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  • Oven Fuel Consumption

    Hi FB Forum,

    I am in the design stage of my oven and have been wondering about how fuel consumption of ovens varies when considering oven material; brick or steel, and fuel type; gas or wood (or even diesel in some immense commercial ovens). Here is a concise report of my search that I would like to share with the forum.

    I would suppose the oven is made of either BRICKS or STEEL and is WOOD or GAS fired.

    If we want to know the fuel consumption of an oven, two factors are to be considered. The first is the ovens material response to being heated, which is known as the specific heat of the material. The second is the heat produced by burning the fuel, which is known as the heating value of the fuel. Knowing these two factors, one can estimate the amount of heat that is required to raise the temperature of the oven to the required temperature, and the amount of fuel needed to produce that heat. That is fuel consumption.

    As mentioned above, one should first know the material the oven is built from; bricks or steel, and its response to being heated, to know that, the concept of specific heat is appropriate.

    Specific heat of a substance is the amount of heat, measured in calories, required to raise the temperature of one gram of a substance by one degree Celsius.

    Q = c m dt

    Q: Thermal energy added (cal)
    c: Specific heat cal/gC
    m: Mass (g)
    dt: Temperature change (C)

    Specific Heat: Brick(0.2), Iron(0.11) cal/gC

    If one wants to know how many calories or Btus are required to bring an oven to pizza temp she or he must know the volume of the active hot chamber of the oven. This can simply be done according to the dome geometry and thickness not to forget the hearth.

    After knowing the volume, the mass can be further determined depending on the density of the material.

    Density (Mass of unit volume): Brick(2.1), Iron(7.87) g/cm^3

    Using the equation above, the left hand side is all known. Just multiply to get the calories.

    NOTE: These calories are the calories already absorbed by your hot chamber. There will be heat loss through the chimney and through poor insulation if any. A rough estimate is to multiply the resulting calories by three to ensure that at least third of your heat source will be absorbed by your bricks (steel).
    That sounds natural, doesn't it? These final resulting calories will be more than sufficient to bring a well built oven to the desired temp.

    To estimate the fuel required to fire your oven. The concept of Heating Value of fuels is appropriate.

    Heating value of a fuel substance is the thermal energy which is produced when a given amount of the fuel is burned under standard conditions.
    Heating value: Cooking gas(11800 cal/g), Dry wood (4000 cal/g), Diesel (9800 cal/g).

    This way you would just estimate the probably sufficient amount of fuel required for every single firing of the oven in question.

    NOTE: The data provided here are subject to change according to ambient temperature and pressure as well as material actual components. Any suggestions or corrections are welcomed.

    Here are some conversions that will help.

    C x 9/5 + 32 = F

    (F - 32) x 5/9 = C


    1 Btu = 252 cal
    1 lb = 454 g

    1 Btu/lb = 0.55 cal/g
    1 cal/g = 1.8 Btu/lb

    Cheers.
    V12spirit.
    Last edited by v12spirit; 03-17-2014, 10:07 PM.
    Why is this thus? What is the reason for this thusness?
    I forgot who said that.

  • #2
    Re: Oven Fuel Consumption

    Damn - I feel like an idiot, I just throw in wood until it gets hot
    Check out my pictures here:
    http://www.fornobravo.com/forum/f8/les-build-4207.html

    If at first you don't succeed... Skydiving isn't for you.

    Comment


    • #3
      Re: Oven Fuel Consumption

      Back in the eighties when gasoline was cheap, I can remember my dad filling the car tank every time he encounters a fuel station. Now I'm using his very same car and the gasoline prices are getting mad. I think twice before filling the car with gas or even using the car altogether. I do most of my mobility on my mountain bike.
      Last edited by v12spirit; 03-17-2014, 10:34 PM.
      Why is this thus? What is the reason for this thusness?
      I forgot who said that.

      Comment


      • #4
        Re: Oven Fuel Consumption

        i think there are too many variables to take into account like moisture content in the wood, heat loss up the flue, heat loss out the door, heat loss into the insulation layer and moisture contained in the refractory. I go with the others burn wood oven gets hot. I did weigh my wood once and found it took 4 Kg to get the thing to clear, but might get a slightly different result if I repeated the test because of the aforementioned variables.
        Kindled with zeal and fired with passion.

        Comment


        • #5
          Re: Oven Fuel Consumption

          The science is for the hi-tech. I subscribe to the notion of putting wood in until it is hot enough. Am in the happy place where wood is plentiful and free so I come from that perspective. I can understand the need for economy where wood is scarce and expensive.

          All of that aside, I doubt that I use more that 4 or 5 kg to get the oven up to clear.
          Cheers ......... Steve

          Build Thread http://www.fornobravo.com/forum/f3/n...erg-19151.html

          Build Pics http://www.facebook.com/media/set/?s...1&l=1626b3f4f4

          Forno Food Pics https://www.facebook.com/media/set/?...1&l=1d5ce2a275

          Comment


          • #6
            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.
            Last edited by v12spirit; 03-18-2014, 04:10 AM.
            Why is this thus? What is the reason for this thusness?
            I forgot who said that.

            Comment


            • #7
              Re: Oven Fuel Consumption

              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

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

              Thank you,
              Adam

              Comment


              • #8
                Re: Oven Fuel Consumption

                I will say that I find charts like these interesting, but not useful beyond general information, which centuries of using black ovens has already provided.

                In addition to variables David S mentioned, there are many others including material types, design ratios, atmospheric influences, etc,etc.
                Old World Stone & Garden

                Current WFO build - Dry Stone Base & Gothic Vault

                When we build, let us think that we build for ever.
                John Ruskin

                Comment


                • #9
                  Re: Oven Fuel Consumption

                  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?
                  Last edited by ATK406; 03-18-2014, 09:43 AM. Reason: punctuation

                  Comment


                  • #10
                    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.
                    Kindled with zeal and fired with passion.

                    Comment


                    • #11
                      Re: Oven Fuel Consumption

                      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

                      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). Ill view your project too.
                      Cheers.
                      V12spirit
                      Why is this thus? What is the reason for this thusness?
                      I forgot who said that.

                      Comment


                      • #12
                        Re: Oven Fuel Consumption

                        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 doesnt help, Ill give you the number of Btus required so it might describe the appropriate burner. Btus required: (76514 )Btu

                        Bests
                        Why is this thus? What is the reason for this thusness?
                        I forgot who said that.

                        Comment


                        • #13
                          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

                          Comment


                          • #14
                            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!

                            Comment


                            • #15
                              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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