Hydrodynamic fluid film oil bearings have been standard in large equipment for over 100 years. More recently, Tilting Pad oil bearings dominate in high energy rotating equipment applications. Additionally, in the past few decades, hydrodynamic gas bearing technology has been accepted in dry gas seal applications. Foil gas bearings are also a type of hydrodynamic bearing and have seen growing acceptance in aviation HVAC, and sub megawatt micro turbines. Foil bearings are fluid film bearings supported on flexible bumps or leaves so there is low load and stiffness but large gaps and high speed capability. In contrast the bearing technology in dry gas seals often uses stiff silicon carbide and high unit loading with small gaps. All of these cases are examples of dynamic bearings; that is, fluid film bearings that generate pressure to support the load from the relative motion between the bearing surfaces. This is what our turbo industries are used to.
Fig.1 – This is a “big picture” chart to conceptualize gas bearing technology in turbo equipment. It is a lift vs load chart for an externally pressurized porous (EPP) gas bearing that has 10 square inches of bearing face, 100 psi is fed to the bearing as the load on the bearing is increased. Resulting air gaps are shown on the bottom axis in microns. The slope of the curve is representative of the film stiffness. The typical operating regions of dry gas seals and foil bearings are superimposed as is the operating area for EPP Bearings. It can be seen that the EPP Bearings operate with a larger gap than dry gas seals, reducing heating and contact issues but still having good stiffness, damping and speed capabilities.
What Don Bently was suggesting is the use of external pressure and compensation both to establish the fluid film and control bearing stiffness and damping, from outside the machine. After all the years of just measuring what was going on with a rotor/bearing or seal, he was demonstrating a way of tuning bearings dynamically to operating conditions in situ. Have a seal running hot? An increase of input pressure directly between the bearing faces will increase the gap proportionally, but not much is needed as shear in the gap is reduced by a cube function of the increasing gap. These are game changing ways of addressing rotor dynamic issues that have not yet been accepted by the turbo industries and this is what Don was suggesting.
Additionally, externally pressurized bearings offer new options in machine architecture. For instance, hydro and aerodynamic thrust bearings have segmented pads for the purpose of having leading edges for the fluid film wedge to develop. Because of these big radial gaps no engineer would consider a dynamic thrust bearing as a seal. Thinking alternatively, externally pressurized thrust bearings have a contiguous 360 degree face that looks just like a dry gas seal face and because the pressure is always highest in the bearing gap, it is already a seal! This gets the engineer thinking “If I could eliminate oil lubrication, combine my thrust bearing with my dry gas seal, I could even use area on the thrust runner to replace the balance piston.” This dramatically reduces moving parts and brings DGS low leakages to balance pistons (the low hanging fruit of improved compressor efficiency). The biggest advantage though, as Don would argue is that the shaft, which is the weak link, stiffens on a cube function as you make it shorter, dramatically improving the rotor dynamics.
Fig 2 – An externally pressurized thrust bearing looks and acts much like a double opposed face seal, when positioned on the high pressure side of a straight though centrifugal compressor, radial real estate on the thrust disc can be used as the balance piston shortening the required length of shaft enough to make a big improvement in rotor dynamics.
Performance Characteristics of Bearing Types
|Oil Journal||Active Magnetic||Bump Foil||Flex/Pivot Hybrid Oil||Pressurized Gas|
|Friction at Start||1||3||1||2||3|
|Shear/Loss and Speed||1||3||2||1||3|
Source for this page: Turbomachinery International, “Scoping Don Bently’s bearing concept” by Drew Devitt, 27 October 2016