The trommel length can be used in a recovery index by assuming that the screening rate is constant along the length of the screen. This assumption is not entirely correct, and results have shown that the screening rate decreases asymptotically along the length of the screen. However, for most practical applications the error is not large, although care must be exercised when using data from very long or very short trommels. The reason for this reduction in effectiveness has been explained elsewhere but is mainly due to the depletion of fine material at the screen surface and then to the subsequent rate at which additional fines become available for screening due to liberation or breakage effects. However, there is little variation in unit recovery along the length of the screen beyond the first metre or so of the trommel. Thus, a recovery per unit length can be calculated from the total recovery to undersize, and the screening length. A more general term could possibly be established if the particle-to-aperture size ratio were used as opposed to data based on discrete size ranges with particular screen aperture sizes. However, results from trommels with significantly different aperture sizes do not compare well. This is not surprising, since the nature of the oversize material can vary substantially with screen aperture size. For example, the oversize fraction of refuse screened at 200 mm consists mainly of paper, textiles and plastic film whilst the oversize fraction of refuse screened at 50 mm will contain metals, dense plastics, and putrescibles in addition to the paper and film plastics. However, if screen sizes are grouped together then there is some agreement between results and this may further improve the design guide. Invariably, performances with screen aperture sizes between 40 and 60 mm compare well in garbage sorting system whilst performances with coarser aperture sizes do not.
