A model of fibre fractionation by slotted screen apertures

Pressure screens improve pulp quality by fractionating fibres for specific processes or products. Fibre passage ratio, P, is an important screening performance parameter since it relates the passage of fibres at individual apertures to the fractionation efficiency of the overall screen. A theoretical expression for fibre passage ratio was derived based on a fibre concentration gradient at the screen plate surface. The theory demonstrates that fibre passage ratio is a function of the dimensionless penetration number, which combines the key screening variables: fibre length, slot width, rotor tip speed and average fluid velocity through the aperture. The effect of these variables on fibre passage was determined experimentally under conditions similar to industrial screening. The experimentally determined fibre passage ratio was shown to be approximately characterized by a single function of penetration number and to agree reasonably well with theoretical predictions over a wide range of conditions.