Impeller pitch blade

A stirred vessel crystallizer is shown in Fig. 6-4. Included are a dual-impeller pitched-blade turbine with a tickler blade (see Section 6.6.1.6), a subsurface addition line, baffles, and a ram-type bottom outlet valve to aid in discharge of slurries. 

Lamade [315] has evaluated the suspension characteristics for three different enameled stirrers (PFAUDLER impeller, pitched-blade, paddle stirrers) in the turbulent flow range. He found a process relationship of the form ... 

Schragblattriihrer pitched blade impeller, pitched-blade fan impeller, pitched-blade paddle impeller, inclined paddle impeller... 

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. 

Axial-Flow Impellers Axial-flow impellers include all impellers in which the blade makes an angle of less than 90° with the plane of rotation. Propellers and pitched-blade turbines, as illustrated in Figs. 18-8 and 18-3, are representative axial-flow 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. 

Figure 21. Discharge co cient for 45° pitched blade turbine V5. impeller Reynolds number.

Hicks et al. [8] developed a correlation involving the Pumping number and impeller Reynolds number for several ratios of impeller diameter to tank diameter (D /D ) for pitched-blade turbines. From this coiTclation, Qp can be determined, and thus the bulk fluid velocity from the cross-sectional area of the tank. The procedure for determining the parameters is iterative because the impeller diameter and rotational speed N appear in both dimensionless parameters (i.e., Npe and Nq). 

Figure 7-15 shows plots of Pumping number Nq and Power number Np as functions of Reynolds number Np for a pitched-blade turbine and high-efficiency impeller. Hicks et al. [8] further introduced the scale of agitation, S, as a measure for determining agitation intensity in pitched-blade impellers. The scale of agitation is based on a characteristic velocity, v, defined by... 

Figure 7-15. Power number and Pumping number as functions of Reynolds number for a pitched-blade turbine and high-efficiency impeller. (Source Bakker, A., and Gates L. , Properly Choose Mechanical Agitators for Viscous Liquids," Chem. Eng. Prog., pp. 25-34, 1995.)...

Axial-flow fan A fan positioned in a cylindrical casing in which the air enters and leaves the impeller in a direction parallel to the casing axis. The fan may have fixed-pitch blades or variable-pitch blades. 

Figure 7.21. Variation in turbine impeller designs (a) Flat blade (b) Disc flat blade (c) Pitched vane (d) Curved blade (e) Tilted blade (/) Shrouded (g) Pitched blade (h) Pitched curved blade (7) Arrowhead...

Contrary to commonly held opinion (e.g. [62, 63]), this comparison shows that axial-flow impellers such as pitched-blade impellers and propellers lead to particularly high stresses at the same specific impeller power. The impeller geometry, such as the impeller-to-tank diameter ratio d/D and the relative blade height h/d (see Fig. 5), also has a distinct influence. 

The three basic types of impeller which are used at high Reynolds numbers (low viscosity) are shown in Figures 10.55a, b, c. They can be classified according to the predominant direction of flow leaving the impeller. The flat-bladed (Rushton) turbines are essentially radial-flow devices, suitable for processes controlled by turbulent mixing (shear controlled processes). The propeller and pitched-bladed turbines are essentially axial-flow devices, suitable for bulk fluid mixing. 

Figure 10.55. Basic impeller types (a) Turbine impeller (b) Pitched bladed turbine (c) Marine propeller...

Fig. 10. Results of LES-based simulations of an agglomeration process in two vessels one agitated by a Rushton turbine (left) and one agitated by a Pitched Blade Turbine (right). The two plots show the agglomeration rate constant fl0 normalized by the maximum value, in a vertical cross-sectional plane midway between two baffles and through the center of the vessel. Each of the two plots consists of two parts the right-hand parts present instantaneous snapshots the left-hand parts present spatial distributions of time-averaged values after 50 impeller revolutions. Reproduced with permission from Hollander et al. (2003).

Note The respective impellers used are a classical Rushton turbine (DT), a hydrofoil impeller (A315) manufactured by Lightnin, and a Pitched Blade impeller (PBT). The cases 1 through 4 all relate to a superficial gas rate of 3.6mm/s only, with impeller speeds varying between 5 and 10/s (gas flow numbers between 0.01 and 0.02) cases 2 and 3 differ in sparger size and position. 

