Vacuum insulation explained: why static evaporation rate matters
What multilayer vacuum insulation actually does, how evaporation rate is measured, and how to read the number on a datasheet.
Cryogenic vessels hold product hundreds of degrees below ambient. The only practical way to keep it there is to remove the heat paths.
Conduction, convection, radiation
A vacuum jacket removes convection. Multilayer insulation between the inner and outer shells reflects radiation. Low-conductivity supports limit the remaining conduction path.
Static evaporation rate
Static evaporation rate (NER) is the percentage of contents lost per day with the vessel full and idle. It is measured under defined conditions and should always be compared like for like.
Reading the number
A larger vessel almost always shows a lower percentage loss because its surface-to-volume ratio is better. Comparing a 175 litre dewar against a 20,000 litre tank on percentage alone is meaningless.
What degrades it
Loss of vacuum, mechanical damage to the outer shell and repeated thermal cycling all raise the evaporation rate. Periodic vacuum checks belong in the maintenance plan.
- insulation
- evaporation
- storage