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  #11  
Old 03-18-2014, 03:40 PM
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Default Re: Oven Fuel Consumption

Quote:
Originally Posted by snyderadam View Post
Hey v12spirit,

I'm currently an engineering student and I give you props for your scientific approach. Your analysis is exploiting the principle of conservation of energy (Qcold=-Qhot) which is totally valid with given constants (found in tabulated data). As you mentioned, this equation will not be precise because of heat loss through the chimney, insulation, etc.. I am not sure if, like you said, a good approximation of the heat needed to saturate the oven will be three times your calculated value (in compensation for heat loss). If possible, you might even be able to analyze the heat loss due to the insulation. See if you can find the R-values of the insulation types you are considering.

However, I due want to stress that this analysis is for ideal conditions with "ideal" constants. For example, specific heat values are generally calculated (through calorimetry) for a given substance at room temperature and at atmospheric pressure. The actual heat capacity and absorption of a given substance will change with an increase in temperature and possibly pressure-based on your design (especially with a high change in temperature assuming you will be cooking at 500F+). Furthermore, you might find it hard to find the corresponding specific heats and R-values for given substances. If you have already found them, more power to you. I personally found it hard to find all of these values (especially from masonry shops who did not even know where their firebricks came from).

To conclude, I think the approximation of heat needed you are calculating will be fairly accurate (as in use of the fuels). However, the heat lost in your oven system should be the biggest concern. If possible, expand a bit on your analysis and extend it to better approximate the heat loss (i.e. individual heat losses for each insulation brands in concern). I look forward to your project and would love to hear on any of your design choices. I too am in the process of designing a pompeii style oven. If anyone wants to see my first design concept and even comment go to:

http://www.fornobravo.com/forum/f17/...ign-20467.html (My Pompeii Design)

or search "My Pompeii Design" in the search bar.

Thank you,
Adam
Hi snyderadam,
I think that heat loss through the insulation of a well built oven will be almost negligible compared to heat loss through the oven opening. So the oven geometry and the door area compared to oven volume will determine most of the heat loss. I hope I find an article that addresses that.
You raised an interesting point when you brought up the fact that specific heat and other parameters change with temperature rise inside the oven “(especially with a high change in temperature assuming you will be cooking at 500F+)” and this has been a confusing question to me. I mean how do I guarantee the validity of the equation Q = c m dt while c is no longer constant? I hope you tell me if you know.
Thanks for being interested in my build. Search the thread (Brick oven reconstructed to a steel one). I’ll view your project too.
Cheers.
V12spirit
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  #12  
Old 03-18-2014, 03:44 PM
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Default Re: Oven Fuel Consumption

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Originally Posted by ATK406 View Post
Interesting discussion for those inclined to over analyze things (like me) and aren't afraid to speculate, fully aware that there are no perfect solution/answers to many such things. For a wood fired oven this would seem to be a purely academic exercise - as stated before -"If it is not hot enough use more wood, if it is too hot wait". However I believe V12 has a more practical purpose. He is trying to scale his oven to match the gas fired heat sources available to him (or vice versa). To that end these calculations might be useful.

To get a better approximation of the heat loss we could place a pan of water (of known mass and temperature) on top of the chimney cap and measure the change in temperature over time. We should also approximate the amount of heat (fuel) we put into the oven. Only a portion of the heat lost up the chimney will be absorbed by the water but it might provide some empirical evidence to support the (3x factor used in the calculations above). I would do this the next time I fire up my oven except I risk breaking my neck trying to get up there with a pan of water and my digital thermometer – and ridicule from my family - but I might do it anyway – in the interest of science.

V12,
What did your calculations yield in regards to the size/qty of gas burners that you will need for your oven?
Hi ATK406

Your guess is quite right. I did the search for running the oven on gas. I appreciate your generous contribution of the water pan experiment and would like to hear the results hopefully you will not hurt your neck.
My calculations regarding gas give only the mass of consumed gas. I got (1634 g) of gas. If that doesn’t help, I’ll give you the number of Btus required so it might describe the appropriate burner. Btus required: (76514 )Btu

Bests
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  #13  
Old 03-18-2014, 04:35 PM
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Default Re: Oven Fuel Consumption

Hey v12spirit,

Heres a link to the derevations of specific heat equations:

2.4 Specific Heats

Thermodynamics involves the use of partial differentials, thinking U as a function of time, volume, etc.. With various conditions (isobaric, isothermal, etc.), one can derive the fitting equation for their specific case. If this webpage looks like a foreign language, or even if you do not know what a partial derivative is, I suggest you not stress too much more on this analysis. I myself am not an expert in thermodynamics, but was just pointing out some flaws in the assumptions which arise from your equations.

I think your best bet is to try to find some empirical evidence to analyze your design. Do some tests. Design your own experiment (similar to the water pan idea, but more controlled if possible). Right now I am gathering various refractory materials to decide which behaves the most fitting for my oven design. I plan to do a simple test of heating and cooling the refractory materials, measuring the temperatures during a time interval, and plotting the data in various ways.

If you can not find your own data, all my suggestions are to find someone who has experience with the same materials. I am definitely new with wood/gas fired ovens and am learning as I find my own information also. I will definitely post my data and information I find out about the refractory materials I will test.

