流体力学与 及传热课件Characteristic curves; head-capacity relation.pptVIP

2.2.4.4 Characteristic curves; head-capacity relation ; 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 ; 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.; ? Characteristic curve can also be changed as the impeller diameter of centrifugal pump is changed. ;The head loss is proportional to the square of the flow velocity. Since flow velocity is directly proportional to the volumetric flow rate, the system head loss must be directly proportional to the square of the volumetric flow rate. ; 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; A mechanical energy balance equation can be written between station 1 and 2 in figure ; Rearrange the equation 2.2-28 ;So the equation can be written as follow ;2.2.4.6 Operating point ; 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 operating point for the centrifugal pump?is designated by the intersection of the pump curve and the system curve.;However, it is impossible for one operating point to meet all desired operating conditions. Several methods can be used to change operating point:;Change the system-head curve Change the centrifugal pump characteristic curve;When the disch