流体力学与传热 ：2-2-4-4 Characteristic curves; head-capacity relation

1、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 curve shows the pumps capacity plotted against total developed head. It also shows efficiency, required power input, a

2、nd so on,The capacity and pressure needs of any system can be defined with the help of a graph called a system curve.,Similarly the capacity vs. pressure variation graph for a particular pump defines its characteristic pump performance curve,Developing a Pump Performance Curve,A pumps performance is

3、 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.,This curve is plotted for a constan

4、t speed and a given impeller diameter.,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.,At any fixed speed the p

5、ump will operate along this curve and no other points. It is not possible to reduce or increase the capacity at a given head unless the discharge is throttled., On pumps with variable-speed drivers such as steam turbines or transducers, it is possible to change the characteristic curve.,The characte

6、ristic curve of a pump can be changed by changing both the revolution speed and diameter of pump.,characteristic curve, Characteristic curve can also be changed as the impeller diameter of centrifugal pump is changed.,characteristic curve,The head loss is proportional to the square of the flow veloc

7、ity. 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.,Developing a System Curve,From this relationship, it is possible to develop a curve of system head loss versus volumetric flow

8、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

9、equation can be written between station 1 and 2 in figure,where the system friction Hf,2.2-28,Rearrange the equation 2.2-28,Let the static difference,and system friction losses,2.2-30,2.2-31,So the equation can be written as follow,The equation is known as system curve, or system-head curve, and sho

10、wn in figure.,system curve,2.2-32,2.2-33,2.2.4.6 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.,A system-head curve can be superimposed on the head-capacity curve of a centrifugal pump.,The po

11、int of intersection between the characteristic curve of centrifugal pump and system head curve of pump flow system is called operating point.,The point A in the figure is the operating point for a given pump at the specific speed.,Operating point change,The point at which a pump operates in a given

12、piping system depends on the flow rate and head loss of that system.,The operating point for the centrifugal pumpis 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 b

13、e used to change operating point:,Change the system-head curve Change the centrifugal pump characteristic curve,It is convenient to change the system-head curve by a valve in discharge.,When the discharge valve is throttled, the system resistance curve shift left and so does the operating point.,The

14、 operating point can be changed as adjusting the opening of valve,The operating point can also be changed as the impeller diameter or revolution speed of centrifugal pump is changed (Figure 2.28).,Normal Operating Range,A typical performance curve is a plot of head vs. capacity for a specific impell

15、er diameter.,The plot starts at zero flow. The head at this point corresponds to the shut-off head point of the pump. The curve then decreases to a point where the flow is maximum and the head minimum. This point is sometimes called the run-out point.,Beyond the run-out point, the pump cannot operat

16、e. The pumps range of operation is from the shut-off head point to the run-out point.,Trying to run a pump off the right end of the curve will result in pump cavitation and eventually destroy the pump.,To minimize pumping system energy consumption, select a pump so the system curve intersects the pu

17、mp curve within 20% of its best efficiency point （ BEP ）.,Operation in parallel and in series of centrifugal pump,In ordertoincrease the volumetric flow ratein a system ortocompensate for largeflow resistances,centrifugalpumps are often used in parallelor in series., Pumps operating in parallel,Two

18、pumps or more discharging into a common line are said to operate in parallel.,The performance curve for two identical pumps operating in parallel, double the capacity of each at the same head.,Conclusion: Two pumps operating in parallel, will be more effective with a flatter system head curve.,Two p

19、umps or more in parallel should be selected in case one by itself can not perform as required, or certain flexibility is necessary.,Otherwise one pump by itself with a spare is, in most cases, the best selection., Pumps operating in series,Centrifugal pumps areusedin series to overcome a larger syst

20、em headlossthan one pumpcan compensate for individually.,Two pumps or more are said to operate in series when the discharge of the first pump serves as suction for the second pump, and the discharge of the second pump as suction for the third one, etc.,For a specific system-head curve, two identical

21、 pumps can be selected to double the head of the first pump at the same capacity. The performance curve for two identical pumps operating in series is in figure.,Conclusion: As shown in Figure , using two pumps in series does not actually double the resistance to flow inthesystem.,The operation of t

22、wo pumps in series a steeper system curve will be more effective than with a flatter system curve,2.2.5 Multistage centrifugal pumps,When a head greater than about 20 or 30m is needed, two or more impellers can be mounted in series on a single shaft and a multistage pump so obtained.,In a multistage

23、 centrifugal pump, the discharge from the first stage provides suction for the second, the discharge from the second provides suction for the third, and so forth.,The developed head of all stages add to gives a total head.,problem,The difference between the theoretical and actual curves results from

24、: ; ; . The methods of adjusting volume flow rate of centrifugal pump are (1) ; (2) ; (3) ; The commonest way in practice is ( ).,When higher developed head is required, the best selection is . A) operation in series B) multistage centrifugal pump C) throttled by valve ( )One pump can develop more H

25、 than that two same pumps which work in series.,Two identical centrifugal pumps operating in series will double developed head ( ) Two identical centrifugal pumps operating in parallel will double capacity ( ),Problem1,Water flows through an orifice of 25mm diameter situated in a 75mm pipe at the rate of 900cm3/s. what will be the difference in level on a water