MIXERS AND IMPELLERS

There is a complete range of flow and fluid shear relationships from any given impeller type. 

Three types of impellers are commonly used in the low viscosity region, propellers, Fig. 1 turbines. Fig. 2 and axial flow turbines. Fig. 3. Impellers used on small portable mixers shown in Fig. 4, are oflen inclined at an angle as well as being off-center to give a good top-to-bottom flow pattern in the system. Fig. 5. Large top-entering drives usually use either the axial flow turbine or the radial flow flat blade turbine. For aerobic fermentation, the radial flow disc turbine is most common and is illustrated in Fig. 6. 

To complete the picture, there are also bottom-entering drives, shown in Fig. 7, which have the advantage of keeping the mixer off the top of all tanks and required superstructure, but have the disadvantage that if the sealing mechanism fails, the mixer is in a vulnerable location for damage and loss of product by leakage. 

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Oldshue (26) provides a detailed procedure for selecting mixing times and optimizing mixer and impeller configurations for viscous and shearthinning materials, which can be adapted for other rheologically challenging... 

Mixer and Impeller Design—Modem approaches employ the use of computational fluid dynamics (CFD) modelling to improve designs of mixer boxes, baffles, and impellers (Giralico et al. 2003 Gigas and Giralico 2002). The choice of whether or not to use mixer-settlers or columns can also influence the efficiencies achievable (Fox et al. 1998). 

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