Baffles propeller mixer

For turbine mixers that the width of a baffle should not exceed more than one-twelfth of the tank diameter and, for propeller mixers, no more than one-eighteenth the tank diameter. With side-entering, inclined or off-center propellers, as shown in Figure 13, baffles are not required. Instead, shrouded impellers and diffuser rings may be used to suppress vortex formation. These devices contribute to flow resistance and reduce circulation by creating intense shear and abnormal turbulence... 

Baffles can be omitted when propeller mixers are top mounted at an angular off-center position (see Figure 5-23D) and vortex swirling is prevented. This is not recommended for large povver systems on large tanks, due to shaft fatigue. 

Portable or fixed mixers up to 5hp normally use propellers and run at either direct drive speeds of 1,150 or l,750rpm or at single reduction gear drive speeds between 300 and 420 rpm. They may either be clamped on the rim of open tanks or mounted with a fixed assembly for open or closed tank operation. These mixers are the most economical and are usually used in tanks without baffles. They are rugged and long-lasting. 

Turbine designs are intermediate between paddles and propellers. Turbines are effective mixers over a wide viscosity range and provide a very versatile mixing tool. The ratio of radial to tangential flow, the predominating parameters with this impeller, increases as the operating speed increases. Pitched-blade turbines are sometimes used to increase axial flow. Baffles must be used to limit swirling unless the turbine is shrouded. 

Liquid-Liquid Mixer Design Many different types of impellers are used for liquid-liquid extraction, including flat-blade and pitched-blade turbines, marine-type propellers, and special pump-mix impellers. With pump-mix designs, the impeller serves not only to mix the fluids, but also to move the fluids through the extraction stages of a mixer-settler cascade. The agitated vessel should be baffled if the vessel is operated with a gas-liquid surface, to avoid forming a vortex. As noted earlier in reference to Eq. (15-172), baffles are not needed if the vessel is operated with the liquid full [Weinstein and Treybal, AIChEJ., 19(2), pp. 304-312 (1973)]. 

POWER CORRELATIONS FOR SPECIFIC IMPELLERS. The various shape factors in Eq. (9.16) depend on the type and arrangement of the equipment. The necessary measurements for a typical turbine-agitated vessel are shown in Fig. 9.7 the corresponding shape factors for this mixer are Sj = DalD S2 = EID S = L/D , S4 == W /Od. S5 = J/D, and Sg = HfD,. In addition, the number of baffles and the number of impeller blades must be specified. If a propeller is used, the pitch and number of blades are important. 

The speed of such mixers is between 1800 and 400 rpm, higher for smaller volume, and lower for larger volumes. Baffles can be used to reduce the vertex formation that diminishes the active volume. For low viscous liquids (10 to 1 Ns/m ) turbines, impellers and propellers are suitable. For viscous liquids, above 10 Ns/m, anchor and helical ribbon agitators are recommended. 

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