Screening is important operations. They rely on separation of particles according to their size. Screening in performed using screens equipped with one or two screen surfaces called also decks. A single screen provides two products.
- The product from the top is called the oversize or plus product (QG)
- The one which passes through the screen is the undersize or minus product (QD)
Material passing one screen surface but remaining on the next one in a double deck screen is an intermediate product.
During ideal screening, the plus product contains only particles greater than the size of openings of the screen surface while minus product consists of particles smaller than the opening.
In real operations the oversized particles are present in the minus product and some undersized particles are in the plus product. It results from adhesion of small particles to large ones and that some particles never get into contact with the screen surface.
The presence of large particles in the undersize product may be caused by greater than nominal openings or rupture of the screen surface.
To Calculate the Screen Opening (%):
Ao = Open area expressed as %
L = Diameter of the wire in mm
L = Hole size for square type holes
A = Angle of inclination of the screen to the Horizontal.
For the inclination
Square opening in Mesh is calculated as
Example:
A screen has 16 mm opening made of 6 mm wire and inclined at 20 degrees to horizontal,
The Open area is without inclination
The Open are with Inclination of 20 degree i.e. net opening area in %age is
Square Opening in Mesh is
