Heat conduction through insulating supports in very low temperature equipment

Abstract

An apparatus is described that is used to meas ure the heat conduction t hrough ins ulating s upports of storage vessels for cryogenic liquids and presents the data obtained from the cond uc t io n measureme nts. Two types of supports were tested: (1) multiple-contact supports in the form of stacks of thin meta llic p lates or spirally wound strips, a nd (2) nonm etallic spheres. The high t herm al resista nce of the multiple-contact supports ari ses from the numerou s r elatively poor contacts between the individual plates. Some special treatments of the plates were tried, and two of these, p erforating and dusting, Kere found to be effective. Pyrex-glass spheres were also found to be excell ent insulators, but, of course, a re not a s rugged as a stack of m etal plates. Of t he un treated plates tested, t hose of stainless steel (0.0008 in ch thi ck) we re found to be the best insulators per unit length of stack. Th e heat conduction through these p lates, at a load press ure of 1,000 pound s per squ ar e inch , was found to be 2 pe rcent of t he conduction by a solid sample of th e same metal having the sam e dimensions.

Optimum apparent areas for thin-layer elements of mechanically-strong insulating stacks

“…Since two deformation processes, asperity distortion and layer bending, occur simultaneously, and each predominates in a different load region, it would appear likely that there exists an optimum area for a given supported load to give maximum resistance of the stack. This, however, contradicts the conclusion of Mikesell and Scott (1956), who recommended the use of the highest loadings per unit area.…”

“…The high series resistance of a stack of thin layers is used in mechanically-strong thermal insulators for cryosystems (Berman 1956, Mikesell and Scott 1956, Probert et al 1967. The thermal or electrical resistance of each interface would be greatest if the contacting materials were perfectly hard, actual contact then occurring only over three infinitesimal areas where the microscopic asperities meet.…”

“…The thicker the layers the greater the fraction of the load required to flatten the layer buckle (Probert and Jones 1966), and so the optimum area will depend upon the layer thickness. Also, for the predominantly asperity-deformation region, both the stack compression (Probert and Jones 1966) and the reciprocal of the thermal resistance per unit length of stack (Mikesell and Scott 1956) vary approximately as the square root of the individual layer thickness. When designing mechanically-strong thermal insulators of the multilayer type, it would appear reasonable that the stack elements of pre-selected thickness (dictated by the load and high elastic modulus of the material) should be shaped so that a pre-determined fraction of the load is used for flattening the layers.…”

Vessels for the storage and transport of liquid hydrogen

“…Preliminary experiments by the NBS Cryogenic Engineering Laboratory h ave indicated that the apparent thermal conductivity of a laminated stainless steel or Mon~l insulating support under high unit loads (1,000 pSI) would be about one-fiftieth that of the metal itself [4] . Fmther experiments [5] design (dusted surfaces and moderate unit lo ads) , ratios as low as one two-hundredth of the base metal can be achieved.…”