Oil free turbo-compressors for CO2 refrigeration applications

•Feasibility of oil-free CO2 refrigeration systems is assessed.•Potential for good efficiency deteriorates with increasing operational pressures.•Main efficiency limiting factor is motor windage.•Foil bearing technology requires optimization of axial loads.•The trade-off between aerodynamic efficiency and non-stage losses must be found.

Feasibility of replacing oil-lubricated compressing equipment in CO2 based refrigeration systems with oil-free turbo-machinery is assessed. Presented concept enables efficient compression for systems ranging from 0.1 to 5 MW of cooling capacity, provided that the operating pressures are low, i.e. 30/10 bar. Performance of the systems with higher operating pressures, i.e. 77/30 bar is penalized in wide range of capacities due to the excessive windage losses, especially pronounced in the systems with cooling capacities lower than 1 MW. In some cases, possibility of using longer motor should be analyzed. This may require special strategies for rotordynamic issues or driving each impeller with separately mounted motor. It is observed that optimal specific speed of the compressor stage does not always result in optimal overall performance. The trade-off between aerodynamic efficiency and non-stage losses must be found.