Numerous industrial processing operations involve the handling and transport of powders, bulk solids, and granules. Silos and bins are widely used to store granular solids, and it is imperative to be able to control the flow rate during the discharge of these solids from such equipment. It is important to study the gravity discharge of fine particles because of its many industrial applications. Most theoretical and empirical equations for the calculation of the discharge rate of fine powders tend to overestimate when particles smaller than about 500 mu m are used. An air stream flows through a silo from an outlet on the top which may cause the discharge process to become unstable and erratic. We used open-top and closed-top silos with different sizes of outlet openings for an experimental study of the discharge behaviors of different-sized silica sands. The impact force, average discharge rate and pressure drop were measured and are discussed. Darcy's law was used to predict the pressure drop and this calculation is compared with the experimental results. In addition, we observed that a bubble phenomenon can occur in very fine-powder silo systems.