Impeller Efficiency

The question of which impeller is the most efficient for blending can be answered by rearranging the blend time correlations. In the turbulent regime, for a vessel 

These conclusions are strongly supported by the theoretical analysis and experimental results of Nienow (1997) and Langheimich et al. (1998). Most surprising is the conclusion concerning the equivalence of different impellers, which is counter-intuitive and contrary to what many vendors claim. 

A similar analysis for the transitional regime leads to some similar conclusions but some which are quite different  

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Fig. 4. Chart for efficiency estimates and curve shapes, where (a) represents curve shapes showing the relationship between efficiency (Eff), head (H), and power (P) as a function of flow (b) specific speed, where the numbers represent flow in nr /s and (c) impeller profiles.

According to HI convention, for double-suction pumps, Q is half of the total pump flow, ie, taken per impeller eye. The value of S is calculated at the best efficiency point (BEP) at maximum impeller diameter. 

In the case of speed changes, the pump efficiency is not affected except for a minor change owing to Reynolds number change, but the diameter cut may reduce the efficiency appreciably on account of increased gap and losses between the impeller OD and a collector (casing or diffusor). 

Suction Limitations of a Pump Whenever the pressure in a liquid drops below the vapor pressure corresponding to its temperature, the liquid will vaporize. When this happens within an operating pump, the vapor bubbles will be carried along to a point of higher pressure, where they suddenly collapse. This phenomenon is known as cavitation. Cavitation in a pump should be avoided, as it is accompanied by metal removal, vibration, reduced flow, loss in efficiency, and noise. When the absolute suction pressure is low, cavitation may occur in the pump inlet and damage result in the pump suction and on the impeller vanes near the inlet edges. To avoid this phenomenon, it is necessary to maintain a required net positive suction head (NPSH)r, which is the equivalent total head of liquid at the pump centerline less the vapor pressure p. Each pump manufacturer publishes curves relating (NPSH)r to capacity and speed for each pump. 

Volute casings take the form of a spiral increasing uniformly in cross-sectional area as the outlet is approached. The volute efficiently converts the velocity energy imparted to the liquid by the impeller into pressure energy. 

However, when comparing different impeller types at the same power level, it turns out that impellers that have a higher pumping capacity will give decreased circulation time, but all the impellers, regardless of their pumping efficiency, give the same blend time at the... 

Numerous factors affect the pump s efficiency. The impeller is one of... 

Impeller diameter - There will be an efficiency reduction with a reduction in the impeller diameter. For this reason, it s not recommended to reduce (trim) the impeller by more than 20%. For example, if a pump takes a fiill sized 10-ineh impeller, don t trim the impeller to less than 8-inches diameter. This would be a 20% reduction. 

Some pump eompanies will design replaceable wear bands for the OD of the impeller eye and the bore of the pump housing. It s said that the pump loses 1.5% to 2% efficiency points for every one thousandths wear in a wear band beyond the factory setting. Therefore, by changing wear bands, the pump is returned to its original effieiency. Becau.se of this, the term wear band is a misnomer. A better term would be effieiency band (Figure 6-19). 

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