It is not known when filtration was first utilized as a means of removing solid particles from water. It is known that the Phoenicians were pouring water through sand for such purpose as early as 4000 B.C. and we are still doing so today. Many methods of removing suspended solids from water are employed today. One is the simple screen.

Screen Structure

For this discussion we will define a screen as a frame or structural support covered with a homogenous network of openings used to separate solid particles from a liquid stream. Screens are generally constructed of metal, usually some form of stainless steel or brass, but may also be made of no-metallic materials such as plastics or nylon. The openings in a screen are of uniform shape and size and can be generally thought of as two-dimensional. Two basic types of filter screens are wedge-wire and weave-wire.

Wedge-wire screens are formed by laying stainless steel wires, having trapezoidal or "wedge-shaped" cross-sections, parallel to one another on a structural frame with a small gap between them. The openings therefore are long slots with the width of the slot the nominal filtration degree as shown in Figure 1. For wedge-wire screens the filtration degree is usually given the units of microns (one millionth of a meter) or thousandths of an inch.

Weave-wire screens are simply threads (wires) that are woven together like cloth. A simple two dimensional square-weave would have rectangular openings. Some manufacturers of square-weave material form a rectangular opening with the length of the rectangle 2.5 to 5 times longer than the width. The best square-weave is one with rectangular openings having the length and width of the rectangle equal defining a true square as shown in Figure 2.

However, most screens today do not use a square-weave but other patterns such as Dutch-weave, Double Dutch-weave and various Modified Dutch-weaves . Figure 3 shows an example of Dutch-weave.

Note that the openings are not rectangles but a three dimensional warped triangle. This type of weave is stronger and provides a higher porosity at small filtration degrees than a simple square weave. Wires tend to shift in square weaves after much use causing some openings to become smaller while others become larger. Dutch weaves tend to maintain opening size during usage.

Two conventions are used to define the filtration degree of weave-wire screens. The first is taken from the textile industry. In that industry the density of a woven material is expressed as the number of threads per linear inch referred to as "mesh." In the field of filtration the term mesh has come to mean the number of pores or openings per linear inch in the screen media. Although still in common use, the term "mesh" is not a true parameter of measurement since the actual opening or pore size depends on the diameter of the threads or wires and the type of weave used in the screen manufacturing process. The second and more preferred convention used to describe filtration degree is an actual linear dimension of the shortest distance across an individual opening or pore of the screen. This is most often given in microns (1 micron = 0.001 millimeters or 0.00004 inches.

Particle Properties

As one can imagine, the properties of solids particles can have a great impact on their retention by filter screens. All particles have three dimensions measures about three axis: X, Y and Z. If all three of these axial dimensions are greater than the filtration degree of the screen (think of a baseball or grain of sand), they will be retained on the surface of the screen regardless of its construction or opening shape. If one of these dimensions is less than the screen's filtration degree (think of a postage stamp or rust flake), there exists an orientation where these particles could slip through a wedge-wire screen slot opening. However, this shaped particle would still be retained by a weave-wire opening. If two dimensions are less than the filtration degree of the screen (think of a toothpick or fiber), two of the XYZ orientations would allow the particle to slip through a wedge-wire slot and one orientation would allow it to pass through a weave-wire opening. Therefore, particle shape has an impact on removal efficiency for both wedge-wire and weave-wire screen constructions but to differing degrees.

Plasticity of solids can play a big role in filter efficiency. A grain of sand is not going to change shape when it encounters a screen surface no matter how great the differential pressure becomes. (Differential pressure is the absolute difference between pressures upstream and downstream of the screen.) However, many solids are plastic or deformable and will change shape when they encounter an object (screen surface) or unequal surface pressures. When lying against a screen surface and the pressure upstream of the screen begins to increase relative to that downstream of the screen surface, a plastic solid will tend to deform and eventually extrude through the screen like Play-Doh pushed through a keyhole. To filter out plastic solids effectively, the differential pressure across the screen surface must be limited to less than the differential pressure that will cause extrusion. The rheological properties of a solid will also influence filtration. This property adds time to the equation relating to how slowly or quickly a solid will change shape under a given differential pressure. If a filter is automatic self-cleaning, plastic solids may not extrude if the filter cleans the screen surface in time intervals (not differential pressure measurements) short enough to prevent extrusion as dictated by the rheological properties of the solids.

To operate under a wide range of solid particle conditions, a filter must include a number of characteristics. It must be automatic self-cleaning with a cleaning mechanism that will not push built-up solids through the screen during the cleaning cycle so suction technology is best. The automatic cleaning cycle should be initiated on an adjustable differential pressure basis and a time basis. Wedge-wire and weave-wire screens should be interchangeable in the filter body for different operating conditions and these screens should come in a variety of filtration degrees. Figure 4 shows an example of a number of such filters on the market.

Orival, Inc.
213 S. Van Brunt Street
Englewood, NJ 07631
(765) 987-7843