3.1 Centrifugal Pumps
Centrifugal Pumps
Centrifugal pumps were in the past used almost exclusively for transporting water from a lower to higher level, e.g. in a mine or in an irrigation installation. Then it was practical to use the concept of delivery head (delivery height) as a criterion of the pumps performance ability. Despite the fact that today pumps are used to a large extent for purposes where the alteration in the height position of the medium is of subsidiary importance, this concept of delivery head still remains.
The process which takes place when a liquid is passing through the pump is adiabatic, i.e. the exchange of heat between the pumped medium and the environment is so small that it can be neglected. An equal amount of mass flows through per unit of time in and out through the pump, the external leakage normally being negligible in comparison with the mass flow through the pump. Such a process is described by the energy equation for steady flow reckoned per unit of mass of pumped medium and using the terms as given in figure 3.6.
Flow velocity is designated by c in this chapter in line with current practice for turbine machines (such as in a centrifugal pump).
Put into words, the energy equation states the following:
The technical work It which is supplied via the pump shaft, increase the internal energy (u) in a medium, with an increase of the static pressure (the term p/ρ, or pressure/density), an increase in kinetic energy (c²/2) and an increase in potential energy (gh).
It is desirable that the energy supplied by the driving shaft, the shaft work (or torque), results in an increase of static pressure in the liquid to as large extent as possible. In principle kinetic and potential energy can be completely converted into static pressure. It is more difficult to make use of the increase in the internal energy. It corresponds to an unnecessary and undesirable increase in temperature of the medium and is reckoned as a loss. It is against this background that the pump’s delivery head H is defined as that useful part of the change in state of the liquid measured in metres of liquid column.
where:
H = Head (m)
p = ststic pressure (Pa)
c = absolute velocity (m/s)
ρ = density (kg/m³)
g = 9,806 N (m/s²)