Impellers flat-blade radial-flow

The 45° axial-flow turbine is more efficient at all Reynold s numbers than the flat-blade radial turbine. However, as the pitch angle decreases below 45°, the impeller becomes increasingly sensitive to high viscosity. 

The effect of impeller elevation C on the power number is small for turbine impellers, especially the radial flow impellers. Figure 6-16 shows this effect for pitched blade (PBT), flat-blade (FBT), and disk flat-blade (DFBT) turbines with... 

Radial-flow impellers include the flat-blade disc turbine, Fig. 18-4, which is labeled an RlOO. This generates a radial flow pattern at all Reynolds numbers. Figure 18-17 is the diagram of Reynolds num-ber/power number curve, which allows one to calculate the power knowing the speed and diameter of the impeller. The impeller shown in Fig. 18-4 typically gives high shear rates and relatively low pumping capacity. 

If a propeller is located quite close to the bottom of a tank, the flow becomes radial like that of the flat blade turbine. In a properly baffled system the propeller flow is axial. WTien dynamic similarity is accomplished, the systems are similar [21]. For a first approximation, placing the impeller at E of liquid height off the bottom is good. 

The most common type of agitator is turbine. It consists of several short blades mounted on a central shaft. The diameter of a turbine is normally 35 15% of the tank diameter. There are four to six blades for perfect mixing. Turbines with flat blades give radial flow. This is good for gas dispersion in the media, where the gas is introduced just below the impeller, is drawn up to the blades and broken up into uniform fine bubbles. 

The three basic types of impeller which are used at high Reynolds numbers (low viscosity) are shown in Figures 10.55a, b, c. They can be classified according to the predominant direction of flow leaving the impeller. The flat-bladed (Rushton) turbines are essentially radial-flow devices, suitable for processes controlled by turbulent mixing (shear controlled processes). The propeller and pitched-bladed turbines are essentially axial-flow devices, suitable for bulk fluid mixing. 

Figure 3.14 Left Axial-flow pattern (marine impeller). Right Radial-flow pattern (flat blade turbine impeller).

A large number of impeller types have been studied over the years, but interest has centered on three designs marine-type propellers, flat-or curve-bladed turbines, and flat paddles, with the first two of greater interest than the third. Propellers produce axial flow of the liquids and are turned in such fashion as to direct flow against the bottom of the tank. Turbines provide radial flow, but in any case the presence of baffles strongly influences the flow pattern in the tank. The effectiveness of these in liquid extraction has not been well established, but it appears that there... 

Figure 7.7b shows a two-flat blade paddle. If the flat blades are pitched, then the liquid flow pattern becomes intermediate between axial and radial flows. Many other types of impellers are used in stirred tanks, but these are not described at this point. 

The impellers used in this study are shown in Figure 1. They are both of the radially discharging type which is characterized by a strong radial jet of fluid that moves out to the vessel walls while entraining fluid from above and below. Near the wall it splits into two circulation zones. The flat blade turbine (FBT) is commonly used in industry and consists of a disc with several paddles fixed normal to the disc which serve to generate the radial flow. The novel impeller design... 

Radial-Flow Impellers Radial-flow impellers have blades which are parallel to the axis of the drive shaft. The smaller multiblade ones are known as turbines larger, slower-speed impellers, with two or four blades, are often called paMes. The diameter of a turbine is normally between 0.3 and 0.6 of the tank diameter. Turbine impellers come in a variety of types, such as curved-blade and flat-blade, as illustrated in Fig. 18-4. Curved blades aid in starting an impeller in settled sohds. 

To disperse gas, the gas is usually injected into the liquid from the bottom of the tank or near the impeller to enhance dispersion. Disk style turbines are found to be most convenient for gas dispersion because the disk disturbs the freely rising gas bubbles. The turbines with flat blades give radial flow and are very useful for gas dispersion where the gas is introduced just below the impeller at its axis and drawn up to the blades and chopped into fine bubbles. 

Three types of impellers are commonly used in the low viscosity region, propellers, Fig. 1 turbines. Fig. 2 and axial flow turbines. Fig. 3. Impellers used on small portable mixers shown in Fig. 4, are oflen inclined at an angle as well as being off-center to give a good top-to-bottom flow pattern in the system. Fig. 5. Large top-entering drives usually use either the axial flow turbine or the radial flow flat blade turbine. For aerobic fermentation, the radial flow disc turbine is most common and is illustrated in Fig. 6. 

Figure 20 illustrates flow pattern in the laminar flow region from a radial flat blade turbine. By using a velocity probe, the parabolic velocity distribution coming off the blades of the impeller is shown in Fig. 21. By taking the slope of the curve at any point, the shear rate may be calculated at that point. The maximum shear rate around the impeller periphery as well as the average shear rate around the impeller may also be calculated.

Impellers available in the pre-1960 era would have been limited to four- and six-blade disc turbines (also known as radial-flow turbines or RFT or Rushton turbines), the four- and six-blade 45° pitch blade turbines (PBT), the four- and six-blade flat-blade turbines (FBT), and the three-blade retreat-curve impellers (RCI). 

FLOW NUMBER. A turbine or propeller agitator is, in essence, a pump impeller operating without a casing and with undirected inlet and output flows. The governing relations for turbines are similar to those for centrifugal pumps discussed in Chap. 8. Consider the flat-bladed turbine impeller shown in Fig. 9.8. The nomenclature is the same as in Fig. 8.11 2 is Ihe velocity of the blade tips V 2 and F 2 are the actual tangential and radial velocities of the liquid leaving the blade tips, respectively and V 2 is the total liquid velocity at the same point. 

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