Design of Cryogenic Ejector Pump for Helium Liquefaction Plant to Produce Temperature Lower than 4 K

Abstract

To produce temperature lower than 4 K in the proposed helium liquefaction (HeL) plant at Institute for Plasma Research(IPR), a pressure lower than atmospheric pressure in a Liquid Helium(LHe) chamber is to be produced by pumping helium vapor from the top of the chamber by the use of cryogenic ejector pump. The other method of creating subatmospheric pressure is by the use of cold compressor, which involves a rotating machine. Whereas the usage of a ejector pump does not involve any rotating parts and hence is a maintenance free equipment. The ejector pump is planned to be installed before the Joule-Thomson(J-T) valve of the helium Plant. The proposed design of HeL plant has two LHe chambers: the bigger one is to get LHe at the downstream of the J-T valve of the main process line whereas the smaller one is to get LHe through a branched process line just before the main J-T valve. The vapor from the smaller chamber is pumped to obtain a sub-atmospheric temperature over its liquid. This vapor is pumped by the ejector pump from below 1 bar to about 2 bar, which is then pushed to the main J-T valve for liquid Helium production at 4.5 K. The ejector pump is planned to produce refrigeration of about 300 W at 4 K. The design of ejector is usually done based on assumption of ideal gas behavior, whereas for the helium ejector working at less than 10 K, this assumption is not valid. A novel approach for the design of the ejector including real gas properties has been proposed, which uses the HEPAC software to obtain helium properties at that temperature range. This approach uses a fundamental approach to calculate the gain in kinetic energy (and thus by velocity of fluid) from enthalpy loss excluding the assumption of constant specific heat and adiabatic ratio required in standard gas dynamics approach. Computational Fluid Dynamics (CFD) simulation for the design obtained from real gas modeling approach is done using NIST REFPROP real gas database in ANSYS Fluent 14.5. The ejector design parameters like convergent angle of mixing chamber are not calculated from the analytical model. So the optimization of convergent angle, diffuser angle and constant area duct diameter is done using ANSYS Fluent 14.5. The present work explores the use of CFD as powerful tool to optimize the thermal design of ejector for low temperature applications.