Specific speed and efficiency of centrifugal impellers

Analysis of the test performance of centrifugal impellers showing a satisfactory correlation of impeller polytropic efficiency vs specific speed based on average impeller density is presented. The use of polytropic efficiency and speed reduces the impeller flow path to an equivalent incompressible frictional path; examination of test impeller efficiency levels and of internal losses indicates that the majority of the losses is frictional from the flow-path geometry and the windage between impeller and stationary shrouds. Test data on improved efficiency correlation are presented on several impeller geometries at low and high Mach numbers and specific speeds. Also, design charts are shown which indicate that the attainable state-of-the-art impeller efficiency levels are dependent on geometry and operating conditions.