Influence of screen parameters optimization on screening efficiency of feed hammer mill

Abstract: Traditional hammer mill generally has the problem that the screening efficiency is lower than the grinding efficiency. In order to improve the screen efficiency of the hammer mill, based on the combination of experiment and numerical simulation method, the influence of the structure parameters of the hammer mill screen and the air flow velocity in the separation device on the screening efficiency was studied, the mechanism of screening was revealed, and the way to improve the screening efficiency of the crusher was explored. According to a type of hammer mill designed by research group, we designed a screening test bench to simulate the air flow field in the separation device of the hammer mill using a blower as the air flow source. In this experiment, 5 kinds of particle sizes through screen were selected, and the influences of 4 factors including the screen hole shape, the screen installation angle, the screen hole arrangement and the air flow velocity on the particle screening efficiency were investigated. In the experiment, 10 kinds of screens were adopted, which were composed of 5 kinds of screen hole shapes and 2 kinds of screen hole arrangements, and there were 11 screen installation angles and 8 kinds of air flow velocity levels selected. The 5 kinds of screen holes were the rectangular hole, the oblong hole, the hexagonal hole, the square hole and the circle hole. The 2 kinds of screen arrangements included "U" type and "T" type arrangement. The experiment showed that the screening efficiency was high and then became low after the air velocity increased; the screen hole shape and screen installation angle had lager effect on the screening efficiency; the arrangement of the screen holes had less effect on the screening efficiency. Also the EDEM-FLUENT software was used to simulate the process of particle screening; the simulation and the experiment results were basically consistent, and the relative error of the two was not more than 4%. Based on this, the research found the optimum combination of the structure parameters and the flow parameters of the screen: when the air flow rate was 6-15 m/s, the screen efficiency of the material was gradually increasing and then decreasing; when the air flow velocity was 6-7 m/s, the phenomenon of material passing through screen was not obvious, mainly because air was not sufficient to drive the movement of particles when the material was under the influence of gravity; when the air flow velocity was more than 7 m/s, the screen efficiency increased, and when air flow speed reached 12.37 m/s, the screening efficiency reached the optimal value, but when the air flow speed continued to increase, the screening efficiency decreased; when the air flow velocity reached 14.21 m/s, the air flow began to negatively impact the particles passing through the screen and the screening efficiency decreased. This was because when the energy of air flow was greater, the particle reached the screen panel with greater speed for the first time, and if they first failed to pass through the screen, they would hit the screen surface to rebound, thereby reducing the secondary screen penetration probability. When the screen installation angle was in the range of 20°-70°, the screening efficiency decreased with the screen installation angle, and the best screen installation angle was 35°-45°; if the screen's opening rate was the same, the screening efficiency of the rectangular hole was the highest among the 5 kinds of screens, and the screening efficiency of circle hole was the lowest of all; the screen hole arrangement had little influence on the screening efficiency. Results suggest that when the air flow velocity was 12.37 m/s, and the screen installation angle was 40°, the rectangular "T" type arrangement had the highest screening efficiency of 0.72-0.74.