Clearance Stirrers

Close-Clearance Stirrers For some pseiidoplastic fluid systems stagnant fluid may be found next to the -essel walls in parts remote from propeller or turbine impellers. In such cases, an anchor impeller maybe used (Fig, 18-6), The fluid flow is principally circular or helical (see Fig, 18-7) in the direction of rotation of the anchor. Whether substantial axial or radial fluid motion also occurs depends on the fluid iscosity and the design of the upper blade-supporting spokes. Anchor agitators are used particularly to obtain irnpro ed heat transfer in high-consistency fluids,... 

The very slow-speed helical ribbon stirrer is a close-clearance stirrer [D/d 1.05) and is so operated that the liquid on the wall is transported downwards. Under these conditions this stirrer is the most suitable of all the stirrer types for homogenizing high viscosity liquids. 

Fluid flow in a stirred tank with anchor stirrer can be characterized as flow past a horizontal plane [310] induced by the vertical arms of the close-clearance stirrer. A numerical algorithm of the two-dimensional Newtonian flow past a horizontal plane was established using an iterative method for the determination of the boundary values of the stream function. The flow profile was determined with measurements and the stirrer power derived therefrom. The correspondence between the measurements and calculations was excellent. 

Fig. 33 Work-sheet for determining optimal operating conditions for heat removal in a vessel with an anchor stirrer with two different wall clearances (D/d <= 1.00 - without wiper blades -and D/d = 1.10) in the laminar flow range (Re < 100) from [59]...

The exponents were found to be independent of the impeller type, vessel size, impeller clearance and impeller to tank diameter ratio. The dimensionless constant Ci accounted for variations in the system geometry (e.g. on dc/di). This would indicate that the basic mechanism leading to minimum suspension may be the same for rather different stirrer geometries. Table 2, which is an update of the one given by Nienow [19], indicates different exponents found in a few other investigations. Baldi et al. [26] made a relatively successful theoretical approach by assuming that the suspension of particles is mainly due to eddies of a certain critical scale comparable to the particle size. From an energy balance it follows that... 

Figure 3. Increase in stirrer speed necessary to keep the particles suspended if gas is fed to the stirred vessel for disk turbines of diameter 4/2 and clearance 4/4 A, 4 = 0.29 m B, 4 = 0.30m C. 4 = 0.91 m D, 4 = l-83m. Barred line ranges for 4 = 0.56 m at dilferent conditions [17].

Chapman et al. [17] proposed an extremely simple relationship from which the increase in stirrer speed required to suspend all particles under aerated conditions over unacrated conditions is given for disc turbines of diameter dc/2 and clearance of height dc/4 ... 

This type of stirrer was developed for use in enamel-coated vessels and thus has rounded stirring arms. The stirrer can be used in vessels with or without baffles. It is generally placed with a small clearance from the bottom, and its diameter is such that dT/d, = 1.5. It can operate in a vessel with strongly fluctuating liquid levels because it mixes even a small amount of liquids very well. 

This low-speed stirrer is normally used for viscous fluids commonly encountered in biological and polymeric reactors. They are well suited for enchancing the heat transfer rate in viscous fluids. The stirrer is generally placed in a vessel with small clearances from the wall (dx/d,-1.05). 

This very low-speed stirrer is normally used for very viscous fluids. It is normally placed in a vessel with small wall clearance (dT/dt 1.05) and operates in such a way that it drives the liquid downward along the wall. The stirrer is best suited for improving homogenization and heat transfer in very viscous fluids. 

The heat transfer is particularly important for the laminar flow regime, which generally prevails for viscous (e.g., polymeric) systems. Zlokarnik (1969) showed that for a vessel with a low-clearance anchor stirrer, the effects of Re and Pr on Nu decrease steadily as the Reynolds number decreases. He suggested a two-parameter equation of the type... 

