14 Extruder.pdf

Screw Sections
Feed section: has deeper flights or with
greater pitch. Feed falls easily into screw
and begins to be conveyed.
Transition or compression region: granular
or sticky mass converted to a uniform
plasticized dough. Product compacted,
mixed, and worked.
Metering section: shallow flights, reduced q
used to thoroughly mix and/or increase T
Other Components
Extrusion stand: base that supports the
drive, bearings, and extrusion barrel and
screw
Drive mechanism: usually an electric motor
operating through gear reduction to obtain
proper speed. Drive attached to screw via
flexible coupling.
Barrel: Usually bolted together sections
containing cylinder within which screw
rotates. Usually hardened surface to limit
abrasion caused by high shear conditions.
Common diameters: 2.54 - 20.32 cm. Long
barrels give more surface for heating/
cooling, longer residence time => more
control. L/D range from 3:1 to 25:1.
Normal operating pressures: 15 - 70 atm.
Hopper
Feeder
FEED
SECTION
COMPRESSION
SECTION
Steam Jacket
METERING
SECTION
Thrust bearing: extrusion screw supported
by bearing at driven end. Rearward thrust
must be absorbed to compensate for force
imparted to extruded food as it moves
forward.
Feed hopper: opening through which feed
materials enter screw. Often with agitators,
offset, and with cooling jackets to enhance
feed. May be dry grits, flours, sticky
doughs, etc.
Barrel temperature control: heat is added/
removed from sections of barrel surface.
Hollow jackets around barrel can accept
steam or cooling water. Temperature of last
barrel most commonly varied to achieve
desired product temperature. Steam
injection sometimes used.
Die