Impellers single-phase liquids agitated

FIG. 15-23 Power for agitation impellers immersed in single-phase liquids, baffled vessels with a gas-liquid surface [except curves (c) and (g)]. Curves correspond to (a) marine impellers, (h) flat-blade turbines, w = dj/5, (c) disk flat-blade turbines witb and without a gas-liquid surface, (d) curved-blade turbines, (e) pitcbed-blade turbines, (g) flat-blade turbines, no baffles, no gas-liquid interface, no vortex. 

Consider a stirred tank vessel having a Newtonian liquid of density p and viseosity p, is agitated by an impeller of diameter D, rotating at a rotational speed N. Let the tank diameter be D, the impeller width W, and the liquid depth H. The power P required for agitation of a single-phase liquid ean be expressed as ... 

The power dissipated by an impeller rotating in a homogenous, single-phase liquid in a baffled vessel is a function of the type of impeller, the flow regime in which the impeller operates (laminar vs. turbulent), which is, in turn, a function of the impeller Reynolds number. Re = plAD /p, and a number of geometric ratios. For an agitated vessel, the impeller power dissipation, P, and the power dissipation per unit liquid mass, , can be calculated, respectively, from ... 

The flow patterns of single-phase liquids in tanks agitated by various types of impeller have been reported in the literature. The experimental techniques used include the use of coloured tracer liquid, mutually-buoyant particles, hydrogen bubble generation and mean velocity measurements using pitot probes, hot-rdm devices and lasers. 

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. 

Since a well-designed turbine impeller provides very thorough mixing of the liquid, it can reasonably be assumed that the concentration throughout the liquid phase in such devices is quite uniform. Therefore the benefits of counter-current flow for continuous operation cannot be had with a single tank and agitator. 

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