Wire mesh (filtration)

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Wire mesh (square mesh)

Wire mesh are sheets with similar openings in regular arrangement, the rectangular through interlacing of warp and weft wires are made on looms. They are used for filtration in many places where defined and reproducible filtration properties are important. Mesh fabrics are open wire constructions, the mesh size of which determines the size of the particles retained by the sieve. Filter mesh fabrics, on the other hand, are wire meshes in which either the warp or weft wires are so close together that no mesh is visible in the projection (zero mesh), so that a very fine filtration is generated.

Mesh fabric

Mesh fabrics are open wire fabrics with openings (meshes) between the spaced-apart wires. A distinction is made between different bonds . The wires usually have a round cross-section, but square wires are also used for special applications.

Plain weave

Plain weave

The plain weave , also called " plain weave", is the most common type of weave in wire mesh. Warp and weft wires are alternately passed over and under the cross-wire. Square meshes fix the wires very well, so that the mesh size is well defined and the screens are dimensionally stable. Typical for the plain weave are filter-effective cross-sections of over 30% and a high number of meshes, which means that the flow resistance remains low. The dirt storage capacity and the regenerability of the sieves is good. Mesh sizes down to about 60 µm can be achieved with sufficient fabric stability  . In addition to filtration, plain weave screens are also used in screen printing . Wire meshes with standardized mesh sizes are used for the sieve analysis in order to determine the grain size distribution of bulk materials.

Special wire mesh with a rectangular mesh shape is available as long or wide mesh, depending on whether the longer mesh size is oriented in the warp or weft direction. For reasons of stability, wires of different strengths are used in the various directions. This type of fabric is used as a vibrating screen or in vibrators.

Twill weave

Pointed or interchangeable twill
Simple twill weave

The twill weave offers many variation options. It is used to advantage when smaller pores are required and the wire would be so thick in relation to the mesh size that, taken individually, it could not withstand the deformation during the weaving process. With the simple twill weave, the weft goes alternately under two warp wires and then over two warp wires. The next weft wire is offset relative to its neighboring wires, so that an overall diagonal pattern (ridge) results on the surface.

The twill weave results in an open wire mesh with smaller mesh sizes from around 20 µm with relatively thick wires. The wires are not as well fixed here as with the plain weave, so that meshes with a slight diamond shape or different side lengths can arise. Under load, the twill weave is less dimensionally stable and shifts slightly in a diagonal direction. At over 25%, the effective filter cross-section, like the number of meshes, is relatively high. The flow resistance is low, but the dirt storage capacity is still good. In order to avoid the problem of diagonal displacement, the direction of the ridge can be changed using an interchangeable body, so that a herringbone pattern is created and the dimensional stability is increased.

Five-shaft twill (satin weave)

Five-shaft twill (underside)
Five-shaft twill (top)

In a five-shaft twill weave , every fifth warp wire is tied in with the weft wire. This type of weave creates a smooth surface on the top with parallel weft wires, while the bottom is open. Since the surface is very smooth, a filter cake that forms can easily be detached. The open underside, however, offers good drainage. The rectangular meshes are usually smaller than the diameter of the wires. The weave allows minimum mesh sizes of around 50 µm. The flow rate is high and the stability of the sieve is high.

Tissue parameters

Wire mesh terminology according to DIN ISO 9044

In addition to the type of weave, various filtration-relevant fabric parameters according to DIN ISO 9044 specify a mesh fabric (dimensions greater than 1 mm are usually given in millimeters, values ​​below in micrometers):

The most important parameter is the mesh size (clear distance between the wires) between two adjacent warp or weft wires, measured in the projection direction. The mesh size defines which particle sizes are retained by the fabric. The wire diameters of the warp and weft, on the other hand, provide information on the stability of the wire mesh.

The filter-effective open sieve area is given in percent and is defined as the area of ​​the mesh divided by the total area. This parameter is important to estimate the pressure build-up or the dirt storage capacity.

The division is the distance between the central axes of two neighboring wires and thus the sum of the nominal values ​​of mesh size and wire diameter . Thus: . The related mesh fineness (number of meshes per unit length) is particularly common in Anglo-American measurement systems. The unit mesh (number of meshes per inch or equivalent ) is used for this. However, this only says something about the real mesh size if the wire size is known.

Weft

In the wefts, either the warp wires or the weft wires are so close together that there are no more open stitches. The filtration takes place through the gaps, some of which are inside the screen. If the warp wires are close together, one also speaks of reverse braid or armored braid.

