Helical ribbon screw impeller

Tecante a., Choplin L., Gas-Liquid Mass Tranffler in Non-Newtonian Fluids in a Tank with a Helical Ribbon Screw Impeller, Can. J. Chem. Engng. 71 (1993) 12, p. 859-865... 

Tecante, A., and Choplin, L. (1993), Gas-liquid mass transfer in non-Newtonian fluids in a tank stirred with a helical ribbon screw impeller, Canadian Journal of Chemical Engineering, 71(6) 859-865. 

Tanguy, P. A., R. Lacroix, F. Bertrand, L. Choplin and E. B. Delafuente, Finite Element Analysis of Viscous Mixing with a Helical Ribbon-Screw Impeller, A.I.CkE. Journal, 38, 939-944 (1992). 

OXYGEN TRANSFER IN NON-NEWTONIAN FLUIDS STIRRED WITH A HELICAL RIBBON SCREW IMPELLER... 

Helical Ribbon Screw impeller (Helical Ribbon Pumping Up at Wall with Screw Pumping Down in Center)... 

The most commonly used impeller for laminar blending applications is the helical ribbon. Other impeller types have been studied, including anchors and helical screws, but the helical ribbon is most effective. A helical ribbon impeller will have a large diameter, typically 90 to 95% of the vessel diameter. This ensures that the fluid is positively displaced by the ribbons. This is important because there is no mixing due to entrainment by eddies in the laminar regime. 

For laminar mixing other impellers are used, some laminar impellers are sketched in Fig. 7.4. To bring the fluid in the entire tank in motion, the diameter of these impellers usually approach the tank diameter since the laminar bulk flow is otherwise relatively low. Laminar impellers with diameter approaching the tank diameter are also called close-clearance impellers. Laminar mixers often have complex geometries, characterized by geometrical variables as the impeller diameter D, the blade width W, the pitch p, the impeller wall clearance C, and the off-bottom clearance Cb- In most applications, baffles are not needed and can in fact cause poor mixing behavior [87], Examples of laminar impellers are helical ribbons, screws, helical ribbon screws and anchor impellers. 

Figure 7.20 shows some of the impellers which are frequently used. Propellers, turbines, paddles, anchors, helical ribbons and screws are usually mounted on a central vertical shaft in a cylindrical tank, and they are selected for a particular duty largely on the basis of liquid viscosity. By and large, it is necessary to move from a propeller to a turbine and then, in order, to a paddle, to an anchor and then to a helical ribbon and finally to a screw as the viscosity of the fluids to be mixed increases. In so doing the speed of agitation or rotation decreases. 

Figure 7,20. Commonly used impellers (a) Three-bladed propeller ( >) Six-bladed disc turbine (Rushton turbine) (c) Simple paddle (d) Anchor impeller (e) Helical ribbon (/) Helical screw with draft tubs...

A vertical helical ribbon blender can be combined with an axial screw of smaller diameter (Fig. 18-25). Such mixers are used in polymerization reactions in which uniform blending is required but in which high-shear dispersion is not a factor. Addition of the inner flight contributes little more turnover in mixing newtonian fluids but significantly shortens the mixing time in nonnewtonian systems and adds negligibly to the impeller power [Coyle etal.. Am. Inst Chem. Eng.J., 15, 903 (1970)]. 

Impeller power requirement, ft Ibf/s or HP Pitch of a propeller or a helical ribbon impeller (forward motion of the impeller in one rotation when moving in an internal screw thread)... 

For high viscosity liquids, anchor impellers with a small wall clearance are often used (see Figure 7.3d). However, such systems do not promote strong top-to-bottom circulation and this drawback can be overcome by the use of helical ribbons, Figure 7.3e). Similarly, the helical screw, with a smaller diameter, often fitted into a draft tube can promote strong axial flow with viscous materials Figure 7.3f). 

The flow in an anchor agitated vessel has been studied in detail by Peters and Smith . The impeller promotes fluid motion close to the vessel wall but the region near the shaft may be relatively stagnant. In addition, there is little top-to-bottom turnover. In order to promote a top-to-bottom motion a helical ribbon may be used and often a screw is added to the shaft to obtain motion in the central regions of the vessel. This combined impeller system would have a ribbon pumping upward near the wall of the vesel with the screw, twisted in the opposite sense, pumping downwards near the shaft. 

Figme 8.24 shows some of the impellers which are frequently used. Propellers, turbines, paddles, anchors, helical ribbons and screws are usually mounted on a... 

It has to be mentionned that a two-dimensional flow model has been developped by Hirose and Murakami to calculate the power consumption in vessels equipped with paddle impeller, anchor, gate, helical ribbon or helical screw [5]. This approach is interesting because a single unified correlation is able to cover both straight and helical blades, but it suffers from an unavoidable complexity. 

Pumping numbers for the helical ribbon and screw impellers are available in the literature. They range from 0.04 to 0.5 and are highly dependent on the geometry. The anchor impeller only pumps along its radial arms, and pumping numbers are not readily available for this impeller. 

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