First, a Little More Theory
Whenever you have have a particle moving through a horizontally-flowing fluid, confined within a certain space, a number of forces come into play. Gravity, the type of fluid involved, how smoothly the fluid is flowing, the size of the particles, the dimensions of the piping or chamber, the bouyancy force, and a few other negligible forces. All in all, there are quite a few things to look into.
First, there was a man named Stokes, who did a great deal of work in these areas. To find out more about him, go to your library or text and read about him. Basically, his one important contribution was a formula for calculating Drag Force in completely laminar environments, with perfectly spherical bodies. And for situations with basically spherical particles and mostly laminar flow, those equations work. The equation, when coupled with one other and trying to solve for Drag Coefficient, works out to this.
Drag Coefficient = 24 / Reynold's Number
However, we (usually) need to go a bit beyond that. I have tried to make this simple, I hope, and so I broke it down into three more subsections, each dealing with a certain type of settling.