A theory has been developed, based on a physical model, to describe the behaviour of non-cohesive granular material inside a vertical screw conveyor. By use of this theory, relationships have been derived between dimensionless numbers for capacity, power consumption and efficiency. These relationships, or characteristics, are compared with the results of experiments carried out with two models of vertical screw conveyors, one of 50.8 mm and the other of 162.0 mm diameter. The agreement between the calculated and the measured values of capacity and power consumption was within 5 and 9% respectively. The investigation was extended to a screw with an inclined screw blade, because one might expect that this would result in a steeper upward motion of the granules and thus would lead to an increased capacity. It appears, however, that this type of screw has no practical advantages over the normal one, and it is therefore not treated here. Two other simpler theories were also developed, one based on a simplified physical model  and the other on the conveying of a single granule . It appears that the simpler theories do not agree with the experiment as well as the one developed in this dissertation does, the theory of the single granule producing the greatest discrepancies. With the latter theory, however, the capacity can be reasonably well approximated when the ‘degree of fullness’ 60%. The influence of the inlet section on the performance of the screw conveyor is discussed. It was found that the capacity of the conveying section is in most cases limited by the inlet and not by the conveying section itself. As the maximum performance of the conveying section can be calculated with the more developed theory, a method is thus available for judging the potential increase in capacity which could be obtained through improved inlet design.