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202 Waste Management Practices: Municipal, Hazardous, and Industrial

FIGURE 7.33 Vortex effect occurring within an air classifier. (Stessel,
R.I., Recycling and Resource Recovery Engineering, Springer, Berlin,
1996. Reproduced with kind permission of Springer-Verlag, Inc.)

FIGURE 7.34 Horizontal air classifier (NASA CR-2526 as cited in Domino, 1979).

All air classifiers use one of two types of air transport to aid in the separation. A positive-pres-
sure air transport system will push the MSW feed through the system. This is accomplished by
attaching a blower to the air classifier housing and creating a higher pressure within the system rel-
ative to the ambient environment. The other method, a negative-pressure air transport system, pulls
the MSW through. An exhaust fan is placed at the end of the system, creating a lower pressure
within the system. This has the same effect as a vacuum.
The extracted materials must be removed from the air stream once they are separated.
Following the air classifier, a cyclone separator is often used to separate the light fraction from the
conveying air. Before being discharged to the outer atmosphere, the conveying air is passed
through a dust collection system, typically a baghouse (Tchobanoglous et al., 1993). Alternatively,
the discharge air can be recycled back to the air classifier. The light fraction is stored in bins or
conveyed to another shredder for further size reduction before storage or utilization as a fuel or
compost feedstock.
In the cyclone (Figure 7.36), particles and air enter the chamber at a tangent, setting up a high-
velocity rotational air movement within the chamber. The solid particles, having greater mass, move
outward toward the inside wall, are slowed down on contact, and eventually drop out of the bottom