Analysis of shaker unit parameters to separate whole-tree wood chips

The quality of whole-tree wood chips has been the focus of much attention during the past few years because of the increased use of lower grade wood über in pulp production. The need to up-grade the quality has resulted in the use of some form of screening system to separate acceptable wood chips from unwanted material at most pulp mills.

In order to upgrade the chip quality, a study was conducted to determine the effect of selected parameters of a separation system with an inclined, vibrating screen on screening efficiency. The parameters studied included screen aperture size and frequency of oscillation. Two screen sizes (3/8 and 1/4 inch apertures) and four different frequencies (1.25, 1.50, 1.75, and 2.00 Hertz) were considered. All other shaker unit parameters were held constant. Statistical analysis revealed that the lowest frequency resulted in the highest screening efficiencies for both the pin chips and fines categories. Screening efficiencies were higher with 3/8 inch screen as opposed to 1/4 inch screen, for all frequencies considered, but at 1.25 Hertz, the screening efficiency of fines was only 6 percent better with 3/8 inch screen.

This analysis revealed that the lowest acceleration vectors produced the best screening efficiencies. Velocity and acceleration vectors were directly related to frequency of oscillation since all other parameters were held constant for this study. A Programmer’s Hierarchical Interactive Graphics System (PHIGS) program was developed to graphically simulate the screen motion and to analyze the maximum velocity and acceleration vectors of the upper swing arm (input link). By interactively changing the shaker unit parameters, the screen motion was animated and viewed, with the calculated vectors used in the statistical analysis.