Wefts in plain weave

Armored tress
Smooth braid
Cross-section parallel weft wire of the smooth braid (above) and parallel warp wire of the armored braid (below)

With the so-called "smooth weft", the weft wires are knit tightly to the fabric when weaving in plain weave, so that zero stitches are created. The warp wires are further apart and are thicker than the weft wires. Due to the strong attack during weaving, the weft wires deform slightly at their crossing points, so that they are slightly flattened at the contact points. The wire spacing of the weft wires is therefore up to 6–12% smaller in the smooth braid than would be expected from the diameter of the wires. Compared to mesh fabrics of the same filter fineness, a braid fabric shows significantly higher mechanical stability and resilience.

In the case of a simple smooth braid, the filtration takes place inside the fabric at triangular openings between the warp and weft. The free effective cross-section is less than 20% and the dirt storage capacity is low. Cleaning is more difficult than with mesh fabrics and is done by backwashing.

By varying the wire thickness and the wire spacing of the warp wires, the filtration properties of the smooth braid can be varied. If you use very thin weft wires, the filtration shifts from the inside of the fabric to its surface. The significantly higher number of openings also increases the dirt storage volume and the flow rate. The effective filter cross-section reaches values ​​of 27 to 42% depending on the version. The ability to regenerate is also improved, so that this type is usually referred to as an optimized smooth weave or high-performance filter weave.

So-called "reverse wefts" or "armor weaves" are also woven in plain weave. In this type of weave, the warp wires are thin and tightly arranged next to one another. The thick weft wires are beaten close together. The openings therefore run obliquely to the tissue surface. The effective filter cross-sectional area is between 25 and 38% depending on the structure. Mechanically, this type of filter is very stable and tear-resistant.

Body stress

Twill sieve

In the twill weave, as in the plain weave, relatively thick warp wires and thin weft wires are used; the latter are woven here in a twill weave. The weft wires are so close together that, compared to a smooth braid, double the number of weft wires are incorporated with the same wire diameter, creating a light-tight fabric. The filter fineness is significantly better than with the smooth weft and filter fineness down to 1 µm can be achieved. The high degree of filtration accuracy (uniformity of the openings) enables a very high degree of selectivity to be achieved. On the other hand, the effective filter cross-sectional area is the smallest with this filter type. The consequences are a reduced dirt storage capacity, accelerated growth of the filter surface and thus a faster pressure build-up. The regenerability of the twill weave is poor, as the dirt primarily gets stuck within the fabric. Mechanically, the twill is very stable.

With the "wide-mesh twill" (also called "open twill"), the weft wire is not twisted so closely together. Therefore this type of braid is not light-tight. The selectivity is lower than that of a normal twill, but it is sufficient for many applications. In return, the flow rate improves considerably and the pressure loss is also lower. At the same time, the regenerability is better. The smooth surface on both sides makes cleaning easier.

In the case of the reversed twill weave (also called "twill armor weave"), the warp wires are thin and the weft wires are thick. Since the weft wire is wrapped in a twill weave, the warp wires are not deformed as much as is the case with the normal reverse weave. They are therefore less mechanically preloaded. The body armor weave withstands high mechanical loads. Flow rate and filter accuracy are good.

Marking of filter cloth fabrics

The weave is identified by the type of weave, the number of warp and weft wires per inch (25.4 mm) and the thickness of the wires used before weaving. However, due to the dense weave, the braid fabrics cannot be assigned any mesh sizes, only filter fineness. A distinction is made between absolute and nominal filter fineness. The absolute filter fineness indicates the diameter of the glass balls which the sieve securely holds back (> 99.98%), while the nominal filter fineness is smaller. It depends on the measuring method and indicates the particle size of round glass spheres in micrometers, a large part of which (e.g. 70%) is retained by the filter. The actual particle size filtered out depends not only on the exact shape and deformability of the particles but also on the pressure difference.

Delivery forms, post-processing and materials

Smooth weft (high-performance variant, calendered) with flattened tips

Single screens are usually delivered as rolled goods or in the form of ready-cut pieces. Some of these are treated appropriately after weaving. After bright annealing in a protective gas atmosphere, the wire meshes are completely flat, clean and mesh-proof. Coarser mesh fabrics , on the other hand, are calendered (rolled smooth). This also makes them flat and more solid. Various wire meshes can then be combined to form a filter package and framed on the side, or complete filter units can be sold. In filter candles, the wire mesh is used as a filter in either a smooth or pleated form.

