Many types of drum screen are used for sorting small-particulate foods
(for example nuts, peas or beans) that have sufficient mechanical strength to
withstand the tumbling action inside the screen. Drum screens are almost
horizontal (5–10º inclination), perforated metal or mesh cylinders. They may be
concentric (one inside another), parallel (foods leave one screen and enter the
next (Figure 5.4)) or series (a single drum constructed from sections with
different sized apertures). All types have a higher capacity than flat bed screens
and problems associated with blinding are less severe than with flat bed
screens. The capacity of drum screens increases with their speed of rotation up
to a critical point. Above this the food is held against the screen by centrifugal
force and results in poor separation. Similarly there is an increase in capacity
with the angle of the screen up to a critical angle. Above this the residence time
is too short and products pass through without separation