Paper media filter sheets

A common surface cartridge is the pleated paper constmction type, which allows larger filtration areas to be packed iato a small space. Oil filters ia the automobile iadustry are of this type. The paper is impregnated, for strength, with epoxy or polyurethane resia. Any other medium ia sheet form, similar to cellulose paper, such as wool, polypropylene, or glass may be used. 

A eonsiderable proportion of hydrauhc filters are of the in-line strainer type (Figure 5.27), with an easily replaceable cartridge element, very often supplied in duplex form so that the element in one can be changed, while the other keeps filtering. The most common medium is a pleated paper or nonwoven sheet, fitted with protective layers front and back to provide the necessary strength (Figure 5.28). 

The filter medium can be fibrous, such as cloth granular, such as sand a rigid solid, such as a screen or a mat, such as a felt pad. It can be in the shape of a tube, sheet, bed, fluidized bed, or any other desired form. The material can be natural or man-made fibers, granules, cloth, felt, paper, metal, ceramic, glass, or plastic. It is not surprising that filters are manufactured in an infinite variety of types, sizes, shapes, and materials. 

Containers, media and seeds. Glass Petri dishes, 100 x 15 cm, were used with two sheets of 75-mm Whatman 41 filter paper as the absorptive medium. Ten seeds of TAM105 wheat were placed in a radial pattern with the micropyle end toward the center between the two sheets of filter paper. Seeds were hand-selected for normal size and absence of damage. TAM105 was selected because it is the variety used in the ongoing field research on conservation tillage practices. The bottom section of each Petri dish cover was covered with a square of kitchen-type plastic wrap to retard moisture loss before the lid was pressed on. 

The basic technique is quite simple. A sheet of cellulose filter paper, such as Whatman No. 1, serves as the separation medium. For one-dimensional paper chromatography (PC), the paper is cut into strips about 5 cm wide and 20 cm long for two-dimensional PC (below), a 20 X 20-cm sheet is commonly used. The papers come in various porosities (fine, medium, coarse) the porosity determines the rate of movement of the developing solvent. Low-porosity paper gives slow solvent movement but good resolution. Thick papers, which have increased sample capacity, are available for preparative separations. 

The resultant porous sheet can be used as a filter medium, but if the untreated paper gets wet then the fibres absorb moisture, with two significant consequences the fibres swell, so that the spaces between them reduce and the paper improves in filtration efficiency, but the mechanical strength drops sharply, making the paper less useful as a filter. To be used as a filter, then, the paper must be fully supported for use in wet filtration (as, for example, in the filter papers used in the laboratory filter funnel) or it must be restricted to dry filtration (such as in building air cleaning). 

The progress of cellulose paper as a filter medium has been considerably affected by the manufacture of fibres of other materials that can be formed into paper-Uke sheets by adapting the conventional paper-making process the outstanding example of this is the variety of glass fibre papers, which are of major importance in filtration. The other has evolved by exploiting the characteristics of the synthetic fibres formed by the extrusion of molten polymers adaptation of this extrusion process enables these fibres to be formed directly into the paper-like sheets of the spun-bonded media discussed below, which have taken a share of the markets formerly supplied by wet-laid media. 

The pad filter (often called a cassette) is a special case of the range of panel filters, all of which are made in standard sizes, to fit air-conditioning installations. The pad, as its name implies, is a thick flat sheet of fibrous filter medium, made either by wet-laying (as in paper) or by dry-laying (as in a felt). Felt pads are the most common, and they can be either as-laid, which would normally be the case for natural fibres such as cotton or wool, or needle-punched, for synthetic fibres. 

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