Oven Fuel Consumption

[v12spirit]

Re: Oven Fuel Consumption

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.

[v12spirit]

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

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.

[jeeppiper]

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.

[snyderadam]

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

[v12spirit]

Re: Oven Fuel Consumption

I did the math for a 65 cm diameter igloo oven with bricks thickness of 5 cm and a temperature rise of 400 C and got a considerably reasonable result (compared to your actual data): 4820 g of dry wood.
Thanks david, and greenman for your input.

I apologize to you guys for this math mistake I did..
I just replaced the sphere volume (4/3)*(PI)*(r)^3
with (3/4)*(PI)*(r)^3
so I got this result: 4820 g
that should have been: 6948 g

The result is that my 3 factor is quite more safe so it can be reduced to a smaller factor that I hope one would one day do it.

[v12spirit]

Re: Oven Fuel Consumption

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

On the same basis, the result given for gas is: 2354 g of gas or (110226) BTU.
So again. Sorry ATK406 for the wrong results. They are now corrected.


Again. This makes the 3 factor more safe but less accurate.
See the very previous post on the previous page..

PS: I don't like to sound like academic people. But I DID find numbers the best way to express the world, even if the world turns out to be a bit different. Moreover. It was from here where I decided that my experimental steel oven should perform as I wanted my oven to perform, ant that is how it was built.

[v12spirit]

Re: Oven Fuel Consumption

Hey v12spirit,


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/my-pompeii-design-20467.html

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

Thank you,
Adam

Hi Adam,
Glad you still following the forum. After my corrections to the math mistake, the approximation of the 3 factor turned out to be safe enough, but unfortunately inaccurate. Hope someone interested would one day provide a "magic formula" for calculating the factor accurately.
Cheers.

[david s]

Re: Oven Fuel Consumption

Remember also that the amount of fire in the oven (fuel consumption) is also directly proportional to the volume of the combustion chamber.This is why even a small thin walled oven can take as long to heat up as a large one.