Solids suspension using an angle-mounted axial-flow impeller in an unbaffled tank relies on proper placement of the impeller in the vessel such that swirling flow does not occur. Even with proper placement, the just-suspended speed of an angle-mounted agitator is often 75% higher than with vertical mounting on the centreline in a baffled tank, leading to torque and power requirements that average more than twice and five times higher, respectively. Angle-mounted turbulent power numbers of axial-flow impellers are generally similar to, but lower than the corresponding power numbers with vertical mounting on the centreline of a baffled tank.La suspension de solides en utilisant un agitateuràécoulement axial monté en angle dans une cuve non cloisonnée repose sur le positionnement approprié de l'agitateur dans la cuve afin d'éviter unécoulement en vortex. Même avec un positionnement correct, la vitesse juste suspendue d'un agitateur monté en angle est souvent soixante-quinze pour cent supérieureà celle obtenue avec un montage vertical sur l'axe central d'une cuve cloisonnée, ce qui amène des exigences de couple et de puissance qui sont en moyenne plus de deux fois et cinq fois supérieures, respectivement. La puissance de turbulence avec montage en angle des agitateursàécoulement axial sont généralement similaires, mais inférieurs, aux puissances correspondantes pour le montage vertical sur l'axe central d'une cuve cloisonnée.
Data taken with six solids at numerous Zwietering loadings ranging from near zero to 67 have been used to determine the just‐suspended speed Zwietering loading exponent (Njs ∝ Xn where n is the Zwietering solids loading exponent). When only the loadings range similar to that studied by Zwietering [Zwietering, Chem. Eng. Sci. 1958, 8, 244] is considered (0 < X < 18), the solids loading exponent averaged over all solids is equal to 0.12, essentially the same as the 0.13 reported by Zwietering. However, when the entire loading range is considered (0 < X ≤ 67), a higher average exponent of 0.17 is found and a single power‐law correlation does not accurately describe the experimental data. A piecewise fit of the data indicates that the solids loading exponent increases from an average value of 0.097 at low solids loadings (0 < X ≤ 5) to 0.22 at intermediate loadings (5 ≤ X ≤ 25) and 0.34 for the highest loadings (25 ≤ X ≤ 67).
A thorough review of the major parameters that affect solid-liquid slurry wear on impellers and techniques for minimizing wear is presented. These major parameters include (i) chemical environment, (ii) hardness of solids, (iii) density of solids, (iv) percent solids, (v) shape of solids, (vi) fluid regime (turbulent, transitional, or laminar), (vii) hardness of the mixer's wetted parts, (viii) hydraulic efficiency of the impeller (kinetic energy dissipation rates near the impeller blades), (ix) impact velocity, and (x) impact frequency. Techniques for minimizing the wear on impellers cover the choice of impeller, size and speed of the impeller, alloy selection, and surface coating or coverings. An example is provided as well as an assessment of the approximate life improvement.
With the impeller placed low in the tank (C/T ¼ 1/3), turbulent blend times produced by radial-flow and down-pumping axial-flow impellers generally increase slowly with increasing liquid level in shorter batches (Z/T < 1), but increase dramatically in taller batches (Z/T > 1). The turbulent blend times produced by up-pumping axial-flow impellers increase slowly with increasing liquid level across the entire spectrum of liquid levels that were studied (up to Z/T ¼ 1.75). This can lead to down-pumping blend times that are twice as long as those with up-pumping operation. These differences can be explained by differences in the velocity field in the agitated vessel. Further, the down-pumping mode can produce blend times in tall tanks that are comparable to those of the up-pumping mode if the down-pumping impeller is placed high in the batch such that its discharge flow is directed into the bulk of the liquid.
The effects of impeller blade width and blade number on the gas dispersion behaviour of disc impellers with semicircular blades have been characterised. Increasing blade width or number increases the ungassed turbulent power number and reduces the drop in power draw upon gassing. While speed and power required for gas dispersion decrease with increasing blade width and number, the dispersion torque is relatively independent of these impeller geometric characteristics. A three-blade impeller exhibits the worst gas dispersion characteristics, while a four-blade impeller with non-uniform blade spacing outperforms a four-blade impeller with uniform blade spacing. Les effets de la largeur des pales d'un agitateur et du nombre de pales sur le comportement de dispersion gazeuse des agitateursà disque avec pales semicirculaires ontété caracterisés. L'augmentation de la largeur et du nombre de pales augmente la puissance des turbulences non gazeuses et réduit la chute de prélèvement de courant lors du gazage. Bien que la vitesse et la puissance requises pour la dispersion gazeuse diminuent avec l'augmentation de la largeur et du nombre de pales, le couple de dispersion est relativement indépendant de ces caractéristiques géometriques de l'agitateur. Un agitateurà trois pales démontre les plus mauvaises caractéristiques de dispersion gazeuse alors qu'un agitateurà quatre pales avec espacement non uniforme des pales produit de meilleurs résultats qu'un agitateurà quatre pales avec espacement uniforme des pales.
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