Miller Magazine Issue: 132 December 2020

54 COVER STORY MILLER / december 2020 brating (vibro) allow reduction in sieve size by increasing the efficiency of separation. Vibrations can make separa- tions between heavier and lighter seeds of the same size or between particles of the same specific gravity but of different sizes. Perforated sheet metal screens are made from steel, tin-plated iron, aluminum, special alloys, cop- per and bronze. Holes may be round or oblong, and the width of the hole is determined by the separation to be carried out. Wire cloths differ in wire gauges (light or heavy), meshes and cross sections. The sieve aperture is selected based on the working principles of the machine and the kind of grain to be sieved. The performance effi- ciency of a screening machine depends on suitable screen surface, appropriate agitating motion and speed, and de- pendable screen cleaning mechanism. Separation is also made according to width. Particles tend to be arranged on a sieve in relation to the sieve movement. Particle passes through a screen opening that is only slightly larger than its width of grain. Addition- ally, the length of grain as axis is another separation di- mension. Particles in a mixture passing over a horizontal sieve that moves with a gyrating or rotary motion in the horizontal plane have no tendency to upend. Particles whose length is less than the diameter of the holes in the sieve fall through sideways, thus making a separation by length regardless of width. Screen positioning is important; grain flow on the back side of the screen surface from which the holes are drilled or punched. This ensures that the kernels are not caught in the screen. Coarse and fine screens with efficient aspi- ration are used to remove light dust and foreign material. Machine capacities and performance depends on design parameters, sieve motion, speed angle of inclination and sieve area. The machine makes three separations by size: foreign material larger than wheat, wheat, and material smaller than wheat. In addition, a fraction lighter than the wheat is separated with the help of help of air currents. The reel (precision grader or ring grader) is a rotating sieve cylinder that separates and sizes granular material by thickness, using cylinders with slotted perforations, and by width, using round perforations. A vibrating trough conveys through to the feed end of the unit, where a discharge spout is located. iii) Cleaning by using Shape Separation based on differences in shape is carried out in a disc or drum/rotary separator (trior, trieur, intended cylin- der). The disc separator comprises a series of discs rotating metal cylinder in which material is picked up by indenta- tions or pockets and thrown into a centrally located trough. Pocket shape influences the separation; howev- er, manufacturers have standardized some pocket shape for economic reasons. To make a good sep- aration with the disc separator, there must be at least 1/16 inch (1.3 mm.) difference in the length between the two separated fractions. Based on expected capacity and the amount of foreign material to be separated from grains, machines vary in disc diameter and number. In the cleaning process, trior or disc separators are divided into long-and short-kernel separations. In the short-kernel trior or disc separator, small seeds and particles that fit into the indented pock- ets are lifted. For example, for wheat kernel, the pockets of a long-kernel disc separator accommodate the size of whole kernels. The longer kernels of grain that do not fit into the trior or disc pockets are conveyed to the end of the equipment. For wheat, those are usually barley, oats or other seeds. At the outlet of a trior or disc separator, an adjustable gate can control the over-tail of the kernels that does not fit into the pockets and the depth of the grain in the equipment. Trieur (trior, indent separator or indent cylinder) is especially designed to separate bulk fractions different in shape is the trieur cylinder. The trieur cylinder, with indentations on its inner surface, is usually inclined to allow the mass of wheat to move by gravity to the end. Particles small enough to lodge in the pockets are raised to a certain point, after which they drop out into a trough and removed by a screw conveyor or different suitable conveyors. The rest of the material in the system tails over the end of the trior cylinder. The maximum capacity of the trieur is reached when the indented cylinder is ro- tating at a certain speed below that at which centrifugal force would prevent the lifted particles from dropping out at all. Two other machines that separate particles using difference in shape or in ‘‘roundness’’ are the band and spiral separators. The first separator is a short length, endless belt running over some rollers, the upper surface being slightly inclined and running uphill. iv) Cleaning by Specific Gravity Specific gravity is one of the most important proper- ty for particles such as stones, dirt (mud balls) or metal about the same size as grain kernels flow with the stream of grain from the sieve. Stones or some other foreign ma- terials heavier than grains may be separated by specific gravity differences, using air or water or vibrating as seg- regation aids. Equipment in which air current/flow, sieve stroke and screen inclination are variables that affect the separation performance. Specific gravity can also be used to separate heavier materials than grain, but additionally materials (ergot, rodent pellets, and heavily insect infest- ed kernels) lighter than grain. v) Cleaning by Air Currents Air channel, regular aspiration, round aspiration and ascending air current system are used mainly to sepa- rate light foreign materials such as dust, chaff, straw, leaves and sound grain. Fluids mechanics and aerody- namic principles are used such as fluidization, pneu-