Improved aerogel-based thermal insulation systems have been developed to provide cost-effective and easier-to-handle alternatives to various types of multilayer insulation (MLI) and evacuated powder insulation now used on cryogenic equipment. The apparent thermal conductivities of the aerogel-based systems are comparable to MLI systems and are well below the thermal conductivities of the other systems.
MLI systems are expensive, structurally complex, and bulky; the insulating properties are anisotropic; and maintaining a high vacuum [10 -4 to 10 -5 torr (about 10 -2 to 10 -3Pa)] is required in MLI for full insulating effectiveness. Evacuated powder insulation is about one order of magnitude less effective than is MLI, but its insulating properties are isotropic, it is generally easier to install, and it requires only a moderate vacuum [10 -2 to 10 -3torr (about 1 to 10 -1 Pa)] to realize its full insulation potential. Unfortunately, the powder in evacuated powder insulation tends to settle in response to vibration and thermal cycling, forming voids that act as heat leaks.
The improved aerogel-based insulation systems are composites which can be manufactured in blanket, sleeve, or clamshell forms to be used with or without evacuation. These composite systems take advantage of the low thermal conductivity of the ultra-low-density aerogels to minimize heat transfer and incorporate a flexible, durable matrix to maximize applicability. The core of the system is aerogels formed at the fiber-fiber contacts of the matrix, forcing solid heat-transfer to occur through the aerogels. This composite configuration improves both the ease of handling aerogels and the overall thermal resistance. The closed-packed structure of the aerogels eliminates the open spaces in the fiber matrix and thereby minimizes convection heat transfer. Excellent thermal resistance has been achieved for both evacuated and nonevacuated insulation systems while maintaining good flexibility. The aerogels can also be produced in an opacified fiber matrix to inhibit radiation heat transfer in the infrared range.
A typical insulating system for use on cryogenic equipment is an integrated, layered structure that includes a backing layer on the cold side and protective layers on the warm side (see figure). One of the protective layers is a tightly woven fabric coated with polytetrafluoroethylene (PTFE), which serves as a vapor barrier to prevent condensation of moisture. Multiple layers of polyester foil coated with aluminum serve as radiation shields and give additional protection from the environment. An outer layer of polyvinyl chloride (PVC) in the form of pipe or a foil jacket provides secondary protection from the environment and protection against mechanical impacts. This system can be designed as a fully flexible blanket type configuration or a pre-formed molded type configuration for installation on a variety of cryogenic storage and transfer equipment.
This work was done by James E. Fesmire of Kennedy Space Centerand Jaesoek Ryu of Aspen Systems, Inc. For further information, please contact Kang P. Lee, President, Aspen Systems, Inc., 184 Cedar Hill Street, Marlborough, MA 01752.
Inquiries concerning rights for the commercial use of this invention should be addressed to
the Technology Programs and Commercialization Office
Kennedy Space Center
Refer to KSC-11903