Specific speed Impeller designs

Another distinction in impellers is the way the liquid traverses and leaves the impeller blades. This is called the Specific Speed, Ns. It is another index used by pump designers to describe the geometry of the impeller and to classify impellers according to their clesign type and application. By definition, the Specific Speed, Ns is the revolutions per minute (rpm) at which a geometrically similar impeller would run if it were of such a size as to discharge one gallon per minute at one foot of head. 

The Specific Speed is a dimensionless number using the formula above. Pump design engineers consider the Ns a valuable tool in the development of impellers. It is also a key index in determining if the pump... 

Figure 3-47. Impeller designs and corresponding specific speed range. (By permission. Standards of the Hydraulic Institute, 10th ed.) Also see [17], Hydraulic Institute, 13th ed., 1975.

Sonic flow, safety relief, 438 Rupture disk, 460, 461 Sub-sonic flow, 461 Sonic or critical flow, 115, 125 Calculations, 125 Velocity, 126 Specific speed, 194-197 Impeller designs, 194 Upper limits, chart, 195-197 Specifications,... 

Figure 8-4 Impeller designs and specific speed characteristics. (From Karassik et al 1976.)...

NOTE Impellers made as forgings or fabrications have machined waterways, w hich may offer improved performance for low suction specific speed designs. 

The vendor shall provide complete performance curves, including differential head, efficiency, water NPSHR, and power expressed as functions of flowrate. Except for low specific speed designs where it would not be feasible, the curves shall be extended to at least 120 % of flowrate at peak efficiency, and the rated operating point shail be indicated. The head curve for maximum and minimum impeller diameters shall be included. The impeller identification number, specific speed and suction specific speed shall be shown on the curves. If applicable, the curves shall indicate viscosity corrections. Minimum flow (both thermal and stable), preferred and allowable operating regions, and any limitations of operation shall be indicated. 

It is often used by designers in sizing and selecting the type of pump or compressor. The performance of centrifugal pumps is related to a parameter called specific speed. It is a correlation of capacity, head, and speed at optimum efficiency that classifies pump impellers with respect to their geometric similarity. Specific speed is a number expressed algebraically as ... 

A rotational speed of 6000 rpm was selected for optimum efficiency at a specific speed of 500. The radial type of impeller element determined by this specific speed was expected to operate satisfactorily at suction specific speeds nss of beyond 8000 and up to 20,000. For reliability, a value of 12,000 was selected. The gap between this capability of the impeller and an ascertained nss requirement of 70,000 (36-in. liquid level at the inducer inlet) was equivalent to 0.75 psi. Therefore, the inducer was designed for a head rating of at least this amount. 

The steepness of the curve between the best efficiency capacity and the shut-off point of the pump depends on the geometrical design of impeller and casing. With so many different designs of pumps, engineers have used nondimensional specific speeds and other parameters, In the International System of Units, the specific speed is defined as ... 

At impeller speeds corresponding to Njs, the value of ksL is independent of the geometry of the vessel, impeller design, or the specific power consumption (Doraiswamy and Sharma 1984). 

If it is turning at suitable speed, the impeller will break the gas up into very fine bubbles, which are swept out radially from the tip. In the imme ate vicinity of the impeller tip, the specific interfacial area is consequently very large. Coalesc-nece of the bubbles in the slower-moving liquid in other parts of the vessel increases the bubble diameter and lowers the local area. For moderate impeller speeds, only the gas fed to the impeller produces the specific interfacial area designated as but at higher impeller speeds, especially in small vessels, gas drawn from the space above the liquid contributes importantly to produce the larger mean specific area a. For disk flat-blade turbines, the specific area can be estimated from [23, 24, 84] ... 

The specific speed, Ng, is merely a design comparative number, used to compare impeller and pump proportions. 

In any event, the users of the mixer can issue a mechanical specification aud determine the speed, diameter of an impeller, aud power with in-house expertise. Or they may issue a processes specification which describes the engineering purpose of the mixing operation and the vendor will supply a description of the mixer process performance as well as prepare a mechanical design. 

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