Feasibility Studies on Gas Foil Journal Bearings in Helium Turboexpander

Abstract

A turboexpander is also referred as rotatory expansion device used in various cryogenic processes such as air separation, cryogenic liquification, etc. The operational objective of a turboexpander is to refrigerate a gas stream, by expanding the gas at the nearly isentropic condition and removal of work from the gas. The high efficiency of cryogenic liquification process is possible at high-speed of the turboexpander. High-speed brings constraints on the selection of bearing. So to meet above requirements gas bearings are found to be suitable for such application. The gases of gas bearings unlike liquid lubricants have an inherently low viscosity, which leads to lower stiffness and damping. Low stiffness and damping are prone to instability at high rotational speed. The gas foil bearings with compliant structure can tailor stiffness and damping. Also, the compliant surface can accommodate the thermal and centrifugal growth of the rotor. In the current research work, a modest attempt has been made to study the feasibility of gas foil bearing for helium turboexpander rotating a 2,55,000 rpm. The feasibility study includes (i) thumb rule for load carrying capacity, stiffness and damping coefficient (ii) aerodynamic analysis of gas foil journal bearings, by solving compressible Reynolds equation and structural equation using Finite Difference Method and (iii) rotordynamic analysis of the vertical rotor. The author believes the detailed steps of the feasibility study will help the researchers around the world to design gas foil bearings for other high-speed turbomachinery.