Two major heat sources at ultra low temperature system: heat conduction and heat transfer from radiation.
(1) Convective heat transfer: due to high vacuum condition (about $10^-4$ torr), convective heat transfer is almost eliminated
(2) Heat conduction: In order to reduce conductive heat, low thermal conductivity materials are used
(3) Radiated heat transfer: using superinsulation materials for radiation barriers
1. what is superinsulation?
- it is also called multilayer insulation(MLI)
- superinsulations usually are made up of multiple layers, which have high reflectivity. These layers are separated by spacers with low conductivity.
- superinsulation is used for preventing heat loss by thermal radiation. In order to achieve that, materials for superinsulation should have a high reflectivity and low emissivity. (e.g. For commercial aluminized mylar, its thickness: 400 anstroms and emissivity: 0.030 [1])
- superinsulations are mainly used for cyrogenics where the typical temperature is below -150 Celcius degree.
(a) installed as a blanket that lines the interior of the cavity in vacuum chamber
(b) used as a wrap around piping and tanks in cryogenic system
- It is also used for spacecraft and satelites.
- Typical number of layers for superinsulation: 60 per inch
- Typical pressure for MIL: a high order of vacuum pressure ($1\times10&(-4)$torr)
- Its performance depends on the number of layers. That is, as the number of layers increases, the thermal insulation is also increased.
1) spacer
- materials for spacer: polyester, nylon, inorganic glass fiber, or mylar
- In order to inhibit conductive heat transfer between neighbor aluminized mylar. So, it should have low thermal conductivity
- These spacers are manufactured with a specific porosity, which results in forming a thermal barrier in interstitial vacuum space.
- the role of spacers: it is to prevent aluminum foils used for superinsulation to be touched each other. If MIL are touched to each other, then, a thermal short circuit will occur and increase the heat transfer.
2) Performance comparison of superinsulation to other insulation techniques [1]
※ for maximum performance, MLI requires at least below 1 milli Torr.
- MLI 0.5-inch thick: 0.13 mW/m-K
- Perlite 6-inch thick: 1 mW/m-K
- Aerogel with carbon 3-inch thick: 0.55 mW/m-K
- Foam (at ambient pressure): 30 mW/m-K
[1] https://www.mtm-inc.com/multi-layer-insulation.html
[2] https://www.csm-cryogenic.com/prodtech/
[3] https://technifab.com/cryogenic-insulation/
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