Imagine a large metal tank containing a hot liquid. Would the surface temperature of that tank get hotter if the insulation was installed with a 100mm air gap between it and the surface as opposed to installing the insulation directly to the surface or wopuld it be the same either way?
esselte said:
Imagine a large metal tank containing a hot liquid. Would the surface temperature of that tank get hotter if the insulation was installed with a 100mm air gap between it and the surface as opposed to installing the insulation directly to the surface or wopuld it be the same either way?
I would have to say that it would be the same either way. Given enough time, and assuming no other heat sources, the surface of the tank will reach the same temperature of the liquid it contains; this will be true regardless of the location of the insulation. It can’t get hotter (if it did, the added heat would be transferred to the liquid) or colder (the liquid would then heat it.) The insulation reduces the amount of heat lost to the surrounding environment, or the other way around (if the ambient temperature is higher,) but doesn’t change the temperature of the conductor.
Well air is not a very good insulator so having the insulation directly on the container would prevent the loss of heat better.
Here you go. Three replies and three different opinions.
The insulation increase by the air gap would be similar to adding a similar thickness of foam. Give or take a bit. So adding an air gap makes the insulation better.
Air is a good insulator in conduction, better than foam. Not so good an insulator in turbulent convection, worse than foam. I have the equations governing free convection somewhere, in summary it depends on the orientation, gap width and temperature difference.
Esselte, can I clarify what you mean by the container surface? If the exterior of the metal, before the insulation or air gap starts, then my original answer stands. If you mean the exterior of the insulation surface, then my answer is not correct and mollwollfumble’s is. PWM seems to misinterpret the question either way (by my reading.)
btm said:
Esselte, can I clarify what you mean by the container surface? If the exterior of the metal, before the insulation or air gap starts, then my original answer stands. If you mean the exterior of the insulation surface, then my answer is not correct and mollwollfumble’s is. PWM seems to misinterpret the question either way (by my reading.)
Exterior of the metal.
Moll’s answer makes sense to me in that the insulation thickness is essentially being increased, even though the extra insulating “material” being used is air. With greater insulation thickness you can expect both the contents of the vessel and the walls of the vessel to maintain a higher temperature. Can you explain what you think is wrong with Moll’s answer?
esselte said:
btm said:
Esselte, can I clarify what you mean by the container surface? If the exterior of the metal, before the insulation or air gap starts, then my original answer stands. If you mean the exterior of the insulation surface, then my answer is not correct and mollwollfumble’s is. PWM seems to misinterpret the question either way (by my reading.)
Exterior of the metal.Moll’s answer makes sense to me in that the insulation thickness is essentially being increased, even though the extra insulating “material” being used is air. With greater insulation thickness you can expect both the contents of the vessel and the walls of the vessel to maintain a higher temperature. Can you explain what you think is wrong with Moll’s answer?
Perhaps I misread the question. I assumed a tank of liquid at a constant temperature higher than ambient; you appear to be describing a Thermos® flask, possibly larger than the ones readily available. In either case, the temperature of the metal will heat up until it reaches the same temperature as the liquid; if the liquid is cooling via heat transfer through the container (as in a Thermos flask) then the added layer of air will reduce the heat loss, as moll said.