Compressed air is air kept under a pressure that is greater than atmospheric pressure. Compressed air is an important medium for transfer of energy in industrial processes. Compressed air is used for power tools such as air hammers, drills, wrenches and others. Compressed air is used to atomize paint, to operate air cylinders for automation, and can also be used to propel vehicles. Brakes applied by compressed air made large railway trains safer and more efficient to operate. Compressed air brakes are also found on large highway vehicles.
Compressed air is used as a breathing gas by underwater divers. It may be carried by the diver in a high pressure diving cylinder, or supplied from the surface at lower pressure through an air line or diver's umbilical. Similar arrangements are used in breathing apparatus used by firefighters, mine rescue workers and industrial workers in hazardous atmospheres.
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.
Air for breathing may be stored at high pressure and gradually released when needed, as in scuba diving. Air for breathing must be free of oil and other contaminants; carbon monoxide, for example, in trace amounts that might not be dangerous at normal atmospheric pressure may have deadly effects when breathing pressurized air. Air compressors and supply systems intended for breathing air are not generally also used for pneumatic tools or other purposes.
Workers constructing the foundations of bridges or other structures may be working in a pressurized enclosure called a caisson, where water is prevented from entering the open bottom of the enclosure by filling it with air under pressure. It was known as early as the 17th century that workers in diving bells experienced shortness of breath and risked asphyxia, relieved by the release of fresh air into the bell. Such workers also experienced pain and other symptoms when returning to the surface, as the pressure was relieved. Denis Papin suggested in 1691 that the working time in a diving bell could be extended if fresh air from the surface was continually forced under pressure into the bell. By the 19th century, caissons were regularly used in civil construction, but workers experienced serious, sometimes fatal, symptoms on returning to the surface, a syndrome called caisson disease or decompression sickness. Many workers were killed by the disease on projects such as the Brooklyn Bridge and the Eads Bridge and it was not until the 1890s that it was understood that workers had to decompress slowly, to prevent the formation of dangerous bubbles in tissues.
Air under moderately high pressure, such as is used when diving below about 20 metres (70 ft), has an increasing narcotic effect on the nervous system. Nitrogen narcosis is a hazard when diving. For diving much beyond 30 metres (100 ft), it is less safe to use air alone and special breathing mixes containing helium are often used.
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:
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. After compressed air cools, then the vaporized water turns to liquefied water.,  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. 
An air brake or, more formally, a compressed air brake system, is a type of friction brake for vehicles in which compressed air pressing on a piston is used to apply the pressure to the brake pad needed to stop the vehicle. Air brakes are used in large heavy vehicles, particularly those having multiple trailers which must be linked into the brake system, such as trucks, buses, trailers, and semi-trailers, in addition to their use in railroad trains. George Westinghouse first developed air brakes for use in railway service. He patented a safer air brake on March 5, 1872. Westinghouse made numerous alterations to improve his air pressured brake invention, which led to various forms of the automatic brake. In the early 20th century, after its advantages were proven in railway use, it was adopted by manufacturers of trucks and heavy road vehicles.Air compressor
An air compressor is a device that converts power (using an electric motor, diesel or gasoline engine, etc.) into potential energy stored in pressurized air (i.e., compressed air). By one of several methods, an air compressor forces more and more air into a storage tank, increasing the pressure. When tank pressure reaches its engineered upper limit the air compressor shuts off. The compressed air, then, is held in the tank until called into use. The energy contained in the compressed air can be used for a variety of applications, utilizing the kinetic energy of the air as it is released and the tank depressurizes. When tank pressure reaches its lower limit, the air compressor turns on again and re-pressurizes the tank.
An air compressor must be differentiated from a pump because it works for any gas/air, while pumps work on a liquid.Air dryer
See Also: Compressed air dryerA compressed air dryer is used for removing water vapor from compressed air. Compressed air dryers are commonly found in a wide range of industrial and commercial facilities.
The process of air compression concentrates atmospheric contaminants, including water vapor. This raises the dew point of the compressed air relative to free atmospheric air and leads to condensation within pipes and the compressed air cools downstream of the compressor.
Excessive water in compressed air, in either the liquid or vapor phase, can cause a variety of operational problems for users of compressed air. These include freezing of outdoor air lines, corrosion in piping and equipment, malfunctioning of pneumatic process control instruments, fouling of processes and products, and more.
There are various types of compressed air dryers. Their performance characteristics are typically defined by the dew point.
