The flow pattern and stresses within the bulk material during discharge from silo, bins, hoppers, etc. depend on internal friction between the particles, friction between particles and wall, and the inclination of the converging section of the vessel. The discharge rate of particulate solids (bulk solids) is influenced by numerous factors, such as:
• particle characteristics: size, size distribution, shape, compressibility, state of aeration, degree of cohesion, moisture content
• vessel characteristics: wall thickness, orifice geometry, orifice orientation, and ratio of particle size to orifice diameter
• pressure drop across outlet (flow assist vs. flow impediment)
• flow pattern
• temperature
Due to differences in the underlying physics, the factors and resulting influences affecting the discharge rate of solids from vessels is not intuitive to an engineer who is only familiar with fluid flow. This article explains the differences between fluid and solids flow, and provides guidance on how to calculate discharge rate for design and troubleshooting.