Application of exergy analysis in designing helium liquefiers

Exergy has proved to be a useful tool to analyze and optimize the design and operation of many systems. Some studies on helium refrigerators and liquefiers based on exergy are available in literature. In this paper, systematic evaluation of important operating and geometric parameters has been done to determine the exergy destructions in components as well as in the entire cycle of Collins helium liquefiers. Grassmann diagram of exergy flow has been shown to be of immense help in understanding relative importance of different components used in the system. Compressor pressure, expander flow rates, heat exchanger surface area are some of the parameters optimized considering both presence and absence of pressure drop in the heat exchangers. Non-dimensionalization of parameters makes the results applicable to plants of any capacity. Exergy-analysis based on Second Law proves to be far superior to the First Law based energy analysis in designing of the helium plant as the former is holistic in approach and capable of deriving some additional conclusions. Results derived on Collins cycle may be applicable in large-scale helium liquefiers by providing basic understanding of the influence of components on the plant performance and reasonable initial guess values in their design and simulation.

► Exergy analysis used to optimize operating pressure, flow rates, heat exchanger sizes. ► Grassmann (exergy flow) diagram for Collins helium liquefaction cycle presented. ► Exergy analysis found to be superior to energy analysis in designing helium plants. ► Influences of pressure drop and Joule-Thomson expansion better explained using exergy. ► Conclusions help in generating the initial design values for large helium liquefiers.