23. April 2020

Gas bearing design for non-air applications

The design of gas bearings for noble or inert gases is an interesting engineer’s challenge, as the design has significantly more variables than for air bearings. The behavior of a gas bearing is strongly dependent on the gas viscosity in the bearing clearance. However, the viscosity is dependent on the gas temperature and the type of gas. The gas bearing must be designed to run stably over the complete operating range of parameters, throughout the entire speed range and for a range of manufacturing tolerances. As an example the smallest gas bearing clearance is critical at high temperatures and maximum speed, while the largest bearing clearance occurs at minimum temperature and at medium speeds.

The specific challenges in designing a high-speed turbo compressor with gas bearings for noble and inert gases:

  • The gas bearing must work robustly for all expected operating conditions, which include:
    • Varying viscosity due to changing gas mixtures within the same compressor. For example, in noble gas cooling applications where the system is not completely sealed then there is a variable percentage of air in the gas mixture.
    • Fluctuating pressure levels resulting from operation at different altitudes. For example changing specific inlet pressure levels in noble gas recirculation applications.
    • Widely varying gas temperatures in the bearing clearance at different operating points. For example internal losses can heat up the gas to 200 °C (392 °F) in the bearing clearance.
  • The gas bearing, the rotor dynamics and the thermodynamic behavior of the system are strongly coupled, therefore, an interdisciplinary design is required to ensure that the total system is fully functional.
  • The overall efficiency of the aerodynamics, motor and the gas bearing must be maximized. This often leads to a different solution than just using the combined individual maximum efficiencies.
  • The manufacturing tolerances must be considered already during the design phase, in order to produce a robust and stable product series.
  • Last but not least the total system has to be economic to produce.

These design challenges increase significantly with increasing rotational speed. The following considerations are needed:

  • Higher rotational speed leads to higher performance but also higher losses. These higher losses subsequently result in higher temperatures, especially in the rotor.
  • The damping required for a stable gas bearing increases with higher rotational speed.
  • Higher rotational speed leads to miniaturization, but the manufacturing tolerances must reduce accordingly.

An air bearing as in the Celeroton’s turbo compressor CT-2X-series for the supply of air to fuel cells, is designed according to the expected viscosity, temperature and pressure range during operation, but only for air. For a gas other than air, e.g. noble gases, which has a different viscosity, the gas bearing behavior is different, and therefore the compressor designed for air is usually not directly applicable. However, the gas bearing can be adapted to the new gas properties and operating specification. For the Celeroton turbo compressor CT-NG-2000, which is operated with different inert and rare gases, there are gas bearing designs already available for several gas types.

A general overview of gas bearings and their design related aspects is found in this link.