Biomedical Engineering Reference
In-Depth Information
8.7.1 Centrifugal and Axial Pump Characteristics
Fluid enters the central portion, called the eye, flows radially outward, and is discharged
around the entire circumference into a casing. During flow through the rotating impeller,
the fluid receives energy from the vanes, resulting in an increase in both pressure and
absolute velocity. Since a large portion of the energy leaving the impeller is kinetic, it is
necessary to reduce the absolute velocity and transform much of this energy to pressure
head. This is accomplished in the volute casing surrounding the impeller or in flow through
diffuser vanes. If a higher capacity is required without an increase in the diameter, the
pump dimensions in the direction parallel to the shaft must be increased (Daugherty and
Franzini, 1977). This becomes a mixed-flow pump and is shown in Figure 8-64 along with
a number of examples of centrifugal pumps.
The configuration of the axial pump, as shown in Figure 8-65, occurs at the limit
where no radius change exists between the streamlines moving from the inlet to the outlet,
and centrifugal action plays no part.
8.7.1.1 Torque, Energy, and Power
The required torque,
τ
(Nm), from the motor can be expressed in terms of the centrifugal
pump velocity,
n
(rpm), as
kn
2
τ
=
(8.7)
where
k
is a constant.
FIGURE 8-64
Common turbo
pump
configurations.
(a) Centrifugal.
(b) Mixed-flow.
FIGURE 8-65
Axial pump type.
