Compressed Air Basics

Atmospheric air is a mechanical mixture of individual gases mainly comprising of Nitrogen and Oxygen traces of other gases, water vapour and particulate matter. 

Compression of a gas is possible due to the large intermolecular distances.  Energy is used forcing the molecules closer together (the container is smaller, thus more collisions, thus a higher pressure).   When the gas is allowed to expand, this energy is released and the intermolecular spacing reverts to its normal conditions.

Hence compressed air can store energy and is one of the safest ways to transmit energy in piping networks.

Compressed air has become an essential facility in many applications from dentistry, scuba diving, throughout industry to space vehicles.  It should be considered in its many forms from vacuum to the highest pressures of over 400 bar(g).

Over 80% of air systems are designed to run at a pressure of around 7 bar(g).  This paper will concentrate on these systems unless stated.

History

Compressed air was probably first used, during pre-historic times, in bellows pumps to increase the intensity of fires.

Its first probable industrial use was in the Cornish tin and copper mines to replace hammers that operated by two men called Jacks.  The pneumatic jackhammer became the basis for many air tools during the early 20th century that helped speed up production of large structures such as ships and bridges.  These included riveting guns and breakers.

Since then the use of compressed air for a plethora of applications has become commonplace and still growing in such as the microelectronic and pharmaceutical industries.

Energy issues

As will be explained in more detail later a compressor rejects around 90% of the input energy in the form of waste heat.  As a rule of thumb with a 7 bar(g) system with an electrically driven compressor the unit power cost can be multiplied by a factor of 10 once the air is used to drive devices.

With compressed air being accepted as a safe and flexible method of transferring energy many misuses often occur where the same duty can be accomplished by differing methods at lower energy costs.

next >