A Discrete Element Methods model of a horizontal screw conveyor was developed and used to study the volume flow rate of granular materials. The volume throughput was monitored as a function of two operating parameters: the conveyor's filling level and the screw's rotating speed. For a steady flow, which is slow enough to avoid particle centrifuging, the volume throughput is linearly proportional to the screw velocity. However, the dependence on the relative filling level is non-linear. The particles, which are lifted laterally by the rotating blade, are often confined to one side, between the shaft and the casing. However, if the particle level is sufficiently high, the particles leap over the middle shaft into the previous screw thread (which is behind the original one), decreasing the throughput. These two major effects have strong impact on the particle levels to the left and the right side of the shaft and the relative velocity in relation to the screw's surface and geometry. Our study aimed to investigate the relationship between the particle volume flow rate and the relative shaft size, as well as the influence of the friction coefficient between particles and walls on the efficiency of the device. The numerical results may be used for determining the optimal operating parameters in order to maximize the conveyor throughput efficiency based on the shaft's relative size. (C) 2016 Published by Elsevier B.V.