Large blade impellers

The results in Fig. 25 for hybridoma cells show that due to the low growth rate, which lies in the range of p < 0.02/h, animal cells could only be cultivated under very moderate stress conditions of it< 0.005 -0.05 N/m. That means that, also for very low-shear impellers such as large-blade impellers, the average power input has to be limited to P/V <30-50 W/m ... 

Large blade impellers generally are used for laminar mixing of highly viscous fluids. One of the commonly used impellers is the two-blade impeller (Figure 1). The ratio (D/T), impeller diameter to tank diameter, can be varied from 0.5 to 0.9. In this case of large diameter impellers, these last ones can be used for heat transfer and can successfully replace anchor impellers. 

For a standard PBT the error is on the order of 5% if the perpendicular distance is used. For the large-bladed impeller with a shallower angle, the errors are up to 70% This D also makes sense when measuring the primary flow with a laser Doppler velocimeter for determination of the flow number (see Chapter 6). It is very important in a mixing installation when one has to be concerned with clearances from the tips of the blades. 

The pumping number is a function of impeller type, the impeller/tank diameter ratio (D/T), and mixing Reynolds number Re = pND /p.. Figure 3 shows the relationship (2) for a 45° pitched blade turbine (PBT). The total flow in a mixing tank is the sum of the impeller flow and flow entrained by the hquid jet. The entrainment depends on the mixer geometry and impeller diameter. For large-size impellers, enhancement of total flow by entrainment is lower (Fig. 4) compared with small impellers. 

Paddle A paddle is similar to a turbine impeller but typically has only two large blades and operates at lower speeds than a turbine. They are primarily used in high viscosity mixing operations. In European and Japanese literature the term "paddle" also is used to describe the flat blade and pitched blade turbines discussed above. The term "turbine" generally is reserved for disk turbines. 

Impellers with a large blade area, whieh rotate at low speeds. 

For reactors with free turbulent flow without dominant boundary layer flows or gas/hquid interfaces (due to rising gas bubbles) such as stirred reactors with bafQes, all used model particle systems and also many biological systems produce similar results, and it may therefore be assumed that these results are also applicable to other particle systems. For stirred tanks in particular, the stress produced by impellers of various types can be predicted with the aid of a geometrical function (Eq. (20)) derived from the results of the measurements. Impellers with a large blade area in relation to the tank dimensions produce less shear, because of their uniform power input, in contrast to small and especially axial-flow impellers, such as propellers, and all kinds of inclined-blade impellers. 

The testing should determine the intensity of agitation necessary to obtain the desired level of suspension uniformity. Suppose that by visual observation and sample analysis, a pitched-blade impeller with a diameter D of 4 in (with four blades, each 0.8 in wide) operating at a speed N of 465 r/min was found to produce the level of agitation necessary for uniform suspension to three-fourths the total liquid level. With data about impeller diameter and agitator speed in a small tank, it should be possible to scale up performance to the large-scale tank. 

In this section, a large two-blade impeller, the height of which is equal to the liquid level, is considered. This impeller scrapes the tank bottom without any friction it is this kind of impeller that was frequently studied in the previous works, assuming 2D flows. In the following, more precise results are presented. 

Choose the impeller type and diameter. A large diameter impeller with a high power number will be best suited for blending a shear-thinning fluid. Choose a pitched blade turbine with Po = 1.75 and D = 1.0 m (or T/2). 

Large mixers running at less than 150 rpm usually operate below the first critical speed. Small mixers operating above 250 rpm usually operate between first and second critical, 1.2Nc to 0.8Nc2, where Nc2 is the second lateral natural frequency. Other frequencies, such as a blade-passing frequency, four times the operating speed for a four-blade impeller with four baffles, can cause mechanical excitations. Structural vibrations at certain fractions of operating speed can also contribute to natural frequency problems. 

Fig. 20. Cavity formations behind impeller blades where (a) illustrates clinging cavities, (b) a large cavity, (c) 3—3 cavities, (d) alternating large and larger...

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