Power curves, impellers

Power curves for many different impeller geometries, baffle arrangements, and so on are to be found in the literature/10111719 2I but it must always be remembered that though the power curve is applicable to any single phase Newtonian liquid, at any impeller speed, the curve will be valid for only one system geometry. 

Figure 7.8. Power curve for pseudoplastic fluids agitated by different types of impeller...

Calculate the theoretical power in watts for a 0.25 m diameter, six-blade flat blade turbine agitator rotating at N = 4 rev/s in a tank system with a power curve given in Figure 5.10. The liquid in the tank is shear thinning with an apparent dynamic viscosity dependent on the impeller speed N and given by the equation fia = 25(N)n 1 Pa s where the power law index n = and the liquid density p = 1000 kg/m3. 

Considering the prediction of the power consumption, classically, the power drawn by the impeller is expressed with power curves, i.e., plots of the power number Np versus the Reynolds number. Re, where ... 

Ibrahim S., Nienow A.W., Power curves and flow patterns for a range of impellers in Newtonian fluids ... 

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 power-versus-flowrate curve can be deduced from the head-versus-flowrate and efficiency-versus-flowrate curves for a given fluid density, using the equation above. It is found that the power curve tends to rise continuously as flowrate increases for a radially vaned impeller, while for a backward-sloped impeller the power rises less steeply (due to the head decrease) and may reach a maximum value and then decrease, which is described as a non-overloading characteristic. The curve shapes are summarized in Fig. 14.1. 

Figure 5-35 shows the velocity field around the impeller blades. The velocity vectors are drawn in the frame of reference of the impeller. It is clear that no flow separation occurs behind the impeller blades. This means that cavity formation under gassed conditions will be reduced. Indeed, visualization studies have shown that gas is captured under the top overhang and dispersed from a deep vortex on the inside of the blade. No large gas-filled cavities have been observed behind the blade. As a result, the BT6 has a gassed power curve that is flatter than that of... 

Any impeller in a vessel capable of pumping fluid and providing shear can produce liquid-liquid dispersions. The impellers commonly used for immiscible liquid-liquid systems include disk turbines, pitched blade turbines, propellers, hydrofoils, paddles, retreat curve impellers, and other proprietary designs. We showed in Section 12-2 that drop size depends on maximum energy dissipation rate. More specifically, eq. (12-23) shows that the power number of an impeller affects drop size. In this section we deal with equipment used for two common industrial applications creating the maximum interfacial area and creating uniformly sized drops. 

Fig. 10.32 Power curves for some typical impellers. (From R. Hemrajani, G.B. Tatterson, Mechanically stirred vessels, in E. L Paul, V.A. Atiemo-Obeng, S. M. Kresta (Eds.), Handbook of Industrial Mixing Science and Practice, DOI 10.1002/0471451452.ch6, (Chapter 6). Copyright 2004 Wiley).

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.

FIG. 15-23 Power for agitation impellers immersed in single-phase liquids, baffled vessels with a gas-liquid surface [except curves (c) and (g)]. Curves correspond to (a) marine impellers, (h) flat-blade turbines, w = dj/5, (c) disk flat-blade turbines witb and without a gas-liquid surface, (d) curved-blade turbines, (e) pitcbed-blade turbines, (g) flat-blade turbines, no baffles, no gas-liquid interface, no vortex. 

Radial-flow impellers include the flat-blade disc turbine, Fig. 18-4, which is labeled an RlOO. This generates a radial flow pattern at all Reynolds numbers. Figure 18-17 is the diagram of Reynolds num-ber/power number curve, which allows one to calculate the power knowing the speed and diameter of the impeller. The impeller shown in Fig. 18-4 typically gives high shear rates and relatively low pumping capacity. 

As the flow from an impeller is increased from a given power level, there will be a higher fluid velocity and therefore a shorter circulation time. This holds true when dealing with any given impeller. This is shown in Fig. 18-18, which shows that circulation time versus D/T decreases. A major consideration is when increasing D/T becomes too large and actually causes the curve to reverse. This occurs somewhere around 0.45, 0.05, so that using impellers of D/T ratios of 0.6 to 0.8... 

Figure 12-46D, Part 1. View toward inlet of 4 Vj-in. diameter brazed aluminum impeller. Note Regardless of the metal of manufacture, enclosed impellers with back-leaning blades are extremely useful in applications requiring a steep head-volume characteristic and the highest attainable efficiency. Applications include parallel operation with other compressors or boosting of another compressor s output. The power-volume curve will show a self-limiting feature at higher volumes. This feature is very beneficial when the driver has limited power available but operation throughout the full capacity range is required. (Used by permission A C Compressor Corporation.)...

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