DESIGN AND FABRICATION OF GAS FOIL JOURNAL BEARING FOR HIGH SPEED SMALL TURBOEXPANDER USED IN CRYOGENIC APPLICATION

Abstract

Oil free turboexpander is one of the most critical parts in large no of cryogenic gas process plants. These turboexpanders in a typical cryogenic liquefier runs at high speed between 1,00,000 and 5,00,000 rpm. To overcome problems of contamination, maintenance and friction losses at high rotational speed, gas bearings are being used. Gas bearing using rigid geometry like tilting pad gas bearing successfully has been developed indigenously, an attempt is made in present investigation to design and develop a compliant surface gas bearing (foil journal bearing) with excellent dynamic properties and possibilities of adaptation to changed conditions at very high speed or demanding operating conditions. The compliant surfaces of gas bearing allow accommodating levels of misalignment, thermal and centrifugal growth that would possibly destroy a rigid surface gas bearing. This paper concentrate on steps of design of gas foil journal bearing 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. On succeeding theoretical studies a pair of gas foil journal bearings is designed and fabricated to accommodate in indigenously developed turboexpander. This paper also describes methods to fabricate 2nd generation gas foil journal bearing. Authors hope that detailed steps of design and development methodology on gas foil journal bearings will be useful to researchers around the world.