A cool way to eliminate lubricants

A refrigerant has successfully been used as a lubricant in compressor bearings, paving the way for cheaper, more efficient compressors which avoid the environmental problems of conventional oil-lubricated bearings.

Traditionally, oil is mixed with compressor refrigerants to lubricate the bearings. However, when this mixture enters the bearing, the refrigerant can "flash" or evaporate, preventing the oil from lubricating the balls and raceways. Another problem is that, because the oil is diluted when it is mixed with the refrigerant, its viscosity is lowered, resulting in poor lubrication.

After several years of development, SKF has shown that a refrigerant can serve as the lubricant in hybrid ceramic bearings, avoiding the problems of evaporation and dilution. This technology also removes the need for separator to collect the oil and inject it back into the bearing to minimise dilution.

The US compressor manufacturer Trane has used the bearings in an oil-free compressor with a direct-drive design that eliminates the need for gears. The CenTraVac compressor is said to have about 40% fewer parts than conventional designs, and to be 5-20% more efficient.

Eliminating oil from the compressor is expected to cut maintenance costs, by eliminating the need for oil checks and filter changes, and avoiding the problems associated with oil leaks and oil disposal.

The hybrid bearings used in the compressor combine steel raceways with silicon-nitride balls, resulting in greater stiffness than all-steel bearings, and the ability to operate longer at higher speeds. The technology is expected to find other applications in areas such as pumping and food processing.