Improving heat-up efficiency - currently only 0.7% efficient?

[thelightningking]

I have a Forno Bravo Casa90 kit installed. I did some calculations and figured out that in order to take the oven up to pizza temperature, about 0.7% of the energy burned out of the oak ends up stored in the walls/floor,. The rest, 99.3%, must go up the chimney or out the front door (very little escapes through the dome). A few details of this calculations are below.

My question is - does a really low efficiency sound right to people? I guess I'm surprised it's so inefficient. What kinds of things can we do to improve that? For example, I hadn't previously insulated my chimney, it's just exposed metal, is that bad?

Does any particular configuration of the startup fire work better for efficient energy delivery to the dome? I usually try to make a fire that's not too wide but fairly high so the flame hits the dome directly and rolls along it as much as possible.

What if one kept the door closed but then had a small opening and had some kind of fan pumping air in to feed the fire? Anyone ever try something like that?

Improving that 0.7% number would dramatically speed up the heat-up process and/or require much less wood.

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Calculation:

I have a Casa90, 2" thick walls and 36" diameter dome. I estimated the specific heat of FB refractory concrete to the 690 J/Kg/K based on the materials composition they posted. From that I get 61 MegaJoules of energy to bring temp from 20 C to 500 C. I assumed oak heat density of 91 trillion Joule per m^3, and that I use 0.09 m^3 of wood (or, 2.5% of a cord) to get the oven up to temperature, so that's 8.3 GigaJoules of energy burned.

It's possible I got something wrong but I've checked it over a few times.

[Petter]

You're math is not yet correct. About 8-12 kg would be expected. Please recalculate.

However, to get the best use of the wood during heat up: Burn ~1.5 kg wood per m2 inner surface area per h. Make a blast door to control the air flow. If you reduce the incoming air, the cooling effect is reduced. If you reduce the air too much, the fire will die out and/or you will get a lot of smoke. A starting point is to reduce the opening size with 50-75 %.

[JRPizza]

Interesting post - I've never thought about how efficient my oven is, but I don't doubt most of the heat goes up the flue during the heating process. The only recommendations I would have to get more heat in the dome is to push the fire to the rear wall as soon as the very center of the dome clears, then move it to the left and right to burn off all the carbon and try to heat the dome slowly and uniformly. You might already be doing this. I also occasionally place my door across the front of the opening at an angle so the fire gets a fast draft and the door heats up, but usually only do it when I am getting ready to shut the oven down and prepare for retained heat cooking.
Lastly, did you include the mass of the floor in your calculations? That is a bit of thermal mass and if not included could skew your numbers a bit.

[Petter]

Make the fire in the middle where the dome is high. If you make a rolling flame with cold walls, the combustion efficiency drops since the wall steal heat from the flame.

[david s]

Having built around 7 pottery kilns over the years my first oven was based on ancient Roman kilns, that is a hemisphere with the flue placed centrally at the top of the dome. This creates an updraft system with excellent draw and no tunnel to have to work past at the oven mouth. The disadvantage is that the flame tends to jump straight to the the flue and lots of energy is lost. Of course in a kiln the chamber is filled with kiln furniture and wares so the flame needs to travel through these before exiting out the flue. However for an oven the chamber is fired empty and therefore a lot of the heat is lost out the flue. By contrast the Pompeii style oven is a cross draft system where the heat from the flame is captured by the dome. If the flame is captured by the dome little heat is lost out the flue. Modern kilns take this even further by placing the flue exit at the base of the chamber, pulling the flame down before exiting, This is called a downdraft system. (see my kiln build here https://community.fornobravo.com/for...kiln#post14564) This system is highly efficient as well as producing good circulation which is required to produce even and consistent temperature for glaze maturation.

The Pompeii style oven is based on that of the bakery oven at Pompeii which uses the cross draft system, however I noticed quite a few ovens in individual homes had very small ovens that were of the updraft system. I was puzzled by this , but after working both types I'm pretty sure the choice of an updraft oven for the private homes was because although there's some efficiency loss, there is no problem with smoke at start up.You wouldn't want to fill your house with smoke. The lack of a tunnel in front of the oven mouth is also a huge advantage. These oven were probably not fired every day and also doubled to warm their homes. By contrast the bakers oven probably never actually got cold, being fired every day and fuel efficiency would be of a far greater concern.

Regarding efficiency I did measure the amount of fuel used to take my small oven up to 350C, the dome was beginning to clear, and it was only 4kg. That's pretty efficient. I think you're 0.7% efficiency calculation is out. The large oven mouth is not that efficient because the flue sucks a high proportion of the air from the front as well as from the chamber. This can be improved by chocking the intake which some folk do and some have called it a "blast door". I think this is not a good idea because when firing an empty chamber the temperature rise is too rapid for the refractory material to cope. It produces uneven temperatures which contribute to thermal expansion stresses and micro and at worst macro cracking. In a kiln this problem is mitigated by the kiln furniture and wares, but an oven chamber is fired empty and the temperature rise can be too rapid for the safety and longevity of the refractory. A safe temperature rise for kilns is usually 100C/hr, his is for the wares to be safely fired, the refractory can take more but the same applies. We fire our ovens at more like 300C/hr. Slower is safer.

