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 Centrifugal pump: He is by the high-speed motor rotation medium (usually less viscous medium: such as water, diesel, kerosene, gasoline, etc.) by the impeller runner to the media rejection Out, there is volumetric pump can be transported medium viscosity is relatively large, such as heavy oil, viscous oil, asphalt and other viscous medium; centrifugal pump can only transport some less viscous medium, and his delivery viscosity can only be less than 3Cst ( Cousins)