Impeller flow number, table

In Eq. (6) Kq/u (=Pg/Pxj) is the gassing factor or relative power demand (RPD) between (otherwise identical) gassed and ungassed systems, g/u depends on the impeller type, Qq, N, and D, and generally decreases with increasing gas flow number, Flo- A summary of typical g/u values for popular impellers at high gas flow rates (Mg = 0.1) is given in Table 3. 

The discharge flow rate from most impellers is expressed in terms of the flow number N. Values are given in Table 9.1. The flow number, Nq, is defined by... 

This analysis has been done for a batch reactor, but it applies equally well to a CSTR. The heat transfer coefficient is the same because the agitator dominates the flow inside the vessel with little contribution from the net throughput. The analysis also applies to heat transfer using internal coils or baffles. The equations for the heat transfer coefficients are similar in form to Equation 5.33. Experimental results for the exponent on the impeller Reynolds number vary from 0.62 to 0.67 and are thus close to the semi-theoretical value of 2/3 used in Equation 5.33. The results in Table... 

Table 5-2 presents the effects of expected performance on various parameters or relationships for mixing. To actually calculate a numerical result of comparing impeller performances, the dimensionless numbers for flow power and force are needed. Note that in Table 5-2 the constant basis is across the horizontal top of the chart and the function to be examined or compared is along the vertical left side. The functions in the body of the table are used as ratios for condition (1) and condition (2), holding the basis constant. 

Table 4 shows the recommended numbers of impellers used in low viscosity working media. In the case of high viscosity media, the impeller spacing, S, normally needs to be decreased, i.e., S = 0.75-1.0 D. Multiple axial impellers are desirable for a synergistic flow pattern. 

There is a different power number for each impeller, reflecting its uniqne shape and drag-producing characteristics. The power number values given in Table 9.1 are only applicable for tnrbulent flow. Mannfactnrers offer a variety of impellers, the characteristics of which are given in Table 9.1. 

Multiple impellers are recommended if H/T 1.2 or if Ap > 150 kg/m. Assuming a less dense dispersed phase, the second or top impeller often is a hydrofoil placed midway between the RDT and the surface of the liquid. This impeller produces high flow at low power, provides excellent circulation, and complements the flow pattern produced by the RDT. The diameter of the second impeller is usually greater than the RDT, typically D/T > 0.45. A good practice is to distribute the total power to f 20% for the hydrofoil and f 80% for the RDT. Since the power number, Np, is known for each turbine, setting the power distribution enables the diameter of the hydrofoil to be determined. The vertical position of the upper turbine must ensure that fluid reaches the lower impeller, but must avoid gas entrainment that could occur if placement is too close to the hquid surface. Flow from a PBT does not complement that from a RDT and is therefore not recommended. Power requirements are discussed in Section 12-7.3. Table 12-6 lists equipment options for different drop sizing objectives (desired result). If ds2 must be less than 30 tim, the use of a stirred tank is not recommended, so other devices are also included in the table. 

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