Impeller INDEX

Another distinction in impellers is the way the liquid traverses and leaves the impeller blades. This is called the Specific Speed, Ns. It is another index used by pump designers to describe the geometry of the impeller and to classify impellers according to their clesign type and application. By definition, the Specific Speed, Ns is the revolutions per minute (rpm) at which a geometrically similar impeller would run if it were of such a size as to discharge one gallon per minute at one foot of head. 

The Specific Speed is a dimensionless number using the formula above. Pump design engineers consider the Ns a valuable tool in the development of impellers. It is also a key index in determining if the pump... 

In discussing equations 9.207 and 9.208 Fletcher 96 has summarised correlations obtained for a wide range of impeller and agitator designs in terms of the constant before the Reynolds number and the index on the Reynolds number as shown in Table 9.11. 

Calculate the theoretical power in watts for a 0.25 m diameter, six-blade flat blade turbine agitator rotating at N = 4 rev/s in a tank system with a power curve given in Figure 5.10. The liquid in the tank is shear thinning with an apparent dynamic viscosity dependent on the impeller speed N and given by the equation fia = 25(N)n 1 Pa s where the power law index n = and the liquid density p = 1000 kg/m3. 

Estimate the stirrer power requirement P for a tank fermentor, 1.8 m in diameter, containing a viscous non-Newtonian broth, of which the consistency index A = 124, flow behavior index n = 0.537, density p = 1050 kg m", stirred by a pitched-blade, turbine-type impeller of diameter d = 0.6 m, with a rotational speed AT of 1 s . ... 

M 21] [P 20] Mixing near a T-junction (channel 750 pm wide and 40 pm deep) is complete at a 0.17 pi min-1 flow rate when using a micro impeller at 120 rpm, as demonstrated by scanometric color index profiles providing line concentration profiles [32]. 

Figure 1.50 Liquid mixing time of micro-impeller mixing in a mixing chamber derived by plotting a mixing index yield by photometry versus time [32]...

Index Entries Distiller s grain slurries rheologic properties wet grains calibration fluids helical impeller. 

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. 

For normal mixing, the Pfaudler agitation-index (y) number for this low-viscosity fluid is 2 ft2/s3. Most stirrers are designed for impeller Reynolds numbers of 1000 or greater. For the impeller specified, the power number tjfn is 0.6 at high Reynolds numbers [2]. 

Many other factors have to be considered for the characterization of a turbine in a chemical or electrochemical reactor. First, the impeller pumping capacity, defined as the liquid flow, is obtained from the revolution volume of the impeller. In addition it is also considered here the circulation flux, conceived as the fluid flowable to drag by the circulation laces generated by the impellers. The renovation time—the time that the entire electrolyte contained in the vessel remains before being drawn across the impeller—has to be also considered. The circulation time is the time that taken by the electrolyte in the reactor to circulate along all the circulation laces (flux pattern of the impeller). Finally, the index of the turbulence is simply the ratio between the mean fluctuant speed in the entire reactor volume from the edge of the impeller. 

The effect of the mixing mode on the power law index is also shown in Table 3. The polymer chains dissolved in a relatively mild impeller mixer have been mechanically broken the least consequently, these solutions have large [1-n] values. The molecules that have been subjected to severe shearing conditions (380 psi across the orifice) exhibit a Newtonian behavior indicating that the polymer chains have been broken down into short segments. 

Casson models were used to compare their yield stress results to those calculated with the direct methods, the stress growth and impeller methods. Table 2 shows the parameters obtained when the experimental shear stress-shear rate data for the fermentation suspensions were fitted with all models at initial process. The correlation coefficients (/P) between the shear rate and shear stress are from 0.994 to 0.995 for the Herschel-Bulkley model, 0.988 to 0.994 for the Bingham, 0.982 to 0.990 for the Casson model, and 0.948 to 0.972 for the power law model for enzymatic hydrolysis at 10% solids concentration (Table 1). The rheological parameters for Solka Floe suspensions were employed to determine if there was any relationship between the shear rate constant, k, and the power law index flow, n. The relationship between the shear rate constant and the index flow for fermentation broth at concentrations ranging from 10 to 20% is shown on Table 2. The yield stress obtained by the FL 100/6W impeller technique decreased significantly as the fimetion of time and concentration during enzyme reaction and fermentation. 

If it is possible to assume that the maximum shear stress is proportional to the tip speed of the impeller, Eq. (18) has power index on the impeller diameter and rotation speed equivalent to those of Eq. (14). 

The simple concept of an average mixer shear rate has been widely used in laboratory and industrial work and in most applications it has been assumed that the shear rate constant, k, is only a function of impeller type. Research is continuing on the possible influence of flow behaviour index and elastic properties, and also on procedures necessary to describe power consumption for dilatant fluids. It should be noted that in all aspects of power prediction and data analysis, power law models (equation 8.12) should only be used with caution. Apparent variability of k, may be due to inappropriate use of power law equations when calculations are made it should be ascertained that the average shear rates of interest (y = k N) lie within the range of the power law viscometric data. 

It is not yet established, however, how strongly the value of kg depends on the rheology, and on the geometrical arrangement of the system. For example, both Calderbank and Moo-Yoimg [1959] and Beckner and Smith [1966] have related ks to the impeller/vessel configuration and the power-law index, n the dependence on, n, however is quite weak. 

Because the head is greater than 16 in. of H2O and the flue gas is not likely to be clean, using Table 16.23, select a centrifugal fan with a straight-radial impeller. From Table 16.24, the head factor, Fn, is 1.45. From Eqs. (16.24) and (16.26), correcting for the cost index, the purchase cost, including the motor, is... 

FIGURE 9.6 Impeller performance index as a function of modified Froude number, Fr. (Reproduced from Zundelevich, 1979 with permission from John WUey and Sons. Copyright 1979 American Institute of Chemical Engineers.)... 

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