What is the difference between positive displacement pump and centrifugal pump?

Volumetric pumps are proportional to the speed and flow, such as gear pumps. Screw pumps, piston pumps, such as their rotation speed of 100 cubic meters when the flow, while the 1000 rpm can be understood as 10 cubic, but the actual efficiency Losses, traffic is about 9.5-9.8 cubic meters. Centrifugal pump by motor high-speed rotation of the medium (usually less viscous medium: water, diesel, kerosene, gasoline, etc.) by the impeller runner to throw out the medium. There are volumetric pumps can transport medium viscosity is relatively large, such as heavy oil, viscous oil, asphalt and other viscous medium; Centrifugal pump can only transport some of the less viscous medium, and his delivery viscosity can only be less than 3Cst ).