Comparing pneumatic conveying system types
In this column, we'll return to a topic covered in the March 1991 column: types of pneumatic conveying systems. The types include vacuum, pressure, and combination vacuum-pressure systems. We'll describe each, along with its advantages and disadvantages, to help you understand what each system includes and how systems of each type are designed.
Before discussing each system, let's examine some common misconceptions about how the systems work. Many people believe that a vacuum system draws material into the conveying line's center and separates the material, and that a pressure system forces material toward the conveying line walls and compacts the material. However, neither is true. At any point in either a vacuum or pressure system, the air's dynamic action on the material is the same. The particles are moved through the conveying line by the airflow, which is established by a pressure differential. In a vacuum system, air flows from atmospheric pressure to a reduced pressure; in a pressure system, air flows from an elevated pressure to atmospheric pressure. In other words, air always flows from a higher pressure to a lower pressure and always expands through the conveying line.