Clearance Impellers

Impeller Clearance Adjustment. AU pumps shall have provisions for adjustment of axial clearance between the leading edge of the impeller and casing. This adjustment shall he made by a precision microdial adjustment at the outboard hearing housing, which moves the impeller forward toward the suction wall of the casing. 

Viscosity Maximum level Number of Impeller Clearance ... 

The exponents were found to be independent of the impeller type, vessel size, impeller clearance and impeller to tank diameter ratio. The dimensionless constant Ci accounted for variations in the system geometry (e.g. on dc/di). This would indicate that the basic mechanism leading to minimum suspension may be the same for rather different stirrer geometries. Table 2, which is an update of the one given by Nienow [19], indicates different exponents found in a few other investigations. Baldi et al. [26] made a relatively successful theoretical approach by assuming that the suspension of particles is mainly due to eddies of a certain critical scale comparable to the particle size. From an energy balance it follows that... 

Viscosity, cP Maximum level, Number of Impeller clearance ... 

Impeller clearance measured from the impeller centerline to the vessel bottom off the vessel bottom (m)... 

Recommended impeller diameter-to-tank diameter D/T) ratios, for liquid-liquid operations vary from 0.25 to 0.40 for radial disk turbines, from 0.4 to 0.6 for hydrofoils and propellers, and from 0.5 to 0.8 for retreat-curve, glassed-steel impellers. Vertical placement of the impeller depends on the vessel shape and the application. For dispersion by continuous addition of a dense phase fluid into a less dense fluid, the impeller should have a relatively small impeller clearance off the reactor bottom, C, with respect to the final height of the dispersion, H, i.e., the impeller should be placed low in the vessel (C H/A to H/S). For dispersion of light liquids, it is good practice to place a single impeller between 0.20 < C/H < 0.50. 

Typical values for the impeller clearance off the vessel bottom are 7/4 for hydrofoil impellers and 7/3 for pitched-blade turbines. Hydrofoil impellers may be a poor choice when solid suspension is accompanied by other mixing duties, such as liquid-liquid dispersion or gas dispersion. For such cases, a multiple impeller system consisting of a high-efficiency impeller in combination with a 45"" pitched-blade impeller should be evaluated. Small pitched blade impellers with a... 

Armenante, P.M. Uehara Nagamine, E. Effect of low off-bottom impeller clearance on the minimum agitation speed for complete suspension of solids in stirred tanks. Chem. Eng. Sci. 1998, 53, 1757-1775. 

Montante G., Lee K.C., Liquid and particle mean flow and turbulence levels in a stirred vessel with low impeller clearance, Instit. of chem. Eng., Sympos. Series No. 146 (1999), p. 305— 316... 

C Impeller clearance, distance between lower tip of the impeller and the bottom of the vessel m ft... 

Impeller clearance affects total discharge flow and its direction. For example, when axial-flow impellers are placed close to the bottom, they produce a radial discharge. Upward-angled, retreat-curve impellers are always located at the bottom of the vessel, where they produce strong radial flow and produce good circulation while handling level changes and solids effectively. 

Impeller clearance also affects power draw, as is shown in Figure 9.6 for six-blade Rushton, PBT, and flat-blade turbines. 

Power Numbers and s Values (Zwietering Equation) for Different Impellers, Clearances (C/7), and Impeller Sizes (D/7)... 

The minimum speed, to form dispersions from two settled phases of different densities is expressed in terms of dimensionless variables. Differences in constant C reflect the effect of impeller clearance. Similarities can be seen between Ajs for solids suspension and for... 

Edwards and Ellis found somewhat similar result in that a T propeller was most efficient with a clearance off the base of centrally placed and without any baffles (compared to a wide range of baffle configurations and impeller clearances using the propeller, a disc turbine and a flat paddle), i.e. they found central vortexing with solids added into the vortex the best. 

From systematic simulations of tracer experiments in a stirred vessel equipped with a Rushton turbine, a height/tank diameter ratio = 1.0, an im-peller/tank diameter ratio = 0.3125, and an impeller clearance/height of liquid ratio = 0.31 the following correlation was obtained ... 

Gray DJ. (1987) Impeller clearance effect on off bottom particle suspension in agitated vessels. Chem. Eng. Commun., 61 151-158. 

Montante G, Lee KC, Brucato A, Yianneskis M. (2001) Numerical simulations of the dependency of flow pattern on impeller clearance in stirred vessels. Chem. Eng. Sci., 56 3751-3770. 

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