostly the skill of the chef, but both top and bottom looked great. 

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Re: Earthstone 110 or FGM?
« Reply #74 on: October 02, 2012, 04:11:17 AM »
Triple sounds impressive, but consider the numbers. (N.B. 2,772 in3 = 1.6ft3)

Let’s say the superheated air is 1500F/1089K

2,772in3 = 0.0454m3

At 1089K, the density of air is .3243kg/m3, so you have 0.0147kg of air in the extra 2”

At 1089K, the specific enthalpy of air is 1,149kJ/kg, so the energy of the extra 2” of air is 16.9kJ or 0.017MJ

There is about 10.4MJ/kg recoverable energy in oak firewood, so the energy in the air in the extra 2” is about the equivalent of 0.0016kg or 0.06oz of oak.

According to this calculator, the ideal flow rate for an 8" diameter 2' high chimney at 800F is .05799 m3/s.  In other words, the superheated air volume that we're discussing (0.0454m3) is being replaced every second (give or take). Based on your numbers, for a six hour preheat, the necessary energy to maintain that 1089K temp in that 2" of volume would translate into 81 lb. of wood. Now, this is an ideal flow rate for one chimney.  Even if the chimney is only drawing 1/4 of that, you're still talking 20 lb. of wood.

That's a lot of wood. A lot of energy.  If the draw keeps that superheated air in contact with the additional 2" wide surface area for a second (or more), over the course of six (or more) hours, that's a substantial amount of heat transfer, especially considering the convective impact of the perpetually moving air.
« Last Edit: October 02, 2012, 04:25:50 AM by scott123 »


 

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