Impellers, various types

Axial-flow turbines are often used in blendiug pseudoplastic materials, and they are often used at relatively large D/T ratios, from 0.5 to 0.7, to adequately provide shear rate in the majority of the batch particularly in pseudoplastic material. These impellers develop a flow pattern which may or may not encompass an entire tank, and these areas of motion are sometimes referred to as caverns. Several papers describe the size of these caverns relative to various types of mixing phenomena. An effec tive procedure for the blending of pseudoplastic fluids is given in Oldshue (op. cit.). 

This chapter reviews the various types of impellers, die flow patterns generated by diese agitators, correlation of die dimensionless parameters (i.e., Reynolds number, Froude number, and Power number), scale-up of mixers, heat transfer coefficients of jacketed agitated vessels, and die time required for heating or cooling diese vessels. 

Figure 5-17 is useful for determination of horsepower during turbulent flow for various types of impellers, and Figure 5-18 is useful for laminar flow. y lso see Figure 5-19. 

The flow patterns for single phase, Newtonian and non-Newtonian liquids in tanks agitated by various types of impeller have been repotted in the literature.1 3 27 38 39) The experimental techniques which have been employed include the introduction of tracer liquids, neutrally buoyant particles or hydrogen bubbles, and measurement of local velocities by means of Pitot tubes, laser-doppler anemometers, and so on. The salient features of the flow patterns encountered with propellers and disc turbines are shown in Figures 7.9 and 7.10. 

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. 

One of the complications in stirred tank flows is the presence of macroinstabilities (i.e., low-frequency mean flow variations) that may affect the mixing performance. Various authors have distinguished between various types and investigated their occurrence and their frequencies under varying operating conditions and with several types of vessels and impellers (Yianneskis et al., 1987 Haam et al., 1992 Myers et al., 1997 Hasal et al., 2000 Nikiforaki et al., 2002). 

Fig. 16. The yield Xg of the product Q of the slower reaction of a set of two competitive parallel reactions in a fed batch reactor plotted vs. impeller speed (in /s). The experimental data are due to Bourne and Yu (1991) the crosses refer to feeding reactant A at the top of the vessel, while the diamonds refer to feeding more closely to the impeller. The various types of lines refer to simulations as specified in the legend. Reproduced with permission from R. A. Bakker (1996).

There are various purposes of using a stirred vessel, and the required mixing effect depends on the purpose. For any mixing purpose, rapid and homogeneous dispersion is required. In a stirred vessel, forced convection by the rotation of an impeller occurs, that is, each element of the fluid has an individual velocity finally, turbulent flow based on the shear stress accelerates the mixing. Therefore, the shape of the impeller has a very important effect on the mixing state. However, there are various types of impeller shapes, and traditional impellers are classified into three types ... 

Until now, bioreactors of various types have been developed. These include loop-fluidized bed [14], spin filter, continuously stirred turbine, hollow fiber, stirred tank, airlift, rotating drum, and photo bioreactors [1]. Bioreactor modifications include the substitution of a marine impeller in place of a flat-bladed turbine, and the use of a single, large, flat paddle or blade, and a newly designed membrane stirrer for bubble-free aeration [13, 15-18]. Kim et al. [19] developed a hybrid reactor with a cell-lift impeller and a sintered stainless steel sparger for Thalictrum rugosum cell cultures, and cell densities of up to 31 g L1 were obtained by perfusion without any problems with mixing or loss of cell viability the specific berberine productivity was comparable to that in shake flasks. Su and Humphrey [20] conducted a perfusion cultivation in a stirred tank bio-... 

Fig. 33. Horizontal contactor and various types of impeller. 1, vessel 2, impeller shaft 3, impeller 4, sparger assembly 5, gland and stuffing box 6, pulley 7, nozzle 8, plan 9, elevation. (After Joshi el al., 1985.)...

Here, P0 is the impeller power, s0 is the impeller speed, d, is the impeller diameter, Pl and v l are the density and kinematic viscosity of the liquids, respectively. The term tf Myr adjusts the actual impeller speed to the speed at which a fan-disk turbine would rotate for the same power input per unit mass. Although no gas was used in this study, the correlation should be useful as a first estimate for Ks in various types of stirred three-phase slurry reactors. 

Figure 4.4 shows various types of impellers that are used in centrifugal pumps. The one in a is used for axial-flow pump. Axial-flow pumps are pumps that transmit the fluid pumped in the axial direction. They are also called propeller pumps, because the impeller simply propels the fluid forward like the movement of a ship with propellers. The impeller in d has a shroud or cover over it. This kind of design can develop more... 

FIGURE 4.4 Various types of pump impellers (a) axial flow (b) open type (c) mix-flow type and (d) shrouded impeller. 

FIGURE 4.5 Various types of pump impellers, continued (a) lobe type (b) internal gear type (c) vane type (d) impeller with stationary guiding diffuser vanes (e) impeller with volute discharge and (f) external gear type impeller. 

Figure 6 Various types of turbine (A, B and C) and paddle (D) impellers.

Fig. 6 Various types of animal cell bioreactors (A) roller bottle (B) rotating disk (C) stirred tank with a marine impeller (D) tank with a pulsating agitator (E) stirred tank with a spin filter (F) airlift (G) fluidized bed and (H) hollow fiber. (View this art in color at WWW. dekker. com.)...

Variation in Impeller Profiles with Specific Speed and Approximate Range of Specific Speed for the Various Types. 

Fig. 16.2-1, in the presence of air that is sucked or fed into the impeller zone where the air is well dispersed owing to the intense agitation in that zone. The air bubbles collide with pardcles and are attached to those that are hydrophobic or heve acquired hydrophobicity. The bubble-particle aggregates hue to the rop of the cell and are removed by skimming. Various types of machine that ase used by the industry have been described in detail by Harris in a recent publication on flotation.1... 

Magnitudes of the constants Kp and for various types of impellers and tanks are shown in Table 9.3. 

The boundaries between the fields of application of the various types of mixers are not sharp. Impeller mixers of the kind described in Chap. 9 are not often used when the viscosity is more than about 2 kP, especially if the liquid is not newtonian. Kneaders and mixer-extruders work on thick pastes and plastic masses impact wheels are restricted to dry powders. Other mixers, however, can blend liquids, pastes, plastic solids, and powders. 

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