Due to the normal conditions of the atmosphere, free air seldom satisfies these requirements adequately. The atmosphere contains both dust and impurities in various amounts and a substantial amount of moisture in vapor form. Solids, such as dust, rust, or pipe scale in pneumatic systems, may lead to excessive wear and failure of components and, in some cases, may prevent the pneumatic devices from operating.
Moisture is also very harmful to the system.
It washes lubrication from moving parts, thereby aiding corrosion and causing excessive wear of components. Moisture will also settle in low spots in the system and freeze during cold weather, causing a stoppage of the system or ruptured lines. An ideal filter would remove all dirt and moisture from a pneumatic system without causing a pressure drop in the process. In some cases, the removal of moisture is done in conjunction with the removal of solids. Some types of air filters are similar in design and operation to the hydraulic filters discussed earlier.