fluid_mechanics

654 Chapter 12 ■ Turbomachines
F I G U R E 12.7
(a) Open impeller, (b) enclosed
or shrouded impeller.
(Courtesy of Ingersoll-Dresser
Pump Company.)
Centrifugal pumps
involve radially
outward flows.
fluid discharges directly into a volute-shaped casing. The casing shape is designed to reduce the
velocity as the fluid leaves the impeller, and this decrease in kinetic energy is converted into an
increase in pressure. The volute-shaped casing, with its increasing area in the direction of flow,
is used to produce an essentially uniform velocity distribution as the fluid moves around the casing
into the discharge opening. For large centrifugal pumps, a different design is often used in
which diffuser guide vanes surround the impeller. The diffuser vanes decelerate the flow as the
fluid is directed into the pump casing. This type of centrifugal pump is referred to as a diffuser
pump.
Impellers are generally of two types. For one configuration the blades are arranged on a hub
or backing plate and are open on the other 1casing or shroud2 side. A typical open impeller is shown
in Fig. 12.7a. For the second type of impeller, called an enclosed or shrouded impeller, the blades
are covered on both hub and shroud ends as shown in Fig. 12.7b.
Pump impellers can also be single or double suction. For the single-suction impeller the fluid
enters through the eye on only one side of the impeller, whereas for the double-suction impeller
the fluid enters the impeller along its axis from both sides. The double-suction arrangement reduces
end thrust on the shaft, and also, since the net inlet flow area is larger, inlet velocities are
reduced.
Pumps can be single or multistage. For a single-stage pump, only one impeller is mounted on
the shaft, whereas for multistage pumps, several impellers are mounted on the same shaft. The stages
operate in series, that is, the discharge from the first stage flows into the eye of the second stage, the
discharge from the second stage flows into the eye of the third stage, and so on. The flowrate is the
same through all stages, but each stage develops an additional pressure rise. Thus, a very large discharge
pressure, or head, can be developed by a multistage pump.
Centrifugal pumps come in a variety of arrangements 1open or shrouded impellers, volute
or diffuser casings, single- or double-suction, single- or multistage2, but the basic operating principle
remains the same. Work is done on the fluid by the rotating blades 1centrifugal action and
tangential blade force acting on the fluid over a distance2, creating a large increase in kinetic energy
of the fluid flowing through the impeller. This kinetic energy is converted into an increase
in pressure as the fluid flows from the impeller into the casing enclosing the impeller. A simplified
theory describing the behavior of the centrifugal pump was introduced in the previous section
and is expanded in the following section.
12.4.1 Theoretical Considerations
Although flow through a pump is very complex 1unsteady and three-dimensional2, the basic theory of
operation of a centrifugal pump can be developed by considering the average one-dimensional flow of
the fluid as it passes between the inlet and the outlet sections of the impeller as the blades rotate. As
shown in Fig. 12.8, for a typical blade passage, the absolute velocity, V 1 , of the fluid entering the passage
is the vector sum of the velocity of the blade, U 1 , rotating in a circular path with angular velocity
v, and the relative velocity, W 1 , within the blade passage so that V 1 W 1 U 1 . Similarly, at the
exit V 2 W 2 U 2 . Note that U 1 r 1 v and U 2 r 2 v. Fluid velocities are taken to be average
velocities over the inlet and exit sections of the blade passage. The relationship between the various
velocities is shown graphically in Fig. 12.8.