Brochure
1616
1. Introduction to Centrifugal Pumps
The impeller’s ability to increase pressure and create flow depends mainly
on whether the fluid runs radially or axially through the impeller,
see figure 1.6.
In a radial impeller, there is a significant dierence between the inlet
diameter and the outlet diameter and also between the outlet diameter
and the outlet width, which is the channel height at the impeller exit. In
this construction, the centrifugal forces result in high pressure and low
flow. Relatively low pressure and high flow are, on the contrary, found in an
axial impeller with a no change in radial direction and large outlet width.
Semiaxial impellers are used when a trade-o between pressure rise and flow
is required.
The impeller has a number of impeller blades. The number mainly depends
on the desired performance and noise constraints as well as the amount and
size of solid particles in the fluid. Impellers with 5-10 channels has proven to
give the best eciency and is used for fluid without solid particles. One, two
or three channel impellers are used for fluids with particles such as waste-
water. The leading edge of such impellers is designed to minimise the risk
of particles blocking the impeller. One, two and three channel impellers can
handle particles of a certain size passing through the impeller. Figure 1.7
shows a one channel pump.
Impellers without a shroud are called open impellers. Open impellers are
used where it is necessary to clean the impeller and where there is risk of
blocking. A vortex pump with an open impeller is used in waste water ap-
plication. In this type of pump, the impeller creates a flow resembling the
vortex in a tornado, see figure 1.8. The vortex pump has a low eciency
compared to pumps with a shroud and impeller seal.
After the basic shape of the impeller has been decided, the design of the
impeller is a question of finding a compromise between friction loss and loss
as a concequence of non uniform velocity profiles. Generally, uniform velocity
profiles can be achieved by extending the impeller blades but this results in
increased wall friction.
Figure 1.6: Radial, semiaxial and
axial impeller.
Figure 1.8: Vortex pump.
Radial impeller Semiaxial impeller Axial impeller
Figure 1.7: One channel pump.