Investigations On Thermal Conductivity Of Powder Insulation With Opacifiers And Under Evacuation By Using Ln2 Boil-Off Calorimeter

Abstract

Heat transport or entropy production is a common problem with all low temperature work . Sometimes heat transport can be undesirable and should be cut to minimum. These unwanted \heat-leaks" are costly, and the lower the temperature the more costly it becomes to remove a given amount of heat and discharge it at ambient temperature. At very low temperatures, it becomes economical to conserve the cold using insulating material rather than to produce it because of the requirement of very large amount of energy transfer. Therefore the cryogenics vessels, transport vessel and transfer lines are insulated. Many insulating powders are used for these purposes. For the estimation of heat transfer, thermal conductivity must be known. A double guarded cylindrical boil o_ calorimeter is an absolute method of determining thermal conductivity. The apparatus consist of a main vessel in the middle and two guard vessels located at top and bottom. This vessel is housed in a cylindrical vessel. For determining the thermal conductivity of powder at different vacuum pressure, the powder is _filled in the annular space and pressure is reduced from . The test and guard vessels are filled with liquid nitrogen. The heat transferred to the main vessel result in formation of nitrogen vapor. From the amount of vapors formed or the liquid nitrogen boil off rate, the heat transfer rate and subsequently the apparent thermal conductivity of perlite powder is estimated. It is known that in high vacuum, radiation plays a major role because gas conduction and convection are negligible. The present experimental work aims to study the influence of evacuation pressure on the apparent thermal conductivity of powder insulation material with different proportion of opacifiers (Al powder) which affects radiation heat transfer within the material. It is observed that by mixing about 50% (by weight) opacifier with perlite powder, the optimum perlite-opacifier combination is achieved.