LIQUID MIXING

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LIQUID MIXING
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• 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

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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 lt6 ft diameter, when lt3 hp. is
  enough for the desired process results.

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Fig Pitched blade turbine
Fig Marine type mixing propeller
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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.

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Fig Side entering propeller mixer
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Off center top entering propeller position Fig
Flow pattern for propeller in angular off
center position without baffles
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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

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Fig Flat blade turbine
Fig Curved blade turbine
Fig Glassed Steel impeller
Fig Anchor impeller
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• 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
  two-sided 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

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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.

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Fig Typical flow pattern for either axial or
radial flow impellers in unbaffled tank
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Fig Typical flow pattern in baffled tank with
propeller or axial flow turbine positioned on
center
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• 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

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Silverson emusifier
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Silverson emulsifier