流体力学与传热课件Characteristic curves; head-capacity relation

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
H p g 2Z22 ug 2 2 p g 1Z12 u1 g 2 H f 2.2-28
2
where the system friction Hf
Figure shows a typical characteristic curve of a centrifugal pump.
At any fixed speed the pump 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.
Hf
l
le
u2
d d2g
p1
1
Rearrange the equation 2.2-28
H p2 gp1Z2Z1 l dle 2 ug 2 2.2-30
Let the static difference
K
p2 p1
g
Z2
Z1
and system friction losses
2.2-31
The curve shows the pump’s capacity plotted against total developed head. It also shows efficiency, required power input, and so on
The capacity and pressure needs of any system can be defined with the help of a graph called a system curve.
2.2.4.4 Characteristic curves; head-capacity relation
Before you select a pump model, examine its performance curve, which is indicated by its headflow rate or operating curve.
to increase D
to decrease D
characteristic curve
Developing a System Curve
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.
Similarly the capacity vs. pressure variation graph for a particular pump defines its characteristic pump performance curve
• 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.
The characteristic curve of a pump ctaondebcerease speed changed by changing both the revolution speed and diameter of pump.
to increase speed
characteristic curve
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.
♣ 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.
l le u2 BQ2 2.2-32
d 2g
So the equation can be written as follow
System curve
HKBQ2
2.2-33
Biblioteka Baidu
system curve
The equation is known as system curve, or system-head curve, and shown in figure.