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 trommel screen 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 whilst performances with coarser aperture sizes do not.
