A centrifugal pump or fan running at half speed consumes only one-eighth of the energy compared to one running at full speed. This is because the torque needed to run a pump or fan is the square of the volume. For instance, reducing the pump speed to 80% only requires 64% of the torque (0.8x0.8). Furthermore, to produce 64% of the torque only requires 51% of the power (0.64x0.8), as the power requirement is reduced in the same way.
The explanation for this lies in the pressure difference across the impeller. When less pressure is produced, less acceleration of air or fluid across the impeller is required. It is the simultaneous reduction of acceleration and pressure that multiplies the savings.
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