Optimization Of Thermal Conductivity For Economical Cryogenic Insulation

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 as work. 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 off 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, the powder is filled in the annular space and pressure is reduced to high vacuum. 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 powder insulation is estimated. The conventional insulating materials like perlite, aerogel, glass microsphere have high initial cost. The agricultural waste like rice husk, saw dust, palm kernel husk, sunflower stalk are readily available, cheaper and have good insulation property. So the cost of the cryogenic insulation can be reduced by using mixture of conventional and non-conventional materials. However, this can lead to increase in thermal conductivity and hence increase in the boil-off rate of the cryogenic liquid. Therefore, a thermo-economic analysis is required to be carried out to find out the optimum proportion of the mixture which will lead to the minimum cost, which is the objective of the present work.