The impact of hydrodynamic stress on animal cells has been reviewed extensively (29,43). Most of the work reported in the literature on cell damage in agitated bioreactors has been done at bench-scale. Kunas and Papoutsakis (44) reported that in 1-2 L bioreactors equipped with a 7 cm diameter pitched-blade impeller, cell damage was not observed until the impeller rate was raised to above 700 rpm (tip speed 513cm/s), as long as air entrapment did not occur. However, it is not clear how these bench-scale observations translate into damaging impeller rates at manufacturing scale. 

The pitched-blade turbine is a reasonably cost-effective impeller in both turbulent and laminar flow. It is also a suitable impeller for applications where the viscosity changes over a wide range causing the flow regime to vary between turbulent and laminar flow. Moreover, it is a cost-effective impeller for solid suspensions. 

Double-impeller combinations Bouaifi et al. (2001) derived the following correlations for stirred gas-liquid reactors with various combinations of double impellers. The impellers used were the lightning axial flow impeller (A-310), the four 45° pitched blade turbine pumping down (PBTD) and the Rushton disk turbine (RDT). Furthermore, the tank was a dish-bottom cylindrical tank equipped with four baffles, while the gas was introduced by a ring sprager. The gas-flow rate ranged from 0.54 to 2.62 L/s, whereas the rotational speed was from 1.66 to 11.67 s. The gas holdup is... 

Estimate the stirrer power requirement P for a tank fermentor, 1.8 m in diameter, containing a viscous non-Newtonian broth, of which the consistency index A = 124, flow behavior index n = 0.537, density p = 1050 kg m", stirred by a pitched-blade, turbine-type impeller of diameter d = 0.6 m, with a rotational speed AT of 1 s . ... 

The last of these methods has been applied particularly to chemical reaction vessels. It is covered in detail in Chapter 17. In most cases, however, the RTDs have not been correlated with impeller characteristics or other mixing parameters. Largely this also is true of most mixing investigations, but Figure 10.3 is an uncommon example of correlation of blend time in terms of Reynolds number for the popular pitched blade turbine impeller. As expected, the blend time levels off beyond a certain mixing intensity, in this case beyond Reynolds numbers of 30,000 or so. The acid-base indicator technique was used. Other details of the test work and the scatter of the data are not revealed in the published information. Another practical solution of the problem is typified by Table 10.1 which relates blend time to power input to... 

Figure 10.5. Power number, Np = PgJN3Dsp, against Reynolds number, NR = ND2p/p, for several kinds of impellers (a) helical shape (Oldshue, 1983) (b) anchor shape Oldshue, 1983) (c) several shapes (1) propeller, pitch equalling diameter, without baffles (2) propeller, s = d, four baffles (3) propeller, s = 2d, without baffles (4) propeller, s = 2d, four baffles (5) turbine impeller, six straight blades, without baffles (6) turbine impeller, six blades, four baffles (7) turbine impeller, six curved blades, four baffles (8) arrowhead turbine, four baffles (9) turbine impeller, inclined curved blades, four baffles (10) two-blade paddle, four baffles (11) turbine impeller, six blades, four baffles (12) turbine impeller with stator ring (13) paddle without baffles (data of Miller and Mann) (14) paddle without baffles (data of White and Summerford). All baffles are of width 0.1D [after Rushton, Costich, and Everett, Chem. Eng. Prog. 46(9), 467 (1950)].

Figure 10.7. Flow number as a function of impeller Reynolds number for a pitched blade turbine with AE = 1.37. D/T is the ratio of impeller and tank diameters. [Dickey, 1984, 12, 7 Chem. Eng., 102-110 26 Apr. 1976)].

TABLE 10.3. Mixing of Liquids Power and Impeller Speed (hp/rpm) for Two Viscosities, as a Function of the Liquid Superficial Velocity Pitched Blade Turbine Impeller... 

For suspension of solids, the tables pertain to particles with settling velocities of 10 ft/min, but data are available for 25 ft/min. The impeller is a pitched-blade turbine. 

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