Thermodynamic and cost-effectiveness analyses of chosen cooling loops for local production of saturated superfluid helium in large cryogenic systems

Abstract

Large scientific facilities applying HeII technologies usually use the Joule-Thomson expansion for the final production of saturated superfluid helium at their cryogenic users. The users are usually supplied with subcooled liquid helium at 4.5 K and 3 to 4 bar(a). Then, to produce HeII, the 4.5 K helium is precooled in a local heat exchanger and throttled to a subatmospheric pressure below 50 mbar(a). This final throttling goes along an isenthalpic line which leads to the zone of wet vapour at the quality of 15.9%. The efficiency of this process can be strongly affected by additional heat loads, which may result in a significantly higher quality of the throttled helium. This imperfection can be partly decreased by using a local subcooler or by splitting the expansion process into two phases with an intermediate point around 1.2 bar(a). However, these solutions require additional components, such as phase separators with some instrumentation and another throttling valve. The paper presents the comparative thermodynamic analysis of the three identified cooling loops. Potential savings due to thermodynamic efficiency improvements are verified against the capital costs for different operation times.