Air compressors and pneumatic technology have been the industry standard in most heavy manufacturing and heavy transportation applications for decades.
The fact that compressed air is vastly more efficient and powerful than many other power sources and generates considerably less waste has made them extremely popular with large industrial operators.
As such the technology has constantly been improving for more than a century now.
Some of the basic air compressor technology still in use today was initially developed around the start of the twentieth century, but there has been a constant effort to improve the technology; improving its efficiency, its price, its portability, and its overall effectiveness.
One of the key challenges for air compressor development has been getting more pressure out of the same general technology.
For example, a basic rotary screw compressor or a basic piston compressor can only compress so much air in each of its cycles and while more compressed air can be piled on top of it in the storage tank; once the stored air reaches a certain level, the technology simply cannot squeeze in any more.
In some cases the answer was to modify the process in some respect, such as the addition of oil into the air.
While this does improve efficiency and less air escapes - allowing more to be piled into the tank - air/oil mixtures are terribly inappropriate for some applications, such as the generation of breathable or medical air.
Another idea to increase the level of air compression has been to create two stage air compressors which compress and then recompress the air a second time.
Essentially two stage air compressors are two separate compressors built into one unit.
The first stage does the initial air compression and stores the air in a tank.
The second compressor takes this already compressed air from the tank and then recompresses it a second time and stores this air in a separate tank.
The result is to effectively compress the air at twice the basic level using the same technology, just through two cycles of compression.
In many cases, operating through two stage air compressors allows compressors that are specifically designed for special purposes to generate far more compressed air - or air compressed at a far higher level - than would otherwise be possible.
Good examples of this include the previously mentioned compressors designed to generate breathable or medical compressed air.
The same may also be said for smaller compressors specifically designed to run silently for office applications.
Adding a second stage of compression simply doubles the level of compression possible using the exact same technology.
Since the key result of using two stage air compressors is double the pressure - the psi, or pounds per square inch of pressure generated - most of these machines are designed specifically for industrial applications.
That is, you will rarely find a two stage air compressor designed for household use or small portable applications.
Instead the increased psi means that these machines are really only applicable to uses where extremely high psi levels are required.
Two stage air compressors also generally cost a lot more than a single stage version of the same machine.
Of course the increased price makes sense in that these compressors is essentially two air compressors in one.
Further, they are not really designed for personal use, but industrial applications, and presumably businesses can afford to pay more for essential machinery than individuals.