Design and Analysis of Aerostatic Bearings of Cryogenic Turbines for Helium Refrigerator/Liquefier

Abstract

Aerostatic bearings are generally used in the field of high speed applications. The Helium Refrigerator/Liquefier (HRL) needs turbines as expansion machines to produce cooling effect which is further used for production of liquid helium. Cryogenic turbines are significantly smaller in size compared to those for room temperature applications but rotational speed is very high, about few hundred thousands of rpm and hence these have contact less gas bearings or magnetic bearings. This project involves the design and analysis of the aerostatic bearings with horizontal shaft configuration. In the aerostatic bearings, pressurized helium gas is passed through the bearings. Based on this pressure and temperature and the rotational speed of the turbines, the shaft of the turbine rotates without contact with bearing wall and the leakage between process gas and bearing gas is minimum. For different normal and offnormal operations, speeds will be different and hence the flow parameters for bearing gas flow will be controlled via control valves and the bearing should be designed to provide such contactless rotation. In this study, a theoretical analysis is presented for the load capacity, stiffness, flow rate of aerostatic journal bearing and thrust bearing with pocketed orifice. Effects of orifice diameter, radial clearance, inlet pressure and outlet pressure on load capacity, mass flow rate and stiffness have been analyzed. Dynamic unbalances like whirling of the shaft have also been covered in this study. Design considerations for limiting dispersion effect, and to avoid pneumatic hammer has also been taken into account. Validation of the analysis has been done by using ANSYS CFX with the numerical results