Transcript SIZE REDUCTION PROCESSES File

SIZE REDUCTION PROCESSES:
GRINDING AND MILLING
Raw materials often occur in sizes that are too large to be
used and, therefore, they must be reduced in size. Size
reduction is the unit operation in which the average size
of solid pieces of food is reduced by the application of
grinding, compression or impact forces.
Size reduction has the following benefits in food
processing:
• There is an increase in the surface-area-to-volume ratio of
the food which increases the rate of drying, heating or
cooling and improves the efficiency and rate of extraction
of liquid components (for example fruit juice or cooking
oil extraction.
• A similar range of particle sizes allows more complete
mixing of ingredients (for example dried soup and cake
mixes).
Grinding
• the most extensive application of grinding in the food
industry is in the milling of grains to make flour, but
it is used in many other processes, such as in the
grinding of corn for manufacture of corn starch, the
grinding of sugar and the milling of dried foods, such
as vegetables.
• Hammer Mill:
• Used for variety of size-reduction or grinding job.
• Consist essentially of a rotating beater and a heavy
perforated screen.
• The material is introduced into the housing and the
beater which consist of series of hammers turning at
1500 to 4000 rpm, beats and pounds the material until
it is small enough to pass trough the screen at bottom.
• Fineness of division is controlled mainly by the size
of holes in the screen, rotor revolutions per minute
and the rate of feed.
• The hammers are rigidly fixed to the shaft or swing.
• There is less danger of swinging hammer causing
damage if large metallic object gets into the mill by
accident.
Advantages :
1) Simplicity
2) Versatility
3) Freedom from significant damage to foreign objects.
4) Freedom from damage when operating empty.
5) Hammer wear does not materially reduce its
efficiency.
Disadvantages
1) Inability to produce a uniform grind.
2) High power requirements
• Burr mill:
• Also called as plate mills or attrition mill.
• Consist of two roughened plates, one stationary, the
other rotating .
• The material is fed between the plates and is reduced
by crushing and shear.
• Plates made of chilled cast iron and are usually made
of chilled cast iron.
• Operating speeds are usually less than 1200 rpm.
• Fineness of reduction is controlled by the type of
plates and by spacing.
• Spacing screw is spring loaded so that the space will
increase in case of overload or if a foreign object gets
into the mill.
Advantages
1)low intial cost .
2) Product may be relatively uniform.
3) Power requirement may be low.
Disadvantages:
1) Foreign object may cause breakage.
2) Operating empty may cause excessive burr wear.
3) Worm burrs yield poor results.
• Roller mill:
Roller mills are similar to roller crushers, but they
have smooth or finely fluted rolls, and rotate at
differential speeds. They are used very widely to
grind flour. Because of their simple geometry, the
maximum size of the particle that can pass between
the rolls can be regulated. If the friction coefficient
between the rolls and the feed material is known, the
largest particle that will be nipped between the rolls
can be calculated, knowing the geometry of the
particles.
Ball Mill:
In a ball mill or pebble mill,
most of the reduction is done
by impact as the balls or
pebbles drop from near the
top of the shell. In a large
ball mill the shell might be 3
m in diameter and 4.25 m
long. The balls are 25 to 125
mm in diameter.
A mill of this kind contains balls of different sizes, all of
which wear and become smaller as the mill is operated. New
large balls are added periodically. As the shell of such a mill
rotates, the large balls move toward the point of maximum
diameter, and the small balls migrate toward the discharge.
The initial breaking of the feed particles, therefore, is
done by the largest ball dropping the greatest
distance; small particles are ground by small balls
dropping a much smaller distance.
When the mill is rotated, the balls are picked up by
the mill wall and carried nearly to the top, where they
break contact with the wall and fall to the bottom to
be picked up again