Firebrick - How Thick Should I go

[Oven_Man]

Hi All

A fabricator is about to start building my Stainless Steel Oven and I already sourced 25mm Thick Firebricks (35% Allumina) - I picked them already and now have 2 minds about going so thin on the firebrick for a Wood Fired Oven.

I've come across this on the FB Page https://www.fornobravo.com/commercia...oor-thickness/ and thinking of going for the 50mm Brick for better heat retention as well as recovery time.
Am I doing the right thing or should I stick with the 25mm Bricks?

By the way, my oven is a Stainless Steel Shell, 70" X 70" internal cooking area with 75mm Insulation between the domes - has a 18" X 9" entrance and internal dome height is 13" above the cooking floor - that will obviously drop to 12" if I change the brick to 50mm.

Any advise would be greatly appreciated

Thanks

[UtahBeehiver]

1" thick floor bricks are too thin to maintain heat (not enough thermal mass). They can be used if you double the layers or use the 50 mmm bricks. Is the inner dome SS as well or brick. SS does not have the ability to retain heat since due to its high K value aka thermal conductivity, ie SS 304 K value is 50, fire brick K is 1.4 so SS will transmit heat roughly 36 times more than fire brick, basically a heat sink.

[Oven_Man]

Thank you UtahBeehiver for responding.

Yes, oven is full Stainless Steel - only oven floor is Firebrick. I guess I'll be swapping the bricks out for the 50mm then - the price difference is like 20% more for the 50mm.

I'm interested to know more about the Thermal Conductivity of Stainless. If I am understanding correctly, the Stainless Steel dome will bounce the heat back inside the oven if there's enough insulation on the outside of that stainless. I'm assuming that's a good thing and that's why most Stainless ovens boast quick heatup time compared to brick oven which can take up 5X longer.

[UtahBeehiver]

the bounce back of the heat is a function of the shape of the oven not the material. The reason brick, as you say, takes 5 times longer, brick will store the heat (known as saturation). If you are going to just make pizzas for a single night and you keep a live fire going the perhaps SS will suit your need. If you want to take advantage of multiday cooking then SS inner dome will not store the heat like brick will You can go to Engineering Tool Box and see K values of various materials. SS ovens may boast quick heat times but not heat retention.

[MarkJerling]

....I'm assuming that's a good thing and that's why most Stainless ovens boast quick heatup time compared to brick oven which can take up 5X longer.

While you'll see quick heat up time, you'll also see very quick cooling due to not having enough thermal mass. I would be interested to know what temperature your oven will get up to.

[Oven_Man]

I plan to make mostly Pizza in the oven - maybe the occasional steak or chicken. Doubt I'm venturing into bread making anytime soon. Pizza loves instant high heat so SS will work for me for now.

Mark, I'm looking in the 700F range but once it's ready I would know for sure how high the oven will go.

Out of curiosity, how would 1 go about retaining heat in this kind of oven build - perhaps some sort of ceramic material on the inside dome - outside can still remain SS?

[P3 Stoaker]

WOW, that does sound like an interesting build. Noting I have little experience with Pizza Ovens, my general thoughts are that with only a small amount of thermal mass the heat source would need remain within a suitable window range, and then remain there whilst the oven is in use. Sounds a little like using a wood fired offset smoker, lots of attention to the fire and keeping ahead of the game so it doesn't cool too much.
Good Luck.

[Kvanbael]

Retaining heat is the combination of:
1. Lots of thermal mass on the inside(steel or any refractory material)
2. Lots of fuel to saturate said thermal mass
3. lots of insulation to keep the heat in the system as long as possible.

as P3 said, with pizza, you don’t need to retain heat that much, since you keep the fire on. But still all 3 elements should be there.

[MarkJerling]

I plan to make mostly Pizza in the oven - maybe the occasional steak or chicken. Doubt I'm venturing into bread making anytime soon. Pizza loves instant high heat so SS will work for me for now.

Mark, I'm looking in the 700F range but once it's ready I would know for sure how high the oven will go.

Out of curiosity, how would 1 go about retaining heat in this kind of oven build - perhaps some sort of ceramic material on the inside dome - outside can still remain SS?

I'm not sure how to do it! You'd want to avoid anything that can fall onto your pizzas or that can impart strange flavours to your food. Bear in mind that stainless steel, when heated, will discolour, so don't expect it to stay all 'silvery', if it's a single layer stainless steel design. The alternative would be a double layer design with furnace insulation between the two stainless steel layers.

[Oven_Man]

Hi Everyone
Thank you for the responses and advices. One of the trade-offs on a Steel Oven Design is thermal mass but for Pizza's you looking for High Heat which I believe Steel Ovens deliver - we'll see when I put mine to use.

I'm at the final design stage with my fabricator and he's having a little difficulty with the front of the oven. Everything on the oven will be locked into place - no welding whatsoever. I want the front to be insulated but with the cutout for the mouth of the oven - this is a little challenging without having gaps and sharp edges which is a definite no no.

He suggested we close of the height of the opening to 200mm (Oven Internal Height is 340mm) and leave the full width of the inner dome as is - see the 2nd picture of how it'll look. My question is, is the width of the opening as important as the height is? Heat Loss is always a concern and I'm trying to avoid that altogether. Picture 1 is what I originally had in mind.

Please advise me - I'm so confused as to what to do.

Take Care