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The weave of any mesh is usually one of six basic types, but there are also some minor variations. Square mesh (Figure 2.9) has each weft wire passing alternately over and under each warp wire. The opening can be square or rectangular (much
less commonly) and so this weave is more correctly referred to as plain or double crimped weave. It is generally satisfactory in strength, from 10 to 60 Mesh, but finer meshes demand the use of such smaller diameter wires that there is an appreciable
loss of strength. The width of the opening, it will be appreciated, is limited by the diameter of the wire and thus finer meshes can only be woven from finer wires.

Twilled weave is the next strongest, with each weft wire progressing one wire at a time, alternately crossing over two and then under two warp wires, producing a marked diagonal pattern (Figure 2.10). This allows the use of larger wire diameters
for a given mesh and thus greater strength, but with proportionately smaller openings. This can be offset to some extent, if necessary, by reducing the number of weft wires, resulting in rectangular apertures.

For filtration purposes, the most widely used forms of woven wire are the Dutch or hollander weaves, wherein the warp and weft are of different diameter, generally with a corresponding difference in the relative numbers of warp and weft wires. If
the warp wires are thicker, the result is the ‘ plain Dutch weave ’ ; the alternative is for the weft wires to be the thicker, giving the ‘ reverse plain Dutch weave ’ .

Plain Dutch weave is the first of the zero aperture weaves (i.e. no openings can be seen at right angles to the mesh surface). It is actually a plain weave, with the larger diameter wires as the warp and straight, while the weft wires are crimped at
each pass (Figure 2.11 – which actually shows warp and weft as the same diameter). The range of cloths produced in this weave extend from 340 um down to about 15 um in aperture size (i.e. coarse to medium). The openings are small in size,
triangular in shape and not straight through the mesh. The cloth itself is firm and compact with good strength. Two variations of this type of weave exist, the first of which uses two warp wires instead of one and which is normally chosen for openings below 14m or when additional strength above 14m is required. The second uses much finer warp and weft wires, in flat groups of three or four, leading to better flow rates and higher contaminant tolerance.

Reverse plain Dutch weave is similar except that the thicker wire is in the weft ( Figure 2.12 ). Reverse plain Dutch weave is substantially stronger, and is in fact the strongest filter weave in commercial production; as a result, coupled with its good flow characteristics and high dirt-holding capacity, it is widely used industrially.

By a similar combination of warp and weft wires of different diameters, two basic forms of twilled Dutch weave are produced. The use of heavy warp wires

results in Dutch twilled weave ( Figure 2.13 ), which permits the production of the very finest grades of woven wire cloths, while also having the advantage of a very smooth surface on both sides; its disadvantage is a relatively high resistance to
flow. With heavy weft wires, twilled reverse Dutch weave is formed; this offers less resistance to flow but with a corresponding decrease in sub-micrometre retention characteristics and with rough surfaces on both sides.

A summary of the main types of weave for wire cloth is given in Table 2.9 , while Table 2.10 compares the performance characteristics of wire mesh media with other forms of metallic filter media.

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