Impellers filling level

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 influence on power number is high for blade-type impellers. For pitched blade turbines or propellers, Ne is constant for filling levels H/T > 0.8. 

A cylindrical tank (1.22 m diameter) is filled with water to an operating level equal to the tank diameter. The tank is equipped with four equally spaced baffles, the width of which is one tenth of the tank diameter. The tank is agitated with a 0.36 m diameter, flat-blade disk turbine. The impeller rotational speed is 4.43 rps. The air enters through an open-ended tube situated below the impeller and its volumetric flow rate is 0.0217 m3/s at 1.08 atm and 25°C. Calculate ... 

According to Table 12.2, a 9y2-ft-diameter tank holds 44.1 gal/in of liquid level. Therefore, 8840 gal will fill the tank to 8840/44.1 = 200 in. The resulting liquid-level-to-tank-diameter ratio is Z/7 = 200/114 = 1.75. The following guidelines for number and location of impellers should be applied ... 

A dished head tank of diameter DT = 1.22 m is filled with water to an operating level equal to the tank diameter. The tank is equipped with four equally spaced baffles whose width is one-tenth of the tank diameter. The tank is agitated with a 0.36-m-diameter, flat, six-blade disk turbine. The impeller rotational speed is 2.8 rev/s. The sparging air enters through an open-ended tube situated below the impeller, and its volumetric flow, Q, is 0.00416 m3/s at 25°C. Calculate the following the impeller power requirement, Pm gas holdup (the volume fraction of gas phase in the dispersion), H and Sauter mean diameter of the dispersed bubbles. The viscosity of the water, //, is 8.904 x 10 4 kg/(m-s), the density, p, is 997.08 kg/m3, and, therefore, the kinematic viscosity, v, is 8.93 x 10 7 m2/s. The interfacial tension for the air-water interface, a, is 0.07197 kg/s2. Assume that the air bubbles are in the range of 2-5 mm diameter. 

Pavlushenko et al. (P2) studied the suspension of screened fractions of sand and iron ore in a variety of liquids, with a 1-ft. diameter unbaffled vessel filled to a depth of one foot. Square-pitch three-blade propellers of 3-, 4-, and 5-in. diameter were used, and most of the observations were made with a 1 to 4 weight ratio of solids to liquid. Thief samples were taken at various levels in the vessel. In some cases, the contents did not become uniform at any impeller speed in other cases the contents became uniform at some impeller speed and remained so at higher speeds in a third type of behavior, the upper part of the vessel reached the over-all vessel average and then exceeded it as impeller speed was increased. Using the observations from the second and third types of behavior, a critical speed was defined as the lowest impeller speed at which the solids concentration at each level, or in the upper layers of the liquid, was equal to the over-all average solids concentration. This critical speed Nc in revolutions per second had the following relation to the operating variables ... 

A glass mixing vessel of 0.20 m diameter was used for the study. It was enclosed in a square plexiglass tank that was also filled with the same liquid as used in the mixing vessel in order to minimize refraction of the light beams. The liquid level was 0.20 m and the impellers were 0.10 m from the bottom. Impellers of approximately 0.064 m diameter were used to minimize wall feedback effects. Water was the working fluid for the results reported here. 

The power number and corresponding power of an anchor impeller are proportional to the height of the vertical arm. Thus, an anchor with a height H equal to 75 percent of the impeller diameter would have a power number equal to 75 percent of the typical values shown in Fig. 18-43. Similarly, a partially filled tank with a liquid level Z that covers only 75 percent of the vertical arm will also have a power number that is 75 percent of the typical correlation value. The addition of scrapers will increase the power requirement for an anchor impeller, but the effect depends on the clearance at the wall, design of the scrapers, processed material, and many other factors. Correlations are not practical or available. 

Consider a hollow shaft connected to a hollow impeller inunersed in a liquid. The space above the liquid level is filled with the gas to be induced (Fig. 9.1a, b, c, d, e). 

The correlation for heat transfer is evaluated with the respective dimensionless groups. With the units stated in the example, the impeller Reynolds number (AIrs = YP Np/fi) requires a conversion factor of 10.7 to make it dimensionless thus, AIrs = 10.7(38) (56)(0.89)/1200 = 642. The Prandtl number N = Cpp/k) requires a conversion factor of 2.42 thus A/p, = 2.42(0.52)(1200)/0.079 = 19,115. The hquid-level-to-tank-diameter ratio Z/T requires determination of the liquid level for a 5000-galbatchin the tank. A standard dished head holds 301 gal and is 15 in deep, from Table 12.2. The remaining 5000 — 301 = 4699 gal fills the cylindrical part of the tank at a rate of 39.6 gal/in of heighL or to a height of 4699/39.6 = 119 in. The total liquid level is Z = 119 + 15 = 134 in and Z/T = 134/108 = 1.24. The impeUer-to-tank-diameterratio D/T is 38/108 = 0.35.Theviscosity ratio will be assumed to be unity (/r//r , = 1) because of lack of data and the very small exponent on the term. 

If for a usual rotoklon efficiency of trapping of a dust was small and sharp depended from fluid level, filled in the apparatus in the presence of impeller blades efficiency trapping in all velocity band of air was high and did not depend on liquid level in the apparatus (See Figures 12.4-12.6). 

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