Helical Impeller design

Close-Clearance Stirrers For some pseiidoplastic fluid systems stagnant fluid may be found next to the -essel walls in parts remote from propeller or turbine impellers. In such cases, an anchor impeller maybe used (Fig, 18-6), The fluid flow is principally circular or helical (see Fig, 18-7) in the direction of rotation of the anchor. Whether substantial axial or radial fluid motion also occurs depends on the fluid iscosity and the design of the upper blade-supporting spokes. Anchor agitators are used particularly to obtain irnpro ed heat transfer in high-consistency fluids,... 

The bullgear design uses a direct-driven helical gear to transmit power from the primary driver to a series of pinion-gear-driven impellers that are located around the circumference of the bullgear. Figure 36.4 illustrates a typical bullgear compressor layout. 

Research on the impeller method using the helical ribbon impeller is well documented (7,8). The impeller method is often employed to measure the rheology of suspensions. Previous researchers assumed that the effective shear rate of such a device is related to the impeller speed by a fluid-independent constant, but this assumption may not be accurate for all impellers (8,9). It has been suggested that a properly designed helical ribbon impeller might be more appropriate for this technique. 

Proximity and nonproximity impellers are the two major designs used in mixing applications. Proximity relates to distance from the vessel wall. Figure 9.50(a) shows a nonproximity impeller typically used for turbulent conditions. Its blades are not close to the vessel wall. Close proximity agitators like anchors and helical ribbons, illustrated in Figures 9.50(b, c), are typically used for high-viscosity applications. 

These impellers are designed to physically turnover the fluids because viscous fluids are difficult to pump. These impellers are typically large in size, nearly the same size as the tank diameter, and provide gentle macroscale blending of liquids at low shear. The most common designs are the anchor and the helical ribbon, shown in Figure 6-31. 

Helical ribbon impellers provide top-to-bottom physical movement of the liquid. In addition to one outer helix, they can be designed with an inner helix pumping in the opposite direction. This is particularly needed for direct-action mixing for high viscosity materials. These impellers can also have two outer helixes. The most commonly used pitch for the helixes is 0.5. A higher pitch... 

Example 14.2 Helical Ribbon Impeller. Determine the process-side heat transfer coefficient for the tank blending design example for a helical ribbon impeller (Bakker and Gates, 1995) ... 

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