Mixed flow pump and centrifugal pump, axial flow pump performance comparison
Francis pumps, also known as mixed-flow pumps, have a design that falls between centrifugal and axial flow pumps. Their pumping mechanism involves the high-speed rotation of an impeller, which generates both centrifugal force—like in centrifugal pumps—and axial thrust, similar to axial flow pumps. This combination of forces allows the pump to move water in a mixed manner, making it suitable for applications where both moderate head and relatively high flow are required.
Compared to centrifugal pumps, mixed-flow pumps operate at a lower head but with a higher flow rate. On the other hand, they have a higher head than axial flow pumps but with a smaller flow capacity. This unique performance range makes them ideal for use in regions with complex terrains, such as hilly areas in our country, where different pumping solutions may be needed depending on local conditions.
The performance of mixed-flow pumps is typically described by several operating parameters, including flow rate, head, power, efficiency, and speed. At a given speed, when the flow rate changes, the head, power, and efficiency all vary accordingly. These relationships are often plotted on curves known as performance curves, which visually represent how these parameters interact under different operating conditions.
The shape of a mixed-flow pump's performance curve lies between that of a centrifugal and an axial flow pump. For high-head mixed-flow pumps, the relationship between flow and head, as well as flow and power, closely resembles that of a centrifugal pump. In such cases, it is advisable to start the pump with the discharge valve closed, as this minimizes the initial power demand and protects the motor from overheating. However, for low-head mixed-flow pumps, the performance characteristics are more similar to those of axial flow pumps. In this case, starting the pump with the valve open is not recommended, as it can cause excessive current draw and potentially damage the motor. Instead, the pump should be started with a partially open valve to reduce the initial load on the motor.
Due to their versatile performance and adaptability, mixed-flow pumps are widely used in irrigation and drainage systems, especially in hilly or uneven terrain where precise control over water flow and pressure is essential. Their ability to handle a wide range of flow rates and heads makes them a valuable choice for various hydraulic applications.
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