Flat Panel Vacuum Thermal Insulation

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Abstract
Evacuated mats of glass fiber made up of fibers of proper size and orientation are capable of supporting a compressive mechanical loading of at least one atmosphere and yet maintain a thermal conductivity of less than 10 microcalories/cm°C sec. The use of such a glass fiber mat as a filler makes possible an evacuated flat‐panel thermal insulation which is comparable to a Dewar flask in insulation efficiency. The rate of heat transfer through a Dewar flask wall was reduced several‐fold at liquid nitrogen temperatures and below by adding a 2‐cm‐thick layer of orientated and evacuated glass fiber mat to the outer surface. This investigation showed that in evacuated glass fiber mats, supporting external atmospheric loading, the fiber to fiber contact area is less than 10−4 the mat area, making the contact pressure about 15 000 kg/cm2. The effective length of the thermal conduction paths along the fibers is about four times the mat thickness. The mean pore size for gas molecule motion in the mat was found to be about 54×10−4 cm and 2×10−4 cm for mean fiber sizes of 14×10−4 cm and 0.2×10−4 cm, respectively. The radiation mean free paths for the same fiber sizes were found to be 150×10−4 cm and 52×10−4 cm, respectively. The thermal diffusivity is about 10−4 cm2/sec, which is much smaller than any other insulating material.

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