Speaker
Description
ABSTRACT:Small-scale helium liquefiers with typical liquid-helium production of the order of tens of liters per day are required for cryogenic applications, such as cryogenic research experiments and operation of superconducting magnets in hospitals. These liquefiers are typically based on 4 K regenerative cryocoolers, including Gifford-McMahon (GM) or pulse-tube (PT) cryocooler. However, due to the heat exchange between the helium being liquefied and the outer wall surface of the cylinder, the precooling process is inefficient, leading to a superheat of helium at the cold head and a low liquefaction rate. To address this challenge, this study proposes a gas-coupled GM/Joule-Thomson (JT) hybrid helium liquefaction system. The system uses the internal helium of the GM cryocooler as the source for JT throttling and liquefaction, resulting in highly efficient precooling and liquefaction. There are different effects of direct current gas flow (DC flow) mass, temperature and pressure on the performance of GM cryocooler and JT stage of the system, which should be comprehensively considered. To determine the impact of interstage pressure, first and second stage precooling temperature, hot end DC flow ratio and recuperator effectiveness on the liquefaction rate, a Pressure-Enthalpy map analysis and thermodynamic calculations were performed. For a GM cryocooler with 1 W cooling capacity at 4.2 K, and a charge pressure of 1.6 MPa, and a secondary cold end temperature of 5 K, it was found that inducing a DC flow rate of only 50 mg/s at the cold end is sufficient to achieve the ideal helium liquefaction rate, corresponding to a sufficiently precooled helium gas, which is not achievable with conventional small-scale helium liquefiers. This DC flow is less than 1% of the actual flow amplitude of the cryocooler, and its effect on the performance of the cryocooler can be deemed negligible. The new helium liquefaction method proposed in this study has the potential to increase the liquefaction rate of existing small-scale helium liquefiers.
