Is Mylar Necessary for Preventing Heat Leak in Low Temperature Apparatus?

[Rajini]

Dear all,
i noticed in all low temp. apparatus like cryo...usually all the internal parts are wrapped with few thin mylar sheets in order to avoid heat leak (heat from surrounding is transferred to cryo)..also some ppl. told me it is for radiation (or thermal) shield..
Even though the cryo is in good vacuum...is these mylar is necessary? (i never tested without mylar)..Is there any good article discussing about these simple things..

thanks

 

[Q_Goest]

Hi Rajini. A conventional, vacuum jacketed (VJ) tank or pipe system is actually two vessels, one inside another. A VJ pipe for example, has one pipe inside another which is supported by some kind of low thermal conductivity spacer. Similarly, a tank will have one tank inside another with some kind of low thermal conductivity suspension system.

Within this annular space is a vacuum. Typical vacuum (warm) is around 10 to 100 micron. Typical vacuum cold (at cryogenic temperature) is less than 1 micron. This vacuum space is intended to prevent any kind of conductive or convective heat transfer. There is no significant convective heat transfer unless the vacuum is lost (ex: due to a leak or out gassing of contaminants). Thermal conductivity is minimized by maximizing the length of the supports and minimizing the thermal conductivity of those supports.

To further reduce heat transfer, one has to do something about the radiation heat transfer. This is accomplished by providing layers of a reflective substance such as aluminized mylar or aluminum foil with a layer of non thermally conductive material that looks and feels like paper of some kind. Typical VJ pipe and tanks have about 60 such layers per inch, and may have anywhere between 10 layers on up to a hundred or so depending on the application. This type of insulation is known as Multi Layer Insulation (MLI). How these MLI systems are designed is generally proprietary, but they are all similar in their construction.

An alternative to MLI is the use of a more conventional insulation such as fiberglass matting or Perlite which is a type of volcanic rock. You can read more about it here:
http://en.wikipedia.org/wiki/Perlite

There are additional things that can be done to further reduce heat transfer, the most important being some kind of shielding which can be combined with either a sacrificial cryogen (typically liquid nitrogen) or a "cold head" which is a type of cryogenic refrigerator that uses helium in a closed loop system similar to a conventional air conditioner.

A good overview of cryogenic systems and MLI insulation in general is covered by a book called "Cryogenic Systems" by Randall Barron.
http://www.oup.com/us/catalog/general/subject/EngineeringTechnology/MechanicalEngineering/?view=usa&ci=9780195035674

 

[Rajini]

Hi thanks for your comments.
Main aim is just to eliminate the radiative heat loss.