food technology

1. SCREENING
Anchal, 18
Food tech ,7th
sem

2.  Separation of mixture of particles of various sizes into two
or more fractions by a screening surface is called
screening.
 This method is based only on the size of the particles.

3. Some general terms…
 Screen : Surface containing a number of equally sized
apertures (plane or horizontal).
 Screen aperture: The distance between two adjacent
wires
 Mesh number: The number of openings per lineal inch.
 Screen interval: The relationship between decreasing
openings in a standard screen series.

4. Screening terminology
 Over size material : material that retain on the screen.
 Under size material: material that passes through the
screen.
 Intermediate material: when two screens are used the
material remain on the second screen .
 Unsized function: a single screen can make a single
separation into two fractions.
 Sized functions: when materials passed through the
series of screens then it is divided into many fractions.

5. Construction
 Metal bars
 Perforated plates

6. Cont..
 Silk or plastic cloth
 Woven wires
Material used for handling food stuffs :
stainless steel, Monel metal ,nylon fibre.

7. Different screen series are in use
 Tyler standards (4-325um)
 British standards (5-300um)
 ASTM-E 11 (4-325um)
 IMMS (5-200um)
 FSS (17-38um)

8. Industrial screens commonly used
Bar screens:
- Used for screening
larger particles
(>25mm).
- Consists of set of
parallel bars, spaced to
the desired separation.

9. Cont…
- Bars are of wedged
shaped to minimize
clogging
- Screens may be
horizontal or inclined,
also vibrating grizzlies
available.
- Feed passing over is in
series of jerks

10. Inclined bar screen

11. Vibrating screen
- Consist of a frame
supporting a wire mesh
or a perforated plate.
- Shaken mechanically or
electromagnetically.
- Inclined or horizontal
- Single screen or multi-
deck.

12. Mechanical Electromagnetically

13. Trommels
- Revolving cylindrical
screens.
- Horizontal or slightly
inclined.
- Screen surface is wire
mesh or perforated
plate.
- Wire mesh or Fabric
screens with powders
and circular perforations
with fruits and vegs.

14. Cont...
- Hexagonal cross
sections used,helps in
separatiopn of fine
materials
- Capacity of trommel
increases with
increasing speed of
rotation until a critical
speed is achieved.
Where
N= number of revolutions per unit
D= diameter of trommel

15. Factors affecting
 Rate of feeding
 Particle size
 Moisture
 Damaged screen
 Blinding of screen

16. Applications
 Regeneration of filtration panels.
 Slotted and perforated screens
for vibrating sifters
 Slotted and perforated screens
for calibrators
 Perforated segments for feeders
 Conveyor belts
 Slotted and perforated segments
for angular filters

17. cont…
 Filtration mats
 Regeneration of basket for starch
centrifugation
 Screens for trays for smokehouse
trolleys
 Arch screens
 Filter segments for cleaning brine
 Perforated screens for air separators
 Screens woven for stone separator

18. References
 http://www.slideshare.net/vicky937/separatio
n-screening-and-
http://www.sswm.com.au/Stainless-Steel-
Woven-Mesh/More-Information
 http://www.kason.com/Centrifugal-Screeners-
Separators/
 Brannon