of water. (3) High-pressure systems use low-velocity high- density air. Positive displacement blowers are usually required for high-pressure systems. System D - G is a true suction system since the pressure is less than atmospheric through the entire conveying distance. Although systems A and B are classed as pressure systems, they are actually combination systems since pressures vary from below to above atmospheric. (4) Fluidized systems employ high pressure air and a high density of granular materials per pound of air so that the materials move as an air-material “ slu rry .” C o n v e y in g velocities are low.