According to the specific speed from small to large, the pumps are divided into centrifugal pumps, mixed flow pumps (slope flow pumps) and axial flow pumps. A low specific speed pump means a high head and small flow, and a high specific speed pump means a low head and large flow.
Low specific speed impellers are narrow and long, high specific speed impellers are wide and short, and the D2 / Dj ratio decreases as ns increases. Low specific speed pumps usually use cylindrical blades, and the high specific speed impeller has a wide inlet. It is assumed that the axial surface speed is evenly distributed along the inlet. Because the u value of each point on the inlet side is different, the inlet flow angle β1 is different. In this case, twisted blades should be made.
The low specific speed pump's head flow curve is prone to hump. This is because on the one hand, the low specific speed pump has a high flow velocity and a large impact loss value; on the other hand, the low specific speed pump uses a larger β2 to reduce disc friction loss The angle is to reduce the outer diameter D2 (the disk friction loss is proportional to the fifth power of the outer diameter). The β2 angle is large, and the theoretical head flow curve is flat, which is prone to hump. High specific speed mixed flow pumps and axial flow pumps have high dead heads and inflection points on the curve. The reasons are more complicated. This will be described in the axial flow pump section.
Low specific speed pump, the shaft power is small at zero flow rate, high specific speed pump (mixed flow pump, axial flow pump), the shaft power is large at zero flow rate, so the former should be closed and the valve should be started.
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