LIQUID MIXING
• For any application, choice of the proper
impeller is critical for success. Impellers
can be divided into two general classes,
• Axial flow, and
• Radial flow
Axial – flow impellers
• Axial – flow impellers include all impellers in
which the blade makes an angle of less than 90
with the plane of rotation.
• Uses: Axial flow impellers are used at high
speeds to promote rapid dispersion and are
used at low speeds for keeping solids in
suspension. It is also used for agitation in tanks
smaller than 1000 gal. or <6 ft diameter, when
<3 hp. is enough for the desired process results.
Fig: Pitched – blade turbine
Fig: Marine – type mixing propeller
Different mounting positions of
propellers:
• Propeller mixers may be clamped on the
side of an open vessel in the angular, off –
center
• position or bolted to a flange or plate on
the top of a closed vessel with the shaft in
the same
• angular, off – center position. This
mounting results in a strong top to bottom
circulation.
Fig: Side – entering propeller mixer
Off center top – entering propeller position
Fig: Flow pattern for propeller in angular off – center position without baffles
Radial – flow impellers:
Radial – flow impellers have blades which are parallel to
the axis of the drive shaft. The smaller multi blade ones
are known as “turbines”; larger, slower - speed
impellers, with two or four blades, are often called
“paddles”
• The diameter of a turbine is normally between 0.3 and
0.6 of the tank diameter.
• Curved blade turbines aid in starting an impeller in
settled solids. A paddle agitator has a diameter usually
greater then 0.6 of tank diameter and turns at a slow
speed. Construction is similar to that of curved – blade
turbine, but with two of four straight blades and a
relatively smaller hub.
• Most large scale agitation of solid – liquid suspensions is
done with top – entering turbines or paddles. The
impeller speed is typically between 50 and 150 r.p.m; but
depending on process conditions, it may go as high as
400 or as low as 15 r.p.m
Fig: Flat – blade turbine
Fig: Glassed – Steel impeller
Fig: Curved – blade turbine
Fig: Anchor impeller
• Radial flow impellers have blades that are parallel to the
axis of the mixer shaft. The smaller, multi-blade types are
called turbines. Larger slow-speed impellers, with two to
four blades are known as paddles, while other larger twosided types are known as gate or anchor impellers.
• For some pseudoplastic systems, stagnant fluid may be
found next to the vessel walls in parts remote from propeller
or turbine impellers. In such cases, an “anchor” impeller
may be used. The fluid flow is principally circular in the
direction of rotation of the anchor. Anchor agitators are used
particularly to obtain improved heat transfer in high –
consistency fluids.
• Uses: Turbines, which come in a variety of types, provide
excellent circulation of fluid throughout the mixing vessel.
The diameter of a turbine impeller is normally between 0.3
and 0.6 of the tank diameter. Paddles are used at slower
speeds and normally have a diameter greater than 0.6 of
the tank diameter. These two types are most often used for
large scale mixing of solid/ liquid suspensions
Unbaffled tanks: (vortexing
phenomenon)
• If a low – viscosity liquid is stirred in an unbaffled tank by
an axially mounted agitator, there is a tendency for a
swirling flow pattern to develop, regardless of the type of
impeller. A vortex is produced owing to centrifugal force
acting on the rotating liquid.
• Rotational speed is difficult to raise as severe air
entrapment may occur once the vortex reaches the
impeller.
• The swirling mass of liquid often generates an oscillating
surge in the tank, which coupled with the deep vortex
may create a large fluctuating force acting on the mixer
shaft. Vertical velocities in a vortexing low viscosity liquid
are low relative to circumferential rates may be obtained
by mounting the impeller off – center.
Fig: Typical flow pattern for either axial – or radial – flow impellers
in unbaffled tank
Fig: Typical flow pattern in baffled tank with propeller or axial flow
turbine positioned on center
• A common baffle width is one – tenth to
one – twelfth of the tank diameter (radial
dimension). For agitating slurries, the
baffles often are located one – half their
width from the vessel wall to minimize
accumulation of solids on or behind them
Silverson emusifier
Silverson emulsifier: