How do the power requirements of axial-flow and mixed-flow pumps change at low capacity?

In the context of pump operations, particularly for axial-flow and mixed-flow pumps, the power requirements can exhibit unique behavior as the capacity decreases. When operating at low capacities, the flow rates into the pump are reduced, leading to a situation where the pump must overcome not just the static head but also friction losses and other inefficiencies that become pronounced at these lower flow rates.

For axial-flow pumps, as the flow rate drops, the performance curve indicates a steeper reduction in efficiency. This means that while the volume of fluid being moved is decreased, the energy required to move that fluid can be disproportionately higher due to the increased losses. The same situation applies to mixed-flow pumps. These pumps rely on a combination of axial and centrifugal forces, and at low capacities, they can also experience increased hydraulic losses, requiring more power to maintain the necessary flow and pressure.

Thus, the power required indeed becomes higher compared to full capacity at low flow rates, as the pumps are less efficient when working away from their design point. This higher energy demand at lower capacities underscores the need for careful matching of pump characteristics to the system requirements to ensure efficient operation across the expected range of flow rates.