Effect of environmental pressure enhanced by a booster on the load capacity of the aerodynamic gas bearing of a turbo expander

Abstract

To improve the load capacity of the gas bearing by changing environmental pressure, this study analyzes the effect of environmental pressure on the load capacity of a bump foil gas aerodynamic bearing at different rotation speeds. Results show that the load capacity of the bump foil gas aerodynamic bearing increases with the increase in environmental pressure. Specifically, the load capacity of the aerodynamic gas bearing increases by 19.97%, 28.89%, and 44.28% at a speed of 60, 100 and 200kr/min, respectively, when environmental pressure increases from 0.1 to 0.5MPa. And the load capacity of bump foil gas bearing changes with the environment at different slenderness ratio is analyzed when rotation speed is 100kr/min and eccentricity ratio is 0.6, the results show that under the same working load, increasing the environmental pressure of gas bearing and improve the load capacity gas bearing can reduce the slenderness ratio of the bearing; consequently, the shaft length is shortened, and the load of the aerodynamic gas bearing is reduced. Moreover, increasing the environmental pressure of gas bearing can shorten the higher the proportion of rotor length for the bigger slenderness ratio of gas bearing. This characteristic is more beneficial to the steady running of rotor at high speed. For a turbo expander that adopts a fan brake, the fan sucks and discharges air from and to the atmosphere directly; hence, the power provided by the turbo expander is wasted completely. This study recommends replacing the fan brake of a turbo expander with a booster and supplying pressurized gas to the gas bearing to enhance the environmental pressure at both ends of the bearing; this measure can improve the load capacity of the aerodynamic gas bearing and thus reduce energy dissipation. In this paper, Numeca software is utilized to analyze the internal flow of the booster impeller of a turbo expander with 69.68kJ/s refrigerating capacity and 66.2kW effective shaft power. On the premise of the power coupling–matching of the expander and booster, when the total pressure ratio of the booster is 3.5, 4.0, and 4.8, the slenderness ratio of the bearing can be reduced 10%, 15%, 25%, respectively, at a speed of 60, 100 and 200kr/min for the turbo expander rotor system supported by gas bearing that the initial slenderness ratio is 1.