Fluid Film Bearings in Low Speed Applications:  Thrust Bearings in Rotary Air Preheaters

A common criticism of using tilting-pad fluid-film bearings in low rotating speed applications is that an adequate  pressure field cannot be created.    However, fluid-film bearing manufacturers have a history of successful operation in these environments when properly designed and paired with an appropriate lubricant.  This talk reviews one such application--thrust bearings in rotary air preheaters.  

Rotary air preheaters are devices installed typically in power plants which reuse the heat of the exhaust gasses to heat the incoming fresh air, thus improving the plants’ efficiency. The bearings  used in these devices range from 580 mm to 2.2 m in diameter and are subjected to continuous loads in excess of 4 MPa at incredibly slow speeds, less than 0.1 m/s at mean diameter. Calculated minimum film thicknesses using a high-viscosity oil (215 mm^2/s at operating  temperature) are on the order of 6 μm or smaller. The standard assumption is that such operating conditions place the application in the mixed-lubrication region on a Stribeck curve.  However, a careful review of the bearing performance demonstrates that these bearings operate in the fully hydrodynamic regime on the Stribeck curve.  

This talk will briefly review rotary air preheaters and then Stribeck curves as functions of the traditional Hersey number and lambda values.  We will review rudementary film-thickness calculations and show how these bearings sit in the fully hydrodynamic region of the Stribeck curve.  Time permitting, we will address using the results of modern high-fidelity FEM codes to calculate minimum film thickness and how the Stribeck curve changes.  Finally, a will end with a reassurance that tilt-pad fluid-film bearings perform well in low-speed applications.