Impellers open-type turbines

Open-type turbines This kind of turbines is also called full-blade turbines (Figure 3.17). The blades are vertical (parallel to the axis of the drive shaft) and could be straight or curved. They are cost-effective impellers for operations veiy near the floor of a tank for agitating the... 

Milk-of-lime transfer pumps should be of the open impeller centrifugal type. Pumps having an iron body and impeller with bronze trim are suitable for this purpose. Rubber-lined pumps with rubber-covered impellers are also frequently used. Makeup tanks are usually provided ahead of centrifugal pumps to ensure a flooded suction at all times. Plating out of lime is minimized by the use of soft water in the makeup tank and slurry recirculation. Turbine pumps and eductors should be avoided in transferring milk of lime because of scaling problems. 

Figure 7.10. Some types of impellers for centrifugal pumps, (a) Open impeller, (b) Semiopen impeller, (c) Shrouded impeller, (d) Axial flow (propeller) type, (e) Combined axial and radial flow, open type, (f) Shrouded mixed-flow impeller, (g) Shrouded impeller (P) in a case with diffuser vanes (V). (h) Turbine impeller.

A modern high efficiency gas-liquid STR commonly consists of a radial gas dispersing impeller in combination with one or more axial down-pumping impellers, as seen in Fig. 2. The type and number of open impellers used in the tank depend on the gassed liquid height and the media viscosity (see Table 4). Recommended axial impellers are 45° four-bladed pitch blade turbines... 

Examples of axial impellers are marine propeller and pitched blade turbine, shown in Fig 7.2. A three bladed marine t3rpe propeller is similar to the propeller blade used in driving boats. The propeller can be a side entering type in a tank or be clamped on the side of an open vessel in an off-center position. Axial flow impellers are used in blending and mixing of miscible liquids. 

Radial flow impellers may either have a disk (Rushton turbine) or be open (FBT) and may have either flat or curved blades (backswept mrbine). Impellers without the disk do not normally pump in a true radial direction since there is pressure difference between each side of the impeller. This is also true when the impellers are positioned in the tank at different off-bottom clearances. They can pump upward or downward while discharging radially. Radial discharge flow patterns can cause stratification or compartmentalization in the mixing tank. Disk-type radial impellers provide more uniform radial flow pattern and draw more power than open impellers. The disk is a baffle on the impeller, which prevents gas from rising along the mixer shaft. In addition, it allows the addition of a large number of impeller blades. Such blade addition cannot be done easily on a hub. A disk can also be used with a pitched blade turbine for use in gas-liquid mixing. 

Both open and disk flat-blade turbines are used extensively, particularly because of the high discharge velocities normal to the flow of gas which they maintain. Especially in the larger sizes, the disk type is preferred. They best specified with dJT — 0,25 to 0.4 and set off the bottom of the vessel a distance equal to the impeller diameter. In some cases, especially designed impellers can be used to induce the gas flow from the space above the liquid down into the agitated mass [81, 115]. 

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