Sieving is a unit operation in which mixture of various sizes of solid particle is separated into two or more fractions by passing over screen. It is applied in industry to determine shape and size of any samples. Sieve analysis has two main objectives.
a. To find out shape of material
b. To find out size of material
In general, there are two types of sieves.
1. Wire woven sieve
2. Perforated sieve
1. Plain: Single wire woven alternately up and down
2. Twilled: Double wire alternately up and down
General terminologies in screening/ sieving
Screen: It is a surface containing number of equally sized apertures or opening. The screen surface may be plain (horizontal or inclined) or it may be cylindrical. Small sized plain screen are called sieves.
Oversized material: material that is retained on screen surface is called oversized material. They are also called tails or plus materials which fails to pass through sieve.
Undersized material: Material that passes through screening surface are called undersized material. They are also called fine or minus material.
Intermediate material: when two screens are used for screening, the material that is retained on the second screen is called intermediated material.
Screen aperture: it is the maximum clear space between the edges of the screen opening. It is usually given in inches or mm. in wire mesh screen, it is the space between two individual wires.
Mesh: Interwoven strands with small openings.
Mesh number: number of aperture per unit linear inch. For example; a 4 mesh size screen means there are 4 little square openings across one inch of screen. A hundred mesh screen has 100 opening per inch and so on. As the number indicating mesh size increases, size of opening (aperture) and thus size of particle captured deceases.
Screen intervals: the relationship between successively decreasing opening in a standard screening series.
Sieves may be designated by opening size and can be classified on the basis of different screen intervals.
1. Tyler standard
2. British standard
3. US – Bureau Standard
Tyler standard: This is widely used series based on a 200 mesh screen having 0.0021 inch diameter wire and screen aperture of 0.0029 inch. The ratio between apertures (screen intervals) in consecutive screen is 1:√2.
British Standard: This series is based on 170 mesh screen having 90 μm diameter wire and screen interval of 4√2.
US- Bureau: In this series, wire of diameter 1 mm is used with screen interval of 4√2. This standard series is adapted by international organization for standardization (ISO). Current sieve designation, unless specified, refer to US- Sieve series. Size of particles are usually designated by mesh size that retains particles that pass through the next larger screen size.
Factors affecting rate of screening:
1. Rate of feeding: If the rate of feed is too high, screen becomes overloaded and materials that are capable of passing remains with oversized materials.
2. Angle of inclination: The angle of inclination of screen influences the residence time. Too steep angle will result in insufficient residence time, while too small angle will cause only gravity flow across the screen.
3. Particle size and shape: Even though sufficiently small, the particle will only pass the screen if its alignment relative to opening is favorable.
4. Moisture: Presence of moisture in feed may cause adhesion of large and small feed particles.
5. Worn or damaged screen: Oversized particles will pass through damaged area and efficiency of separation will be impaired.
6. Binding / clogging of screen: Binding or clogging of opening is likely to occur when size of particle is near to that of screen aperture.
It is the measure of success of screen in closely separating materials. Let ‘A’ and ‘B’ be two closely separating materials. If screen functions properly, all the material of ‘A’ would be in overflow and all the material of ‘B’ would be in underflow. A common measure of screen effectiveness is the measure of ratio of oversized material ‘A’ actually in overflow to the amount of A + B entering the feed.
Screen effectiveness = A (overflow)/ A+B (feed)
A process in which an aggregate is separated into its various sizes by passing it through screen of various size openings for the purpose of determining the distribution of quantities separated. It is accomplished by passing known weight of sample successively through finer sieves and weighing the amount collected on each sieve to determine % weight in each size fraction.
Cut off diameter: An ideal screen would sharply separate the feed mixture in such a way that the smallest particle in the overflow would be just larger than the largest particle in the underflow. Such an ideal separation defines cut- off diameter (DPc) that marks point of separation between the fractions.
Fineness modulus: in 1925, Duff Abram introduced the concept of fineness modulus (FM) for estimating the proportion of fine and coarse aggregates in concrete mixtures. Thus FM is an index to the coarseness or fineness of an aggregate. Mathematically, it is defined as sum of cumulative percentage retained on standard sieve divided by 100. A smaller value indicates finer aggregates. Fine aggregates ranges from 2.0 to 4.0 and coarse aggregate (smaller than 38.1 mm) ranges from 6.5 to 8.0 FM.
Type of sand on basis of fineness modulus (FM)
|Type of sand||Fineness Modulus|
|Fine sand||2.2 – 2.6|
|Medium sand||2.6 – 2.9|
|Coarse sand||2.9 – 3.2|
Average particle size / mean particle size:
Average particle size gives the average diameter of the particle in the given sample. It is evaluated by carrying out screen analysis through set of series of sieves. The feed sample is fed to the top most sieve and material collected in each sieve is taken and weighted to find out mass fraction of the material on each sieve. Particle size of material retained on any sieve (n) is the mean of screen opening of that sieve and that of sieve above it (n-1). Fraction size is obtained by multiplying mass fraction of sieve (n) with particle size of sieve (n). The total sum of fraction size is the mass mean diameter of the particle.
ASTM, E. (2013). Standard specification for woven wire test sieve cloth and test sieves. ASTM Standards.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.