Power requirements upon suspension

Weinspach [581] has determined the power characteristics for the downwards conveying propeller stirrer and for the on both sides partly covered double turbine in suspension operation. Here the density and the dynamic viscosity of the suspension were defined as follows  

It was found that up to = 0.25-0.30 suspensions behave as homogeneous mixtures, i.e. up to these values no Nesus(.(/ y) dependence was found. If the Newton and Reynolds numbers are formulated with the material properties valid for suspensions, see (5.7) and (5.8), and are represented as Nesus(Resus), the curves for Re 5 X 10 and / 0.25-0.30 coincide within experimental accuracy. 

Einenkel [114] pointed out that in the suspension process it cannot be assumed that the average density and viscosity values hold sway according to equations (5.7) and (5.8). On the contrary, the controlling density in the region of the stirrer can deviate considerably from it. This relationship is shown in Fig. 5.7, in which the quotient Pgg/Pi from the measured stirrer power and the stirrer power in the pure liquid is plotted against the stirrer speed for different values as a parameter. In the lower right comer of the figure Peg/Pl values are indicated, which correspond to the Psusp/Ft values. 

With regard to the starting of stirrers in settled suspensions, see [278, 383]. 

The stirrer which operates most favorably energetically is that which expends the lowest stirrer power at the critical stirrer speed. In the turbulent flow range, for which Fr = const applies, see (5.4), this condition is described by the relationship NeFr = min. With Ne = 0.35 and 5.0 for the propeller and turbine stirrers respectively (see Table in Fig. 2.2) and the above-mentioned constants for equation (5.4) it can be calculated that the propeller stirrer only requires 28% of the power of the turbine stirrer. From an energetic standpoint stirrer types which convey the liquid axially downwards are clearly superior to axially operating ones. 

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The situation is much more complicated for calculation of the hydrodynamic regularities upon suspension in stirred tanks, because here the radial acceleration may not be ignored. (This is evident from the fact that the specific stirrer power required for suspension decreases with increasing tank diameter ) Zehner developed a theoretical concept, which took into consideration the radial acceleration in the stirred tank and in this way realized relationships, which described the measured in tanks of different sizes well. 

The study carried out by Geisler et al. [152], which considered the dimensioning of the specific stirrer power per unit mass of the suspension e = FlpV, indicated the paramount importance of the D/dp parameter. In industrially-sized tanks (D/dp > 500) this quantity consists of two sum terms from an Ejs, which is required for maintaining that vertical flow rate n>t, which is equal to the terminal sinking velocity of the particle swarm Wss, and from the term edre, which covers the frictional losses of the flow upon reversal of the flow direction ... 

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