Woven wire and screens

Media made from metals have already been mentioned, in Section 2C, in the form of sheets and tubes made from metal fibres and powders, usually sintered together to retain their structure. By far the greatest amount of filter media made from metal, however, is in the form either of woven wire or perforated sheets, which are covered in this part of Section 2, with the conmaon features of high strength, and corrosion and abrasion resistance conveyed by the basic material of construction. The other conunon feature, of great importance to their use for filtration, is that 

It should be noted, however, that the edge filters described in Section 2E are also mainly made of metal and made with precise apertures. It should also be noted that every type of filter medium described in this part as made from metal can also be made of plastic materials, with almost as much strength and almost as much precision in aperture size. Corrosion resistance may actually be higher with plastic wire screens or sheets than for some metals. 

There are two major appUcations for which these metallic media are primarily used the separation of solid particles by size, and the coarse screening of gas or liquid flows ahead of some finer processing stage. In the coarser, macrofiltration processes these are very versatile materials all of the dry classification (sieving, sifting) operations are covered here, as are almost all of the applications of filters for straining and coarse filtration, both of which rely upon the precise size and shape of the apertures in the mesh or sheet. 

There are three broad classes of media covered under the above heading woven wire meshes, sheets perforated with a variety of holes, and elements made up from preformed materials such as rods or bars. Some overlap obviously exists between the woven meshes of this sub-section and the woven monofilament materials of Section 2C. 

In a woven wire mesh, each warp or weft wire bends where it passes over or under a wire of the other kind. This crimping of the wires occurs as part of the weaving process for fine wires, but the crimp has to be imposed on the wire above 

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Data for dry screening on a 20-mm square aperture vibrating screen (8) indicate that the screen is relatively efficient, giving an apparent bypass value of 0.5%, sharpness index of 0.8, and a cut size of 20 mm. On the other hand, results (9) from a plant operating an 8 ft x 20 ft (2.4 m x 6.1 m) double-deck screen with 16 mm woven wire bottom screen deck gave an apparent bypass of 4%, sharpness index of 0.56, and a cut size of 17 mm. Data (10) for smaller... 

For non-Newtonian fluids in slow flow, friclion loss across a square-woven or fuU-twill-woven screen can be estimated by considering the screen as a set of parallel tubes, each of diameter equal to the average minimal opening between achacent wires, and length twice the diameter, without entrance effects (Carley and Smith, Polym. Eng. Set., 18, 408-415 [1978]). For screen stacks, the losses of individual screens should be summed. 

Screen Surfaces The selection of the proper screening surface is veiy important, and the opening, wire diameter, and open area should all be carefully considered. The four general types of screening surfaces are woven-wire cloth, silk bolting cloth, punched plate, and bar or rod screens. 

Flexible metallic media are especially suitable for handling corrosive liquors and for high-temperature filtration. They have good durability and are inert to physical changes. Metallic media are fabricated in the form of screens, wire windings, or woven fabrics of steel, copper, bronze, nickel and different alloys. Perforated sheets and screens are used for coarse separation, as supports for filter cloths or as... 

Sustained commercial application, however, did not occur until 1997, when Zheng et al. (7) described the successful stripping of oxygen from water used in secondary oil field recovery. In 1999 Trent et al. (8,9) introduced the first commercial application involving simultaneous absorption, reaction, and stripping. Both of these involve gas/liquid contact using a woven wire screen for the rotor internals. 

Purpose of the screens is primarily twofold (1) to change the melt s spiraling motion, caused by the screw rotation and (2) to filter contaminants out of the melt. Most plastics contain contaminants and these particles can be conveniently removed by means of a screen placed after the extruder barrel and before the melt flow reaches the extrusion die. The simplest means for filtering plastic melts are woven wire mesh disks of about the same diameter as that of the extruder barrels. Several... 

The broad classes of packings for vapor-liquid contacting are either random or structured. The former are small, hollow structures with large surface per unit volumes that are usually randomly dumped into the tower. Structured packings are normally layers of elements fabricated form expanded metal, woven wire screen, or sheet metal, and are stacked into the tower very carefully. 

For highly exothermic and fast reactions the catalyst is often deposited on the outer surface of the support which is usually of very low porosity (e.g. FjOj on SiC for o-Xylene oxidation) (Ellis, 1972). In other applications (e.g. ammonia oxidation converters) the catalyst is in the form of a woven wire screen (or gauze) which is usually supported on a non-catalytic pad to prevent premature ignition (Gillespie, 1970). 

The material on solids was abstracted in part from Gilson Co. literature by permission. Sieves for most laboratoiy work are round and vary from 3 to 12 inches in diameter. They are made of brass to be less static and magnetic. The screen itself is either woven wire, electroformed metal, woven cloth, or machined plastic. Each sieve is designated by its mesh. Mesh is the number of holes per linear inch of screen. A 10 mesh screen would contain 10 holes along 1 inch. Caution A 10 mesh screen has 1 Ox 10 or 100 holes per in not just 10 holes as you might suspect. 

SCREEN ANALYSIS STANDARD SCREEN SERIES. Standard screens are used to measure the size (and size distribution) of particles in the size range between about 3 and 0.0015 in. (76 ram and 38 pm). Testing sieves are made of woven wire screens, the mesh and dimensions of which are carefully standardized. The openings are square. Each screen is identified in meshes per inch. The actual openings are smaller than those corresponding to the mesh numbers, however, because of the thickness of the wires. The characteristics of one common series, the Tyler standard... 

Industrial screens are made from woven wire, silk or plastic cloth, metal bars, perforated or slotted metal plates, or wires that are wedge shaped in cross section. Various metals are used, with steel and stainless steel the most common. Standard screens range in mesh size from 4 in. to 400-mesh, and woven metal screens with openings as small as 1 m are commercially available.f Screens finer than about 150-mcsh are not commonly used, however, because with very fine particles other methods of separation are usually more economical. 

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