The more specific term screening is used instead of the more general size separation to avoid confusion. As is apparent in other chapters, the size of an object affects its behavior in other unit operations. For example, air
classification separates by the aerodynamic behavior of the object.
Size separation is one of the most important tools in bulk materials processing. As with others, it has its origins in other areas of activity. For example, in the wood products industry, it is common to use scrap to fuel boilers for heat and energy at remote lumber mills. Trees, when felled and dragged to the lumber mill, pick up large amounts of dirt. Scrap consists of branches and leaves not destined to be become lumber. Elaborate processing of this material would involve grinding in a tub grinder and pelletizing into a material that can then be fed into a boiler designed to accept coal. All the dirt and gravel must be removed, first, however; requiring screening. It is interesting to note that this example illustrates another industry, antecedent to modern resource recovery, wherein a waste material was processing to form an alternative energy source in the form of a coal substitute.
There are three types of screen in common employ: the shaker screen, the disc screen, and the trommel. The trommel far dominates the field, although the other two have clear areas of application. The discussion begins with a
discussion of the first two types of screening, together with early theory. The next part of the chapter discusses trommel concepts and theory. Finally, principles of trommel operation are elucidated in the final section, tying together theory and analyses conducted using that theory.
Two types of flat screens predominate: the disc screen and the vibrating screen. There are two cases where flat screens are better choices than trommel screens:
- where the finer material to be removed is more dense than the larger materials from which it is to be separated, and the larger materials have a relatively spherical configuration that will not prevent passage of the smaller materials towards the screen;
- where breakage is a significant problem. An example of the first case is the removal of sand from gravel. An example of the second case is removal of dirt from glass bottles; breakage of the glass bottles might seriously affect the success of subsequent processing and, as a consequence, sale value.
That made clear, one must make certain that flat screens are not being employed solely because they are perceived as simpler or less troublesome than tromrnels.
The two basic types of flat screen could not be more different in terms of complexity and retention of the inherent advantages of flat screens. Of course, in all cases, the engineer must both be familiar with all the variations on the
market, and must be willing to specify variations if such design changes are likely to better meet processing requirements. The most damaging procedure is to use a cheaper screen, be unaware of the differences in attributes, and conclude on the basis of that experience that no screening will work.
Shaker Screen. The simplest screen is the shaker screen, adopted in most cases as the first thought. Shown in Fig. 1, it shares mechanical principles with the vibratory conveyor. A motor with an eccentric weight on a shaft causes oscillatory behavior (the motor is not shown in the figure). The motor’s rotational speed, the mass of the eccentric weight, and the length and rate of the springs shown are all calculated to minimize motor power. In other words, the motor merely works against friction to cause the screen to continually oscillate at its natural frequency. The pivoting screen suspension shown may be replaced by a rolling trolley arrangement. The screen is inclined from the feed to discharge ends.
The screen oscillates along the direction of material motion. The oscillating mass is calculated to include the average feed load on the screen. Thus, if one is processing MSW, one cannot simply directly adopt a screen used for screening gravel from rock; the latter are far heavier for the same burden depth. The oscillation helps prompt the feed to move down the screen area in the direction of the incline. The vibration provided by the oscillation also mixes the material to prompt passage of the smaller particles.
It is clear that the shaker screen is a clear example of a device that would function better with items larger than the hole being sufficiently spherical to allow relatively easy passage of materials small than the hole size. Feed is
loaded into the screen from above at the left of the views shown in Fig. 1. The feed end of the screen has a wall to prevent material from exiting the rear of the screen because of the oscillation of the bed. The common error is to cause breakage by the end of the feed conveyor too high above screen.
Disk Screen. The disc screen has several advantages compared to the shaker screen. It has one big disadvantage: mechanical complexity. As shown in Fig. 2, the “screen” area of the disc screen actually consists of an array of
disks, several spinning on common shafts. The complexity lies in the shafts, their bearings, and the drive system that permits them to all turn in the same direction. As the shafts are all part of the screen, and therefore at the bottom
of the bed of feed, their bearings are all exposed to considerable grit, and must be shielded to prevent wear.
adjustable disc screen permits real-time adjustment of screen size to accommodate different loads arriving during the day.