Fabrication Methodology of Gas Foil Thrust Bearings used in High-speed Cryogenic Turboexpander

Abstract

Cryogenic turboexpander is considered as the heart of modern gas liquefier for its high thermodynamic efficiency and high reliability. The operating speed of small-sized turboexpander is usually greater than 80,000 rpm. Such high rotational speed brings constraints in the selection of the bearings. Oil-free gas foil bearings are a novel approach to keep the process gas contamination-free and better rotor dynamic performance. The gas bearings for turboexpander are precession components of turboexpander as the permissible clearance between stator and rotor is in the range of 20 to 40 m. So accurate fabrication methodology of these bearings is essential to preserve the performance of the turboexpander. In the current research, a pair of gas foil thrust bearings are fabricated for a vertical turboexpander used in nitrogen liquefier. The thrust bearings consist of a bearing base, bump foil, top foil and a mechanism to attach the foils to the bearing base. The bearing base is divided into four sectors and each sector is consisting of a smooth foil and a corrugated bump foil, which are mechanically attached with a bearing base using grub screws. The material of both foils are phosphorus bronze and their thickness is 100 m. The bumps are fabricated using metal dies and die material is SS302. This paper explains in details about the fabrication of smooth foil using wire EDM, fabrication of bump foil by forming process using metal dies. The author explains the method of assembly and the necessary jigs and fixture required for the assembly.