The material of the wires depends on the intended use. The choice of material depends in particular on the chemical and mechanical specifications of the wire mesh and on a cost-benefit analysis. Simple wire meshes consist of bare or galvanized steel. Stainless steels offer a certain degree of acid or chemical resistance. Wires made of spring steel have three to four times the strength of normal steel wires and can be chosen to be much thinner with the same mesh size. For special applications, however, non-ferrous metals such as brass , bronze , copper, aluminum or nickel, and occasionally precious metals such as gold, silver or platinum and materials such as titanium or nickel-based alloys such as Hastelloy are processed.

Properties and areas of application

Wire mesh and, in particular, braid mesh are constructed in such a way that they have a uniform and exact filter geometry over the entire surface, which results in a very good selectivity. Compared to other filters, wire mesh has excellent mechanical stability and strength. They are also heat-resistant, largely chemical-resistant and durable. In addition, many wire meshes can be cleaned and regenerated.

Wire mesh is used in very different industries and processes: chemistry, paints and varnishes, plastic and synthetic fiber production, food industry, industrial and cooling water filtration , oil and natural gas extraction, hot gas filtration , laboratory and analysis, aluminum casting and many others. In addition to the application of filtration, wire mesh is also used for sieving or as sieves in screen printing .

literature

  • Gerhard Schönbauer: Wire mesh (screen mesh) for filtering plastic melts . In: Gesellschaft Kunststofftechnik, Association of German Engineers VDI (publisher): Filtration of plastic melts (plastic technology) . VDI Verlag, Düsseldorf 1981, ISBN 978-3-18-404080-2 , p. 163-186 .
  • DIN ISO 9044: Industrial wire mesh - Technical requirements and testing (ISO 9044: 2016) . Beuth Verlag, Berlin.
  • Philipp Kopf: Modeling of dust filtration on a micro and macro level under the influence of complex boundaries . Ed .: Institute for Mechanical Process Engineering at the University of Stuttgart. Logos Verlag Berlin, Berlin 2015, ISBN 978-3-8325-3898-9 ( google.de ).

Individual evidence

  1. a b c d e f Gerhard Schönbauer: Wire mesh (screen mesh) for filtering plastic melts . In: Gesellschaft Kunststofftechnik, Association of German Engineers VDI (publisher): Filtration of plastic melts (plastic technology) . VDI Verlag GmbH, Düsseldorf 1981, ISBN 978-3-18-404080-2 , p. 163-186 .
  2. 3D model of the square mesh. (animated PDF) Spörl oHG Precision Wire Weaving, accessed on May 15, 2017 .
  3. a b c d e f g h i Metal wire mesh / types of weave. Wiremesh ProTec GmbH, accessed on May 17, 2017 .
  4. a b c Basics 1: Metal wire mesh for sieves and filters. (PDF) PACO, Paul GmbH & Co. KG, accessed on May 17, 2017 .
  5. a b 3D model for twill weave. (animated PDF) Spörl oHG Precision Wire Weaving, accessed on May 15, 2017 .
  6. a b c d Wire mesh - Technical list. Wire mesh terminology according to DIN ISO 9044. Haver & Boecker, accessed on May 21, 2017 .
  7. 3D model of the smooth braid. (animated PDF) Spörl oHG Precision Wire Weaving, accessed on May 15, 2017 .
  8. 3D model of the armored tress. (animated PDF) Spörl oHG Precision Wire Weaving, accessed on May 15, 2017 .
  9. 3D model of the twill weave. (animated PDF) Spörl oHG Precision Wire Weaving, accessed on May 15, 2017 .
  10. GKD - Gebr. Kufferath AG: Köpertressenweb 3D PDF. In: gkd.de. GKD - Gebr. Kufferath AG, February 1, 2015, accessed on September 7, 2017 .
  11. a b c d e Basics 2: Filter cloth. (PDF) PACO, Paul GmbH & Co. KG, accessed on May 17, 2017 .
  12. Definition of terms. (No longer available online.) Schwegmann Filtrationstechnik GmbH, archived from the original on July 4, 2017 ; accessed on July 1, 2017 .
  13. VDI 3677 sheet 3: 2012-11 Filtering separators; Hot gas filtration (filtering separators; high-temperature gas filtration) . Beuth Verlag, Berlin. P. 20.
This version was added to the list of articles worth reading on December 23, 2017 .