Speed – Affinity Laws

The affinity law describes the speed dependency of the delivery parameters of centrifugal pumps. It can be derived from the general similarity relationships as a special case.
Speed – Affinity Laws

The following applies:

1. Model law

2. Model law

3. Model law

Q - flow rate
H - delivery head
P - power consumption
n - speed
The indices relate to the respective speed.

The affinity laws apply exactly to frictionless, incompressible flows. For technical applications, they are to be regarded as an approximate solution.

In general, these affinity laws are independent of how the speed change is technically implemented. Traditionally, multi-speed functions of small and medium size pumps are realized by changing the motor windings configurations to achieve stepped speed variations. Meanwhile, these have been largely replaced by frequency converters.

Slow-running electric drives are very expensive for larger centrifugal pumps, so that reduction gears are used for these cases.

Combustion engines are used for some mobile applications. They are also variable in speed within a specified range.

More articles on this topic

Mains frequency

Mains frequency (utility frequency) is the nominal frequency of the alternating voltage provided by the energy supplier.

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Pump Performance Curve

The pumping performance of a centrifugal pump is expressed in the form of an H-Q curve, depicting the Flow Q (e.g. in m³/h) and the Head H (e.g. in m) of the pump.

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System Curve

The system H-Q curve is essentially a parabolic curve representing the relation between the head (pressure) required to handle the respective rate of flow within a pipe system.

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