Feasibility Studies on Gas Foil Journal Bearings In Small and High Speed Oil-Free Cryogenic Turboexpander

Abstract

Oil free turboexpander is one of the most critical active element in large no of cryogenic gas process plants. The turboexpander in a typical cryogenic liquefier operates at high speed ranging between 1,00,000 to 5,00,000 rpm. To overcome problems of contamination, maintenance and friction losses at high rotational speed, gas bearings are being used. However gas bearing using rigid geometry because of inadequate damping could not satisfactorily operate during critical speeds or during shock load conditions giving rise to instabilities. For better rotor dynamic stability, it becomes necessary to use bearings with excellent dynamic properties and possibilities of adaptation to changed conditions. The targets of present investigation is to study feasibility of gas foil journal bearings in vertically oriented cryogenic turboexpander and redesign the gas foil journal bearings to replace pivot less tilting pad gas journal bearings currently being used in indigenous nitrogen liquefaction plant. The compliant surfaces of gas bearing allow accommodating levels of misalignment, thermal and centrifugal growth that would destroy a rigid surface gas bearing. This paper concentrates on steps of feasibility study like (i) thumb rule for load carrying capacity, stiffness and damping coefficient (ii) aerodynamic analysis of gas foil journal bearings, by solving compressible Reynolds equation using finite difference method to find the pressure distribution, gas film thickness profile and load carrying capacity etc. and (iii) rotodynamic analysis using transfer matrix method for critical speed, mode shapes, imbalance response etc. In succeeding theoretical studies, a pair of gas foil journal bearings is designed to replace pivot less tilting pad gas bearings. Necessary modification is being done in bearing housing to maintain over all compactness of the system. Authors hope that detailed steps of feasibility study on gas foil bearings will be useful to researchers around the world.