FRIB helium refrigeration system commissioning and performance test results

Abstract

At its maximum capacity condition, the 4.5 K refrigerator system for the Facility for Rare Isotope Beams (FRIB) accelerator supports a 180 g/s 30 K 1.16 bar cold compressor return flow, a 14 g/s 4.5 K liquefaction load, a 4 kW 4.5 K refrigeration load, and a 20 kW 35-55 K shield load. Five additional design conditions, ranging from liquefaction to refrigeration and a stand-by/reduced load state, were specified for the sizing and selection of its components. The cold box system is comprised of a 300-60 K vertical cold box that incorporates a liquid nitrogen pre-cooler and a 60-4.5 K horizontal cold box housing seven turbines that are configured in four expansion stages including one Joule-Thompson expander. This cold box system, operates using the Ganni-Floating pressure process, automatically adjusting to the linear accelerator (Linac) load with the cold box supply (and compressor discharge) pressure varying from 6 to 21 bar, without introducing additional (artificial) loads or throttling turbine inlet valves (or other exergy loss mechanisms), and with minimal liquid nitrogen usage. This paper will briefly review the salient 4.5 K system design features and discuss the recent commissioning 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.…”

“…However, the experimental system superconducting magnets are supported with the supercritical helium flow from the JT-expander. A wide range of cold box performance testing (including LN usage and cold box efficiency characterization) was done earlier and is discussed in [5]. The performance testing has demonstrated that the FRIB refrigerator has good efficiency for a wide capacity range and load type.…”

“…The SRF cavities are designed to be operated at sub-atmospheric pressure (nominally at 2.0 K, or 30 mbar-absolute, accomplished using cryogenic compression process), while the rest of the loads are to be supported at 4.5 K. A helium cooled thermal shield (around 55 K) is used in all these loads for intercepting heat inleak from thermal radiation. In total, the FRIB helium refrigeration system is required to support the following type of loads during accelerator operations -(a) the return from the cryogenic (cold) compressors (at around 30 K), (b) 4.5 K refrigeration, (c) 4.5 K liquefaction and (d) 35-55 K helium thermal shield [5].…”

“…The FRIB helium refrigeration system was commissioned in late 2017 and is designed based on the Ganni Floating Pressure cycle and offers a wide range of operational flexibility. The testing for the design-operating modes and performance mapping was completed in early 2018 [5]. In the present paper, testing, and analysis of the stand-by operating modes for the FRIB helium refrigeration system is discussed.…”