流体力学与 及传热课件Positive-displacement Pumps.pptVIP

2.2.3 Positive-displacement Pumps ;Reciprocating pumps ;Most piston pumps are double-acting with liquid admitted alternatively on each side of the piston.; Often two or more cylinders are used in parallel with common suction and discharge header to minimize fluctuations in the discharge rate .;For higher pressure plunger pumps are used. Plunger pumps are single acting and usually are motor-driven. ;Volumetric efficiency;Because of the constancy of volume flow, plunger and diaphragm pumps are widely used as “ metering pumps”; Rotary pumps;gear pump;Screw pump;cam pump;spur-gear pump; Reciprocating and rotary pumps can be used to very high pressures, whereas centrifugal pumps are limited in their head and are used for lower pressures. ; Centrifugal pumps deliver liquid at uniform pressure without pulsations and can handle liquids with large amounts of suspended solids. ; In general, in chemical and biological processing plants, centrifugal pumps are primarily used. ;2.2.4 Centrifugal pumps; The energy changes occur by virtue of two main parts of the pump, the impeller and the volute. ; The impeller is the rotating part that converts driver energy into the kinetic energy. ; The liquid enters the suction nozzle concentric with the axis of a high-speed rotary element called impeller then into eye of a impeller. ; Liquid flows outward in the spaces between the vanes and leaves the impeller at a considerably greater velocity with respect to the ground at the entrance to the impeller. ;As liquid leaves the eye of the impeller, a low-pressure area is created and causing more liquid to flow toward the inlet. ;Conversion of Kinetic Energy to Pressure Energy ; The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid. ; The first resistance is created by the casing that catches the liquid and slows it down. In the discharge nozzle, t