Minimum Viscosity for Bearing Reliability in Rotary Compressors

To ensure bearing reliability, it is traditional for compressor manufacturers to specify a minimum oil sump temperature difference above the saturated discharge temperature in a high-side compressor and suction temperature in a low-side compressor. This minimum oil sump temperature specification is, in essence, a surrogate measure of the minimum viscosity the compressor manufacturer reqtrires to ensure adequate lubricant film thickness under system flooding conditions. The minimum viscosity required to maintain an adequate lubricant film thickness in the bearings depends on the bearing geometry, the load and the speed. Because the load, and to a much lesser extent, the speed vary from point to point over the compressor operating envelope, so v.i.ll the minimum viscosity required to maintain adequate film thickness. This paper describes a method for specifying the minimum safe viscosity over the entire operating envelope of high-side rotary compressors based on film thickness calculations.