Steel Dome thermal mass question

[Beardy Mike]

I have been reading avidly and am particularly taken with the idea of a steel dome based WFO. It seems like a simple and fast to build solution with some potential advantages of very even dome temperature as the result of high conductivity. The question of thermal mass seems to have been addressed by multiple people using sectioned home-brew refractory casting over the dome, but I wanted get opinions as to whether just having a retaining "cylinder" around the dome and filling the inside (and surrounding the dome), with loose basalt chips or coarse sand would work? If you don't intend to move the oven, then perhaps this would work OK?
Any thoughts?
thanks
Mike

[david s]

It might be worth a try. But loose grains of either sand or aggregate leaves you with poor contact between the grains which results in much lower conduction as well as air spaces between the grains which acts as an insulator. Steel is highly conductive in comparison to brick or refractory, leading to faster heat up but also faster heat loss. This results in difficulty in maintaining consistent temperature. Brick or refractory use enables far less temperature fluctuation.

[Beardy Mike]

Thanks David. Yes I appreciate the need for some thermal mass to be tied to the steel dome. I also appreciate the lack of thermal contact between the sand/loose media and the dome, but I wonder whether its much worse than a moulded home-brew refractory cement that has been plastered on the outside and then got loose as the result of cracking away from the metal. Wiley's steel oven and others clearly work so I wonder if the high radiation output from the steel makes up for the poor thermal contact. just musing

[Beardy Mike]

Steel shot?

[UtahBeehiver]

Here are some engineering facts on Thermal Conductivity K factorl("the quantity of heat transmitted through a unit thickness of a material - in a direction normal to a surface of unit area - due to a unit temperature gradient under steady state conditions"), Carbon Steel 31.2, Fire Brick 0.27, Concrete Dense ( 0.6 - 1.04). So the numbers show quickly the heat will move out of the steel dome vs other materials by a factor of 100 on fire brick and a factor of 30-50 on concrete.

[david s]

Thanks David. Yes I appreciate the need for some thermal mass to be tied to the steel dome. I also appreciate the lack of thermal contact between the sand/loose media and the dome, but I wonder whether its much worse than a moulded home-brew refractory cement that has been plastered on the outside and then got loose as the result of cracking away from the metal. Wiley's steel oven and others clearly work so I wonder if the high radiation output from the steel makes up for the poor thermal contact. just musing

You could also use thicker steel to create more thermal mass. It would also take longer to rust away.

[Disco Stu]

I tried making an oven with a 1/4" inner steel dome. Over that was a 3.5" thick layer of 1/4" basalt screenings as the heat bank contained by a lighter steel centre dome. Insulation over that and then a 1/8" corten steel outer dome. I don't notice any difference in how it retains heat compared to my brick or cast oven domes. One advantage of this design is that I can screw in an eye bolt to crane the whole thing into position, or take it away. I've moved on to making cast domes in a steel three piece form now, but the steel / basalt oven was an interesting experiment.
Stu

[david s]

That’s a very interesting build. I imagine it would be well suited as a mobile oven, not having the problems of heavy brittle materials like brick or refractory. Interesting that you’ve not noticed any difference in heat retention, how about heat up times?

[Disco Stu]

Hi David, I haven't tried them side by side but I really don't notice any difference. The main thing I was curious about was how slowly it would cool down, it held the heat really well.

[david s]

I tried making an oven with a 1/4" inner steel dome. Over that was a 3.5" thick layer of 1/4" basalt screenings as the heat bank contained by a lighter steel centre dome. Insulation over that and then a 1/8" corten steel outer dome. I don't notice any difference in how it retains heat compared to my brick or cast oven domes. One advantage of this design is that I can screw in an eye bolt to crane the whole thing into position, or take it away. I've moved on to making cast domes in a steel three piece form now, but the steel / basalt oven was an interesting experiment.
Stu

I suppose as the thermal mass is largely contained in the 1/4" basalt screening, then conductivity would be similar to brick or refractory. The air spaces between the basalt aggregate would act somewhat as an insulator. If they were also filled with sand it would increase conductivity as well as increased thermal mass.

[Kvanbael]

It looks amazing!

[hillscp]

Can you touch the outside?

[Disco Stu]

Thanks Kvanbael!
Hillscp, it's only just warm to touch apart from the front two segments that feed the flue. I used two layers of 1" superwool as insulation.
The next oven that I made has a similar outer shell but an external flue at the front, and I insulated the outer edge of the bed bricks as well.
Stu

[gazzton]

Great build Stu!

Where on Earth did you get that inner dome from?! Lookes Middle Age-ish...