-- 5: Screening --
Screening is used for a variety of operations including cleaning,
and removal of solids from liquids. This topic discusses screening
from the viewpoint of the separation of solids with a variety of sizes
into two or more fractions each with less size variation.
Screening terminology
Undersize = fines or minus (-) material, material passing through a given screen
Oversize = tails or plus (+) material, material retained on a given screen
Screen aperture = the space between the wires of a screen mesh.
Screens are sometimes denoted by mesh number (number of wires per inch).
In this case, aperture actually depends on the wire diameter,
but for standard screens, wire size is specified.
Screen interval = the relationship between successively decreasing openings
in a standard screen series.
In the Tyler Standard the ratio between screen apertures in successive screens
is 20.5, though at finer sizes a screen interval of 20.25 is used.
In British Standards and American Society for Testing Materials,
the screen interval is 20.25.
Diameter of a sieve fraction = the average diameter of the fraction
passing one screen, but retained on the next larger screen. This is normally
taken as the arithmetic average of the two screen apertures.
Diameter of solid particles = this is defined as the particle dimension
controlling its retention on a particular sized screen. Since particles
are usually irregularly shaped, so an average diameter is used. A number of methods
of measurement may be used. In practice, average particle diameter is often
determined by sieve analysis, with a spherical shape assumed for the sake of calculation.
Types of industrial screens
Grizzlies or bar screens: Consist of a set of parallel bars, used for
screening larger particles - greater than 25 mm. The bars are often wedge shaped
to minimise clogging. They may be used horizontally, though are normally inclined
at an angle (up to 60o), and may be shaken mechanically or electromagnetically.
Vibrating screens: Consist normally of a frame supporting a wire mesh
or perforated plate, though a bar screen similar to a grizzly at closer spacings
may be used (eg "wedgewire" screen). They can be shaken mechanically
or electromagnetically. They may be organised as multi-deck units.
One of the best known suppliers of vibratory screening equipment is
SWECO -- (look under "Products").
Reels or trommels: Revolving cylindrical screens, mounted almost horizontally.
The capacity of a trommel increases with increasing speed of rotation
until a critical speed is reached. At greater speeds the material does not
cascade properly over the screen, but is carried around due to centrifugal force.
The critical speed of a trommel is given by:
N = 42.3 / D0.5
where
N is the number of revolutions per minute
D is the diameter of the trommel in metres
Factors affecting the efficiency of a screening system
Rate of feeding: If feed rate is too high, there is insufficient residence time.
The screen becomes overloaded, and some "fines" leave with the oversize.
Particle size: Large particles can impede the path of smaller ones,
and a preliminary separation may be required if a high proportion
of larger particles are present
Moisture: Moisture can cause adhesion of small particles to larger ones,
so some undersize leave with the oversize
Worn or damaged screens: Oversize may fall through damaged areas
Blinding (clogging) of screens: Particularly likely when the size
of particles is very close to the screen aperture.
Result can be undersize leaving with oversize.
Electrostatic charge: When screening dry powders, surfaces can become charged,
resulting in small particles clumping together and leaving with the oversize.
Grounding of screens may be necessary.
-- 6: Questions --
1. You need to monitor particle size of product from a flour milling operation.
Describe the steps and procedures which would be required (that is, where
and how product would be sampled, etc.)
.
.
.
.
.
.
2. You wish to measure the size distribution of fat globules
in homogenised milk. Suggest ways in which this may be done.
.
.
.
.
.
.
3. For the "Sieve Analysis" practical, you are required to perform
a sieve analysis on a sample of milled material. Comment on possible
sources of error when using this method of analysis.
What steps should be undertaken to prevent or minimise these errors?
.
.
.
.
.
.
-- 7: References --
Brennan, J.G., Butters, J.R., Cowell, N.D., and Lilley, A.E.V.
-- Food Engineering Operations (Elsevier Appled Science, London & New York, 1990),
Chap. 4, pp 85 - 90
Svarovsky -- Powder testing guide: Methods of measuring the physical properties
of bulk powders (1987). Published on behalf of the British Materials Handling Board
by Elsevier Applied Science, London.
Toledo, R.T. -- Fundamentals of Food Process Engineering (1994) pp 538 - 540