Power number for

Power is defined for a well-agitated vessel with a mechanical stirrer then read power number for turbulent flow from Figure 6.6, Chapter 6 ... 

The values of VP are approximate. At low Reynolds numbers, about 300, the power number curves for baffled and unbaffled tanks are identical (McCabe et al., 1993). For higher NAV, the power number for unbaffled tanks is lower than the values for baffled tanks. 

Power number for several types of impellers and vessel characteristics (Perry and Green, 1984,... 

There are well-estabhshed empirical correlations for stirrer power rcquircrnenls [6, 7]. Figure 7.8 [6] is a log-log plot of the power number for imgassed hquids versus the stirrer Reynolds number (Re). These dimensionless numbers are defined as follows ... 

The computation of the thermal energy dissipated by a stirrer requires knowledge of the power number (Ne) and of the geometry of the stirrer. Some examples of power numbers for common stirrers are given in Table 2.6. 

Determine the turbulent power number for impeller geometry. Power number N P is a dimensionless variable [5] which relates impeller power P to such operating variables as liquid density p, agitator rotational speed N, and impeller diameter D as follows ... 

A conversion factor (see below) is used when working with English engineering units no factor is necessary for SI metric units. For a given impeller geometry, the power number is a constant for conditions of turbulent agitation. Values of turbulent power numbers for some agitator impellers are shown in Fig. 12.1. 

The viscosity power factor for lVRe = 290 is found to be 1.2fromFig. 12.2. The power number for the impeller described in the example is the viscosity factor times the turbulent power number (from the previous step) Np = 1.2(1.43) = 1.72. 

Estimate viscous power number for the helix impeller. A helical-ribbon impeller, also called a helix impeller, is used primarily when high-viscosity fluids are being processed. Most of the power data on such impellers have been obtained in the laminar and transitional flow ranges. The effect on power of common geometry factors, i.e., impeller diameter D, tank diameter T, helix pitch P, impeller height H, and helix (blade) width W, can be incorporated into a correlation for a (dimensionless) viscous power number ... 

The power number for individual impellers should be used in this calculation. If different style or size impellers were used at different locations, separate calculations for each impeller would be required. 

Ungassed power requirements were fully covered earlier. The turbulent regime power numbers for the 6BD and the CD-6 impeller are given in the following tabulation ... 

Power numbers for different impeller types depend upon the impeller Reynolds number. Representative relationships of Power number versus Reynolds number for several types of impellers are given in Fig. 15-54. For additional information on a variety of impellers, see Sec. 6 and Hemrajani and Tatterson, Chap. 6 in Handbook of Industrial Mixing, Science and Practice, Paul, Atiemo-Obeng, and Kresta, eds. (Wiley, 2004). 

FIG. 18-43 Power numbers for anchor impellers typical two-arm impeller anchors 95 percent of tank diameter T and 90 percent of T three-arm impeller anchors 95 percent of T and three-arm impeller anchors 90 percent of T, similar to two-arm impeller that anchors 95 percent of T. 

FIGURE 9.1 Reynolds number vs. power number for six turbine impellers. (Modified from Rushton et al., 1980. With permission.)... 

There is a different power number for each impeller, reflecting its uniqne shape and drag-producing characteristics. The power number values given in Table 9.1 are only applicable for tnrbulent flow. Mannfactnrers offer a variety of impellers, the characteristics of which are given in Table 9.1. 

The mixing power per unit volume P/ C is a function of impeller rotational speed co, impeller diameter /). and the Power number /(, for the type of impeller and vessel geometry ... 

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. 

For dissolving polymer in solvent, the major problem is the small clumps of polymer formed in the viscous fluid. These clumps are difficult to break up. We need sufficient shear combined with axial flow in order to break the polymer quickly and immediately spread the polymer into the liquid for subsequent dissolution. Leave a small gap between the baffle and the tank wall in order to avoid the dead corner of undissolved polymer. If multiple impellers are used, then to save on power consumption, the bottom impeller might supply axial flow plus shear (as an open turbine) with the impeller above supplying axial flow. The Power number for the open turbine might be, for example, 1.2, whereas for axial flow the Power number value might be about 0.3. 

For preliminary estimates, the value of the power number for mixing can be estimated from correlations... 

The power number reported for Wemco cells varied from 2.9 for laboratory scale (2 lit) to 1.45 for 1.75m (Arbiter et al. 1976). Kind (1976) has reported the power number for vessel diameters from 1 to 3.5m. These varied over a relatively narrow range from 3.3 to 4.4. Since the larger sizes have better geometrical uniformity, the range given by Kind (1976) is more reliable. 

The power number for a plain anchor-type agitator similar to Fig. 3.4-2b but... 

The power number for a helical-ribbon agitator for very viscous liquids for Nrc < 20 is as follows (H2, P3). 

TABLE 7-7 Typical Power Number for Mixers in Turbulent Flows... 

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