Determination of Apparent Thermal Conductivity of Expanded Perlite and to Evaluate the effect of Evacuation Pressure

Abstract

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 to produce very low temperature. Therefore the cryogenic storage ves- sels, transport vessels and transfer lines are insulated. Perlite powder at atmospheric pressure and under vacuum is a common insulating, material 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 for di_erent vacuum pressure, the powder is _lled in the annular space and pressure is reduced from 0.8 mbar to 1_10􀀀6 mbar . The test and guard vessels are _lled 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-o_ rate, the heat transfer rate and subsequently the apparent thermal conductivity of perlite powder is estimated. The present experimental study undertaken to study the inuence of evacuation pres- sure on the apparent thermal conductivity of perlite powder found that the value of the thermal conductivity increased with an increase in the gas pressure. The lowest values of ka are obtained when the interstitial gas pressure is below 0.001 mbar. How- ever lowering the interstitial gas pressure below this value has no appreciable e_ect on the heat ux. The apparent thermal conductivity of perlite powder was found to be between 12.9 to 2.9 mW/m K for an evacuation pressure ranging from 0.8 mbar to 1 _ 10􀀀6 mbar . The value of apparent thermal conductivity is comparable with those evaluated by Kropschot and Burgess[1].