radial impellers have low specific speeds, mixed flow ("diagonal") impellers have a higher specific speed range and axial impellers have the highest specific speeds. The fluid flow through the impeller changes with increasing specific speed, i.e. The values to be inserted in the above equation are the optimum head Hopt for one stage in the case of multistage pumps, and the optimum flow rate Qopt for one impeller half in the case of double-entry impellers.
The following relationship exists between the numerical values of the dimensional and dimensionless coefficients: N in s rpm -1 Rotational speed Q opt in m 3/s Flow rate at η max H opt in m Head at η max g = 9.81 m/s 2 Acceleration due to gravity Q opt in m 3/s Flow rate at ηmax H opt in m Head at ηmax n in rpm Pump speed n s in rpm Specific speedĪ dimensionless characteristic coefficient in accordance with DIN 24260 can be established using the following equation: The specific speed can be used to classify the optimum impeller design and the corresponding pump characteristic curves.ĭefined as the theoretical rotational speed at which a geometrically similar impeller would run if it were of such a size as to produce 1 m of head (H opt) at a flow rate (Q opt m 3/s at the best efficiency point, the specific speed is expressed in the same units as the rotational speed: The specific speed (ns) is a characteristic coefficient derived from the similarity conditions which allows a comparison of impellers of various pump sizes even when their operating data differ ( flow rate and head at best efficiency point, rotational speed).
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