Type Impellers

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 pumping number is a function of impeller type, the impeller/tank diameter ratio (D/T), and mixing Reynolds number Re = pND /p.. Figure 3 shows the relationship (2) for a 45° pitched blade turbine (PBT). The total flow in a mixing tank is the sum of the impeller flow and flow entrained by the hquid jet. The entrainment depends on the mixer geometry and impeller diameter. For large-size impellers, enhancement of total flow by entrainment is lower (Fig. 4) compared with small impellers. 

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. 

Obviously shear rate in different parts of a mixing tank are different, and therefore there are several definitions of shear rate (/) for average shear rate in the impeller region, oc V, the proportionaUty constant varies between 8 and 14 for all impeller types (2) maximum shear rate, oc tip speed (%NU), occurs near the blade tip (3) average shear rate in the entire tank is an order of magnitude less than case / and (4) minimum shear rate is about 25% of case 3. 

Fig. 9. Flow patterns with different impeller types, sizes, and liquid viscosity (a) FBT (b) hydrofoil (c) PBT (d) PBT, large diameter (e) PBT, high...

Fig. 12. Common impeller types (a) axial, (b) radial, (c) hydrofoil, and (d) closeclearance.

The correlation for based on experimental data has been weU researched. The pioneering work was carried out in 1958 and a correlation with different impeller types and tank sizes was developed (5) ... 

High Speed Fluid Energy Mills. This type of equipment is used primarily for preparation of relatively low viscosity mill bases for inks and paints. The first is an impeller type which achieves dispersion by the appHcation of shear. The second type is in the form of a rotor—stator, and the dispersion is achieved by impingement or impact. 

FIG. 10-79 Two -impeller type of rotary positive-displacement blower. 

It is seldom possible, either economically or timewise, to study eveiy potential mixing variable or to compare the performance of many impeller types. In many cases, a process needs a specific fluid regime that is relatively independent of the impeller type used to generate it. Because different impellers may require different geometries to achieve an optimum process combination, a random choice of only one diameter of each of two or more impeller types may not tell what is appropriate for the fluid regime ultimately required. 

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. 

When comparing different impeller types, an entirely different phenomenon is important. In terms of circulation time, the phenomena shown in Figs. 18-18 and 18-19 stiU apply with the different impellers shown in Fig. 18-5. When it comes to blending another factor enters the picture. When particles A and B meet each other as a result of shear rates, there has to be sufficient shear stress to cause A and B to blend, react, or otherwise participate in the process. 

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... 

Impeller Type Horsepower (Op. cost mix. heat added) N rpm Torque Initial cost... 

However, the range of validation of the reaction model was limited. Only one impeller type and one reaction system were studied. In the future, the model should be tested for a wider range of geometries and reaction systems and, if necessary, modified to increase its validity. 

Grootseholten etal. (1982) diseuss the influenee of different impeller types on seeondary nueleation rates. There is some evidenee (Serutton etal. 1982) that seeondary nueleation rates are partieularly sensitive to small elearanees between the stirrer and a draft-tube by whieh the stirrer is shrouded. 

Figure 3-15. Impeller types. Open impeller for corrosive or abrasive slurries and solids. (Courtesy Goulds Pumps, Inc.)...

The specific speed of a centrifugal pump correlates the basic impeller types as shown in Figure 3-47. 

Impeller types usually used with mixing and listed in decreasing order of high volume ability (hence in increasing order of high head ability or requirement) are paddle, turbine, propeller, sawtooth impeller or propeller, cut-out impeller disc (no blades), colloid mill. 

This is dependent on the impeller type, speed, diameter, and the geometry of the installation. 

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]. 

The pattern of the fluid motion is a function of the fluid system, impeller, vessel configuration, and location of the impeller in the fluid system relative to the vessel walls and/or bottom. The patterns illustrated in Figures 5-23A-5-23K indicate that almost any pattern can be established provided the particular impeller type is located in the proper position. This is easier to accomplish in some s) stems than others. 

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. 

Because the most common impeller type is the turbine, most scale-up published studies have been devoted to that unit. Almost all scale-up situations require duplication of process results from the initial scale to the second scaled unit. Therefore, this is the objective of the outline to follow, from Reference [32]. The dynamic response is used as a reference for agitation/mixer behavior for a defined set of process results. For turbulent mixing, kinematic similarity occurs with geometric similarity, meaning fixed ratios exist between corresponding velocities. 

A.i.Ch.E. Standard Nomenclature for Mixing from A.i.Ch.E. Standard Testing Procedure for Mixing Equipment (Impeller Type), American Institute of Chemical Engineers. 

Figure 12-46F. Open radial blade impeller, Type R. (Used by permission Bui. Centrifugal Compressors Single Stage. A C Compressor Corporation.)...

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