Compressed air is used extensively in process plants. It's a clean, safe and efficient utility that can be employed where other energy types (such as electricity) might pose a potential hazard. Compressed air is the motive force for many instruments, actuators, control valves and equipment. In addition, some processes require compressed air.
Most air compressors are electrically driven but engine-driven units also are used.
The "air" is naturally contaminated with solid particles such as dust, soot, salt, etc. It also contains water vapor, which if not removed can cause significant corrosive damage under the compression. Many applications require completely dry and oil-free air, to avoid risk of contaminating sensitive pneumatic equipment.