How many BTU or Kcal/hr for my brick oven?

[Ramiro]

Hi, I'm installing gas for my brick oven that is still under construction and I'd like to know how many BTU or Kcal/hr do I need for my oven to heat up and reach 400?C. My oven internal size is: 90cm diameter and 45cm hight.

Thank you!

[enz]

Re: How many BTU or Kcal/hr for my brick oven?

I'll give it a shot.

90 cm is close to 35", which I'll use since all my data is in Btu. Assuming 3" walls, you have about 10780 cubic inches of brick, which weighs about 884 lbs @ 0.082lb/cuft. Brick has a specific heat of .200 Btu/lb*DegF. Average temperature of the brick is tricky, but previous threads have put the outside of the brick at 100F at the beginning of "operating" temp. I'll assume 800F inside temp, for an average of 450F. Assume a cool day 50F to start and you have a delta T of 400F. So, 884 lbs*400 DegF *0.200 Btu/lb*DegF =70720Btu.

Enz

[Balty Knowles]

Re: How many BTU or Kcal/hr for my brick oven?

GOOD FOR YOU ENZ. 70,000 btu is surprisingly light but theres no way I'm going to try to figure it out. My thermo dynamics class was right after lunch & several beers, I used it to recover from my partying the night before.

[Ramiro]

Re: How many BTU or Kcal/hr for my brick oven?

Thanks Enz. An italian builder recommended me 200.000BTU. Sounds too much, don't you guys think?

[Balty Knowles]

Re: How many BTU or Kcal/hr for my brick oven?

Would have guessed that 200k btu was more like it but check out the attached site. Smaller ovens approx. 60,000 btu. commercial around 140,000 btu. looks like ENZ was right on the money.

Foodservice Equipment Reports: Evaluations

Rgds

Balty

[jengineer]

Re: How many BTU or Kcal/hr for my brick oven?

Instead of re inventing the wheel although we often do that I will point you to a link form one of our resident, north american experts.

http://www.fornobravo.com/forum/f2/how-many-btus-2104.html?highlight=natural#post11454

followed up by your needing to get in touch with the moderator of the board, James, who will get you in touch with Alf who probably has may be able to provide pointer for you.

Safety with gas in a non-vented dome (which is what these ovens are when a fire is not present) is paramount. Gas accumulation, before the gas is burned can be catastrophic. I once received 2nd degree burns when a furnace did not light off properly, my removal of the access panel created enough draft for the gas to waft over the pilot light and whompf a 3 feet flame engulfed me. No eyebrows, eylashes, hair on arms for a few months and crisped skin was my reward.

[enz]

Re: How many BTU or Kcal/hr for my brick oven?

Hi all,
Sorry I haven't followed up. I was casting an oven. See my thread under tools, tips and techniques "Casting Refractory....". Phew!

So just to clarify, 70,000 Btu is what the brick needs to receive, not the burner spec. You will need to allow for a lot of heat to go up the chimney during your burn. I think 200k Btu is pretty reasonable.

Good luck,

Enz

[v12spirit]

Re: How many BTU or Kcal/hr for my brick oven?

Hi ENZ
You mean that the heat absorbed by the bricks should be 70,000 BTU so a higher heat supply say 3 times more is needed which is about 200,000 BTU. Is that what you mean?
If so, I'll suppose that 200,000 BTU heat source inside the oven will bring it to pizza temp. Cooking gas has a fuel efficiency of 11.8 Kcal/g which is 46.8 BTU/g. This implies that the 200,000 BTU inside the oven will require 4.274 Kg cooking gas. Is not that very much? It is the third of the medium size cooking gas bottle!! I would like someone to correct these calculations if they were wrong.
Cheers.

[david s]

Re: How many BTU or Kcal/hr for my brick oven?

I doubt whether you'll get any response as the last previous post on this thread was over six years ago.

[v12spirit]

Re: How many BTU or Kcal/hr for my brick oven?

Ah! I was not aware that this post is 7 years old. I hope Wiley will reply. He has a good background in thermodynamics, and his daughter is pursuing a master's degree in thermodynamics and is currently teaching in the university of Washington. He must have the answer.

[jeeppiper]

Re: How many BTU or Kcal/hr for my brick oven?

10-points shall be awarded to david-s for paying close attention to details!!!

[Wiley]

Re: How many BTU or Kcal/hr for my brick oven?

V12Spirit,
My daughter is the "expert" what little I know is from a course I took many many years ago and what has "rubbed off" in discussions with her. The person I believe you need to inquire of is "Mklingles" who has a good grasp of the mathamatics of what is going on inside a WFO. I do not know if he is still on this forum if he is not it's unfortunate as he seemed to know his material. We communicated a bit concerning some questions I had. He is helpful and easy to approach. The discussion you posted this on is one I did not follow and probably would not have. Right off in reading thru the thread I had lots of issues with assumptions made by contributors. People can get quite defensive if one questions their assumptions and I'm not here to fight.

