Comparative studies on journal bearings for turbo expanders of a dual mode LOX-LIN plant at BARC, Mumbai

Medium and large capacity Liquid Oxygen (LOX) and Liquid Nitrogen (LIN) plants are available in the international market. However, a dual mode air separation plant (either LOX or LIN) is not readily available. Hence, a turbo expander based dual mode air separation plant is conceptualized by BARC, Mumbai. The process employs a single distillation column for both LOX and LIN modes. LOX output of 50 l/hr with 96% purity and LIN output of 75 l/hr with 99% purity is expected from the plant. The design speed of the turbo expander comes out to be 1,00,000 to 1,30,000 rpm for various modes of operation. This eliminates the use of rolling contact and oil-lubricated bearings as these are prone to wear and tear at high speeds. Hence, specialized gas bearings are required for these rotors. This article describes a comparative study on various kinds of aerodynamic and aerostatic journal bearings for the turbo expander. The aim is to find a suitable bearing which can provide adequate stiffness and damping to the turbo expander rotor for all operating scenarios. Reynolds equation for compressible flow is used to formulate the air flow between the rotor and the bearing, and pressure profiles are generated which are used to compute stiffness and damping coefficients. The stiffness and damping characteristics of different bearings with varying operational parameters are compared in this article. It is expected that the development of this LOX-LIN plant would generate hands on experience and knowledge base in indigenous air liquefaction technologies. (author)