A centrifugal pump is a rotodynamic pump with radial flow, while a rotary pump is a broad category of positive displacement pumps. The centrifugal pump (Image 1, left) is a kinetic type of pump that continuously imparts energy to the pumped fluid with a rotating impeller (Image 1, right). The kinetic energy is converted into potential energy (pressure head) in the discharge collector after the liquid exits the impeller. The head developed by this pump type is limited by the diameter and speed of rotation.
Rotary pumps are a displacement type of pump that traps and displaces a volume of liquid through the pump with meshing screws, gears, vanes, lobes or other similar mechanisms. For this reason, these pumps will provide nearly constant flow regardless of pressure, and if you vary the speed of the pump, the output flow will change linearly with the change in speed. This is a fundamentally different pumping action than the centrifugal pump’s rotating impeller.
For more information on pumps, refer to the Hydraulic Institute’s Introduction to Pump Fundamentals training at training.pumps.org.