Regenerative desiccant dryers, often called "regens" or "twin tower" dryers
Membrane dryersWater vapor is removed from compressed air to prevent condensation from occurring and to prevent moisture from interfering in sensitive industrial processes.Compressed-air vehicle
A compressed-air vehicle (CAV) is a transport mechanism fueled by tanks of pressurized atmospheric gas and propelled by the release and expansion of the gas within a Pneumatic motor. CAV's have found application in torpedoes, locomotives used in digging tunnels, and early prototype submarines. Potential environmental advantages have generated public interest in CAV's as passenger cars, but they have not been competitive due to the low energy density of compressed air and inefficiency of the compression / expansion process.Compressed-air propulsion may also be incorporated in hybrid systems, such as with battery electric propulsion. This kind of system is called a hybrid-pneumatic electric propulsion. Additionally, regenerative braking can also be used in conjunction with this system.Compressed air car
A compressed air car is a compressed air vehicle that uses a motor powered by compressed air. The car can be powered solely by air, or combined (as in a hybrid electric vehicle) with gasoline, diesel, ethanol, or an electric plant with regenerative braking.Compressed air energy storage
Compressed air energy storage (CAES) is a way to store energy generated at one time for use at another time using compressed air. At utility scale, energy generated during periods of low energy demand (off-peak) can be released to meet higher demand (peak load) periods. Small scale systems have long been used in such applications as propulsion of mine locomotives. Large scale applications must conserve the heat energy associated with compressing air; dissipating heat lowers the energy efficiency of the storage system.Double Bay Compressed Air Ejector Station
The Double Bay Compressed Air Ejector Station is an heritage-listed former sewage pumping station and now decommissioned sewerage infrastructure at Cross Street, Double Bay in the Municipality of Woollahra local government area of New South Wales, Australia. It was designed and built from 1895 to 1896 by the New South Wales Department of Public Works. It is also known as Double Bay Sewage Ejector Station No. 1 (decommissioned). The property is owned by Sydney Water, an agency of the Government of New South Wales. It was added to the New South Wales State Heritage Register on 18 November 1999.Fireless locomotive
A fireless locomotive is a type of locomotive which uses reciprocating engines powered from a reservoir of compressed air or steam, which is filled at intervals from an external source. They offer advantages over conventional steam locomotives of lower cost per unit, cleanliness, and decreased risk from fire or boiler explosion; these are counterbalanced by the need for a source to refill the locomotive, and by the limited range afforded by the reservoir.
Typical usage was in industrial switching where a conventional locomotive was too noxious or risky, such as in a mine or a food or chemical factory; they also saw use where the source of air or steam was readily available.
They were eventually displaced by diesel and battery electric locomotives fitted with protective appliances; these are described as flame-proof locomotives.Gas duster
Gas duster, also known as canned air, is a product used for cleaning or dusting electronic equipment and other sensitive devices that cannot be cleaned using water.
The products are most often a can that, when a trigger is pressed, blasts a stream of compressed gas through a nozzle. Despite the name "canned air", the cans actually contain gases that are compressable into liquids. True liquid air is not practical, as it cannot be stored in metal spray cans due to extreme pressure and temperature requirements. Common duster gases are 1,1-difluoroethane, 1,1,1-trifluoroethane, or 1,1,1,2-tetrafluoroethane. Hydrocarbons, like butane, were often used in the past, but their flammable nature forced manufacturers to use fluorocarbons.When inhaled, gas duster fumes may produce psychoactive effects and may be harmful to health, sometimes even causing death.Gas lift
Gas lift or bubble pumps use the artificial lift technique of raising a fluid such as water or oil by introducing bubbles of compressed air, water vapor or other vaporous bubbles into the outlet tube. This has the effect of reducing the hydrostatic pressure in the outlet tube vs. the hydrostatic pressure at the inlet side of the tube.
Devices using this type of lift mechanism:
Coffee percolators use vaporized water to lift hot water
Airlift pumps uses compressed air to lift water
Pulser pumps use a subterranean chamber of air for an airlift pump
Suction dredges use an airlift pump to vacuum mud, sand and debris
Mist lift pumps uses vaporized water to lift seawater in Ocean thermal energy conversionHydropower
Hydropower or water power (from Greek: ὕδωρ, "water") is power derived from the energy of falling water or fast running water, which may be harnessed for useful purposes. Since ancient times, hydropower from many kinds of watermills has been used as a renewable energy source for irrigation and the operation of various mechanical devices, such as gristmills, sawmills, textile mills, trip hammers, dock cranes, domestic lifts, and ore mills. A trompe, which produces compressed air from falling water, is sometimes used to power other machinery at a distance.In the late 19th century, hydropower became a source for generating electricity. Cragside in Northumberland was the first house powered by hydroelectricity in 1878 and the first commercial hydroelectric power plant was built at Niagara Falls in 1879. In 1881, street lamps in the city of Niagara Falls were powered by hydropower.
Since the early 20th century, the term has been used almost exclusively in conjunction with the modern development of hydroelectric power. International institutions such as the World Bank view hydropower as a means for economic development without adding substantial amounts of carbon to the atmosphere,
but dams can have significant negative social and environmental impacts.Jackhammer
A jackhammer (pneumatic drill or demolition hammer in British English) is a pneumatic or electro-mechanical tool that combines a hammer directly with a chisel. It was invented by William Mcreavy, who then sold the patent to Charles Brady King. Hand-held jackhammers are generally powered by compressed air, but some are also powered by electric motors. Larger jackhammers, such as rig mounted hammers used on construction machinery, are usually hydraulically powered. They are typically used to break up rock, pavement, and concrete.
A jackhammer operates by driving an internal hammer up and down. The hammer is first driven down to strike the back and then back up to return the hammer to the original position to repeat the cycle. The effectiveness of the jackhammer is dependent on how much force is applied to the tool. It is generally used like a hammer to break the hard surface or rock in construction works and it is not considered under earth moving equipment, along with its accessories (i.e., pusher leg, lubricator).
