Flow resistance of screen plate apertures

Pulp screens remove fibre bundles, plastic specks and other oversize contaminants from pulp suspensions before the pulp is made into paper. Removal efficiency increases as the size of the apertures in the screen plate is reduced. Smaller apertures do, however, increase the hydraulic resistance of the screen plate and may cause an excessive loss of screen capacity. There is considerable incentive to reduce hydraulic resistance so that smaller, more efficient, apertures can be used. The objective of this study was to learn what determines the hydraulic resistance of screen plate apertures, and how resistance can be reduced. Hydraulic resistance was assessed using the nondimensional pressure drop coefficient (K) across the screen plate, and K was studied by three methods. Computational fluid dynamics (CFD) was used to predict how flow variables and aperture geometry affect K in an idealized screening configuration. Experiments with a flow channel were used to confirm the theoretical CFD findings and explore how fibre accumulations at the screen aperture would affect K. Finally, trials with an industrial pulp screen showed how industrial variables influenced K. This study determined that the recirculating zone at the slot entry has a dominant influence on K. Both the size of the recirculating zone and K could be reduced by increasing the ratio of slot velocity to upstream velocity. In one typical case, an increase in this velocity ratio (V[sub]N) from 0.2 to 1.0 caused K to decrease from 10.8 to 2.2. Optimizing the shape of the slot entry was also found to reduce K. When a simple recess was made at the slot entry, the value of K (at V[sub]N = 1) decreased from 2.2 to 1.5. A hydraulic model of a commercial screen was used to assess the relative contribution of the screen plate resistance to the total pressure loss through the screen. For water flowing through a particular screen, the resistance of the screen housing was more than six times that of the screen plate. For pulp flow through the same screen, where apertures may have been partially blocked by fibres, the resistance of the housing was still double that of the screen plate.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat