Page 196 - Fluid mechanics, heat transfer, and mass transfer

MIXING
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dissolving applications, and for liquid-phase chemi- over finite sections, having blades either vertical or
cal reactions. set at an angle less than 90 with the vertical plane,

& Disadvantages, compared to paddle and turbine mix- usually 45 .
ers, are higher cost and sensitivity of operation to the & Blades may have different configurations, curved or
vessel geometry and location within the tank. As a flat. Impeller has generally more than four blades in
general rule, propeller mixers are installed with the same plane of rotation. Number of blades is in the
vessels having convex bottom. They should not be range of 2–8, with 6 blades being common.
used in square tanks or in vessels with flat or concave & The predominantly radial flow from the impeller
bottoms. impinges onto the vessel walls, where it splits into
& Propeller mixers are not suitable for suspending rapid two streams.
settling substances and for the absorption of gases. & Generally smaller than paddles, measuring 30–50%
For suspensions, the upper limit of particle size is of the vessel diameter. Speeds of rotation are
0.1–0.5 mm, with a maximum dry residue of 10%. 30–500 rpm.
& Figure 6.5 illustrates three designs of propellers, & Baffles and pitched blades may be used in a similar
namely, marine, saw-toothed, and perforated types. way as with paddles.
& Marine type circulates by axial flow parallel to the & Velocities of the liquid are relatively high and fluid
shaft and its flow pattern is modified by baffles, currents travel throughout the vessel, with vertical
normally a downward flow. flow often being set up by the deflection of currents
➢ Operates over wide speed range. from the vessel walls.
➢ Can be pitched at various angles, most common is & When operated at high rotational speeds, both radial
the three blades on square pitch (pitch equal to and tangential flows become pronounced, along with
diameter). vortex formation. In such situations, it is preferable to
➢ Shearing action is very good at high speed, but not use baffles to ensure a more uniform flow distribution
generally used for this purpose. throughout the mixing vessel.
➢ Power consumption is economical. & Vaned disk impellers may be used to disperse gases in
liquids. Suited for liquids up to 100,000 cP at high
➢ Generally self-cleaning.
pumping capacity and particularly effective in mod-
➢ Relatively difficult to locate in vessels to obtain
erately viscous liquids. Used for fairly high shear and
optimum performance.
turbulence. Better than axial flow units for tanks with
➢ Not effective for viscous liquids.
conical bottom of >15 angle, to lift material from
& Sawtooth design displaces a large amount of liquid
bottom of the cone and mix with bulk of the liquid.
and combines cutting and tearing action. Suitable for & Curved blade turbines effectively disperse fibrous
fibrous materials.
materials without fouling. The swept back blades
& Perforated propeller is sometimes recommended for have a lower starting torque than straight ones, which
wetting dry powders, especially those that form is important when starting up settled slurries.
lumps. & Shrouded turbines consisting of a rotor and a stator
. What are the characteristic features of turbine mixers? ensure a high degree of radial flow and shearing
& Turbine impellers have a constant blade angle with action and are well adapted to emulsification and
respect to a vertical plane, over its entire length or dispersion.

Marine, saw-toothed, and perforated propellers.
FIGURE 6.5