Multi-layer Insulation – Super Insulation
One common radiation barrier used in cryogenic applications is known as Multilayer Insulation (MLI), or Super insulation. The space program encouraged the development of MLI around 1960. The MLI generally contains multiple layers of reflective material separated by spacers having low conductivity.
MLI consists of many radiation shields stacked in parallel as close as possible without touching one another. MLI will typically contain about 60 layers per inch. MLI is anisotropic by nature, making it difficult to apply to complex geometries. MLI is generally very sensitive to mechanical compression and edge effects, requiring careful attention to details during all phases of its installation. Accordingly, performance in practice is not typically as good as theoretically possible.
Each layer is isolated from the other by spacer material such as polyester, nylon, or Mylar. The aluminum foil is carefully wrapped around the container such that it covers the entire surface of the inner vessel. Spacer material, as described, is placed between the layers to completely prevent the separate coverings of foil from contacting. Should they touch, a thermal short circuit will occur and increase the heat transfer. The layers can be applied manually as blankets. These are hand cut to fit and wrapped over the vessel and vessel ends. Tape that has low out-gassing properties is then used to hold the blanket layers in place. Another method of applying the layers is by “orbital wrapping”. This method is used where high-volume vessels are being manufactured. Special equipment is required that wraps the alternating layers much like the wrapping of a spool of string.
As the number of layers increase the insulation capability is also increased. Typically layers adding up to about one inch in total thickness is applied in the liquid nitrogen temperature range described.
MLI is designed to work under high order vacuum, i.e., pressure below about 1 X 10-4 torr. To obtain this vacuum generally requires lengthy pumping along with heating and purging cycles. Chemical gettering materials are required to absorb the out-gassed molecules to maintain the vacuum over extended periods.
Super insulation advancements are being made at the Kennedy Space Center. A new-layered cryogenic insulation system is being developed. This insulation is different from others due to its superior thermal performance in “soft” vacuum conditions. This system overcomes some of the typical shortcomings of super insulation discussed above. Those shortcomings include exhibition of different insulation properties when measured in different directions and sensitivity to mechanical compression.
Foam insulation is generally not favored in cryogenic applications. Such insulation is likely to crack due to thermal cycling and environmental exposure. Cracks permit incursions of moisture and humid air that will form ice and greatly increase the surface area for heat transfer.
Other Insulation Systems
Other types of cryogenic insulation systems include those where the evacuated annular spaces (space between an inner and outer vessel) contains bulk filled materials i.e. glass fiber, silica aerogel, or composites. As with MLI, these systems require vacuum levels of around 1 X 10-4 torr to be effective.