Modeling of Cold Compressor Pump Down Process

Abstract

The Facility for Rare Isotope Beams (FRIB) will use a sub-atmospheric helium refrigeration process operating at 2 K (31 mbar) to support the superconducting radio frequency (SRF) Niobium structures (known as cavities), which are housed within ‘cryo-modules’. The cryomodules are large containers whose exterior forms a vacuum chamber that serves as a thermal shield. The cryo-modules, and the superconducting devices contained within, are used to accelerate charged particles. The accelerator at FRIB is comprised of three separate linear segments, separately or collectively, called a linear accelerator or ‘LINAC’. The helium used as the working fluid to cool the SRF Niobium cavities is supplied from a 4.5 K refrigerator, but the sub-atmospheric condition will be produced by ‘pumping-down’ the LINAC using cryogenic (cold) centrifugal compressors to remove mass, thus reducing the pressure within the SRF Niobium cavities. The initial condition of liquid helium before starting a ‘pump-down’ can range from a 2 K sub-cooled liquid to a saturated liquid at around 1 bar. These initial condition extremes will result in pump-down processes that are different. This variability of initial conditions increase the complexity of the overall process. As such, a process model can provide considerable insight into the best approach to use for a particular pump-down. This research has developed a simplified model of sub-atmospheric components downstream of the 4.5 K cold box. The initial condition of the helium within the SRF Niobium cavity is assumed to be a saturated mixture at near atmospheric pressure and remain a saturated mixture as the pump-down proceeds. The prime mover in this study is a single radial centrifugal cold compressor removing mass from the Niobium SRF cavities. A model for the return transfer line is incorporated to simulate pressure drop, heat in-leak, and mass accumulation of the sub-atmospheric helium returning from the LINAC back to the cold compressor. A counter flow heat exchanger is also a part of the model. This heat xchanger uses the sub-atmospheric helium stream leaving the SRF cavity to the cool the supply stream from the 4.5 K cold box. The model accounts for the non-constant thermal capacity rates present in this heat exchanger. The sum of the SRF cavities are modeled as a single dewar process, with a non-flowing two-phase mixture. The dewar process involves heat transfer to the liquid, and mass and energy depletion. The model is used to study the time to achieve a desired final within the dewar for a given set of system parameters. The component models are individually validated. The overall process can be extended and validated and compared to the FRIB process after such commissioning is complete. This model serves as the foundation for further process studies