Pressure Screening - The Distributions of Pulp Properties in the Screen Basket, 1998 Papermakers Conference Proceedings

This paper reports on changes in pulp properties in the screen basket. The experimental work reported here was performed with pilot-scale pressure screen apparatus with an axial feed and equipped with two axially and radially movable pipe samplers which allowed pulp to be sampled in the inner parts of the screen basket. In addition to these movable samplers, three fixed sampling pipes were located on the surface of the screen. The properties of the pulp were found to undergo a change in both an axial and a radial direction with respect to the screen basket. The axial change took the place because probabilities of the various fibre fractions passing through the screen vary, leading to penetration by smaller particles in an earlier part of the screen basket than by larger particles, which tended to remain inside the basket for longer. The radial variation in pulp properties was due to centrifugal force, which compressed the fibre network against the screen plate, whereas water and fines passed through. According to the probability theory of pressure screening, the properties of the pulp will also change in an axial direction in the accept chamber, along with the changing pulp properties inside the screen, thereby affecting the screening efficiency. An understanding of the axial and radial distributions of pulp properties in the screen gives us an insight into its functioning and a means for evaluating this.