流体力学与传热（伍钦）08-9chapter2-4.pptVIP

2.2.4.4 Characteristic curves; head-capacity relation Before you select a pump model, examine its performance curve, which is indicated by its head-flow rate or operating curve. The capacity and pressure needs of any system can be defined with the help of a graph called a system curve. Developing a Pump Performance Curve A pumps performance is shown in its characteristics performance curve where its capacity is plotted against its developed head. The pump performance curve also shows its efficiency, required input power, NPSH, speed, and other information such as pump size and type, impeller size, etc. It is generated by tests performed by the pump manufacturer. Pump curves are based on a specific gravity of 1.0. Other specific gravities must be considered by the user. Figure shows a typical characteristic curve of a centrifugal pump. ? On pumps with variable-speed drivers such as steam turbines or transducers, it is possible to change the characteristic curve. ? Characteristic curve can also be changed as the impeller diameter of centrifugal pump is changed. From this relationship, it is possible to develop a curve of ?system head loss versus volumetric flow rate. ?The head loss curve for a typical piping system is in the shape of a parabola. In addition to the pump design, the operational performance of a pump depends upon factors such as: ? the load characteristics, ? upstream and downstream ? pipe friction, and valve performance 2.2.4.6 Operating point The pump suppliers try to match the system curve supplied by the user with a pump curve that satisfies these needs as closely as possible. The point of intersection between the characteristic curve of centrifugal pump and system head curve of pump flow system is called operating point. Operating point change The point at which a pump operates in a given piping system depends on the flow rate and head loss of that system. The operating point for the cent