Good luck,
Adam
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  #14  
Old 03-18-2014, 06:29 PM
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Default Re: Oven Fuel Consumption

I just want to say, a good engineer knows when they have hit the point-of-diminishing returns is when it comes to trying to define the thermodynamic characteristics of something as fundamental as a wood-fired oven....If you need an equation, it all boils it down to this:

combine firebrick + mortar + insulation with some blood/sweat/tears = WFO

then add: (wood + oxygen + ignition source) to the WFO

Done!
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  #15  
Old 03-18-2014, 07:10 PM
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Default Re: Oven Fuel Consumption

Hey jeeppiper,

I like your formula haha. You have to realize as an engineering major I deal with the physics of things. It only comes naturally to me to try to exploit mathematical approximations, theories etc. in a design process. I realize many designs are proven completely functional and appropriate. However, my thoughts are always how a system could be improved in some area of concern. If you want a matrix equation corresponding to your equation :

(firebrick, mortar, insulation) + (blood, sweat, tears) s.t. (blood, sweat, tears). Solve this equation for the eigenvalues and corresponding eigenvectors and you end up with a result of WFO. Just kidding, this is just a pure math joke.

Adam
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  #16  
Old 03-19-2014, 12:09 AM
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Default Re: Oven Fuel Consumption

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Originally Posted by david s View Post
There is also a further consideration to complicate matters and that is the principle that heat loss increases with temperature. For example when firing a kiln less fuel is used from ambient to 1100 C than is used from 1100 _1260 C. The same thing applies to a WFO as we all notice the oven consumes wood at a much faster rate after the dome has gone white.if you check the R values or thermal conductivity of materials the figures are different as the temperature rises.
That is the same interesting point that snyderadam has raised. This has been actually puzzling me and I used to express it this way:
What guarantees the equation Q = c m dt to be valid so long as c changes as the temperature rises?

Thanks david for your careful notice. I hope somebody can find the answer.
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  #17  
Old 03-19-2014, 12:49 AM
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Default Re: Oven Fuel Consumption

Quote:
Originally Posted by snyderadam View Post
Hey v12spirit,

Heres a link to the derevations of specific heat equations:

2.4 Specific Heats

Thermodynamics involves the use of partial differentials, thinking U as a function of time, volume, etc.. With various conditions (isobaric, isothermal, etc.), one can derive the fitting equation for their specific case. If this webpage looks like a foreign language, or even if you do not know what a partial derivative is, I suggest you not stress too much more on this analysis. I myself am not an expert in thermodynamics, but was just pointing out some flaws in the assumptions which arise from your equations.

I think your best bet is to try to find some empirical evidence to analyze your design. Do some tests. Design your own experiment (similar to the water pan idea, but more controlled if possible). Right now I am gathering various refractory materials to decide which behaves the most fitting for my oven design. I plan to do a simple test of heating and cooling the refractory materials, measuring the temperatures during a time interval, and plotting the data in various ways.

If you can not find your own data, all my suggestions are to find someone who has experience with the same materials. I am definitely new with wood/gas fired ovens and am learning as I find my own information also. I will definitely post my data and information I find out about the refractory materials I will test.

Good luck,
Adam
Hi snyderadam,
Thanks for the link. I'm supposed to manage with differential equations (I hope that) so long as I am currently developing a simulator for ski driving and it involves differential equations.. I opened the page but it will take a while to pick up some useful information from it due to being kind of sophisticated.

I took a glance at your oven plan. It looks like being well engineered.

Bests.
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  #18  
Old 03-19-2014, 07:53 AM
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Default Re: Oven Fuel Consumption

Adam, I hope I didn't offend you. I was just trying to keep you from spending a lot of time on something that might end up steering you down the wrong direction.

Believe it or not, the WFO is a very complex and dynamic system. and trying to characterize it with some equations might mislead you and cause you to build an oven that doesn't function as expected.

If you want to better understand your design, I would recommend downloading a copy of OpenFOAM and learning it. OpenFOAM is a free, open-source CFD package that will allow you to simulate the airflow in your oven. I would first start by a simple aerodynamic flow through your oven (no heat). This will allow you to see your mean flow path and where your might have stagnant flow (cold-spots). You can then expand your to estimate your heat transfer on the dome by simulating a combustion source. I'm sure OpenFOAM will have some sort of thermodynamic/heat-transfer capability.

Doing a simulation will get you closer, however you will still need to understand its limitations. The WFO is a very dynamic system. The heat produced is highly coupled to the airflow. What I mean by this is that the more airflow you have, the more oxygen is sucked in, the more fuel is combusted, which creates more heat. The more heat in the chamber/chimney, the more temperature differential, the more pressure differential....E-I-E-I-OOOOO!

So good luck finding an equation that will characterize the WFO! Personally, I would take advantage of the experience and know-how of the folks in the WFO community. Hope this helps.
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  #19  
Old 03-19-2014, 11:02 AM
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Default Re: Oven Fuel Consumption

Hey jeeppiper,

You didn't offend me, don't worry about that haha. I will definitely look into OpenFOAM. I never did computer modeling of thermodynamics. Actually, I might even use a program next semester for my thermodynamics course. Thanks for the advise.

Adam
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