Classifiers
Gravitational - Inertial
Operating Principles
Gravitational-Inertial classifiers utilize patented classifying principles combining gravitational, inertial, centrifugal and aerodynamic forces. Figures 3 and 4 graphically illustrate the operating principles.
Air or gas-entrained feed material enters the classifier primary air inlet (1). Mechanically fed material is introduced at point (1A). Primary air inlet velocity ranges from 3500 to 6000 feet per minute.
The curtain of feed material drops in front of the air outlet (2) which is provided with widely-spaces vanes (3) almost reversing the gas flow introduced through the primary air inlet (1). Prior to passing through the vanes, the friction of the relatively high velocity causes the particles to flow in a counterclockwise current (eddy) (5) in the chamber (4). The eddy is reinforced by gas entering through the secondary air inlet (6) located just above the coarse discharge outlet (7).
Each particle entering the classifier has a gravitational force (Fg) proportional to its mass, which is in turn proportional to the cube of its diameter. As the particle is introduced into the classifier primary gas stream, it is further subjected to an inertial force (Fi) also proportional to its mass.
Because the gas stream flows in a downward direction, the inertial and gravitational forces (Fi) and (Fg) complement each other. As it passes through the vanes, the gas stream changes direction, exerting a drag force (Fd) proportional to the diameter of the particle and approximately opposite in direction to the gravitational and inertial forces. As the particle is influenced by the drag force (Fd) and changes direction, it is subjected to a centrifugal force (Fc) proportional to is mass, which directly opposes the drag force (Fd). Under design condition, the resultant force (R) acting on a particular particle diameter (K) (cutpoint) is of a magnitude and direction to cause the particle to either be swept through the vanes and be thrown back into the feed curtain. The resultant force (R) on particles larger than (K) is in a direction at small variance with the gravitational-inertial forces. The particles will either impinge on the vanes and be knocked out or fall directly into the coarse discharge (7).
The eddy current (5) flowing downward, parallel to the place formed by the vanes, provides a moving wall containing the curtain of feed material in the classifying zone without the detrimental frictional drag effects of a solid wall.
The particles not swept through the vanes fall onto an inclined baffle plate (8) located at the bottom of the gas outlet (2) directly beneath the primary gas inlet (1). The coarse product is scrubbed by the secondary air as it slides off into the coarse discharge outlet. Secondary air flow dislodges any fines adhering to coarser particles. They join the stray fine particles entrained by the eddy current and are returned to the classifier inlet point (9) and reintroduced into the classifying zone (10).
Buell Gravitational-Inertial classifiers separate particles at any desired cutpoint from 200 to 50 mesh (74 to 297 microns). The cutpoint is controlled by the air velocity through the vanes which determines the magnitude of drag force (Fd) and the primary air inlet velocity which determines inertial force (Fi). Varying cutpoint requirements are met by regulating the inlet velocity while keeping the total air volume, i.e., vane velocity, constant.
Power requirements are extremely low. The minimal energy loss is due solely to the change of direction of the air stream as it is exhausted through the vanes. Resistance to gas flow or draft loss to effect classification ranges from 0.5" to 3" W.G., depending on 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 normal volume of gas-to-feed ratio is 300 cfm per ton per hour of classified feed.