Probably the best way to feed any fine light powder is to pass the material through a mass flow channel of sufficient cross section and residence time to allow it to de-aerate, to a feeder with a cross section larger than the critical arching size. This will secure reliable flow and a consistent state of density. The wall angle of the flow channel is determined by wall friction tests with the powder against a surface as of the hopper construction. A suitable designed screw feeder gives the flow and large cross section benefits of a Vee shape with a long slot outlet. The design of the supply hopper is a crucial feature. If the headroom is restricted for a conventional mass flow design there are various ways to expand the mass flow path by way of inserts or cone-in-cone fittings. Further compaction of the bulk solid can also be achieved in the feeder by change of the screw geometry and/or incorporating a plug restriction in the discharge tube.

Agitators are sometimes used to stimulate flow to screw feeders and assist in the de-aeration process. A disadvantage of this approach is that the flow regime is not predictable. This uncertainty of flow route is often compounded by the employment of screws with a relatively uniform construction. Such screws fill in the initial portion of the axis and cannot extract material from the hopper further along the screw axis, leaving regions of the supply hopper to be ineffective. This will then selectively extract material from the supply hopper, negating the prospect of mass flow, whatever the design of the hopper itself. If the size of opening required for flow cannot be secured with a single screw, then double or triple screws may be used to achieve the necessary cross section for reliable gravity flow with a fully predictable flow channel.

Some care should be exercised in the selection of make-up systems for loss-in-weight feeders for these duties, to avoid re-dilating the bulk material in the short replenishing period. Typically, such a system provides a two-minute usage supply in 10 to 15 seconds, so it is necessary to use a device that can ‘dump’ this quantity quickly, without unduly disturbing the bulk condition. One method that is economical in headroom is a twin-screw feeder that has right and left hand flights to a central outlet. This gives the effect of a quadruple screw capacity, which can be run at a fairly slow speed to discharge a large volume quickly and gently. Another technique is a large diameter, cone-type valve with a shallow focussing section underneath. This provides a large, vibrated cross sectional area to initiate flow of the powder in a dense condition and mild acceleration of the converging flow stream.

Lyn ■