The Flow Resistance of Slotted Apertures in Pulp Screens

Pulp screens remove fibre bundles, plastic specks and other oversize contaminants from pulp suspensions before the pulp is made into paper. Within the pulp screen is a screen cylinder that acceptable fibres pass through but oversize contaminants do not. Apertures in the screen cylinder are in the form of holes or slots, and their size is perhaps the most critical variable in screening. Smaller apertures increase the removal efficiency of contaminants, but also lead to a reduction in screen capacity. The development of screen plate “contours” in the early 1980s led to a revolution in pulp screen design. By locating apertures within recesses on the screen plate surface, smaller, more efficient, apertures could be used without a significant loss in capacity. Various theories have been proposed to explain the action of these contours. It may be that contours increase the turbulence at the aperture entry, which fluidizes the pulp and clears fibres from the aperture. It may be that the contours streamline the flow through the aperture to reduce hydraulic resistance. Alternatively, contours may alter the streamlines through the aperture to reduce the tendency for fibres to become immobilized at the slot entry, which is a precursor to blockage. What is clear is that understanding the action of the contours is critical to the full exploitation of this important development in screening technology. The objectives of the present study were: (1) to create a frame-work for assessing the flow resistance of screen plate apertures; (2) to learn how aperture size, screen plate contours, fibre blockages and other factors of practical importance affect resistance; and (3) to develop a more fundamental understanding of what determines resistance, and how this knowledge could be used to increase screen performance. Flow resistance was assessed using the non-dimensional pressure drop coefficient (K) across the screen plate, and K was studied by three methods. Computational fluid dynamics (CFD) was used to predict how aperture geometry and flow variables 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 affect K. Finally, trials with an industrial pulp screen showed how industrial variables such as pulp consistency and pressure pulsations influence K. CFD analysis determined that the vortex at the slot entry has a dominant influence on K for water flow. The size of the vortex was reduced by increasing the ratio of slot velocity to upstream velocity, a quantity termed the “velocity ratio” (VN). The relationship between K and VN was defined by two regimes: a “descending regime”, where K decreased rapidly with increased VN, and a “constant regime” where K was relatively independent of VN. Examination of the flow patterns revealed that for smooth slots, the vortex on the upstream side of the slot diminished in size in the diminishing regime. The flow then approached a pattern that was relatively unaffected by further increases in VN (constant regime). The presence of a contour at the slot entry led to the expected reduction in K. This study showed that the effect of the contour is dependent on VN. At low values of VN, the contour actually caused K to exceed the value for a smooth slot. The precise dimensions of the contour are critical to its effect. For a step-step type of contour and VN = 0.5, the optimal contour for simple hydraulic resistance had a depth of 0.25 mm and step-width of 0.50 mm. An increase in the contour depth to 1.0 mm caused K to double and to exceed the value for when there was no contour at all. Experimental measurements of K were made for steady flow through slots in a plexiglas channel. Good agreement with the CFD findings was obtained for both smooth and contour slots. To assess the influence of fibre accumulations, the instantaneous value of K was monitored as a fibre accumulation grew. In one typical case, a fibre accumulation filled half of the slot width and caused K to increase from 4.3 to 9.8. This finding underlines that flow resistance is due to both the hydraulic resistance of the slot and the added resistance due to fibre accumulations within the slot. Pilot plant tests were conducted to assess K in an industrial-scale pulp screen. The screen was modelled as a series of resistances combined with the pumping effect of the rotor. One could thus infer the resistance of the screen apertures by measuring the overall pressure differential across the pulp screen, and then by discounting the influences of the screen rotor and housing. The findings for water flows were in agreement with the CFD and flow channel work: The descending-constant form of the K-VN relationship was again found, and the values of K were comparable to those in the CFD and flow channel work. The use of a 1.5% pulp suspension instead of water caused K to double in one typical case, indicating substantial accumulation of fibre within the slots. This study has defined K as the essential measure of flow resistance and shown how it can be measured through theoretical, flow channel or pilot plant tests. The value of K was found to depend on both hydraulic resistance and the degree of fibre blockage in the screen slot. Screen plate contours were found to reduce K by streamlining the flow, although it is recognized that they may also reduce the tendency for fibres to accumulate within the slot. Values of K can thus be used in the development of improved screening technology, and to compare the performance of screen cylinders from different suppliers. The use of K is also important for process control routines that estimate the extent of fibre blockages in screen plate apertures and act to prevent screen failure.

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