Flow, head and power vary with speed and impeller diameter.
Similar pumps follow: Q∝N D³, H∝N²D², P∝N³D⁵. Changing speed from N₁ to N₂ scales flow by the ratio, head by square and power by cube. Used with VFDs for energy saving and to estimate performance of different sizes.
Reducing speed by 10% cuts power by about 27%.
The affinity laws let you predict how a pump changes when its speed or impeller size changes, which is useful for variable speed control.
Flow, head and power vary with speed and impeller diameter.
The affinity laws let you predict how a pump changes when its speed or impeller size changes, which is useful for variable speed control.
Flow changes in proportion to speed, head changes with the square of speed, and power changes with the cube of speed.
Power falls with the cube of speed, so a modest speed reduction gives a large energy saving compared with throttling.
Typical applications include predicting the new flow of a pump after reducing speed with a drive, estimating the power saving when trimming an impeller and scaling a pump test from a model to a full-size machine.
Study "Pumps" first (topic 63 of 105 in Thermodynamics & Fluids). It covers the groundwork this topic builds on.
Specific speed. Trim impeller. Law validity near same efficiency.
Reducing speed by 10% cuts power by about 27%.
The next topic is "Cavitation and NPSH", which builds directly on this one.
It is classed as Intermediate in Thermodynamics & Fluids (topic 64 of 105). It is an intermediate topic. Work through the beginner topics in Thermodynamics & Fluids first.
Want to practise? Open the interactive course for quick checks, quizzes, simulations and progress tracking.