Design and Analysis of Internal 20 K Helium Gas Purification System for the kW Class Helium Refrigerator/Liquefier

Abstract

The Helium refrigerator/Liquefier (HRL) is normally operated with helium gas having purity better than 99.999% by volume which is equivalent to having 10PPM (parts per million) impure gas in helium gas. Although sufficient precautions and impurity removal procedures are used, still, in the process of gas transfer or due to some other processes before reaching to liquefaction, impurity level sometimes can go as high as 500 PPM averaging to about 100 PPM. These impurities mainly consists of gases present in the air, like N2,O2,Ar,H2,H2O,CO,CO2 and the traces of Ne. Hydrocarbons of the lubricating oil can also be sources of H2.These gases condense at significantly higher temperature compared to LHe (4.5K).If such high level of impurity enters the process equipment placed inside the cold box of HRL, then it can condense and choke the pipelines and the valves leading to large pressure drop and inefficient liquefaction process.20k to remove H2.This project is about the design and analysis of 20K purification system to be used for the planned indigenous helium plant of equivalent cooling capacity ~2 kW at 4.5 K. The impurity level of H2 in helium gas stream is expected to about 10 to 100 PPM. Activated charcoals are to be used to adsorb impure gases from cold helium gas at 20K and 80K at ~14 bar. The nominal flow rate of helium, entering the 20 K purifier system, is 100 gm/sec.Activated charcoal is used for adsorption process.At the outlet of the 20K purifier, the total impurity should be 1PPM. At the downstream of the adsorber bed, fine filters of 30 micron pore size should be placed to trap the charcoal dust which can come sometime through the helium stream. This purifier will be a part of the indigenous helium development project. In this context, this work also involves the manufacturing, assembly, test, repair aspect of these purifiers.