Compressed air is air kept under a pressure that is greater than atmospheric pressure. It serves many domestic and industrial purposes.
Industrial use of piped compressed air for power transmission was developed in the mid 19th century; unlike steam, compressed air could be piped for long distances without losing pressure due to condensation. An early major application of compressed air was in the drilling of the Mont Cenis Tunnel in Switzerland in 1861, where a 600 kPa (87 psi) compressed air plant provided power to pneumatic drills, increasing productivity greatly over previous manual drilling methods. Compressed air drills were applied at mines in the United States in the 1870s. George Westinghouse invented air brakes for trains starting in 1869; these brakes considerably improved the safety of rail operations. In the 19th century, Paris had a system of pipes installed for municipal distribution of compressed air to power machines and to operate generators for lighting. Early air compressors were steam-driven, but in certain locations a trompe could directly obtain compressed air from the force of falling water. 
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.
Compressed air is used for many purposes, including:
- Pneumatics, the use of pressurized gases to do work
- Vehicle propulsion (see compressed air vehicle)
- Energy storage (see compressed air energy storage)
- Air brakes, including:
- Underwater diving, for breathing and to inflate buoyancy devices
- Refrigeration using a vortex tube
- Air-start systems in engines
- Ammunition propulsion in:
- Cleaning dust and small debris in tiny spaces
- Sandblasting in machine shops
- Injection molding
- Food and beverage capping and fermentation
Design of systemsEdit
Compressor rooms must be designed with ventilation systems to remove waste heat produced by the compressors. 
When air at atmospheric pressure is compressed, it contains much more water vapor than the high-pressure air can hold. Relative humidity is governed by the properties of water and is not affected by air pressure. Management of the excessive moisture is a requirement of a compressed air distribution system. System designers must ensure that piping maintains a slope, to prevent accumulation of moisture in low parts of the piping system. Drain valves may be installed at multiple points of a large system to allow trapped water to be blown out. Taps from piping headers may be arranged at the tops of pipes, so that moisture is not carried over into piping branches feeding equipment.  Piping sizes are selected to avoid excessive energy loss in the piping system due to excess velocity in straight pipes at times of peak demand, or due to turbulence at pipe fittings. 
- Leino, Raili (24 February 2009). "Paineilma hukkaa 15 hiilivoimalan tuotannon" (in Finnish).
- "Compressed Air System Audits and Benchmarking Results from the German Compressed Air Campaign "Druckluft effizient"" (PDF). Archived from the original (PDF) on 2011-12-24.
- Lance Day, Ian McNeil (ed.), Biographical Dictionary of the History of Technology, Routledge, 2002, ISBN 1134650205,p. 1294
- Peter Darling (ed.), SME Mining Engineering Handbook, Third Edition Society for Mining, Metallurgy, and Exploration (U.S.) 2011, ISBN 0873352645,p. 705
- Yuan, C., Zhang, T., Rangarajan, A., Dornfeld, D., Ziemba, B., and Whitbeck, R. “A Decision-based Analysis of Compressed Air Usage Patterns in Automotive Manufacturing”, Journal of Manufacturing Systems, 25 (4), 2006, pp.293-300
- "Applications - Working With Compressed Air - CAGI - Compressed Air And Gas Institute". www.cagi.org. Retrieved 2017-01-12.
- "Some Like It Hot…Your Compressor Room Doesn't". Compressed Air Tips from Kaeser Talks Shop. 5 May 2015. Retrieved 2017-01-12.
- COMPRESSOR INLET PIPING by Hank van Ormer, Air Power USA, Compressed Air Best Practices, 06/2012 Page 26, column 2, Note 12.
- "Plant services (2005 - 2006 Collection) "Eliminate Mr. Tee"". p. 5.
- Merritt, Rich (May 2005). "Top 10 Targets of a Compressed Air Audit" (PDF). Plant Services magazine. p. 31.
- World's largest glossary of terms relating to compressed air
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- Compressed Air and Gas Institute
- Improving Compressed Air System Performance by U.S. Department of Energy
- History of compressed air
- Compressed Air: Piping, Drying, Filtering, Regulating, Lubricating
- Troubleshooting your compressed air system