Impeller dimensions

Equation (9.99) shows the total pressure increase as a function of volume flow for given impeller dimensions and rotational speed at fixed U j. The blade angle 2 the exit is taken as parameter. This function is a linear one. When /B, > 90°, increases for 2 0°, p ot.s remains constant and for... 

The recommended ranges for impeller dimensions are given in Table 5.4-27. A high-efficiency turbine or four-bladed flat turbine with D/Dr as advised in Table 5.4-27 and C/D., equal to 0.75 seem to be the most versatile agitators in reactors for multiproduct plant. 

Here, k is the fluid consistency index and m the flow behavior index. While no data for a gassed system under laminar and transition flow were reported, Pg is expected to depend on impeller dimensions in these regimes. 

The affinity laws are valid only for compressors where the Mach number (defined in terms of the exit velocity from the stage) is less than 0.8, or, under vacuum conditions, the mean free path does not approach the impeller dimensions. Outside these limits, no simple correlations with speed exist, and the full characteristics must be stored rather than a single curve for each section. Fortunately, most industrial compressors will stay within these limits. 

Geometrical Features of the Reactor/Impeller (Dimensions and Geometric Configuration as per Section 7A.10 and Figure 9.9, Respectively)... 

VESSEL AND IMPELLER DIMENSIONS, 435 AGITATION AND AERATION CONDITIONS, 435 K a DETERMINATION, 435... 

For now, the characteristic length scale Lc is assumed to scale with the impeller diameter, not the tank diameter. If geometric similarity is observed and all impeller dimensions are scaled with the impeller diameter (including details such as blade thickness), the characteristic length scale (CpD) will scale any of the impeller dimensions equally well only Cl will change. The constants and Cl are a function of the impeller and tank geometry selected. For now, however, we retain them as constants. 

Vortex Depth In an unbaffled vessel with an impeller rotating in the center, centrifugal force acting on the fluid raises the fluid level at the wall and lowers the level at the shaft. The depth and shape of such a vortex (Rieger, Ditl, and Novak, Chem. Eng. ScL, 34, 397 (1978)] depend on impeller and vessel dimensions as well as rotational speed. 

Mass-transfer coefficients seem to vary as the 0.7 exponent on the power input per unit volume, with the dimensions of the vessel and impeller and the superficial gas velocity as additional factors. A survey of such correlations is made by van t Riet (Ind Eng. Chem Proc Des Dev., IS, 3.57 [1979]). Table 23-12 shows some of the results. 

When equipment receives impulses at its own natural frequency of vibration, excessive vibration (resonance) occurs, and this can lead to rapid failure. A control valve was fitted with a new spindle with slightly different dimensions. This changed its natural frequency of vibration to that of the impulses of the liquid passing through it (the frequency of rotation of the pump times the number of passages in the impeller). The spindle failed after three months. Even a small change in the size of spindle is a modification [24]. 

This available value of NPSHa (of the system) must always be greater b) a minimum of two feet and preferably three or more feet than the required NPSH stated by the pump manufacturer or shown on the pump curves in order to overcome the pump s internal hydraulic loss and the point of lowest pressure in the eye of the impeller. The NPSH required by the pump is a function of the physical dimensions of casing, speed, specific speed, and type of impeller, and must be satisfied for proper pump performance. The pump manufacturer must ahvays be given complete Suction conditions if he is to be expected to recommend a pump to give long and trouble-free service. 

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). 

Power is the external measure of the mixer performance. The power put into the system must be absorbed through friction in viscous and turbulent shear stresses and dissipated as heat The power requirement of a system is a function of the impeller shape, size, speed of rotation, fluid density and viscosity, vessel dimensions and internal attachments, and posidon of the impeller in this enclosed system. 

Figure 5-17. Power consumption by impeller type/dimensions for turbulent flow conditions. Knowing impeller type, diameter, speed and batch density connect RPM with diameter. The Intersection with A, connected to the density scale, makes an intersection on B. A line from this point to the impeller scale intersects the horsepower scale at the correct value. By permission, Quillen, C. S., Chem. Engr., June 1954, p. 177 [15].

Geometric similarity requires all corresponding dimensions of a new system to have the same ratio with a test model which has proven acceptable. These dimensions should include vessel diameter and liquid level, baffle width and number in vessel, impeller diameter, number of blades and width ratio. For example, a tank four times the diameter of the original model also requires a turbine ten times the diameter of the original turbine. 

Physical depth or height of turbine mixer, ft or in., consistent with other dimensions. Figure 5-34 = Impeller blade wndtii, ft = width of baffles in vertical tank. Figure 5-34. 

Term (4) is effectively a Reynolds number with the velocity the peripheral speed and the characteristic dimension being usually the maximum impeller diameter. 

A bacterial fermentation was carried out in a reactor containing broth with average density p = 1200kg/m3 and viscosity 0.02N-s/m2. The broth was agitated at 90rpm and air was introduced through the sparger at a flow rate of 0.4 vvm. The fermenter was equipped with two sets of flat blade turbine impellers and four baffles. The dimensions of vessel, impellers and baffle width were ... 

These galvanic corrosion processes take place when one or more elemental constituents of an alloy is leached, often leaving a weak, porous structure, although the component dimensions often are unchanged. Dealloying particularly affects equipment constructed of cupronickels, bronzes, brasses, and gunmetal, such as FW heaters, strainers, valves, and pump impellers. 

The area for flow is however, A = (constant) L1, where L is the characteristic linear dimension of the system. In mixing applications, L is usually chosen as the impeller diameter D, and, likewise, the representative velocity u is taken to be the velocity at tire tip of impeller (ttDN), where N is revolutions per unit time. Therefore, the expression for inertial force may be written as ... 

Considering a stirred vessel in which a Newtonian liquid of viscosity p, and density p is agitated by an impeller of diameter D rotating at a speed N the tank diameter is DT, and the other dimensions are as shown in Figure 7.5, then, the functional dependence of the power input to the liquid P on the independent variables (fx, p, N, D, DT, g, other geometric dimensions) may be expressed as ... 

The output from a pump is a function of its linear dimensions, the shape, number, and arrangement of the impellers, the speed of rotation, and the head against which it is operating. From equation 8.12, for a radial pump with ft — nil and sin ft = 1 ... 

It is common practice to use geometric similarity in the scaleup of stirred tanks (but not tubular reactors). This means that the production-scale reactor will have the same shape as the pilot-scale reactor. All linear dimensions such as reactor diameter, impeller diameter, and liquid height will change by the same factor, Surface areas will scale as Now, what happens to tmix upon scaleup ... 

For reactors with free turbulent flow without dominant boundary layer flows or gas/hquid interfaces (due to rising gas bubbles) such as stirred reactors with bafQes, all used model particle systems and also many biological systems produce similar results, and it may therefore be assumed that these results are also applicable to other particle systems. For stirred tanks in particular, the stress produced by impellers of various types can be predicted with the aid of a geometrical function (Eq. (20)) derived from the results of the measurements. Impellers with a large blade area in relation to the tank dimensions produce less shear, because of their uniform power input, in contrast to small and especially axial-flow impellers, such as propellers, and all kinds of inclined-blade impellers. 

---