Impeller power number

Np = impeller Power number P = Impeller power draw, W Pj,[ = density of slurry, kg/m ... 

Power can be calculated by rearranging the definition of power number see the following example. A value for the appropriate power number must be obtained from empirically derived data for geometrically similar impellers. Power number correlations for anchor impellers are shown in Fig. 18-43. The typical anchor impellers have two vertical arms with a blade width W equal to one-tenth of the impeller diameter D, and the arm height H equal to the impeller diameter D. Correlations are shown for typical impellers 95 and 90 percent of the tank diameter. The clearance C is one-half of the difference between the impeller diameter and the tank diameter, or 2.5 and 5.0 percent of the tank diameter for the respective correlations. An additional correlation is shown for an anchor with three vertical arms and a diameter equal to 95 percent of the tank diameter. The correlation for a three-arm impeller which anchors 90 percent of the tank diameter is the same as that for the typical anchor 95 percent of the tank diameter. 

Chudacek, M.W. Impeller power numbers and impeller flow numbers in profiled bottom tanks. Ind. Eng. Chem. Des. Dev. 1985, 24, 858-867. 

Power Number, Np The power number, Np, sometimes referred to as Po, is a measure of the relative drag of the impeller. Streamline curved blades, like hydrofoils and retreat-curve impellers, have less drag than flat blades consequently, their power numbers are lower than those for flat-blade impellers. Power numbers of some of the more popular impellers are given in Table 9.1. The calculation of power from impeller diameter, speed, and liquid density is given by Equation (9.1). 

The impeller power number for the agitator in a gas-liquid system = 0.4 times the power number for liquid system of the same configuration. 

Nienow AW, Miles D. (1971) Impeller power numbers in closed vessels. Ind. Eng. Chem. Process Des. Develop., 10 41-43. 

Hub diameter. Shafts in small vessels tend to be overdesigned compared with large vessels consequently, the hubs fitted to small impellers can be excessively large. This can sometimes be important, particularly when investigating the flow patterns generated by the impeller. However, due to the term in the power draw expression, the effect on the impeller power number may not be significant. 

The triangular mesh on the impeller blade is shown in Figure 5-34. The blades have a concave shape, which consists of three curves of different radii and length. The top part of the blade is longer than the bottom part. The back side of the blade is rounded. After the flow field was converged, the torque on the impeller was calculated by integrating the pressure on the impeller blade surfaces. From the torque, the impeller power number, based on the nominal diameter at the impeller disk level, was calculated to be 2.3, which is in excellent agreement with experiments. 

The boundary of the cavern can be defined as the surface where the local shear stress equals the fluid yield stress. If it is assumed that the predominant flow in the cavern is tangential [and EDA studies suggest that this is a reasonable approximation (Hirata et al., 1994)] and that the cavern shape, fluid yield stress, and impeller power number are known, the cavern size may be determined. A right circular cylinder of height He and diameter Dc centered on the impeller is a good model for the cavern shape, which allows for the effect of different impellers (Elson et al., 1986). Thus,... 

The correlation for Cd (Uke the friction factor and the impeller power number, Np) covers several hydrodynamic regimes. The corresponding ranges for Rep and the correlating expression for Cd are shown in Table 10-1 for three hydrodynamic regimes. 

Froude number impeller power number particle Reynolds number impeller Reynolds number... 

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