Impellers bottom clearance

Type of impeller and its geometric proportions with respect to the vessel. These include the impeller diameter to tank diameter, (D/7), and the impeller bottom clearance to tank diameter, (C/7), ratios (Fig. 7A.1), width of the blade and angle of pitch of the blade in case of pitched blade turbines and propellers, diameter of the hub through which the impeller is connected to the shaft, etc. 

A survey of the published literature indicates that the ratio of the maximum to mean energy dissipation rate in the vessel, Smax/ m can vary substantially but typically in the range 10 to 100 [85]. Recent measurements [100] of the turbulent flow properties with a range of impellers and vessel configurations indicate that the differences between the reported ratios of Smax/Cm re partly due to differences in the geometrical variables. For example, detailed factorial designs of experiments showed significant effects of impeller diameter to tank diameter ratio and off-bottom clearance to impeller diameter ratio on the value of emax/Cm-... 

The above correlation was obtained from the experimental data in a 14.5 cm diameter flat-bottom tank with four baffles over its entire height and a four-blade impeller with d, = dT/2 and with an off-bottom clearance of d,/3. Oguz and Brehm (1988) also proposed the Yagi-Yoshida correlation for both aqueous and organic liquid systems as... 

The effect of off-bottom clearance (C) is pronounced for all impellers, as indicated in Figure 10.7. For a 6BD (Rushton) impeller, the power draw (P) decreases as the impeller is moved closer to the vessel bottom from the typical impeller location of C/D = 1 for a 4BF turbine, P initially decreases as the impeller is moved down from C/D = 1, reaches a minimum at about C/D = 0.7 and then rises again as C/D drops below 0.7 and for a 4BP, the power draw continually increases as the impeller moves down from C/D = 1. 

P.M. Armenante and E. Nagamine, Solids Suspension in Agitated Vessels with Impellers Having Small Off-Bottom Clearances, paper presented at the 1996 AIChE Annual Meeting, Chicago, 1996. 

Results described so far suggest that the snapshot approach can be used to make a priori predictions of the complex flow generated in stirred vessels for impellers of any shape. A number of industrial stirred tank reactors make use of two or more impellers mounted on the same shaft. When more than one impeller is used, the flow complexity is greatly increased, especially when there is interaction between the flow generated by the two impellers. The extent of interaction depends on relative distances between the two impellers (and clearance from the vessel bottom). In order to examine whether the computational snapshot approach can be used to simulate... 

The Pfaudler impeller stirrer was developed for use in enamel-coated vessels [438] and thus has rounded stirring arms. It is installed with small bottom clearance at a D/d ratio of 1.5 and can be used both with and without baffles. Due to the small bottom clearance it can be used with strongly fluctuating filling levels (e.g. during emptying), since it can efficiently mix even small liquid volumes. 

Tank diameter Impeller diameter Bottom clearance Blade width (along blade) Blade angle Number of blades Agitator mass Shaft length Steel density Young s modulus Liquid density Liquid viscosity... 

As already suggested, there are many empirical conelations, and also theoretical equations with supporting experimental data in the literature. In general, all are different. However, the correlation of Zwietering , who carried out a dimensional analysis of the important variables and covered experimentally a very wide range of impeller types, sizes and off-bottom clearances, vessel sizes and physical properties, is broadly similar to a large number of them. The equation is... 

Axial flow impellers used in low bottom clearance tanks can also create radial flow if the direction of the flow is downward. In this case, the flow can leave the impeller zone only by flowing in the radial direction (Tatterson, 1991). This phenomenon is usually not observed since the standard bottom clearance for low viscosity impellers in gas-liquid dispersions is between one impeller diameter and one half the tank diameter (Ulbrecht and Patterson, 1985). The hydrofoil impellea-... 

Geometry For a stirred tank, the geometry is cylindrical, with a small aspect ratio (the height of fluid in the tank [H], is one to three times the tank diameter [T]). Although many industrial vessels have a dished bottom (especially if solids suspension is involved), simulations to date have used the simpler flat bottom geometry. The impeller, of diameter D = T/4 to T/2, is placed at the desired off-bottom clearance (C). Around the tank walls, two to four rectangular baffles are evenly... 

Usually impellers are installed in the lower part of a vessel. When axial impellers are chosen, the power number increases with decreasing bottom clearance due to the flow deflection at the vessel bottom. When radial impellers are used, the power number decreases with decreasing bottom clearance as there is no recirculation below the impeller. 

This scaling, however, introduces a factor of (D/T). This may work well where the bulk characteristics of the flow dominate, but it is not an accurate measure of turbulence if local characteristics are needed. For the same power input per unit tank volume, or holding eq. (2-18) constant with variations in impeller type, diameter, and off-bottom clearance, Zhou and Kresta (1996a) provided an extensive set of data and showed that the local dissipation can vary by up to a factor of 100. This is illustrated for the Intermig on the Visual Mixing CD affixed to the back cover of the book. The best order-of-magnitude estimate of the maximum dissipation uses the swept volume of the impeller instead of the total tank volume ... 

The exact shape of the velocity profile in the outflow of an impeller does not depend solely on the impeller. It is also affected by such variables as the impeller Reynolds number, impeller off-bottom distance C/T, and impeller diameter D/T. If the flow is fully turbulent (i.e.. Re > Kf ), the impeller outflow profiles are typically independent of Reynolds number. If the flow is flansitional or laminar, however, care should be taken so that the velocity profiles used were either measured at a similar Reynolds number, or that the prescribed velocities are being interpolated from data sets measured over a range of Reynolds numbers. Similarly, for impeller off-bottom clearance and diameter, if data for various C/T and D/T values are available, interpolations can be used to obtain the prescribed velocities for the actual conditions. 

Radial flow impellers may either have a disk (Rushton turbine) or be open (FBT) and may have either flat or curved blades (backswept mrbine). Impellers without the disk do not normally pump in a true radial direction since there is pressure difference between each side of the impeller. This is also true when the impellers are positioned in the tank at different off-bottom clearances. They can pump upward or downward while discharging radially. Radial discharge flow patterns can cause stratification or compartmentalization in the mixing tank. Disk-type radial impellers provide more uniform radial flow pattern and draw more power than open impellers. The disk is a baffle on the impeller, which prevents gas from rising along the mixer shaft. In addition, it allows the addition of a large number of impeller blades. Such blade addition cannot be done easily on a hub. A disk can also be used with a pitched blade turbine for use in gas-liquid mixing. 

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