The angle of repose of a material is the steepest angle that can be formed between the sloped edge of a free standing pile of material and the horizontal axis. Each material will have its own angle of repose specific to that material. In general ‘sticky’ materials will have a steeper / larger angle of repose, while more free flowing materials will have a shallower / smaller angle of repose.
The angle of repose affects the silo’s sizing because the top of a pile of material will form a cone where the cone angle is related to the angle of repose for that material. In center fill silos, this cone will form within the silo, with the point of the cone at the center of the silo. When the silo is ‘full’ the tip of the cone will be at the top of the main cylindrical section of the silo. This will leave some free space around the silo periphery at the top of the silo. If this is not correctly accounted for, your silo will be undersized and will not hold the anticipated amount of chemical.