In this paper the term 'bin' refers to the section of a storage vessel with parallel sided walls, 'hopper' is the angled portion below. A storage vessel or silo therefore consists of both bin and hopper. Many different shapes of hopper and bin are routinely used but in each case the design intent is the same: reliable, steady powder discharge, at the required rate. Selecting an appropriate outlet size and hopper half angle, the degree of incline from the vertical, of the hopper walls, achieves this aim. The resultant flow regimes can be broadly divided into two – mass flow and core or funnel flow.