Mixing impeller types

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. 

Axial flow impeller (propeller) or mixed flow type (horizontal or vertical)... 

The above covers most conventional mixers there is another class of mixers, called pump-mix impellers, where the impeller serves not only to mix the fluids, but also to move the fluids through the extraction stages. These are speciahzed designs, often used in the metals extraction industries. For these types of impellers, a knowledge of the power characteristics for pumping is required in addition to that for mixing. For a more detailed treatment of these special cases, the reader is referred to Lo et al. 

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. 

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

Figure 3-11. Vertical single-stage mixed fiow type pump, iiquid iniet and impeller. (Courtesy Peerless Pump Div., FMC Corp.)...

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. 

Figure 5-18. Laminar flow mixing. For known impeller type, diameter, speed, and viscosity, this nomograph will give power consumption. Connect RPM and diameter, also viscosity and impeller scale. The intersection of these two separate lines with alpha and beta respectively is then connected to give horsepower on the HP scale. By permission, Quillen, C. S., Chem. Engr., June 1954, p. 177 [15].

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. 

Ruszkowski, S., A Rational Method for Measuring Blending Performance, and Comparison of Different Impeller Types . Proceedings of the 8th European Conference on Mixing, Cambridge, UK, pp. 283-291 (1994). 

Zwietering constant values for, 16 694t Impeller types, in mixing and blending,... 

Dispersions may be classified into two types, based upon size range of the droplets formed. Turbulence creators (mixing impellers, mixing valves, eductors, orifice plates) will produce fine emulsions of micron-size droplets. Nozzles, perforated plates, bubble caps, tower packings, etc., can form discrete drops of relatively large size which will quickly settle through the continuous phase. 

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. 

It is seldom possible, either economically or time-wise, to study every potential mixing variable or to compare the performance of many impeller types. 

The pumping capacity of a mixing impeller is specified by either the flow from the impeller or the total flow of the tank. Flow varies for any impeller as the speed and diameter cubed. Table VI gives some for constants in the equation Q — KND3 for various impeller types. The radial... 

First ask yourself if there is any role for fluid shear stresses in determining and obtaining the desired process result. About half of the time the answer will likely be no. That is the percentage of mixing processes where fluid shear stresses either have no effect or seem to have no effect on the process result. In these cases, mixer design can be based on pumping capacity, blend time, velocities and other matters of that nature. Impeller type location and other geometric variables are major factors in these types of processes. 

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