Air compressors can vary from small portable units used to operate only one piece of equipment, to very large built-in systems that provide compressed air throughout a large workshop or factory. Fixed compressors are normally powered by electric motors, while some portable units, especially those used on construction sites, may be diesel or petrol powered. For this discussion, petrol and diesel powered mobile compressors will not be covered. This web page will concentrate on the typical built-in compressed air systems commonly found in many factories, shops and garages. A simplified typical compressed air system is shown here.
For larger compressed air systems intended to operate multiple tools, such as in a large repair shop, the system air compressor, receiver tank and manifolds must have the capacity to deliver compressed air at adequate flow rates and pressures to operate several tools at the same time.
Regardless of size, a compressed air system has several common components:
A source of energy and means to compress the air (air compressor)
The source of energy to compress the air is usually an electric motor, although internal combustion engines can also be used. Small low pressure/low volume systems may use diaphragm pumps, but most shop and factory compressors are piston-type compressors.
A piston compressor works like a piston engine in a car, but instead of the piston generating power from burning fuel, the external energy from the electric motor uses the pistons to squeeze the air into a smaller volume in the piston chambers, in other words, to compress the air. As the air is compressed, it gets hot. Thus, special high temperature compressor oil must be used to lubricate the pistons. Use of improper oil, or solvents to clean compressor parts could create an explosion hazard in the air receiver.
Piston valves release the hot compressed air into a tank or “receiving chamber,” typically called the air receiver. The receiver stores the compressed air, and acts as a reservoir of compressed air to supply the compressed air system. Typically, the compressed air in the receiver will range from a pressure of 100 to 150 pounds per square inch (psi) (690-1035 kPa).
Because the receiver is a pressure vessel the receiver must be fitted with a safety valve. The safety valve serves to prevent over pressurising and possible rupturing of the receiver. If the receiver pressure exceeds 110% of the maximum allowable working pressure, the safety valve vents off excess air pressure. For the safety valve to function properly, it must be directly connected to the receiver. No other valve or fitting should be placed between the air receiver and the safety valve.
Compressed air receivers used for operating pneumatic tools are fitted with a readable pressure gauge to allow monitoring of the receiver air pressure. Pressure gauges monitor the air pressure in various parts of the system. A simple system may only have one pressure gauge, while large systems may have a pressure gauge at every connection point.
The pressure of the air in the system is commonly given in units of kilopascals (kPa) or pounds per square inch gauge (psig).
NB: gauge pressure = (pressure inside system) – (atmospheric pressure) Atmospheric pressure is 101.4 kPa (14.7 psi).
The maximum air pressure in the air receiver and manifold is usually higher than the air pressure needed to operate pneumatic tools. An air pressure regulator can be manually adjusted to reduce the air pressure from the manifold to the operating pressure of the pneumatic tool.
Filter bowl (oil /water trap)
An in-line air filter and a liquid drain bowl are fitted into systems to remove contaminants and collect condensed water and oil. The filter bowl is usually mounted at the outlet of the compressed air tank on small portable compressors. On larger built-in systems, filter bowls are usually located along the compressed air manifold at each airline hose connection point.
Smaller compressors may connect directly to an air hose and tool. However, larger systems distribute the compressed air through a rigid pipe system, termed an air manifold. A manifold system typically has multiple air hose connection points. The air manifold is sometimes coloured blue for easy identification.
Compressed air hoses are usually made up of three layers. An inner rubber lining on the inside of the hose, a layer of fabric or wire reinforcement, and an outer layer of rubber. Formerly, most airline hoses had threaded end fittings. At present the safer and more convenient quick disconnect self-closing fittings (couplings) are commonly used.
Air powered devices
Air powered (or pneumatic) tools are simply a wide variety of hand tools and devices powered by the energy of compressed air. This subject is covered more fully during the next pages.