Compressed air is kept under a pressure that is greater than atmospheric pressure. It serves many domestic and industrial purposes. In industry, compressed air is so widely used that it is often regarded as the fourth utility, after electricity, natural gas and water.
However, compressed air is more expensive than the other three utilities when evaluated on a per unit energy delivered basis.
Pneumatics, the use of pressurized gases to do work
Pneumatic post, using capsules to move paper and small goods through tubes.
HVAC control systems
Vehicle propulsion (see compressed air vehicle)
Energy storage (see compressed air energy storage)
Air brakes, including: railway braking systems, road vehicle braking systems
Scuba diving, for breathing and to inflate buoyancy devices
Refrigeration using a vortex tube
Air-start systems in engines
Ammunition propulsion in: Air guns, Airsoft equipment, Paintball equipment