Impellers action

An impeller in a tank functions as a pump that delivers a certain volumetric rate at each rotational speed and corresponding power input. The power input is influenced also by the geometry of the equipment and the properties of the fluid. The flow pattern and the degree of turbulence are key aspects of the quality of mixing. Basic impeller actions are either axial or radial, but, as Figure 10.4 shows, radial action results in some axial movement by reason of deflection from the vessel walls and baffles. Baffles contribute to turbulence by preventing swirl of the contents as a whole and elimination of vortexes offset location of the impeller has similar effects but on a reduced scale. 

The agitator in each stage is provided with two impellers. The impeller that is located at the liquid surface causes surface aeration. It creates turbulence at the liquid surface and generates liquid flow in the downward direction. Because of this impeller action, the oxygen from the gas space is dispersed... 

Centrifugal Pumps. These have been used as mixers in extraction work, although their field of applicability is small. In the ordinary arrangement, the two liquids to be mixed are fed to the suction side of the pump, and the action of the impeller provides the dispersion at the same time the liquids are pumped to the settler which follows. If the interfacial tension and density difference are low, the violence of the impeller action is quite likely to produce an emulsion, while if the density difference is large, stratification... 

As the mechanical integrity of the pump system changes, the amplitude of vibration levels change. In some cases, in order to identify the source of vibration, pump speed may have to be varied, as these problems are frequency- or resonance-dependent. Pump impeller imbalance and cavitation are related to this category. Table 10-11 classifies different types of pump-related problems, their possible causes and corrective actions. 

To produce foam in batch processes, mechanical agitators are used almost exclusively. The gas can either be introduced through the free surface by the entraining action of the impeller or alternatively sparged beneath the impeller. In such batch operation, the liquid... 

Mechanical agitation is needed to break up the gas bubbles but must avoid rupturing the cells. The disk turbine with radial action is most suitable. It can tolerate a superficial gas velocity up to 120 m/h. (394 ft/h) without flooding, whereas the propeller is limited to about 20 i7i/h (66 ft/h). When flooding occurs, the impeller is working in a gas phase and cannot assist the transfer of gas to the liquid phase. Power input by agitation and air sparger is 1 to 4 W/L (97 to 387 Btu/[fF-h]) of liquid. 

When the layer of graphite and corrosion products is impervious to the solution, corrosion wdl cease or slow down. If the layer is porous, corrosion will progress by galvanic behavior between graphite and iron. The rate of this attack will be approximately that for the maximum penetration of steel by pitting. The layer of graphite formed may also be effective in reducing the g vanic action between cast iron and more noble alloys such as bronze used for valve trim and impellers in pumps. 

This type of impeller would be used in a somewhat conventional appearing pump to perform a chopping, grinding, or macerating action... 

Axial flow, in which the liquid enters the impeller and discharges along a parallel path to the axis, is shown in Figure 9. The radial and longitudinal components are primarily responsible for the derived mixing action. The tangential component is important when the shaft has a vertical orientation and is positioned near the center of the tank. 

Do not confuse NPSH vdth suction head, as suction head refers to pressure above atmospheric [17]. If this consideration of NPSH is ignored the pump may well be inoperative in the system, or it may be on the border-line and become troublesome or cavitating. The significance of NPSH is to ensure sufficient head of liquid at the entrance of the pump impeller to overcome the internal flow losses of the pump. This allows the pump impeller to operate wfith a full bite of liquid essentially free of flashing bubbles of vapor due to boiling action of the fluid. 

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