Impellers fluid characteristics

Process design Fluid mechanics of impellers Fluid regime required by process Scaleup hydraulic similarity Impeller power characteristics... 

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. 

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. 

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. 

The action of the impeller design produces flow of the fluid, head on the fluid, or shear in the fluid, all to varying degrees depending on the specific design. A general identification of these characteristics for several types of impellers is given by [27]. (Note Use consistent dimensions). 

The application of draft tubes as related to various mixing operations is showm in Figures 5-231 and 5-24A-5-241. The draft tubes are basically a tube or shell around the shaft of the mixer including the usual axial impeller, which allows a special or top-to-bottom fixed flow pattern to be set up in the fluid system. The size and location of the tube are related to both the mechanical and mixing performance characteristics as well as peculiar problems of the system. Usually they are used to ensure a mixing flow pattern that cannot or w ill not develop in the system. Weber gives the followdng points for draft tubes [23] ... 

The general operation of a centrifugal compressor is like a centrifugal pump, except that the fluid is compressible. Theoretically, the head developed by a centrifugal impeller or wheel is the same regardless of the characteristics of the... 

You want to control the flow rate of a liquid in a transfer line at 350 gpm. The pump in the line has the characteristics shown in Fig. 8-2, with an 5 in. impeller. The line contains 150 ft of 3 in. sch 40 pipe, 10 flanged elbows, four gate valves, and a 3 x 3 control valve. The pressure and elevation at the entrance and exit of the line are the same. The valve has an equal percentage trim with the characteristics given in Table 10-3. What should the valve opening be to achieve the desired flow rate (in terms of percent of total stem travel) The fluid has a viscosity of 5cP and a SG of 0.85. 

Propeller A propeller is a three- or four-bladed flow impeller, having helically shaped blades. The flow is primarily axial (discharge flow parallel to the agitator shaft) and is most effective in low-viscosity fluids. The marine-type propeller (Figure 3.15) is characteristically operated at relatively high speed, particularly in low-viscosity liquids (Treybal, 1980). 

The first requirement is obvious and easy to accomplish, but the second is difficult to understand and also to accomplish and needs explanation. For example, if forces that may act on a fluid element in a fermenter during agitation are the viscosity force Fv, drag force on impeller F0, and gravity force FG, each can be expressed with characteristic quantities associated with the agitating system. According to Newton s equation of viscosity, viscosity force is... 

When liquids are being pumped, it is important to keep the pressure in the suction line above the vapor pressure of the fluid. The available head measured at the pump suction is called the net positive suction head available (NPSHA). At sea level, pumping 15°C (60°F) water with the impeller about 1 m below the surface, the NPSHA is about 9.1 m (30 ft). It increases with barometric pressure or with static head, and decreases as vapor pressure, friction, or entrance losses rise. Available NPSHA is the characteristic of the process and represents the difference between the existing absolute suction head and the vapor pressure at the process temperature. The required net positive suction head required (NPSHR), on the other hand, is a function of the pump design (Figure 2.121). It represents the minimum margin between suction head and vapor pressure at a particular capacity that is required for pump operation. Cavitation can occur at suction pres-... 

During the mixing of a single-phase liquid in a stirred tank, the important parameters are the rate of power input, P, the impeller stirring speed, N, gravitational acceleration, g, the fluid density, p and viscosity, p, the impeller diameter, D and the other geometric characteristic lengths of the vessel (i.e. baffle width and depth, impeller shape,... 

Flow characteristics in a mixing vessel can influence process performance. The impeller is a device which imparts motion to the medium in which it operates. The characteristics of the flow which are of greatest interest are the mean fluid velocity at all points within the fluid and the turbulent fluctuations superimposed on the mean velocity. Paul and Treybal ( ) have discussed how the detailed flow characteristics can influence process performance. This paper will show how impeller style can influence the flow characteristics. 

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