[thelightningking]

Interesting post - Lastly, did you include the mass of the floor in your calculations? That is a bit of thermal mass and if not included could skew your numbers a bit.

I did include it, yes. Given my oven dimensions I calculated ~0.097 m^3 of volume in the walls and floor, 0.0323 of which is in the floor.

Your other ideas are interesting, some things to try.

[thelightningking]

You're math is not yet correct. About 8-12 kg would be expected. Please recalculate.

However, to get the best use of the wood during heat up: Burn ~1.5 kg wood per m2 inner surface area per h. Make a blast door to control the air flow. If you reduce the incoming air, the cooling effect is reduced. If you reduce the air too much, the fire will die out and/or you will get a lot of smoke. A starting point is to reduce the opening size with 50-75 %.

Perhaps I'm overestimating how much wood I use, though I do think I use more than 8-12 kg to get my oven up to temperature. Assuming 750 kg/m^3 density of dry oak, 10 kg works out to 0.0133 m^3. I plugged that into my calculations and got an efficiency of 5.1%. Obviously way higher than 0.7% but still pretty low.

My surface area (walls+floor) is 1.9 m^2, so your rule of thumb implies 2.85 kg per hour. That's definitely not what I do - I load in a large amount of wood to make a big fire to get it up heated quickly. So perhaps you are you suggesting a much slower buildup in temperature as more efficient? That actually does make some sense.

[thelightningking]

Having built around 7 pottery kilns over the years my first oven was based on ancient Roman kilns, that is a hemisphere with the flue placed centrally at the top of the dome. This creates an updraft system with excellent draw and no tunnel to have to work past at the oven mouth. The disadvantage is that the flame tends to jump straight to the the flue and lots of energy is lost. Of course in a kiln the chamber is filled with kiln furniture and wares so the flame needs to travel through these before exiting out the flue. However for an oven the chamber is fired empty and therefore a lot of the heat is lost out the flue. By contrast the Pompeii style oven is a cross draft system where the heat from the flame is captured by the dome. If the flame is captured by the dome little heat is lost out the flue. Modern kilns take this even further by placing the flue exit at the base of the chamber, pulling the flame down before exiting, This is called a downdraft system. (see my kiln build here https://community.fornobravo.com/for...kiln#post14564) This system is highly efficient as well as producing good circulation which is required to produce even and consistent temperature for glaze maturation.

The Pompeii style oven is based on that of the bakery oven at Pompeii which uses the cross draft system, however I noticed quite a few ovens in individual homes had very small ovens that were of the updraft system. I was puzzled by this , but after working both types I'm pretty sure the choice of an updraft oven for the private homes was because although there's some efficiency loss, there is no problem with smoke at start up.You wouldn't want to fill your house with smoke. The lack of a tunnel in front of the oven mouth is also a huge advantage. These oven were probably not fired every day and also doubled to warm their homes. By contrast the bakers oven probably never actually got cold, being fired every day and fuel efficiency would be of a far greater concern.

Regarding efficiency I did measure the amount of fuel used to take my small oven up to 350C, the dome was beginning to clear, and it was only 4kg. That's pretty efficient. I think you're 0.7% efficiency calculation is out. The large oven mouth is not that efficient because the flue sucks a high proportion of the air from the front as well as from the chamber. This can be improved by chocking the intake which some folk do and some have called it a "blast door". I think this is not a good idea because when firing an empty chamber the temperature rise is too rapid for the refractory material to cope. It produces uneven temperatures which contribute to thermal expansion stresses and micro and at worst macro cracking. In a kiln this problem is mitigated by the kiln furniture and wares, but an oven chamber is fired empty and the temperature rise can be too rapid for the safety and longevity of the refractory. A safe temperature rise for kilns is usually 100C/hr, his is for the wares to be safely fired, the refractory can take more but the same applies. We fire our ovens at more like 300C/hr. Slower is safer.

Very interesting context. I will more accurately measure/weigh how much wood I actually use to get up to temp, perhaps I'm off. But I think the efficiency is still low.

What's your dome radius? You called it "small", so I can calculate the efficiency if I know the radius. I imagine it will still be low but perhaps that's just the unavoidable nature of the refractory concrete and firebrick not wanting to suck in heat very fast.

[Petter]

If you read the topics here on the forum about wood consumption on Pompeii ovens, you'll see that people on average report wood weights that would translate to 60 % efficiency during heat up.

The value of 1.5 kg / m2 comes from the mass rocket stove community. It gives the best balance of heat transport through the brick and wood consumption. More wood heats the oven faster but the losses increase more than the heat up time decreases since the heat soak in the brick can't keep up.

Regarding the draft door: It shall not be used to accelerate the incomming air to boost combustion. Then the heat up rate becomes too high as David S points out. It shall be used to control the burning rate of the wood. Long slow flames with no smoke is what you aim for. Start with a large opening and reduce it as the oven temperature increases. Start with thin logs, they burn cleaner in a cold oven than thick ditto. Increase the log size as the oven heats up. Keep the wood weight per h constant.