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RATE OF VELOCITIES – DEGREE OF MIXING
Low radial velocities - The extent of mixing becomes inadequate, as indicated by the loss
in the spiral rope like motion of the particles.
Rate at which the particles traverse the spray zone is prolonged and could potentially lead
to excessive agglomeration.
Caking is another serious problem encountered during powder layering at low radial
velocity.
High radial velocities - particle to particle and particle to wall frictional forces become
intense with a very rapid pellet turnover. Generally, this condition leads to an
uncontrollable, wobbly bed, resulting in severe particle attrition.
Large number of fines generated, but some of the attrite particles may agglomerate to form
nuclei that are subsequently layered upon.
Inert starter seeds and those newly formed nuclei are different and may create content
uniformity problems.
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Screw-feed extruders: These extruders have screws that rotate along the horizontal axis and
hence transport the material horizontally; they may be axial or radial screw extruders.
Axial extruders, which have a die plate that is positioned axially, consist of a feeding zone, a
compression zone, and an extrusion zone.
Radial extruders, the transport zone is short, and the material is extruded radially through
screens mounted around the horizontal axis of the screws.
Gravity-feed extruders: These extruders include the rotary cylinder and rotary gear extruders,
which differ primarily in the design of the two counter-rotating cylinders. In the rotary-cylinder
extruder, one of the two counter-rotating cylinders is hollow and perforated, whereas the
other cylinder is solid and acts as a pressure roller.
In rotary gear extruder, there are two hollow counter-rotating gear cylinders with counter
bored holes.
Ram extruders: In ram extruders, a piston displaces and forces the material through a die at
the end.
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An increase of the
mean pellet weight can
be obtained in three
different ways:
1) Fines could layer
onto the pellets.
2) Pellets could
coalesce.
3) Small pellets could
abrade to particles
fines
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