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As the term is most commonly used in filtration technology, a screen serves two prime functions: as a large strainer, especially for water intakes and in wastewater treatment, and as a device for separating mixtures of solid particles into two or more sizes, either in the dry state or in liquid suspensions. In formal terms, the strainers described in the previous chapter are also screens (as is shown in Table 3.1), the main difference being size, although, as with any attempt to draw hard and fast lines in a classification, some overlap between ‘ strainers ’ and ‘screens’ is unavoidable. Strainers are essentially protective devices, against the presence of oversize particles in a flowing system, while screens have the equally important size classification function (although the protective role in, say, water intakes is obvious). It should be noted that there are plenty of applications in which screens act like a coarse filter, to separate solids from liquids (in particular, dewatering wet solids).

In the filtration spectrum, screens (and strainers) are characterized by having filter media made from perforated plates, woven wire or wedge wire bars, usually with fairly coarse apertures – although woven wire-based media are available down to 500 Mesh (25 um), especially in test sieves. The material of the screen is usually metallic, although any form of metallic screen can be reproduced in plastic, with some loss of strength, but gain perhaps in material cost, and, in some applications, corrosion resistance.