Modifications to JLab 12 GeV Refrigerator and Wide Range Mix Mode Performance Testing Results

Testing and analysis of stand-by operating modes for FRIB helium refrigeration system

“…Mode 3 indicated an increase in cold box supply (HP) pressure but overall reduction in supply flow. The FRIB 4.5 K refrigerator is designed for a liquefaction dominated load [5,8,9], where major share of the thermal load exergy is the cold compressor flow, which can be up to 180 g/s, supplied at 4.5 and returned at 30 K. The turbo-expander nozzle flow coefficients are selected based on this load. For an isothermal refrigeration dominated load (as is the case in these tests), the T3-T4 turbo-expander string recycles more flow than is optimal, due to the fixed flow coefficient of the turbo-expander nozzles, resulting in a colder operating range.…”

“…Flow rates at the cold box interface and each turbo-expander string inlet were measured using venturi elements and corrected using real gas properties. Based on operational experience and observations during 4.5 K refrigerator performance testing [5,7,8], the expander efficiencies remain fairly constant over a wide operating range. The expander outlet temperatures are assumed to be accurate, and the inlet temperatures are calculated (and corrected) based on the refrigerator vendor provided expander efficiency and measured pressure ratio.…”