The fill of a horizontal screw conveyor is normally related to the nature of the bulk material, where the relevant factors are flowability, in regard to the absence of cohesion, and the contact friction value on the flight surface. 45% of the swept volume is generally the maximum fill level recommended for very easy-flow, low friction bulk products, such as grain. Intermediate bearings are an obstacle so it is prudent to de-rate slightly, and arasive materials are generally handled at lower fill and certainly low speed. Low inclinations call for more conservative fill values and a crude rule is that at 30 degree slope the capacity is reduced by a third. Steeper angle conveyors require more specialised assessment. Many manufacturers publish data on fill recommendations for different products, but care is needed if significant moisture is present. Do not use standard screw conveyors for materials that cake and sticky bulk materials also require attention.

The design of screw feeders is more involved as it demands an integrated approch with hopper design, therefore independent advice on the screw power is not appropriate without a comprehensive study. ■