I have a submerged capsule under sea ice. I have one layer of concrete and a steel capsule. Inside this object there is led lighting which keeps this object’s inside temperature optimal. Now I find myself pondering whether the steel capsule should face the cold water or the layer of concrete. Can you help me with this?
My intuition says that steel should be on the warm side - concrete outside. But I also think either way, the result would be the same…
My intuition says - does not matter. Insulation is measured as speed at which heat migrates from hot to cold. AFAIK, don’t recall exactly from physics 40 years ago, this is not really temperature dependent - it’s a simple characteristic of the material - heat migrates as so many calories per cm of material per hour.
The only difference is whether being in pressurized water will affect the concrete’s characteristics? Will the water change the concrete’s specific heat index and the speed at which heat migrates? In that case, steel on the outside is better provided it’s not leaking.
Yes, if the light from the LED is warming up the object inside the capsule directly, then you want the interior surface of the capsule to be low emissivity. Although, if the heat transfer is dominated by conduction and convection (which I suspect it is), then the surface property won’t make much difference.
Protecting the steel might be important as well, depending on the type of steel. Then again, I suspect a protective coating could be applied to either concrete or steel?
Line the inside with reflective foil faced insulation like Tuff-R. 4" of concrete only has an R value of 0.8. Steel doesn’t have enough insulation value to even consider. 1" of Tuff-R will give you an R value of 6.5 and it’s cheap and easy to work with.
You don’t need LEDs unless they are needed for something else. Electrical resistance heat can come from anything, and it’s all equivalent. Use a couple of resistors.
If you have to have steel, you want it on the inside. As mixdenny points out, steel is even more thermally conductive than concrete. An outer steel layer will have way more surface area than an inner one would. An outer concrete layer will dump lots of heat into the seawater, but an outer steel layer will dump even more heat.
Why so coy about what’s actually going on? Why not use better materials?
In the ideals physics world, as long as two insulators are in series the overall insulation is the same no matter which goes first.
Now to the real world :
have you considered that steel and concrete may have different thermal expansion coefficients ? The rebars in the reinforced concrete may expand (contract) differently than the aggregate causing cracks in the concrete?
Have you considered impact resistance of the metal used in the concrete and the steel chamber ? Typically impact testing is recommended below -20F ? Normal Rebar or carbon steel has a propensity to crack below -20F ?
Have you considered the differential expansion between the two chambers ? Assuming the concrete is outside and the steel chamber is inside and somehow there’s a leak and the inside chamber cools down : will there now be a gap between the two chambers ?
Once you do the above checks, I am guessing the concrete will require appropriate gaps to account for temperature changes. This will require that it is protected by a metal shell or the water will seep through defeating the purpose.
I’m guessing cost is some sort of factor, otherwise you’d just have the concrete sandwiched between two steel layers. If cost does matter, clearly steel in the center will be cheapest.
You should look into a third layer on the outside, like a silicone jacket or at least latex paint. Something flexible at the required temperature that can keep the concrete from getting too wet. It would help keep the R-value stable, and probably make the whole thing more durable. How long is this intended to endure in these conditions?
I wouldn’t expend too much mental energy on what’s actually going on. OP has a history of unusually specific questions about the physics of constructing domes and/or living under the sea. The questions are often interesting per se, but as I recall he has always been coy about the purpose or reality of any actual construction project. Sci fi novel maybe?
Steel and concrete have pretty close to the same thermal expansion coefficients, that’s why steel rebar is useful in the first place.
From an engineering standpoint, I’d have to ask what he considers an “optimal” temperature and a reasonable amount of LEDs. Because no amount of LEDs or small power source is going to keep a large dome/capsule at human comfortable temperatures when submerged in cold water. Your best bet is to have it really large and start it off warm, and just go with the thermal mass of concrete and steel to keep you warm for a while. Years/indefinite, it’s going to take a lot of energy input.
edit: no amount of LEDs being used for human habitation lighting, of course LEDs can just be packed in as heating elements, but still would require the large power supply…
Concrete is inherently porous, water will infiltrate through eventually without a sealer, but depending on the quality / thickness of the concrete, it may be slow enough to be negligible over the life of the design.
Thanks for the context! I see your point that the questions are interesting on their own merits. The coyness made me wonder what information was being left out and whether the missing info might change the answer. But if that’s just the OP’s modus operandi, that’s fine too. Thanks again!
To the OP: you probably want to use a thermally insulating concrete, like maybe one containing vermiculite. Its thermal conductivity is only about 5% that of conventional concrete.
Also, you don’t need to use reinforced concrete. The steel rebar in reinforced concrete is there to bear tensile stresses, and there aren’t any in your container.
Your container will be (or at least should be) shaped like a cylindrical or spherical pressure vessel. That means that the water pressure at any depth will put the entire container under uniform compressive stress; there will be no net tensile stress for the rebar to bear. So adding rebar only makes your concrete more thermally conductive and, in the end, weaker.
Putting the steel on the inside would minimize the surface area of the most thermally conductive material (good), but if you use an insulating concrete of some sort, it wouldn’t matter much. Also, the steel would provide a better pressure seal than the porous concrete, as someone pointed out upthread. Plus, concrete on the inside would keep the steel shell from buckling (getting crushed) under the pressure loads.
So: steel on the outside, insulating concrete on the inside. Thick walls of either material would prevent buckling, but thick insulating concrete and relatively thin steel walls would probably be the most practical given the parameters you’ve provided. At least, that’s how I see it.