Stirrer types and their operating characteristics

The stirring operations discussed in the introduction can obviously not be performed with a single type of stirrer. There are many types of stirrers appropriate for particular stirring operations and particular material systems. In this section only those stirrer types will be discussed which are widely used in the chemical industry and for which reliable design guidelines exist. The dimensions of stirrer types have also been standardized to a large extent [161]. 

In Fig. 1.4 the stirrer types are arranged according to the predominant flow pattern they produce, as well as to the range of viscosities over which they can be effectively used. 90% of all stirring operations can be carried out with these standard stirrer types. The flow patterns obtained with typical radially and axially conveying stirrers are shown in Fig. 1,5. 

Of the stirrer types which set the liquid in a radial motion - or into a tangential flow in the case of high viscosities - only the turbine stirrer ) (so-called Rushton turbine , a disk 2d/3 in diameter supporting 6 blades each d/5 high and d/4 wide [474]) belongs to the high speed stirrers. It can be sensibly utilized only with low viscosity liquids and in baffled tanks. Its diameter ratio D/d is 3-5. The turbine stirrer causes high levels of shear and hence is well suited for dispersion processes. 

The Pfaudler impeller stirrer was developed for use in enamel-coated vessels [438] and thus has rounded stirring arms. It is installed with small bottom clearance at a D/d ratio of 1.5 and can be used both with and without baffles. Due to the small bottom clearance it can be used with strongly fluctuating filling levels (e.g. during emptying), since it can efficiently mix even small liquid volumes. 

Cross-beam, grid and blade stirrers are slow-speed stirrers and are used at D/d = 1.5 to 2 both with and (in the case of viscous liquids) without baffles. They are particularly suitable for homogenization. 

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