Classifiers
Gravitational
Operating Principles
Feed
material is dropped in a continuous feed curtain and enters the
classifier at the top of the unit (1). Low velocity air enters the
classifier at inlet (2) and is forced through the feed curtain (3)
which is dropping in front of the air outlet (4) provided with widely
spaced vanes (5). The air stream enters the feed curtain
perpendicularly, but is changed to an almost parallel, but reverse,
direction to the feed curtain as it passes through the vane.
Each particle entering the classifier has a gravitational force (Fg)
proportional to its mass, which in turn is proportional to the cube of
its diameter. The particle is also subjected to a drag force (Fd)
proportional to the square of its diameter, created by the air flow
through the feed curtain. As the particle is influenced by the drag
force (Fd) and changes direction, it is subjected to a small,
centrifugal force (Fc) proportional to its mass, directly opposing drag
force (Fd). Under set conditions, the resultant force (R), acting on a
particular particle diameter (K) referred to as the cutpoint, will be
of a magnitude and direction such that the particle will either be
swept through the vanes by the gas stream or impinge on the vanes,
throwing it back into the feed curtain. The resultant force (r) on
particles larger than (K) is in a direction at small variance with the
gravitational force (Fg). These particles will either impinge on the
vanes and be knocked out or will fall directly into the coarse
discharge outlet (6). Smaller particles will have a resultant force (R)
almost parallel to the drag force (Fd), permitting them to be swept
through the vanes and collected by a Buell cyclone.
Buell gravitational classifiers separate particles at any
predetermined cutpoint between 100 mesh and 10 mesh (150 to 1651
microns). The cutpoint is controlled by the air velocity through the
vanes which determines the magnitude of drag force (Fd)
Power requirements are extremely low; most energy lost is due to the
change of direction of the air stream as it is exhausted through the
vanes. To effect classification, the resistance to gas flow or draft
loss ranges from 0.5" W.G. to 4.0" W.G., depending on the cutpoint and
feed-to-air ratio. The higher the cutpoint, the higher the draft loss;
the gas flow through the vanes must be increased to provide a high drag
force. The volume of gas-to-feed ratio ranges from 150 to 300 cfm per
ton per hour of classified feed.