Rotordynamic Analysis of High-Speed Rotor Used in Cryogenic Turboexpander Using Transfer Matrix Method

Cryogenic turboexpander is considered as the heart of modern gas liquefier for its high thermodynamic efficiency and high reliability. The operating speed of small- and mid-sized turboexpander is usually greater than 50,000 rpm. Such high rotational speed brings constrain in the selection of the appropriate bearings. In turboexpander, gas bearings are found suitable to use, where process gas is used as a lubricant to avoid contamination. The study of rotordynamic behaviour of such high-speed turbomachinery is essential to avoid resonant conditions and predict unbalance vibrations. In the current application, transfer matrix method (TMM) is used to predict critical speed, mode shapes and unbalance response of a rotor using gas foil journal bearings. The predicted critical speeds are compared with experiments to validate the same. The rotational speed of the designed rotor is 80,000 rpm with an unbalance of 40 mg-mm at each journal bearing.