Ideal and Actual Screens

An understanding of the process of screening requires that ideal screening and actual screening are studied comparatively. The aim of a screen is to receive a feed consisting of a mixture of particles of different sizes and separate it into two fractions, an underflow that is cleared through the screen, and an overflow that is refused by the screen. Either one, or both, of these streams may be a product , and in the presentation that follows no discrimination is made between the overflow and underflow streams. 

An ideal screen in essence sharply separates the feed mixture in such a fashion that the smallest particle present in the overflow would be just bigger that the biggest particle present 

The closest separations are acquired by standard testing screens and with spherical particles. Even in this case, an overlap occurs between the largest particles in the underflow and the smallest particles in the overflow. The overlap is particularly marked when the particles are needlelike or fibrous, or where the particles tend to form collections or groups that act as big particles. Some long, thin particles may hit the screen sidewise and be retained. Industrial screens commonly provide poorer separations as compared to testing screens of the same mesh opening while working on the same mixture. 

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