Power impeller types

Each equation is independent of impeller type. As pointed out eadier, the absolute kpi values vary considerably from Hquid to Hquid. However, similar relationships have been found for other fluids, including fermentation broths, and also for hold-up, 8. Therefore, loss of power reduces the abiHty of the Rushton turbines to transfer oxygen from the air to the broth. 

The power number depends on impeller type and mixing Reynolds number. Figure 5 shows this relationship for six commonly used impellers. Similar plots for other impellers can be found in the Hterature. The functionality between and Re can be described as cc Re in laminar regime and depends on p. N in turbulent regime is constant and independent of ]1. 

Additional power data for other impeller types such as anchors, cui ved-blade turbines, and paddles in baffled and unbaffled vessels are available in the following references Holland and Chapman, op. 

It turns out that in low-viscosity blending the acdual result does depend upon the measuring technique used to measure blend time. Two common techniques, wliich do not exhaust the possibilities in reported studies, are to use an acid-base indicator and inject an acid or base into the system that will result in a color change. One can also put a dye into the tank and measure the time for color to arrive at uniformity. Another system is to put in a conductivity probe and injecl a salt or other electrolyte into the system. With any given impeller type at constant power, the circulation time will increase with the D/T ratio of the impeller. Figure 18-18 shows that both circulation time and blend time decrease as D/T increases. The same is true for impeller speed. As impeller speed is increased with any impeller, blend time and circulation time are decreased (Fig. 18-19). 

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... 

Figure 5-17. Power consumption by impeller type/dimensions for turbulent flow conditions. Knowing impeller type, diameter, speed and batch density connect RPM with diameter. The Intersection with A, connected to the density scale, makes an intersection on B. A line from this point to the impeller scale intersects the horsepower scale at the correct value. By permission, Quillen, C. S., Chem. Engr., June 1954, p. 177 [15].

When comparing flow (or pumping) per power, we determine that it is dependent on the impeller type, speed, diameter, and geometry of the installation. The mixer is not fully specified until torque, x, and lateral loads (fluid force, F) are included in the analysis [29]. 

There is no constant scale-up factor for each specific mixing system/process [29]. The two independent impeller variables come from speed, diameter, or power, because once the impeller type/style has been selected. 

The strong influence of the specific impeller power P/V and of the impeller type on the disintegration of floes can be seen from these curves, so that the effects of the operating conditions and the reactor type can be determined with satisfactory accuracy. The disintegration kinetics are complex, and show an exponential decrease in particle size with time. 

Fig. 4. Influence of impeller type on stress Reference floe diameter dpv in dependency on specific impeller power P/V 4 baffles w/D = 0.1 H/D = 1 D = 0.4 m...

Both large-scale motion (mass flow) and small-scale motion (turbulence) are usually required to bring about effective mixing (R5). Different ratios of mass flow to turbulence can be obtained for a given impeller type for the same power input large ratios for large values of d/T and slow speed, small ratios for small values of d/T and high speed. The requirements peculiar to batch liquid extraction have not been established, but for other services d/T = 0.2 to 0.5 is usually recommended for baffled tanks. 

The power put into a fluid mixer produces pumping Q and a velocity head H. In fact all the power P which is proportional to QH appears as heat in the fluid and must be dissipated through the mechanism of viscous shear. The pumping capacity of the impeller has been measured for a wide variety of impellers. Correlations are available to predict, in a general way, the pumping capacity of the many impeller types in many types of configurations. The impeller pumping capacity is proportional to the impeller speed N and the cube of the impeller diameter D,... 

FIGURE 12 Reynolds number-power number curve for several impeller types D, impeller diameter N, impeller rotational speed p, liquid density 11, liquid viscosity P, power and g, gravity constant. 

Change impeller type with equal tip speed and adjusted torque per volume. Calculations similar to those of step 6 can be used to keep both tip speed and torque per volume constant when the impeller type is changed. Because volume does not change, that factor is removed from the calculations, but the change in impeller type means that power number must be included. In addition, a hydrofoil impeller is more efficient at creating liquid motion than a pitched-blade turbine therefore, the torque required by the hydrofoil impeller will be assumed to be half that of the pitched-blade turbine. The subscript 1 will be used for the pitched-blade turbine and the subscript 2 will be used for the hydrofoil impeller. 

The distinguishing characteristic of each reported study is, usually, the physical design of the apparatus involved. In Table I, a brief outline of the published information on power requirements in one-liquid-phase systems is presented. This table gives the significant mechanical characteristics of the systems studied the range of liquid viscosities and the range of values for the impeller Reynolds number, which will be discussed below. In most of these studies the general objective was to relate the power consumption to tank diameter, impeller type and diameter, rotational speed, and liquid properties. Other variables studied are also indicated in the table. The major features of this work will now be reviewed. 

Mass-Transfer Models Because the mass-transfer coefficient and interfacial area for mass transfer of solute are complex functions of fluid properties and the operational and geometric variables of a stirred-tank extractor or mixer, the approach to design normally involves scale-up of miniplant data. The mass-transfer coefficient and interfacial area are influenced by numerous factors that are difficult to precisely quantify. These include drop coalescence and breakage rates as well as complex flow patterns that exist within the vessel (a function of impeller type, vessel geometry, and power input). Nevertheless, it is instructive to review available mass-transfer coefficient and interfacial area models for the insights they can offer. 

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