Design and analysis of the helium purification system for the NSRRC cryogenic system

Abstract Helium is an expensive consumable in cryogenic facilities and is used widely in space, medical and energy research. At NSRRC, liquid helium is used as a coolant for superconducting magnets and SRF cavities . Minor contaminants such as nitrogen, oxygen, moisture and oil will be picked up when liquid helium circulates in large scale cryogenic systems and such contaminants can crystalize and cause damage to the cold box turbo expanders resulting in system damage and failure . Therefore, a helium purification system is designed as an integral part of the cryogenic system to conserve helium by providing 99.9995% pure helium to the liquefier after eliminating contaminants. The NSRRC helium purification process is based on two principles, the first one being a cryo-sorption device using activated charcoal and a molecular sieve and the other being a cryo-condensation unit using a tubular heat exchanger. The purifier has been designed to purify impure helium with overestimated contaminants of as much as 2.5% nitrogen and 2.5% oxygen with a mass flow rate of 475 nm3/hr and delivering a pressure of 17 bar(a) of impure helium to the purifier, the actual impurity will be much lower than the actual design contaminants. In this paper, calculation and design of the helium purification system and components composed of one double pipe counter flow heat exchanger, one vessel and tube heat exchanger, one pre-cooler and one charcoal vessel will be discussed together with charcoal mass requirement calculations and design of other components. In NSRRC, helium is liquefied and is used as a coolant for the SRF system and for cryogenic undulators. During a cryogenic cycle in the cryogenic system, helium may pick up contaminants such as moisture, oxygen, oil, and nitrogen, which have a higher freezing point than liquid helium and will crystalize. These frozen impurities will then affect plant capacity and operation such as alternating flow characteristics, damaging moving parts like turbines in the cold box causing the overall cooling efficiency to drop. Eliminating the contaminants is therefore very important for the cryogenic system.