Francois's description of the use of twin screws may benefit from a more detailed explanation.

This useful technique is described in my book 'Guide to the design, specification and application of Screw Feeders'. The basic principle is to employ two, cantilever-mounted screws, facing each other and almost touching centrally, in a common casing that is fitted under a hopper outlet slot. The screw drives are at each end of the casing that extends from the hopper and an outlet is fitted at each end of the casing, which is at least two screw diameters from the nearest point of the hopper outlet. The length of the hopper outlet slot to serve a mass flow hopper is approx. three times the screw diameter.

The key to effective operation is that each screw drive is reversible and will rotate at half the revs/min of the alternative screw when that screw is delivering material to its own outlet. i.e The separate speed selection of the screw drives are rated to deliver the amount of material that is required at the outlet served by the nearest screw and half the output that is required at the alternative outlet.

The benefits of this system are: -

-That both screws extract material from the hopper outlet when in use.

-That material can be delivered to either outlet independently, or to both outlets at the same time.

-That the rate delivered to any outlet under any of the above circumstances is the design value.

-That there are no dead regions of extraction in the hopper outlet and an even 'draw' of material is extracted by each screw from each half of the hopper outlet length.

-The hopper is emptied completely, without dead regions of extraction.

-The hopper flow and self-clearing performance are not compromised by the fitting of separate outlets or feeders.

There are certain design requirements to be fulfilled in terms of the screw geometry, its accuracy of construction and the hopper interface. However, this is a most powerful technique for the controlled delivery of material to either, or, or both outlets from a single outlet, whether the hopper is of Mass Flow or non-Mass Flow operation. If the hopper is not a Mass Flow type it is practical to extend the length of the outlet slot again, subject to careful design considerations. Overall, it is best to secure the services of a specialist screw manufacturer, such as Ajax Equipment Ltd, to derive the maximum, trouble-free benefits from such arrangements. An extension of the principle to four outlet points allows controller discharge to be made from a single outlet to any one, two, three or all-four delivery points. ■