Condution in low pressure vacuum

[tjwalker11]

Does the conductivity coefficient between surfaces change is the conduction takes place in a low pressure vacuum? Does the lack of atmosphere significantly change conduction?

I can imagine if two surfaces had 100% contact, that the atmosphere would have no effect, but most surfaces have some level of micro-roughness that prevents 100% contact over the overlapping area.

Any leads on this would be appreciated?
The materials I'm interested in would be a heated or chilled roller (probably steel or chrome plated) to stainless steel, polyester, and polyimide films.

 

[chief]

Air is an insulation medium, This is the reason they use hollow walls in house building. I should imagine if you remove the air, you will improve heat transfer properties in the vacuum space.

Offshore Engineering&Design

 

[tjwalker11]

To clarify, I'm asking about two surfaces in contact and the conduction heat transfer. Specifically heating a foil or film wrapping a heated roller. Is there a difference in the coefficient of thermal conductivity if this is done in atmosphere vs. in vacuum. I'm not talking about two surfaces separated by an air or vacuum space, but two surfaces in contact, but with non-ideal contact due to roughness.

 

[MintJulep]

The conductivity of a material is a function of the material. So no, a vacuum won't change it.

But you are correct about the roughness of the contact surfaces between two materials.

The heat transfer is not pure conduction. Radiation and convection are are also in the mix.

A vacuum would reduce convection, so I suspect that overall heat transfer would decrease.

Chief,

So how do you explain the Dewer Vacuum Flask, aka the Thermos Bottle?

 

[IRstuff]

Your question is a bit too arbitrary to answer fully.

Consider Al to Al; the air path is probably irrelevant, because Al is such a good conductor that air doesn't really help it much. Consider alumina to alumina, the air path is probably irrelevant again, because the bulk alumina is such a poor conductor that air wouldn't do much to help.

TTFN

FAQ731-376

 

[Onno]

As you say the heat transfer takes place between a heated or chilled roller and different type of films. The roughness of the roller combined with the (most probably) stretching force in the film would certainly produce some relevant heat transfer capable contact area. The film is pressed against the roller. Apart from this the contact time between film and roller would seem to be important. A longer contact time (i.e. a large coverage angle between film and roller or a slow turning roller) would enhance heat transfer. A low herat transfer coefficient between the two surfaces could be compensated by the low thermal mass of the film combined with an optimum contact angle and/or roller speed.

So, following variables are of importance
thermall mass of film
film/roller temperature
area of contact
roller speed
film tension
film/roller surface roughness

Please fill us in on the details

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