Classifiers
Centrifugal
Operating Principles
The patented design and unique operating functions of the Centrifugal Classifier are detailed in Figures 5, 6
and 7.
Feed material and gas (usually air) enter the classifier inlet (1). The connecting duct for pneumatically conveyed
material can be positioned from a vertical to a horizontal position to suit layout requirements (see dashed outline
Figure 5). Gas inlet velocity is approximately 4,000 feet per minute, dependent upon feed material physical characteristics
and cutpoint. Conveying velocity may be higher. For pneumatically fed, open air systems, the classifier inlet is
flared and the feed dropped directly into the air stream.
The sharp bend (2) behind the inlet separates feed material from the gas steam by centrifugal action. The resultant
"clean" gas stream passes behind a baffle plate (3) against which the feed material is sliding. The air stream then
crosses the curtain of feed material (4) producing an intense scrubbing action which separates fine particles form
the tailings, breaks up agglomerates and subjects all particles to an equal drag force.
Gravitational force immediately precipitates any very large particles to the bottom of the classifier. Intermediate
and finer particles flow with the gas stream in a spiral path around the exhaust orifice (5) and are classified.
The baffle plates (3, 6), the classifier outer casing (7), and side plates (8) form a flat, cylindrical classifying
chamber (9) through which the gas stream spirals inwardly in a two-dimensional flow.
Each particle is subjected to centrifugal force (Fc) proportional to the cube of its diameter, causing the particle
to move toward the periphery of the chamber. Concurrently, the gas stream is exerting a drag force (Fd) directly
proportional to the diameter of the particle. Under design conditions, the centrifugal force (Fc) equals the drag
force (Fd) of a particular particle diameter ( K) (cutpoint). These two opposing forces which are in equilibrium
only at cutpoint separate feed particles into two groups. Centrifugal force (Fc) causes particles larger than cutpoint
(greater mass) to be propelled outwardly, impinging on the peripheral walls; their velocity is slowed; gravity causes
them to settle to the bottom of the classifier. Drag force (Fd) causes particles smaller than cutpoint (lower mass)
to be swept inwardly and discharged with the gas stream through the orifice (5) where they are collected by a cyclone.
To obtain sharp separation, forces acting on cutpoint particle (K) must be in equilibrium through the classifying
chamber assuring that every introduced particle is subjected to the same separation influence. The centrifugal classifier
uniquely meets this requirement with its patented method of introducing feed material and gas flow into the classifying
chamber to form a controlled vortex with a constant velocity profile. Appropriate classifying chamber proportions
and specialized orifice design eliminate detrimental frictional drag effect from the chamber side walls (8) without
moving parts.
Centrifugal classifiers separate at any desired cutpoint from 15 to 100 microns (1 micron = 1/25,400 inch). The
cutpoint is controlled by the vortex flow path steepness, the tangential velocity (Vt) and the absolute dimensions
of the classifying chamber. The flow path is controlled by the ratio of exhaust orifice diameter to the classifying
chamber diameter and the amount of secondary air introduced at the bottom of the unit (11). Customers' varying cutpoint
requirements are met by regulating secondary air flow.
Power requirements are low. The resistance to gas flow or draft loss to effect classification ranges from 1"
to 7" W.G. depending on cutpoint, classifier size andapplication. The volume of gas feed ratio averages between
300 and 500 cfm per ton per hour of classifier feed.