Page 213 - Materials Chemistry, Second Edition

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184 Waste Management Practices: Municipal, Hazardous, and Industrial
paper, aluminum, and glass containers. Removal of coarse or abrasive components early in the process
will reduce the load on a shredder (see below) which will therefore reduce shredder maintenance.
Waste behaves within the trommel in distinctly different ways depending on the speed of rota-
tion. Waste rotating very slowly within the cylinder will travel only slightly up the sides and will
immediately slide back, thus missing most openings. This is known as cascading (Figure 7.13).
Waste that is rotated more rapidly within the cylinder will rise up farther, and then tumble and slide
back. This cataracting motion causes substantial turbulence of the waste. At even higher speeds the
material may adhere to the inside of the trommel and will not effectively tumble or fall by gravity
through the screens, i.e., it tends to centrifuge. The so-called “critical speed”, i.e., the frequency of
rotation at which the force of the trommel on the waste holds the materials against the wall through-
out a complete revolution, is given by the equation (Vesilind et al., 1988)

2
n (g/4 r) ½ (7.6)
c
2
where n is the critical speed (rotations/sec), g the acceleration due to gravity (cm/sec ) and r the
c
radius of trommel (cm).
The ideal trommel rotation speed is that immediately prior to the point where the waste starts
to centrifuge; in other words, it climbs the side of the trommel and then falls immediately upon
reaching the zenith of rotation, which is the upper limit of the cataracting type of motion. This cre-
ates the greatest opportunity for waste particles to fall through the screen openings.

EXAMPLE 7.4
Calculate the critical speed for a trommel screen having a diameter of 3.2 m.

2
n (g/4 r) ½
c
2
(980/[4 (3.14) (320/2)]) ½
0.39 rotations/sec
Some trommels are equipped with horizontal lifter bars along the inside which help to carry
waste part-way up the side of the drum. Any additional upward motion depends on the rotational
speed of the drum (Pfeffer, 1992).
Separation Efficiency with Trommel Screens
The speed of rotation plays a role in the trommel’s separation efficiency via agitating the waste
input. The tumbling action of waste within the trommel efficiently brings about separation of
individual items that may be attached to each other, or of one material contained within another. The

FIGURE 7.13 Cascading, cataracting, and centrifuging of waste input in a trommel screen (Stessel, R.I., Recycling
and Resource Recovery Engineering 1996. Reproduced with kind permission of Springer-Verlag, Inc., Berlin).