Compressed air, commonly called Industry's Fourth Utility, is air that is condensed and contained at a pressure that is greater than the atmosphere. The process takes a given mass of air, which occupies a given volume of space, and reduces it into a smaller space. In that space, greater air mass produces greater pressure. The pressure comes from this air trying to return to its original volume. It is used in many different manufacturing operations. A typical compressed air system operating at 100 psig (7 bar) will compress the air down to 1/8 of its original volume. (figure CA1-1)

Why Use Compressed Air?

Compressed air supplies power for many different manufacturing operations. At a pressure of 100 psig (7 bar), compressed air serves as a utility. It supplies motive force, and is preferred to electricity because it is safer and more convenient. There are numerous industries that use compressed air for various applications.

Industrial Plant Maintenance: Air tools, such as paving breakers, are used to fix cement floors, to open up brick walls for assorted service lines, and other comparable work. Caulking and chipping (fig. CA1-2) can be done using smaller air hammers.

For other maintenance work, plants can use air-operated drills, screwdrivers, and wrenches, provided that the air outlets are well placed throughout the plant. Painting can be done using paint-spraying systems.

Sprinkler systems are controlled by air pressure, which keeps water from entering the pipes until heat breaks the seal and releases the pressure. Air jets speed up the process of cleaning machines, floors, remote ceiling areas, move heavy loads and overhead pipes. Air pressure also efficiently cleans boiler tubes. Tuck pointing of brick walls and metalizing of worn parts are two other compressed air uses.

On the Production Line: Pneumatic tools are convenient for industrial production because they have a low weight-to-power ratio, and they may be used for long periods of time without overheating and with low maintenance costs. Chipping and scaling hammers are used in railroads, oil refineries, chemical refineries, shipyards, and many other industries for general application. They are also used in the foundry for cleaning large castings, and to remove weld scale, rust, and paint in other industries. Additionally, these hammers are good for cutting and sculpturing stone.

Pneumatic drills can be used for all classes of reaming, tapping, and drilling anytime that the work cannot easily be carried to the drill press and for all classes of breast drill work. These air-powered drills (fig. CA1-3) are also often used for operating special boring bars, and in emergencies, for independent drive of a machine tool where required horsepower is within their capacity.

Grinding, wire brushing, polishing, sanding, shot blasting and buffing are performed efficiently with compressed air in the automotive, aircraft, rail car, locomotive, vessel shops, shipbuilding, other heavy machinery, and other industries. The primary goals are to finish surfaces and prepare them for finishing operations. Two of the most basic assembly operations, driving screws and turning up nuts, are performed more efficiently because of pneumatic screwdrivers and nut runners.

Air Motors, Vacuum, & Other Auxiliary Devices: Air motors are often used as a power source in operations involving flammable or explosive liquids, vapor, or dust, and can operate in hot, corrosive, or wet atmospheres without damage. Their speeds may be easily changed; they will start and stop rapidly and are not damaged by stalling and overloading. Air motors power (fig. CA1-4) many hand-held air tools and air hoists. They are used in various applications in underground tunnels and mines and in industrial areas where there are flammable liquids or gas. They also drive many pumps used in construction and many positioning apparatuses used in manufacturing.

Vacuum has numerous applications in production. A vacuum pump is a compressor in which the desired effect is the intake vacuum, not the pressurized air. For vacuum chucking, the pump holds a vacuum in a tank located close to the machine, while bleeder holes under the part to be machined are opened to hold the part in place.

Pneumatic auxiliary production equipment is used extensively. Positioners, feeders, clamps, air chucks, presses, air knives and many other devices powered by air cylinders increase production efficiency. Pneumatic cylinders plus ratchets or stops provide reciprocating or rotating interrupted motions much more economically than by traditional mechanical tools. In finishing and packaging areas, pneumatic devices are used for many applications, such as dry powder transporting and fluidizing, liquid padding, carton stapling, and appliance sanding. Blast cleaning and finishing are other widely used compressed air applications.