Laminar regime (Re < 102). For stirrers with very small wall clearances, such as the anchor or helical ribbon mixer, the laminar regime prevails for Rc < 100. In this regime, the viscous force dominates. The effect of inertial forces (density) is negligible, and thus the baffles are unnecessary. In this regime,... 

Sample Preparation Accurately weigh an amount of sample equivalent to 4.8 g of cellulose gum on the dried basis, and record the actual quantity required, in grams, as S. Transfer an accurately measured volume of water equivalent to 240 - S g into the sample container. Position the stirrer in the sample container, allowing minimal clearance between the stirrer and the bottom of the container. Begin stirring, and slowly add the sample. Adjust the stirring speed to approximately 900 100 rpm. Mix for exactly 2 h. Do not allow... 

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. 

The slow-speed anchor stirrer is generally utilized with close wall clearance (D/d < 1.05) to intensify heat transfer in high viscosity liquids. 

The proportionality constant fe depends upon the stirrer type. For propeller stirrers fe = 10 ]68], for turbine stirrers fe = 11.5 and pitched-blade stirrers fe = 13 [104]. For blade stirrers k = 2.5, for cross-beam stirrer fe = 4.1 and for helical ribbon stirrer fe = 6.0 [411]. Calderbank [66] found that when turbine stirrers were used with Bingham and pseudoplastic fluids fe = 10 and when used with dilatant liquids fe = 12.8 (d/D) 5. Lower fe values were found as the viscoelasticity of fluids increased [104]. In the case of close-clearance anchor stirrer fe depended upon the wall clearance [24]. 

The power P of a given stirrer type and given installation conditions (vessel diameter D, liquid height H, stirrer bottom clearance h) in a homogeneous liquid depends upon the stirrer diameter d (as the characteristic length), the degree of baffling, the material parameters of the liquid (density p and dynamic viscosity v) and upon the stirrer speed n. 

This graphical representation enables an interesting comparison to be made between the power characteristics of different stirrer types. In the first place it is evident that the Ne value for Re = 1, i.e. NeRe = const) increases with the surface area of the stirrer, it being with the anchor stirrer a factor of 10 greater and with the helical ribbon stirrer a factor of 25 greater than that of the propeller stirrer. The smaller the wall clearance of the stirrer, the longer laminar flow conditions remain. With gate and cross-beam stirrers these are maintained up to Re 10, with anchor stirrers and helical ribbon stirrers even up to Re 10. ... 

In [24] it was found that correlation of the stirrer power of anchor stirrers required that the wall clearance s = (D-d) has to be taken into account in addition to RCeff according to expression (1.47). The power characteristic found for the laminar range was ... 

In Fig. 3.13 below, the authors [363] plotted their own measurements for the helical ribbon stirrer for different wall clearances d/D. It shows (see the table below right), that the stirrer power of the helical ribbon stirrer decreased rapidly with increasing wall clearance and that, to a smaller extent, the nd values also decreased. This stirrer type, which up to now has proved to be the most suitable for homogenization in the laminar flow range, should if at all possible be utilized with d/D = 0.9. 

Fig. 5.2 Normalized distribution of solids (glass beads) over the relative layer height b bs is the bottom clearance of the stirrer. For the experimental conditions see [114]... 

Evaluation of his experimental data together with those from [395] is presented in Fig. 5.17. To eliminate the effect of different geometries (stirrer type, bottom clearance), in the ordinate the particular stirrer speed is related to that n, which applies for d/D = 1/3. 

Turbine and propeller stirrers with a low bottom clearance (h/d = 0.25-0.5) are regarded as optimal stirrers. A perforated ring should be used as gas sparger. The required stirrer power is lower, if the baffles are only installed in the upper half of the liquid height [592]. 

Measurements [364] established, that the lowest stirrer speed for complete dispersion also depended upon the bottom clearance h. In the range d/D = 0.2-0.3 it was found for the 6-blade turbine stirrer that for ... 

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