This invention relates to a centrifugal separator of the type which includes a centrifuge bowl mounted for rotation about an axis of the bowl, means for feeding a material to be separated to a base of the bowl and a peripheral wall of the bowl over which the materials flow, the peripheral wall being shaped to effect separation of heavier materials for collection from lighter materials which discharge over an open mouth of the bowl. 
One example of a centrifugal separator of this general type is shown in U.S. Pat.  No.  5,338,284 of the present inventor issued Aug. 16, 1994 which discloses a bowl having a plurality of recesses at axially spaced positions along the peripheral wall.  Each recess has a plurality of holes through the peripheral wall for the injection of fluidizing water into the bowl to fluidize the materials within the recess.  Each recess has in addition a plurality of discharge openings which allow the heavier collected materials in the recess to discharge radially outwardly through the peripheral wall for collection. 
The bowl is generally of the type shown in U.S. Pat.  No.  4,608,040 and in U.S. Pat.  No.  4,846,781 of the present inventor. 
Another example of centrifuge bowl of this type is shown in U.S. Pat.  No.  3,823,869 of Loison which discloses a conical peripheral wall so that the feed material moves along the peripheral wall to a discharge slot which can be opened to release the heavier materials.  A futher example of an arrangement of this type is shown in U.S. Pat.  No.  4,824,431 of McAlister.  A yet further example is shown in U.S. Pat.  No.  1,882,389 of Mclsaac. 
A further patent of McAlister which is WO 93/13864 shows the same construction as shown in his above earlier United States patent in which the collected material is discharged radially outwardly under control from a plurality of pinch valves. 
The separation technique disclosed in the above patents of Knelson using the plurality of axially spaced recesses with the injection openings at the base of the recesses has been established as a particularly effective separation technique which allows a high throughput while maintaining losses at an acceptably low rate. 
However one disadvantage of the technique using the plurality of axially spaced rings is that of a high requirement for water for injection into the bowl through the fluidizing openings.  This requirement for a high quantity of water has the disadvantage that the water must be later separated from the slurry discharged from the mouth of the bowl and also limits or prevents the use of the technique where the required amount of water is not available.