Automation
The field of automation has been impacted by pneumatics. For instance, air circuitry and pneumatic controls allow the integration of traditional and special air tools and auxiliary devices into single automatic machines. One system has a high degree of interchangeability of pneumatic tools and controls. Because of fluidics, we have simple devices for pneumatic control at lower pressures and with almost no moving parts. Pneumatic positioners have been created that are capable of positioning parts to within 1/1000" without the use of mechanical stops.

Compressed air is also used for the pneumatic transportation of materials, such as substances in granular, chip, pelletized, or powdered form and liquids where inertness is not required. Painting is another frequently automated application that uses air circuitry and pneumatic controls in robotic machines and paint spray systems. Compressed air is often used in automatic packaging machinery for sealing, locating the work, and actuating arms that fold paper to wrap the work. Vacuuming machines also perform similar tasks, such as picking up and transferring materials.

Automated Assembly Stations
Compressed air is speeding up operations in the automotive, appliance, electronics, communications, and business machines industries. Common air-powered tasks in automatic machines include the following: tightening threaded fasteners to specified torque; pressing of hammering plugs, pins, and rivets with air; feeding fasteners or parts; actuating positioning cylinders, slides, or work heads, blow-offs, operating indicator lights; and transmitting signals to recording computers.

Common compressed air applications:

Rules of thumb:compressed air. These rules apply to the design and installation of the system:

There are several rules of thumb regarding

• All compressors produce heat during the compression process. This heat must be removed from the compressor room for proper operation of the compressor. Be sure to provide sufficient ventilation for all equipment that may be installed in the compressor room. All compressor manufacturers publish allowable operating temperatures.
• Leave sufficient space around the compressor to permit routine maintenance. It is also suggested to provide space for the removal of major components during compressor overhauls.
• An air receiver near the compressor should be located to provide a steady source of control air, additional air cooling, and moisture separation. In the distribution system, there may periodically be large volume demands, which will rapidly drain the air from surrounding areas, and cause pressure levels to fall for surrounding users. However, strategically located receivers in the system can supply these abrupt demands and still provide a consistent air flow and pressure to the affected areas.
• Select piping systems that have low pressure drop and provide corrosion free operation. When selecting the main air header, size for a maximum pressure drop of 1 to 2 psi (.07 to .14 bar). A good rule is to use a header pipe size at least one size larger than calculated. This will provide additional air storage capacity and allow for future expansion.
• It is suggested that all piping in a loop system (fig. CA1-5) be sloped to accessible drain points. Air outlets should be taken from the top of the main line to keep possible moisture from entering the outlet. Drip legs or drain valves should be installed at all low points in the system where it is possible for moisture to accumulate.

• One gallon per CFM of capacity is the minimum amount of storage recommended. Systems with sharp changes in demand should have a minimum amount of storage of three gallons per CFM of capacity. An efficient control system will help to accommodate these abrupt changes in demand. A Load/No Load control will help system efficiency because it operates the compressor at either full load or no load. The motors continue to run in the unloaded state, but the inlet valves to the compression chamber are left open, keeping air from being compressed. The motor still does a small amount of work even when no air is compressed.
• Position filters and dryers in the air line before any pressure-reducing valve (highest pressure) and after air is cooled to 100°F (38°C) or less (lowest temperature). (fig. CA1-6)

These rules give measurements at which a standard system operates:

• Every 1 psig pressure drop increases compressor power required by .5%.
• At discharge pressures of 100 psig, most water-cooled aftercoolers will need about 3 gpm per 100 CFM of compressed air.
• The water vapor content at 100°F (37.78°C) of saturated compressed air is equal to about two gallons per hour for each 100 CFM of compressor output.
• In saturated compressed air, for every 20°F (-6.67°C) temperature drop the water content of the air drops by 50%. (fig. CA1-7)

• Every 100 CFM of air compressed to 100 psig produces 20 gallons of condensate per day under normal conditions.