The improved pneumatic rice polisher system consisted of no moving part. Basically, it comprised of two parts - an upper cylindrical part and a conical bottom part; similar to that of a cyclone separator as shown in Figure 1. In the present study, a system having diameter of the cylinder and height of the polisher as 0.2 m and 1.06 m respectively was selected. The cylindrical part consisted of an inlet port (feed inlet) provided with 180° scroll. This facilitated two-phase tangential flow of air and brown rice at the entry. The inside surface of the polisher was layered with three grades of hard abrasive particles (Concord, India). These were categorized as fine (FN), medium (MD) and coarse (CR) depending upon their grit sizes as commonly expressed in industry; like 100, 60 and 36, respectively. Corresponding average particle dimension were 122, 254 and 483 m. Air velocity of 30 m s-1 was used. This flow rate was adequate for generating high centrifugal force on the rice grains (terminal velocity of rice is around 6.0 ms-1) for its movement along the abrasive surface.
The final stream of bran and polished rice mixture at the outlet passes through the bran separator where bran particles escape through the perforations and deposited in the outer chamber of the cylinder. Partially polished rice is collected at the bottom of the system. Due to less residence time of rice inside the polisher, required degree of polishing cannot be achieved in a single pass. The collected rice was recycled for 60 times and the broken content and degree of polishing was measured after every 10 passes. The air escaped at the top and carried very fine bran particles with it. Experiments were conducted with brown rice of 10, 11, 12, and 13% (wb) moisture content.
This study was intended to identify and define important parameters required for assessing the separation performance of a tray-type paddy separator and to determine the optimum conditions of the inclination of separator-table. The results of the study were summarized as follows : (1) Separation performance can be given by use of many parameters found from the measurement of separator outputs. In addition, their modes of variation due to a varied inclination of the separation table did not indicated in the same direction . However, the efficiency of separator had an advantage to indicate the overall characteristics of the separator performance with a clear optimum inclination condition of the separator table. (2) The optimum feed rate and maximum efficiency of the separator did not occur at the identical inclination of the separator table. Thus, for its practical operation it is necessary to compromise between the qualitative and quantitative performances of the separator. (3) The optimum setting of the separator table tested in the study showed to occur at its inclination of approximately for the front and for lateral.