In British English, electromechanical versions are colloquially known as "Kangos".Kunze-Knorr brake
The Kunze-Knorr brake (Kunze-Knorr-Bremse or KK-Bremse) is an automatic compressed-air brake for goods, passenger and express trains. It was the first graduated brake for goods trains in Europe. When it was introduced after the First World War, goods train brakes switched from hand operation to compressed-air in various European countries. The Deutsche Reichsbahn alone put the cost of equipping German goods wagons with Kunze-Knorr brakes between 1918 and 1927 at 478.4 million Reichsmarks. The operating cost savings from faster goods services and having fewer brakemen was assessed by the Reichsbahn at almost 96.3 million Reichsmark annually.Pneumatic motor
A pneumatic motor (air motor) or compressed air engine is a type of motor which does mechanical work by expanding compressed air. Pneumatic motors generally convert the compressed air energy to mechanical work through either linear or rotary motion. Linear motion can come from either a diaphragm or piston actuator, while rotary motion is supplied by either a vane type air motor, piston air motor, air turbine or gear type motor.
Pneumatic motors have existed in many forms over the past two centuries, ranging in size from hand-held motors to engines of up to several hundred horsepower. Some types rely on pistons and cylinders; others on slotted rotors with vanes (vane motors) and others use turbines. Many compressed air engines improve their performance by heating the incoming air or the engine itself. Pneumatic motors have found widespread success in the hand-held tool industry, but are also used stationary in a wide range of industrial applications. Continual attempts are being made to expand their use to the transportation industry. However, pneumatic motors must overcome inefficiencies before being seen as a viable option in the transportation industry.Pneumatics
Pneumatics (From Greek: πνεύμα) is a branch of engineering that makes use of gas or pressurized air.
Pneumatic systems used in industry are commonly powered by compressed air or compressed inert gases. A centrally located and electrically powered compressor powers cylinders, air motors, and other pneumatic devices. A pneumatic system controlled through manual or automatic solenoid valves is selected when it provides a lower cost, more flexible, or safer alternative to electric motors and actuators.
Pneumatics also has applications in dentistry, construction, mining, and other areas.Railway air brake
A railway air brake is a railway brake power braking system with compressed air as the operating medium. Modern trains rely upon a fail-safe air brake system that is based upon a design patented by George Westinghouse on March 5, 1868. The Westinghouse Air Brake Company was subsequently organized to manufacture and sell Westinghouse's invention. In various forms, it has been nearly universally adopted.
The Westinghouse system uses air pressure to charge air reservoirs (tanks) on each car. Full air pressure signals each car to release the brakes. A reduction or loss of air pressure signals each car to apply its brakes, using the compressed air in its reservoirs.Self-contained breathing apparatus
A self-contained breathing apparatus (SCBA) sometimes referred to as a compressed air breathing apparatus (CABA), or simply breathing apparatus (BA), is a device worn by rescue workers, firefighters, and others to provide breathable air in an immediately dangerous to life or health atmosphere (IDLH). When not used underwater, they are sometimes called industrial breathing sets. The term self-contained means that the breathing set is not dependent on a remote supply (e.g., through a long hose). If designed for use under water, it is called SCUBA (self-contained underwater breathing apparatus).
An SCBA typically has three main components: a high-pressure tank (e.g., 2,216 to 5,500 psi (15,280 to 37,920 kPa), about 150 to 374 atmospheres), a pressure regulator, and an inhalation connection (mouthpiece, mouth mask or face mask), connected together and mounted to a carrying frame.A self-contained breathing apparatus may fall into two different categories. These are open circuit and closed circuit.Tata OneCAT
Tata OneCAT (Compressed Air Technology) was advertised as an upcoming compressed air car in 2008. India's Tata Motors was said to be collaborating with Air engine developer Guy Nègre of MDI to produce the vehicle.The vehicle contains air tanks that can be filled in four hours by plugging the car into a standard electrical plug. MDI also plans to design a gas station compressor, which would fill the tanks in three minutes.The OneCAT is a five-seat vehicle with a 200 litre trunk. There is no reliable data on range and top speed for this prototype available.
As of August 2009 there was no mentioning of the prototype or planned production of the TATA OneCAT on the TATA or MDI homepages. In December 2009 Tata's vice president of engineering systems confirmed that the limited range and low engine temperatures were causing difficulties.
Meanwhile, all links, articles and archives relative to MDI have disappeared from Tata's website.
In February, 2012 Tata released information that their compressed-air vehicle would debut in August, 2012.In February 2017, news report about the TaTa 'AirPod' claims it will be in production in 2020.Water rocket
A water rocket is a type of model rocket using water as its reaction mass. The water is forced out by a pressurized gas, typically compressed air. Like all rocket engines, it operates on the principle of Newton's third law of motion. Water rocket hobbyists typically use one or more plastic soft drink bottle as the rocket's pressure vessel. A variety of designs are possible including multi-stage rockets. Water rockets are also custom-built from composite materials to achieve world record altitudes.