Exergy based analysis on different expander arrangements in helium liquefiers

Abstract

In helium liquefiers, refrigeration stages have expanders connected in parallel (reverse Brayton stage) and also in series with heat exchangers between them (modified Brayton stages). Options of splitting and combining Brayton stages into modified Brayton stages are evaluated and compared by exergy analysis. Results show that as two Brayton stages are combined to make two modified Brayton stages, the performance deteriorates. When one Brayton stage is split into two modified Brayton stages, the performance shows improvement with the total heat exchanger surface area remaining unchanged. When the stage operates at lower temperatures, such splitting has led to more improvement. Each stage, whether it is Brayton or modified Brayton, has been found to behave as an independent refrigeration stage allowing more additions of heat exchanger area. In a one-to-one comparison, a Brayton stage has been found to be superior to a modified Brayton stage at any temperature of operation. The impact of replacing Brayton stage with modified Brayton stage has been found to be more pronounced when heat exchangers in the configuration are less balanced in mass flow.