5. Size reduction Flashcards
Size reduction involves creating….
smaller mass units from larger mass units of the same material.
What’s mode of reduction?
Types of forces applied to subject stress on certain mass
Explain modes of reduction!
- cutting: sharp blades
- compression: crushed (between rolls)
- shear: shearing (gerus)
- impact: strikes against hammer (rod)
- attrition: rubbing action (between 2 surfaces) diamplas
what force is generally used for coarse crushing of hard materials?
compressive force
which force is used to mill a variety of materials, including fibrous foods?
impact forces
what’s cutting method utilized for?
fibrous foods
what’s the usage of shear force?
best applied to relatively soft materials, again including fibrous foods
in roller mills, …. is the most dominant force
in hammer mills, …. is the dominant force
shear force is dominant in …
compression
impact force
grinder and disc attrition mills
Size reduction equipments:
- roller mills or grinder
- hammer mill
- crusher
- slicer/cutter
- chopper
- shredder
- dicer
real example of roller mills’ usage
for wheat milling.
how does roller mills work?
- based on compression but wheat milling force is combined
- shear and compression forces
- F-S-F-S pattern (different speed)
- double roll to peel husk, roller surfaces are covered in rubber.
Types of roller mills
- two roller mills
simplest. material is crushed between the 2 rollers before final destination. spacing can be adjusted by operator. thinner spacing -> smaller pieces - four roller mills
2 sets of rollers. grain goes through wider gap (seed and husk separated, husks leave almost undamaged and leaves large grits). flour is sieved out of cracked grain. coarse grist and husks are sent through second set of rollers -> further crush the grist w/o damaging the crusts. 3 rollers is similar, however one of the rollers is used twice - five and six roller mills
3 sets of roller. first set of rollers crush the whole kernel, output is divided to 3 pathways. flour is immediately sent out to the mill, grits without a husk proceed to the last roller, and husk (possible contains parts of the seed) go to the second set of rollers. from second roller, flour is directly output, as are husks and any possible seed still in them, and husk-free grits are channeled into last roller.
5 roller mills are 6 roller mills in which one of the rollers performs double-duty
Wheat milling principles:
Passes through >12 set of reduction and rolls and sieving.
final products:
1. white flour
2. white germ
3. white feed
4. bran
explain the concept of pin disk mills/pin mills/vertical pin disk mills
- type of grinders
2.known as centrifugal-impact-mills - high impact mills -> consisting of a feed inlet, a set of 2 disks with concentric rings of pins and a product outlet
- particle size of as low as 2mm can be achieved with the use of finest quality of pin mills
- pin mils are capable of very fine grinding w/o the need for screens and provide a uniform product size
[gambar pin disk mills]. has several metallic pins, placed symmetrically and concentrically on the disk surfaces. how does it work?
- consists of 2 round disks parallel to each other
- material is fed continuously in the gap between the rotating disks
- depending on the type of the machine, one or both disks can rotate. in cases where both disks rotate, the second disk rotates in counter direction
- feeding of the product is usually done by means of a chute, shifting the product in the hole center of one of the 2 disks
- to reduce the temperature increase, air is blasted during grinding or in construction to avoid oxidation, cooled inert gas is used. blast of inert gas helps prevent accumulation of particles in between the pins and on the disks
