Inducers are similar in design to an Archimedes screw and are located upstream of the impeller inlet. See the figure below. Inducers increase the liquid pressure at the inlet to the impeller. The average diameter of the inducer vanes is smaller than the average diameter of the inlet portion of the impeller vanes. The net result is lower relative inlet velocity as the vanes contact the liquid, and lower energy levels means less NPSH required to suppress cavitation. In addition, fewer vanes are used in the inducer, two or three versus four or seven in the impeller. Fewer vanes means less blockage of the flow passages by vapor bubbles resulting in less NPSH required to suppress cavitation. Consequently pumps with inducers can operate with lower values of NPSHR. Cavitation damage to inducers at rated NPSHR should be comparable to damage to impellers at rated NPSHR.
It is also possible to add inducers to impellers to increase the margin of NPSH available and thereby reduce cavitation damage to the impeller.
Not all pump configurations can accept inducers.