Multilayer Insulation—Part 1: Overview
December 17, 2009
Multilayer insulation (MLI), also called superinsulation, is often used in cryogenic applications. When properly designed and installed, multilayer insulation systems can provide a highly thermally effective insulation system. Multilayer insulation is used within a vacuum environment with vacuum pressures of 10-5 to 10-7 torr required for best performance. The use of vacuum in the insulation of cryogenic equipment, with or without the use of multilayer insulation, is one reason Meyer Tool has become as specialist in the manufacture of both types of equipment.
One of the key challenges to the designer of a cryogenic system is to control the “heat leak” into the system from the exterior environment. The use of a vacuum space between the cryogenic temperatures and the exterior eliminates heat transfer via gaseous conduction and convection. This leaves only two major contributors to “heat leak”, conduction from structural members and radiation from the temperature difference of the cryogenic interior and the room temperature exterior. Multilayer insulation reduces the heat transfer via radiation.
Multilayer insulation “blankets” generally consist of alternating layers of a low-emittance radiation shield and a low conductivity spacer material. The low-emittance radiation shield most commonly used is a mylar substrate with a vacuum deposited aluminum coating on a single or both sides of the plastic sheet. Meyer Tool utilizes a half mil thick double aluminized mylar. The spacer material can vary widely; Meyer Tool utilizes 4 mil thick spunbound polyester. Meyer Tool fabricates our own multilayer insulation blankets using a large double roller apparatus.
Other blanket configurations exist, including dimpled or crinkled radiation shields that eliminate the spacers and configurations with holes to improve vacuum pump-down performance.
Cryogenic engineers typically use an apparent thermal conductivity for calculating the heat transfer allowed by a multilayer insulation system. The value of a multilayer insulation blanket apparent thermal conductivity assumes a well-evacuated vacuum system and is based on a combination of theory, the literature and experience. For example, values used by Meyer Tool for a 45-60 layer multilayer insulation blanket insulating from 300K to 80K might vary between 1 to 1.5 W/m depending upon geometry.
The actual effectiveness of a multilayer insulation system is heavily dependent upon the correct installation of the system on the cryogenic surfaces. Poor installation technique including edge effects (the thermal conductivity of MLI is 3 times greater parallel to the layers versus through the layers), lack of interleaving and overlapping of seams, compaction, and direct contact to warm surfaces can negate the best theoretical design. New Meyer Tool technicians receive practical training in proper technique from technicians with over 20 years experience in applying MLI. Installation techniques are codified in a series of written Standard Operating Procedures.