Impellers characteristics

The rotative speed of a pump is dependent upon the impeller characteristics, type fluid, NPSH available and other factors for its final determination. The most direct method is by reference to manufacturer s performance curves. VVTien a seemingly reasonable selection has been made, the effect of this selected speed on the factors such as NPSH required, suction head or lift, fluid erosion and corrosion, etc., must be evaluated. For many systems these factors are of no concern or consequence. 

Tbe intersection of tbe system curt e with the pump impeller characteristic curve is the operating point corresponding to the total head, H. This point will change only if the external system changes. This maybe accomplished by adding resistance by partially clos-... 

The last of these methods has been applied particularly to chemical reaction vessels. It is covered in detail in Chapter 17. In most cases, however, the RTDs have not been correlated with impeller characteristics or other mixing parameters. Largely this also is true of most mixing investigations, but Figure 10.3 is an uncommon example of correlation of blend time in terms of Reynolds number for the popular pitched blade turbine impeller. As expected, the blend time levels off beyond a certain mixing intensity, in this case beyond Reynolds numbers of 30,000 or so. The acid-base indicator technique was used. Other details of the test work and the scatter of the data are not revealed in the published information. Another practical solution of the problem is typified by Table 10.1 which relates blend time to power input to... 

For highly turbulent flow (i.e., constant Kfg), the required pump head is a quadratic function of the flow rate Q. This relation, which is superimposed on the pump characteristic curves (see line SI in Figure 5.9), is the operating line for the system. The actual pump head and the resulting flow rate are determined by the intersection of the operating line and the pump impeller characteristic curve. For the specified flow rate, the best (least cost) pump/impeller/motor combination that will provide this flow rate should be selected. 

To design an effective stirred tank, an efficient impeller should be chosen for the process duty. More than one impeller may be needed for tanks with high aspect ratio (Z/T > 1.5). Sizing of the impeller is done in conjunction with mixer speed to achieve the desired process result. The appropriate size and type of wall baffles must be selected to create an effective flow pattern. The mixer power is then estimated from available data on impeller characteristics, and the drive size is determined. The mixer design is finalized with mechanical design of the shaft, impeller blade thickness, baffle thickness and supports, inlet/outlet nozzles, bearings, seals, gearbox, and support structures. 

Correlations of nucleation rates with crystallizer variables have been developed for a variety of systems. Although the correlations are empirical, a mechanistic hypothesis regarding nucleation can be helpful in selecting operating variables for inclusion in the model. Two examples are (/) the effect of slurry circulation rate on nucleation has been used to develop a correlation for nucleation rate based on the tip speed of the impeller (16) and (2) the scaleup of nucleation kinetics for sodium chloride crystalliza tion provided an analysis of the role of mixing and mixer characteristics in contact nucleation (17). Pubhshed kinetic correlations have been reviewed through about 1979 (18). In a later section on population balances, simple power-law expressions are used to correlate nucleation rate data and describe the effect of nucleation on crystal size distribution. 

Practically, the NPSH required for operation without cavitation and vibration in the pump is somewhat greater than the theoretical. The actual (NPSH)r depends on the characteristics of the liquid, the total head, the pump speed, the capacity, and impeller design. Any suction condition which reduces (NPSH ) below that required to prevent cavitation at the desired capacity will produce an unsatisfactoiy installation and can lead to mechanical dimculty. 

The above covers most conventional mixers there is another class of mixers, called pump-mix impellers, where the impeller serves not only to mix the fluids, but also to move the fluids through the extraction stages. These are speciahzed designs, often used in the metals extraction industries. For these types of impellers, a knowledge of the power characteristics for pumping is required in addition to that for mixing. For a more detailed treatment of these special cases, the reader is referred to Lo et al. 

In order to make the pilot unit more like a commercial unit in macro-scale characteristics, the pilot unit impeller must be designed... 

Not only is the type of flow related to the impeller Reynolds number, but also such process performance characteristics as mixing time, impeller pumping rate, impeller power consumption, and heat- and mass-transfer coefficients can be correlated with this dimensionless group. 

J. Y. Oldshiie, T. A. Post, R. J. Weetman, Comparison of Mass Transfer Characteristics of Radial and Axial Flow Impellers, BHRA Proc. 6th European Conf. on Mixing, 5/88. 

It is the energy in the liquid rec]uired to overcome the friction los.ses from the suction nozzle to the eye of the impeller without causing vaporization. It is a characteristic of the pump and is indicated on the pump s curve. It varies by design, size, and the operating conditions. It is determined by a lift test, producing a negative pressure in inches of mercury and converted into feet of required NPSH. 

Boyce, M.P., A Practical Three-Dimensional Flow Visualization Approach to the Complex Flow Characteristics in a Centrifugal Impeller, ASME Paper No. 66-GT-83, June 1983. 

Rodgers, C., Influence of Impeller and Diffuser Characteristic and Matching on Radial Compressor Performance, SAE Preprint 268B, January 1961. 

The forces applied by an impeller to the material contained in a vessel produce characteristic flow patterns that depend on the Impeller geometry, properties of the fluid, and the relative sizes and proportions of the tank, baffles and impeller. There are three principal types of flow patterns tangential, radial and axial. Tangential flow is observed when the liquid flows parallel to the path described by the mixer as illustrated in Figure 7. 

Figure 7-15 shows plots of Pumping number Nq and Power number Np as functions of Reynolds number Np for a pitched-blade turbine and high-efficiency impeller. Hicks et al. [8] further introduced the scale of agitation, S, as a measure for determining agitation intensity in pitched-blade impellers. The scale of agitation is based on a characteristic velocity, v, defined by... 

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