IMHO heating a WFO is a complex combination of variables: how hot the heat source, the time the heat has to transfer; the transfer rate (absorption rate of the oven material); the transfer (conduction) rate within the material (as the heat source is not uniform over the inside surface); It also gets more complex as transfer rates within a uniform solid (a brick) are different from the transfer rate from one brick to another (and even more complex if one adds in the transfer rate of any mortar between bricks); and the heat loss of the oven material (insulation on one side and loss thru radiation on inside and consequential absorption of the re-radiated heat). And those are some variables for the physical WFO add the variables for the stuff one is burning and the whole question becomes an issue I may ponder over a glass of wine or beer with a friend but it is not something I want to spend time crunching numbers over.

My WFO is a source of fun for me. For me to dissect it to a point that it is all numbers and times is for me to lessen the fun, the art, the joy. Not saying that it is wrong, just not for me.
Bests,
Wiley

[v12spirit]

Re: How many BTU or Kcal/hr for my brick oven?

Hi wiley,
Thanks for directing me to Mklingles. I PM him, and hope he will reply. BTW I was not questioning what people said in the thread. I was just wondering about the calculations I did and wanted and still want someone to correct to me if I was wrong. I know you don't like to get involved in doing maths but I was wondering if you would show the thermodynamics question to your daughter.
Cheers.

[Wiley]

Re: How many BTU or Kcal/hr for my brick oven?

OK V12Spirit When I see her I'll ask but I'm pretty sure of her answer: Too many unknowns. I'll try to make this short (which for me can be a problem).

Heating something like a WFO is a matter of heat and time. In order to compute the amount of energy required to heat an individual oven one needs IMHO to know the absorption rate/transfer rate of the material of which it is constructed. A WFO cast of home brew will vary by the density (even if constructed using the same recipe) simply by whether it was vibrated or not when poured. If built of bricks even the amount of mortar between the joins will have an effect, to say nothing of varying properties of different brick manufacturers. Until your WFO is built the transfer /absorption rate for your WFO is an unknown.

Basically if one tries to heat an oven at a rate faster than the the material it is constructed of it can absorb, the excess heat will be wasted up the chimney. Heating at a slower rate than the maximum absorption rate will result in longer heat up time but less overall energy used. So any statement regarding actual total calories needed is IMHO quite possibly bogus.

Using the figures of Kcal/g you gave for the gas IMHO is using unreliable information. They might be nice averages for a classroom and help to understand what is theoretically going on but in the real world things aren't so tidy. I'm guessing when you refer to bottled gas you are saying the Butagaz type stuff which is basically a volatile gas consisting primarily of butane but with varying amounts of heptane and other contaminants (volatile yet they have different heat energy). Using the gas I've personally used in Egypt and France the quality varied from bottle to bottle. By that I mean by the time one used a bottle up and replaced it the gas was often noticeably different. I did no scientific tests I just know the flame looked different and water took longer or less long to boil.

So right off two things we don't know: how fast your WFO will absorb heat and how much heat your gas will actually produce. There are other things that we don't know about the WFO you are proposing, many of which can dramatically effect how well it performs: chimney (both length and diameter and material of which it is constructed); insulation; interior arrangement of gas jets; and so on. Too many unknowns.

Just like a boat or an airplane or countless other things until it's actually constructed you wont know how well it going to perform in the real world.

It would seem you have looked thru the archives which I think is a good thing and something I would suggest to anyone wanting to build a WFO. One detail you may have noticed is that some people still have problems with their WFO even after they built as close to the plans as possible. And sometimes these problems can be traced back something as simple as the orientation of the opening/entrance to the local prevailing wind.

Who'd a thunk it?

I have a friend with a restaurant that has a beautiful fireplace which draws well until someone opens the back door in the kitchen then it smokes like all get out... into the main dining room.

Who'd a thunk it?

Now I'm not trying to get on your case, but didn't this start out as simply wanting a simple WFO (with gas option) that heats up fast for an occasional pizza?

You're suggesting building a "one off" WFO, a bit of an experiment really, and there are going to be some small risks (maybe large risks if you go with the gas option and get it wrong). It should be fun.

Hope this helps,
Life's an experiment really,
Bests,
Wiley

Hi ENZ
You mean that the heat absorbed by the bricks should be 70,000 BTU so a higher heat supply say 3 times more is needed which is about 200,000 BTU. Is that what you mean?
If so, I'll suppose that 200,000 BTU heat source inside the oven will bring it to pizza temp. Cooking gas has a fuel efficiency of 11.8 Kcal/g which is 46.8 BTU/g. This implies that the 200,000 BTU inside the oven will require 4.274 Kg cooking gas. Is not that very much? It is the third of the medium size cooking gas bottle!! I would like someone to correct these calculations if they were wrong.
Cheers.

[v12spirit]

Re: How many BTU or Kcal/hr for my brick oven?

Thank you wiley for being patient .