Specific Speed

The flow rate, head, and impeller speed at the maximum or "best" efficiency point (BEP) of the pump characteristic can be used to define a dimensionless group called the specific speed:

Although this group is dimensionless (and hence unitless), it is common practice to use selected mixed (inconsistent) units when quoting the value of Ns, i.e., N in rpm, Q in gpm, and H in feet. The value of the specific speed represents the ratio of the pump flow rate to the head at the speed corresponding to the maximum efficiency point (BEP) and depends primarily on the design of the pump and impeller. As previously stated, most centrifugal pumps operate at relatively low heads and high flow rates, e.g., high values of Ns. However, this value depends strongly on the impeller design, which can vary widely from almost pure radial flow to almost pure axial flow (like a fan). Some examples of various types of impeller design are shown in Fig. 8-4. Radial flow impellers have the highest head and lowest flow capacity (low Ns), whereas axial flow impellers have a high flow rate and low head characteristic (high Ns). Thus the magnitude of the specific speed is a direct indication of the impeller design and performance, as shown in Fig. 8-5. Figure 8-5 also indicates the range of flow rates and efficiencies of the various impeller designs, as a function of the specific speed. As indicated in Fig. 8-5, the maximum efficiency corresponds roughly to a specific speed of about 3000.

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