Fluidfoil impellers

To complete the current picture, when composite materials are used, the airfoil can be shaped in any way that is desirable. The A6000 (Fig. 9e) illustrates that particular impeller type. The use of proplets on the end of the blades increases flow about 10% over not having them. An impeller which is able to operate effectively in both the turbulent and transitional Reynolds numbers is the A410 (Fig. 9f) which has a very marked increase in twist angle ofthe blade. This gives it a more effective performance in the higher viscosity fluids encountered in mixers up to about 3 kW. 

One characteristic of these fluidfoil impellers is that they discharge a stream that is almost completely axial flow and they have a very uniform velocity across the discharge plane of the impeller. However, there is a tendency for these impellers to short-circuit the fluid to a relatively low distance above the impeller. Very careful consideration of the coverage over the impeller is important. If the impeller can be placed one to two impeller diameters offbottom, which means that mixing is not provided at low levels during draw off, these impellers offer an excellent flow pattern as well as considerable economies in shaft design. 

The fluidfoil impellers are often called high efficiency impellers , but that is true only in terms of flow, and makes the assumption that flow is the main measure of mixing results. Flow is one measure, and in at least half of the mixing applications is a good measure of the performance fliat could be expected in aprocess. These impellers are low in efficiency in providing shear rates—either of the macro scale or the micro scale. 

The use of computer generated solutions to problems and computational fluid dynamics is also another approach of comparing impellers and process results. There are software packages available. It is very helpful to have data obtained from a laser velocity meter on the fluid mechanics of the impeller flow and other characteristics to put in the boundary conditions for these computer programs. 

As an example of other types of programs that can be worked on, Fig. 13 shows a velocity profile from an A410 impeller, Fig. 14, a map of the kinetic energy dissipation in the fluid stream and in the third one (Fig. 15) model of heavier than the liquid particles in a random tracking pattern. 

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Top-Entering Impellers For vessels less than 1.8 m (6 ft) in diameter, a clamp- or flange-mounted, angular, off-center fluidfoil impeller with no baffles should be the initial choice for meeting a wide range of process requirements (Fig. 18-14). The vessel straight-side-height-to-diameter ratio should be 0.75 to 1.5, and the volume of stirred liqiiid should not exceed 4 m (about 1000 gal). 

Axial-Flow Fluidfoil Impellers For vessel volumes of 4 to 200 m (1000 to 50,000 gal), a turbine mixer mounted coaxiaUy within the vessel with four or more baffles should be the initial choice. Here also the vessel straight-side-height-to-diameter ratio should be 0.75 to 1.5. Four vertical baffles should be fastened perpendicularly to the vessel wall with a gap between baffle and wall equal to Df/24 and a radial baffle width equal to Df/12. 

The fluidfoil impellers in large tanks require only two baffles, but three are usually used to provide better flow pattern asymmetiy. These fluidfoil impellers provide a true axial flow pattern, almost as though there was a draft tube around the impeller. Two or three or more impellers are used if tanks with high D/T ratios are involved. The fluidfoil impellers do not vortex vigorously even at relatively low coverage so that if gases or solids are to Be incorporated at the surface, the axial-flow turbine is often required and can be used in combination with the fluidfoil impellers also on the same shaft. 

Data are not currently available on the dispersion with the newer fluidfoil impellers, but they are often used in industrial mixer-settler systems to maintain dispersion when additional resonance time holdup is required, after an initial dispersion is made by a radial- or axial-flow turbine. 

The fluidfoil impellers (shown in Fig. 18-2) usually give more flow for a given power level than the traditional axial- or radial-flow turbines. This is also thought to be an advantage since the heat-transfer surface itself generates the turbulence to provide the film coefficient and more flow should be helpful. This is true to a limited degree in jacketed tanks (Fig. 18-34), but in helical coils (Fig. 18-35), the... 

Fluidfoil impellers Axial flow impellers in which the blade shape and profile is patterned after airfoil concepts. The blade normally has camber and has a twist in toward the shaft with a rounded leading edge to pro-... 

There are now several varieties of fluidfoil impellers in use. The A310 is an effective impeller for the low viscosity region and has a negative response to viscosity at a Reynolds number of approximately 600. As shown in Fig. 3, the angle that the flow stream makes with the vertical starts to become greater than with the A200 impeller, so we can say effectively that the Reynolds number limitation on the A310 is approximately 200. 

In order to carry this concept of fluidfoil impellers at a uniform velocity of discharge further, the A312 Impeller (Fig. 4) was developed and is used primarily in paper pulp suspensions. Carrying it further is the A320 Impeller (Fig. 5). The A320 has been studied particularly in the transitional area of traditional Reynolds numbers. This is shown in Fig. 6. This figure shows its performance and Reynolds numbers between 10 and 1,000. 

The fluidfoil impeller, shown in Fig. lc, is often designed to have about the same total pumping capacity as the axial flow turbine (Fig. la). However, the flow patterns are somewhat different. The fluidfoil impeller has an axial discharge, while the axial flow turbine discharge tends to deviate from axial flow by 20-45°. Nevertheless at the same total pumping capacity in the tank, the tank shear rates are approximately equal. However, the axial flow fluidfoil turbine requires between 50 and 75% of the power required by the axial flow turbine. This results in a... 

The lower horsepower is an important factor in the efficient design of axial flow or fluidfoil impellers. Such lower horsepower must be considered in the efficient design involving fluid velocity and overall macroscale mixing phenomena. On the other hand, if the process involves a certain amount of microscale mixing, or certain amounts of shear rate, then the fluidfoil impeller may not be the best choice. 

The use of the new type of fluidfoil impeller has reduced the power required for solids suspension to about one-half to two-thirds of the values formerly used with 45° pitch blade turbines. 

When using a draft tube, the back flow possibility in the center of the impeller requires the use of a large-diameter hub. This is not normally desirable in fluidfoil impellers used in open tanks. The system head for a draft tube circulator is a function primarily of the design of the entrance and exit of the draft tube, and considerable work has been... 

Mass-transfer characteristics on large-scale equipment seem to be quite similar, but the fluidfoil impellers tend to release a larger-... 

Mass-transfer characteristics on large-scale equipment seem to be quite similar, but the fluidfoil impellers tend to release a larger-diameter bubble than is common with the radial-flow turbines. The blend time is one-half or one-third as long, and solid-suspension characteristics are better so that there have been notable improved process results with these impellers. This is particularly true if the process requires better blending and there is solid suspension. If this is not the case, the results from these impellers can be negative compared to radial-flow turbines. 

To look at these impellers in a different way, three impellers have been compared at equal total-pumping capacity. Figure 12 gives the output velocity as a function of time on a strip chart. As can be seen in Fig. 12 the fluidfoil impeller type (A310) has a very low velocity fluctuation and uses... 

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