The flow path of multistage centrifugal compressors is characterized by two 180-degree turns per stage: the inlet turning bend connecting the radial inflow return channel with the radial outflow impeller and the cross-over bend connecting the radial outflow diffuser with the return channel. Due to higher flow velocity and larger width to turning radius ratio, the turning losses are substantially higher in the inlet bend than in the return channel bend. Performance measurements were taken using different annular through-flow turning vane arrangements designed to reduce the inlet turning losses and increase the overall efficiency of the multistage centrifugal compressor. The experiments have shown consistent efficiency gains with corresponding capacity increases by adding multiple annular turning vanes in the inlet bend. The performance improvement potential of the vanes depends strongly on the positioning of these vanes in the flow passage. Based on these results, an empirical turning loss model was developed with the capability to predict the performance improvement achievable with correctly positioned single or multiple turning vanes in the impeller inlet bend area.

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