Page 192 - Fluid mechanics, heat transfer, and mass transfer

MIXING
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baffles, 90 apart. The baffles should be 1/12th the & Low speeds: 35–420 rpm for liquids under the fol-

tank diameter in width and should be spaced off the lowing conditions:
wall by 1/5th the baffle width. The off-wall spacing ➢ When the mixture is thick, viscous, or slippery,
helps eliminate dead zones. so that small high-speed propellers tend to
& If baffles are used, the mixer should be mounted in the channel rather than propel the entire mass into
vertical position in the center of the tank. If baffles are circulation.
not used, the mixer should be mounted at an angle of ➢ When the mixture contains particulates such as
about 15 to the right and positioned off-center. This crystals, sliced fruit, or other foods that will be

breaks up the symmetry of the tank and simulates broken up at high speeds.
baffles although not nearly as good as baffles. ➢ When the mixture is of foamy nature and the foam
& The purpose of baffles is to prevent solid body is undesirable.
rotation, that is, all points in the tank move at the ➢ When colloids such as milk and cream are dam-
same angular velocity and there is no top-to-bottom aged at high speeds.
turnover. The formation of a large central vortex is a . Give a diagram of a baffled draft tube mixer showing
characteristic of solid body rotation.
standard dimensions.
& However, small vortices, which travel around the
& Figure 6.2 illustrates standard recommended dimen-
fluid surface, collapse, and reform, are more a func-
sions for a baffled draft tube mixer.
tion of the level of agitation. Violent and vigorous
& Arrangement of heat transfer surfaces is also shown.
agitation will have thesevortices present. In fact, they
are desired for processes that require solids addition & A dished bottom requires less power than a flat one.
from the liquid surface. & When a single impeller is to be used, a liquid
& The impellers are located at different positions de- level equal to the diameter is optimum, with
pending on the design. the impeller located at the center for an all-liquid
system. Economic and manufacturing considera-
& Axial flow impellers should be positioned between
tions, however, often dictate higher ratios of depth
0.5 and 1.5 impeller diameters off the bottom of the
to diameter.
tank. Radial flow impellers can be positioned just few
centimeters off the bottom. & Except at very high Reynolds numbers, baffles are
& If multiple impellers are used, the spacing will needed to prevent vortexing and rotation of the liquid
mass as a whole.
depend on the liquid height to tank diameter ratio.
Care must be taken to prevent impeller spacings of & A baffle width w ¼ D i /12 is used.
one impeller diameter. This can lead to a cancellation & Baffle length extending from one-half the impeller
of flow. diameter, d/2, from the tangent line at the bottom to
& Torque is one of the most important factors. A large the liquid level is normally used, but sometimes
diameter slow spinning impeller is much better for terminated just above the level of the eye of the
blending than a small diameter fast spinning impeller uppermost impeller.
at equal power levels. & When solids are present or when a heat transfer jacket
& Impeller diameters for relatively low viscosities is used, the baffles are offset from the wall a distance
should be between 0.25 and 0.45 times the tank equal to one-sixth the baffle width. Four radial baffles
diameter with some exceptions. at equal spacing are used in standard designs. Heat
& For turbulent mixing, large diameter, low-speed, and transfer surfaces are often installed inside the vessel
axial flow impellers are preferable. and jackets (both sidewall and bottom head) so that
the vessel wall and bottom head can be used as heat
. “It is easier to mix a high-viscosity liquid with a low-
transfer surfaces.
viscosity liquid than the other way round.” True/False?
& Helical coils are attached to wall baffles, as shown
& True.
in Figure 6.2, with tube diameters typically equal to
. What are the speed ranges for high- and low-speed
Z/30 and tube row spacing equal to one tube diameter.
impellers? Give the applications with respect to speeds.
& Six baffles are only slightly more effective and three
& High speeds: 1150–1750 rpm for liquids such as
are appreciably less effective.
aqueous solutions.
& When the mixer shaft is located off-center (one-
& Medium speeds: 450–1150 rpm for liquids of medi-
fourth to one-half the tank radius), the resulting flow
um viscosity such as varnishes, medium oils, syrups,
pattern has less swirl, and baffles may not be needed,
and so on.
particularly at low viscosities.