Which spacing gives better thermal insulation?

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Offline Atomic-S

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Which spacing gives better thermal insulation?
« on: 05/01/2015 02:14:37 »
Consider two flat plates separated by a small distance filled by a gas, the edges being sealed. Which will result in better resistance to the flow of heat between them:  having the spacing between the plates less than the mean free path of the molecules of the gas (but not touching), or having the spacing significantly larger than that?
 

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Offline chiralSPO

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Re: Which spacing gives better thermal insulation?
« Reply #1 on: 05/01/2015 15:48:22 »
How are we changing these parameters? Are we changing the distance between the plates keeping the amount of gas constant or the pressure constant? Are we just changing the pressure of the gas while keeping the distance constant?


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Offline alancalverd

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Re: Which spacing gives better thermal insulation?
« Reply #2 on: 05/01/2015 22:51:41 »
Two extreme situations. If you can tolerate a reasonable vacuum between the sheets, then the bigger the better. If you can't, then you need a separation less than the minimum required to establish convection in the filler gas. For dry air at atmospheric pressure and ambient temperature ranges, this is empirically around 2 cm.
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Offline Atomic-S

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Re: Which spacing gives better thermal insulation?
« Reply #3 on: 06/01/2015 03:39:28 »
I was thinking of situations in which the average temperature of the gas, and its density, were the same in both cases.