Mixing impellers Anchor

External jackets, 326-328 Helical coils, 312, 326, 327 Vertical coils, 326, 327 Mixing impellers, 290-297 Anchor, 290-329 Blending, 324, 326 Characteristic curves, 306 Chart to examine turbine applications, 296 Efficiency of propellers, 299 Flow of propellers. 298, 299 Flow patterns, 309-312 Gas-Liquid contacting, 324, 326 General list impellers, 291 Helical, 290, 329 Liquid-liquid dispersion, 326 Multiple, 297... 

Liquid mixing impellers propeller,turbine, anchor... 

Slow speed close-clearance impellers are used when mixing high viscosity materials. Helical or anchor type close-clearance impellers are used in this application at speeds from 5 to 20 rpm. Table 1 compares the pow er required and cost for conventional axial flow turbines and the helical type. 

Figure 5-39. Close-clearance anchor and helical impellers. By permission, Oldshue, J. Y., Fluid Mixing Technology, 1983, Chemical Engineering McGraw-Hill Publications Co., Inc. [29].

Figure 7.20 shows some of the impellers which are frequently used. Propellers, turbines, paddles, anchors, helical ribbons and screws are usually mounted on a central vertical shaft in a cylindrical tank, and they are selected for a particular duty largely on the basis of liquid viscosity. By and large, it is necessary to move from a propeller to a turbine and then, in order, to a paddle, to an anchor and then to a helical ribbon and finally to a screw as the viscosity of the fluids to be mixed increases. In so doing the speed of agitation or rotation decreases. 

For turbine agitators, impeller to tank diameter ratios of up to about 0.6 are used, with the depth of liquid equal to the tank diameter. Baffles are normally used, to improve the mixing and reduce problems from vortex formation. Anchor agitators are used with close clearance between the blades and vessel wall, anchor to tank diameter ratios of... 

The impeller is the part of the agitator that impacts force to the material being mixed. Propellers, turbines, gates, anchors, and paddles are all types of impellers. Typically, the impeller is a single propeller or turbine blade connected to a shaft that is driven by an electric motor at a fixed speed. There are two classes of impeller agitators axial-flow and radial-flow, and the mixing characteristics are shown in Figure 3.14. 

Gate and anchor impellers, long used advantageously for the mixing of vis-... 

Close-Clearance Impellers There are two close-clearance impellers. They are the anchor impeller (Fig. 18-6) and the helical impeller (Fig. 18-7), which operate near the tank wall and are particularly effective in pseudoplastic fluids in which it is desirable to have the mixing energy concentrated out near the tank wall where the flow pattern is more effective than with the open impellers that were covered earlier. 

Double- and Triple-Shaft Mixers The planetary mixer is an example of a double shaft mixer. However, many different combinations of mixing actions can be achieved with multi-shaft mixers. One variation on planetary motion involves replacing one anchor-style impeller with a high-shear impeller similar to the one shown in Fig. 18-47. The high-shear mixer can be used to incorporate powdered material effectively or create a stable emulsion leading to a final batch of viscous paste or fluid. 

Many types of multishaft mixers do not require planetary motion. Instead the mixers rely on an anchor-style impeller to move and shear material near the tank wall, while another mixer provides a different type of mixing. The second or third mixer shafts may have a pitched-blade turbine, hydrofoil impeller, high-shear blade, rotor-stator mixer, or other type of mixer. The combination of multiple impeller types adds to the flexibility of the total mixer. Many batch processes involve different types of mixing over a range of viscosities. Some mixer types provide the top-to-bottom motion that is missing from the anchor impeller alone. 

At transitional flow conditions, larger diameter impellers are used to improve mixing times, but they require more power than equivalent mixing rates under turbulent conditions. Baffles improve mixing rates for Re > 300, but have the opposite effect for Re below this point. Helical ribbon-, anchor-, and gate-type impellers are commonly used at the low end of the transitional flow range. 

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