"Modular cryogenic insulation" denotes a chaff for installation in the partially evacuated annular spaces between the inner and outer walls of tanks used to store and transport liquid nitrogen and other cryogenic liquids. The insulating chaff was developed to replace low-density powders (e.g., perlite, silica aerogel, carbon black, or diatomaceous earth) that have been used to insulate cryogenic tanks. Thermal cycling of a tank causes some of the powder to fall to the bottom of the annular space; over time, the powder accumulates in the bottom and becomes compacted there, with consequent loss of thermal-insulation performance. In addition, the compaction generates stresses that can damage the tank. The chaff was selected from among several candidate materials that were investigated to find one that would not settle with thermal cycling, would be readily manufacturable, would offer the requisite thermal-insulation performance, would thermally expand and contract along with the tank walls, and could be installed easily.

The starting material for the manufacture of the chaff is Mylar (or equivalent polyethylene terephthalate) coated on both sides with thin film of aluminum. The material comes in rolls and is conventionally used to form multilayer insulation (MLI) blankets for some cryogenic applications. The rolled Mylar is unwound and processed through a cross-cut shredder to produce the chaff. Like an insulating powder, the chaff is poured into the annular space of a tank through a hole on the top of the outer tank wall.

The thermal performance of the chaff approximates that of perlite powder within 10 percent. Although the initial cost of the chaff is higher than that of perlite powder, the life-cycle cost of the chaff is lower, because settling of the powder makes it necessary to replace the powder during the life of a typical cryogenic tank, whereas there is no need to replace the chaff because it does not settle. Moreover, because the chaff does not settle, a tank insulated with the chaff does not exhibit the increased heat leakage observed in a powder-insulated tank after a few years of the useful life of the powder. Yet another advantage of the chaff is low density — less than 10 percent of the density of a typical insulating powder.

This work was done by Richard L. Jetley of Aerospace Design & Development, Inc., for Kennedy Space Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to

the Technology Programs and Commercialization Office,
Kennedy Space Center,
(407) 867-6373.

Refer to KSC-11912.

NASA Tech Briefs Magazine

This article first appeared in the February, 2001 issue of NASA Tech Briefs Magazine.

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