Cryogenic storage dewars and EM waves

[ProjectFringe]

I was researching cryogenic storage dewars and read that, "All dewars have walls constructed from two or more layers, with a high vacuum maintained between the layers. This provides very good thermal insulation between the interior and exterior of the dewar, which reduces the rate at which the contents boil away."

I thought that heat was a type of EM wave (infrared), and as I know EM waves can travel through a vacuum. So how does a vacuum provide good thermal insulation?

 

[mfb]

Conduction (and convection, if possible) is far more efficient than thermal radiation. Making the walls of the vacuum reflective in the infrared helps, too.

 

[ProjectFringe]

Got it! Thanks

 

[f95toli]

Indeed, if you cut a dewar in half you will find that the vacuum space is not actually empty but includes multiple layers of metal covered films ("space blanket").
While it is certainly true that convection and conduction typically will dominate heat transport, radiation is still very much something you need to consider when designing a dewar (or, more generally, a cryostat).
This is also one reason the inside of cryostats are typically very shiny (often gold plated)

 

[Bystander]

more efficient than thermal radiation.

"Break even" is around 500 K; higher temperatures "run-away" as T⁴.

 

[Bandersnatch]

Indeed, if you cut a dewar in half you will find that the vacuum space is not actually empty but includes multiple layers of metal covered films ("space blanket").

I used to work in a small cryogenics factory. I'd tell everyone I was 'a cryogenics technician specialising in installation of radiation shielding', since it sounded so much better than 'wrapping pipes in aluminium foil'.

 

[sophiecentaur]

Indeed, if you cut a dewar in half you will find that the vacuum space is not actually empty but includes multiple layers of metal covered films ("space blanket").

That's the flashy modern stuff. The Vacuum Flasks I used to keep my tea in were blown glass with internal silvering. I'm amazed at just how good the domestic stainless steel flasks are, compared with the old 'Thermos'. And you can drop them too!