Department Editor: Scott Jenkins

Wire Meshes

in Filtration

Fabricated metal-wire mesh is one of the major categories of filter media. Woven metal cloths are often used

to support a filter aid coating or a bed of granular or porous material. Proper selection of wire cloth can have a large impact on the cost and efficiency of the system. This column provides information on the options available for wire meshes in industrial filtration applications.

Key questions for selectionBefore selecting a wire mesh for filtration, consider the following questions:

Is the mesh product capable of withstanding the system pressures encountered in the given application? Is the mesh designed to avoid clog-ging during filtration? In the event of clogging, will the mesh withstand the added strain or is it likely to collapse? Is the mesh sized such that particles are retained in normal use? Will the mesh allow the desired flow? Has the system been designed to minimize pressure loss? Have the corrosion properties of the fluid been considered relative to the mesh material? Are proper alloys available and specified? Will the product fit the filtration system without requiring field modification? Can the mesh be successfully back-washed, or must it be removed for cleaning? If it must be removed, is it easy to take out and put in again? What is the expected lifetime of the mesh?

Mesh sizeWire mesh is defined by the diameter of wires used and the spacing of the openings. Wire sizes may range from 1-in. in diameter to wire sizes finer than human hair. Mesh opening sizes range from 4 in. and larger down to one micron. Mesh is the number of openings

per linear inch. Square meshes have the same mesh count in both the horizontal and vertical direction. For example, a 40 40 mesh cloth has 40 openings per linear inch in both directions. A rectangular (off-count) mesh has a differ-ent number of openings in one direction than the other.

Computing meshTake the wire diameter and the size of the desired opening space. Add the two dimensions together and divide the total into one. For example, given a wire di-ameter of 0.035 in. and an open space dimension of 0.048, the total is 0.083. 1/0.083 12, which corresponds to the mesh number, or spaces in the wire cloth across one linear inch.

Weave typeWire cloth is produced on looms similar to those used for fabrics. Most wire cloth is made with a square configuration (square weave) with the same count verti-cally and horizontally (Figure 1). Dutch weave and twilled Dutch weave are also widely used (Figure 2). Dutch and twilled Dutch weaves do not produce square openings, but rather wedge-shaped open-ings which are not parallel to the surface of the cloth. These cloths serve in centrifu-gal and precoat filters, and make up the majority of the ultrafine wire cloths. One of the most commonly used wire meshes is 24 110-mesh size Dutch weave. It provides the strength, retention and surface characteristics for most filter-aids, and can withstand backwash and surface cleaning. It is rigid and has a smooth surface for ease of cake release.

Opening size In most applications, the function of the screen is to keep the filter media in place, while facilitating fluid flow through the bed. For example, in ion exchange applications, the resins are tiny beads of a uniform size. If a specification calls for 0.012-in. openings, it is important to maintain that size. An opening of 0.015 in. is too large, allowing the resin to leak out. On the other hand, 0.008 in. is too restrictive, impeding flow and leading to

clogging. Either situation will involve a noticeable performance drop.

Widths of wire clothWire cloth is traditionally woven in rolls 100 ft long and either 36 or 48 in. wide. In some cases, cloths as wide as 96 in. are woven. Finer wire cloths are usually available in standard 36-in. widths. Cloths of any desired dimen-sions can be produced by welding sections together.

Materials of construction Wire cloth may be woven of any mal-leable metal or alloy. The most common cloths are stainless steel, copper, brass and Monel. Aluminum, bronze, phosphor bronze, nickel, Nichrome, the Hastelloys, galvanized or tinned steel and steel are also readily available. Noble, rare and refractory metals are also used: gold and gold-plated metal; platinum; silver; tung-sten; molybdenum; columbium; tantalum; and titanium.

CalendaringWire cloth for filtration is often cal-endered, a process wherein it is fed through rollers to flatten and smooth the material. Calendaring improves retention and cake-release properties. The process involves feeding filter cloth through rollers to flatten and smooth the material.

Editors note: The editor would like to acknowl-edge Marvin Greenstein for supplying information for this column. Greenstein is the director of engi-neering for Newark Wire Cloth Co. (Clifton, N.J.; www.newarkwire.com), a supplier of wire cloth for multiple industries, including pharmaceutical, chemical, food, environmental and others.

FIGURE 1. Wire cloth offers a iltration sur-

face to separate solids from liquids. Most wire

cloth is made with a square coniguration

with the same count vertically and horizon-

tally, although many variations are available

FIGURE 2. Wire cloths employing Dutch

weave and twilled Dutch weave are the most

common, behind square weave

Newark Wire Cloth Co.

Newark Wire Cloth Co.