shaker screen performance

Material Factors

Particles in dry bulk materials are noted in a variety of shapes, sizes, surfaces, densities, and moisture content. Each condition must be taken into consideration when attempting to predict screen performance, through its effect on capacity in terms of weight passing a given screen opening per unit area. The combined effects on screen performance, or “screen ability”, of particle shape, surface texture, and surface or internal moisture, are beyond the reach of empirical solutions based only on size and density, independent of these variables. More exact information on their influence needs to be gained from actual laboratory testing.
 

SIZE AND SHAPE

The shape of an individual granule may be angular, spherical, acicular, ovaloid, flaky, or slabby. They can be combined with the same material, as sawdust in wood flakes. Separation cutpoint sizes in most screening applications range downward from 4” to 325 meshes (.0018”). The cutpoint defines the smallest particle size retained on the screen, and the maximum undersize particle passing. Unless the particle is acicular, platy, ovaloid or a perfect sphere, it will be probably (but not necessarily) be sized by its largest dimension5.
 

DENSITY

For any given shape and size distribution, bulk density in lb./cu. ft. (PCF) for any material will be precisely proportional to its specific gravity. Screening is essentially a volumetric measurement, but capacity, or the rate of passage through the screen, is typically charged in units of weight per unit time, based on a standard bulk density of 100 PCF. The actual rate for a material of different bulk density has to be adjusted by the ratio PCF:100. Tables of bulk density for various materials can be discovered in most material handling publications6.
 

MOISTURE

Moisture in granular particles may be absorbed, adsorbed, or both. Either condition can impair screen ability, but tolerance is much greater for internally absorbed than for exterior surface moisture. Surface moisture causes particles to stick together, resisting stratification. Allowable surface moisture for the unimpaired tedious screening of inorganic granular or pelletized particles ranges from bone dry for screen openings below 20 mesh, to 3% for 1/4” openings. Absorbed moisture in permeable soils such as ground clay can block the screen openings with cumulative buildups of extreme fines attached to the screen wires. Absorbent grains such as corn, soybeans, wheat etc. will screen freely after drying to 13-15% internal moisture. Screening of wood chips, flakes and sawdust are unimpaired up to about 30% of internal moisture; however, in laboratory tests with sawdust, efficiency was reduced by almost 60% when moisture was increased to 68%.
 

SIZE DISTRIBUTION

The size distribution of particles in a bulk granular material is the primary characteristic that governs the rate of undersized passage through a screen opening that is larger than the smallest particle and smaller than the largest particle in a representative sample of the material. Size distribution is measured by sieve analysis, using a series of standardized wire mesh sieves with square openings that progress, in the commonly used Tyler standard scale7, at the fixed rate of  √2 from 1.05” to.0029” (200 mesh). The size distribution is given in the weight percent of each fraction between successive sieves in a series. If the weight is plotted on the y-axis against the mean size of each fraction of the x -axis, the result will resemble a frequency distribution curve.