Self-Inducing Impellers

Self-inducing impellers are also used for indirect sparging purposes. The spinning action of the impeller creates a low pressure region at the impeller intake. Orifice holes are exposed to this low pressure region and connected by a hollow shaft to the atmosphere or head space (for dead-end systems) (Patwardhan and Joshi, 1999 Vesselinov et al., 2008). As such, the gas flow rate is a function of the impeller speed. The atmospheric pressure can be adjusted by pressurizing the shaft entrance such that additional gas is pumped into the system (Patwardhan and Joshi, 1999). 

These impellers have both advantages and disadvantages. Type 12 and 22 impellers force the gas phase to travel through a high shear volume, creating smaller average bubble diameters and higher interfacial area in the outlet flow. 

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Joshi (1980) modified the pipe impeller design. Holes were drilled near the tip of the impeller and on the hub (Fig. 9b). In this design, the increase in the liquid flow increases the rate of gas induction by 30-60% at practically the same power consumption. Martin (1972) used a flattened cylindrical tube so as to simulate the aerofil design (Fig. 9c). In flotation applications, the Denver and Wamco type of self-inducing impellers has been widely used for several decades. Zundelevich (1979) has modified the Denver design so as to increase the rate of gas induction by about 100% (Fig. 9d). 

Figure 6.20 Self-inducing impeller types (Patwardhan and Joshi, 1999).

Forrester SF, Rielly CD. (1994) Modelling the increased gas capacity of self-inducing impellers. Chem. Eng. Sci., 49 5709-5718. 

PatU SS, Mundale VD, Joshi JB. (2005) Mechanism of gas induction in self-inducing impellers. Ind. Eng. Chem. Res., 44 1322-1328. 

Fig. 4. Sectional view of one stage of a new type of gas-liquid contactor without pressure drop. (1) Vaned-disk impeller (2) stator (3) contacting tank (4) impeller shaft (5) gas inlet pipe (gas is self-induced through this pipe into the dispersion) (6) gas outlet thermometer (7) thermometer pocket (8) drainage tank (9) lid [after Gal-Or (G4)].

Self inducing agitators have been on the market for many years These communicate the low pressure region behind the blades with the head space Figure 15.4a) or a gas supply source via a hollow shaft and therefore use some of the input shaft energy to draw in the gas. Often they have multiple blades, sometimes curved like a centrifugal pump impeller,... 

Since agitated vessels are so common, it is worth noting some points arising from recent work that lead to recommended designs for turbulent systems. Most of this work has been with sparged systems, bnt the remarks on impeller blade shapes may also apply to self-inducers. 

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