Impeller selection

In this section we describe turbine impellers used in transitional and turbulent flow apphcations. High viscosity applications and appropriate impeller types are discussed in Section 6-6. Further discussions of impellers may be found in Chapter 21 and in Dickey et al. (2001). A number of video clips illustrating the effects of impeller selection are included on the Visual Mixing CD. 

There are four types of turbine impellers, which are characterized by the flow patterns and level of shear they create axial flow, radial flow, hydrofoil, and high-shear impellers. They have the widest use in low and medium viscosity liquid applications, solids suspension, liquid-liquid emulsification, and gas dispersion. Turbine impellers can have blades varying from 2 to 12 in number. Two blades are normally unstable mechanically, while it is difficult to install more than six blades on a hub. Axial flow impellers generally have three or four blades, and radial flow impellers are designed with six blades. 

A pitched blade turbine consists of a hub with an even number of blades bolted and tack-welded on it. It is lighter in weight than a propeller of the same diameter. The blades can be at any angle between 10 and 90° from the horizontal, but the most common blade angle is 45°. The flow discharge from a pitched blade impeller has components of both axial and radial flow velocity in low to medium viscosity liquids, and is considered to be a mixed-flow impeller. Most 

Radial flow impellers may either have a disk (Rushton turbine) or be open (FBT) and may have either flat or curved blades (backswept mrbine). Impellers without the disk do not normally pump in a true radial direction since there is pressure difference between each side of the impeller. This is also true when the impellers are positioned in the tank at different off-bottom clearances. They can pump upward or downward while discharging radially. Radial discharge flow patterns can cause stratification or compartmentalization in the mixing tank. Disk-type radial impellers provide more uniform radial flow pattern and draw more power than open impellers. The disk is a baffle on the impeller, which prevents gas from rising along the mixer shaft. In addition, it allows the addition of a large number of impeller blades. Such blade addition cannot be done easily on a hub. A disk can also be used with a pitched blade turbine for use in gas-liquid mixing. 

The Rushton turbine is constructed with six vertical blades on the disk. Standard relative dimensions consist of blade length of D/4, blade width of D/5, and the disk diameters of 0.66 and 0.75D. The backswept turbine has six curved blades with a power number 20% lower than the Rushton turbine. The backswept nature of the blades prevents material buildup on the blades. It is also less susceptible to erosion. Typical applications include general waste and fiber processing in pulp and paper industries. 

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Normally the proper impeller selections and horsepower requirements are handled in a cooperative manner with the manufacturer of this equipment in order to obtain the best analysis of a given application. There is no substitute for performing the proper test runs to evaluate... 

Figure 7-8. Impeller selection. (Source Penny W. R. Guide to trouble free mixers, Chem. Eng., 77(12), 171, 1970.)...

The final process utilizes simultaneous addition of A and B over a 5 hour period to a seed bed from the previous batch ( -—15%). The impeller selected was the Ekato Intermig for the same reasons as in Example 10-1. 

The power delivered to the agitator decreases by as much as 60%, depending on the impeller selected. 

The discussion now turns to the individual resistances comprising Uq and how it is affected by impeller selection and surface area. The overall heat-transfer coefficient for a jacketed vessel can be obtained from the individual resistances by use of Equation (9.83) ... 

Good impeller selection will ensure that the power input to the agitator provides the correct balance between flow and head. 

Nonuniformity of blend insufficient tumover/not enough time/improper impeller selection/pumping capacity < design/volume > design/relative difference in viscosity increases or differs from design. 

Insufficient heat transfer improper impeller selection/fouled tubes/no baffling/ tube bundles poorly located so as not to supply good baffling. 

Colhgnon M-L, Delafosse A, Crine M, Toye D. (2010) Axial impeller selection for anchorage dependent animal cell culture in stirred bioreactors methodology based on the impeller comparison at just-suspended speed of rotation. Chem. Eng. Sci., 65 5929-5941. 

Brito-de la Fuente, E., L. Choplin, and P. A. Tanguy (1997). Mixing with helical ribbons effect of highly shear-thinning behaviour and impeller selection, Trans. Inst. Chem. Eng., 75, 45-52. 

Impeller Selection. Design for liquid-liquid contactors includes impeller geometry, number of impellers required, D/T ratio, and location in the vessel. Commonly used impellers are classified as producing shear or flow. If the application requires high interfacial area (small drop diameters), a high-shear impeller, such as the Rushton turbine shown on the left in Figure 12-28, is a... 

A narrow drop size distribution of the emulsion is desired for best shear thickening properties. The impeller selection, therefore, should be based on providing narrow distribution of shear. 

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