Jan 28, 2008· Dear all, Imagin to have a vibrating feeder under a hopper, with a certain cross section at the outlet. 1) Does anyone know if there is a ratio to respect between the height of the outlet and the max particle size? 2)A feeder under the hopper full of material with a wide range in grain size (from dust to big rocks) will have enough "extraction force" to empty it or it is better to use an Apron ...