What is Compressed Air?
At it’s most basic level, compressed air is “potential energy” – very much like a compressed spring. It costs energy to compress a spring (force in the form of pressure must be applied). When that force or pressure is released, the potential energy stored in the compressed spring (or in this case, the air) is also released. That energy can then be used to produce “work”. The value of doing that work with compressed air is found in the readily available supply of air for compression (air is all around us), and it’s ease of distribution (via piping) throughout a facility. For this reason, compressed air is called the production facility’s “fourth utility”: after electricity, water, and natural gas. If the compressors in a production facility shut down, or can’t keep up with demand, then production grinds to a halt. Compressed air is a must-have in all industries.
What kind of “work” can be done with compressed air? As compressed air is expanded, it can be used directly as the motive force to power tools or motors in classified (hazardous) environments, as opposed to their more expensive or maintenance-heavy electric counter-parts. Expanding compressed air is also used to move pistons, power diaphragm pumps, to cool sensitive electronics, and to push materials down a pipe, or spray paint or media for abrasive blasting. Air under pressure can also be forced through membranes or molecular sieves to separate its component gases, such as Oxygen and Nitrogen – allowing a production facility to produce its own pure gases for production in a far more cost-effective way versus direct purchase from bulk gas suppliers. The uses of compressed air are limitless.
How is air compressed?
There are two main ways in which compression can take place. The most common is called “Positive Displacement” – whereby a given volume of air is drawn in from the atmosphere, and that volume is then mechanically reduced to “squeeze” the air into a smaller volume, thereby increasing its density. This is an exothermic (heat producing) process – and it’s what is taking place with a piston (reciprocating) or rotary screw compressor.
At left: an example of positive displacement, where a given volume of air at atmospheric pressure is “squeezed” by mechanical force into a smaller volume of “compressed air”. The air will naturally want to expand once the force holding it under pressure is released. That rapid expansion is used to produce “work”.
The other means of pressurizing air is called “Dynamic Displacement”, and it involves imparting velocity to the atmospheric air and driving it by sheer force into a system to build back pressure. Again, this is an exothermic process. It’s the kind of compression that’s generally used more at higher flows and pressures. Centrifugal (Turbo) and Axial compressors are examples of this kind of machine.
What about the Heat of Compression?
As noted above, compressing air is an exothermic process- it produces heat. That heat can be captured (in the case of a fan-cooled or water-cooled compressor) and reused elsewhere in the production facility – to reduce winter heating costs. Conversely, that heat must also be removed from the compressor room to ensure that the compressors continue to operate at an optimal ambient temperature. Furthermore, because hot air can hold a lot more water (in the form of vapor) than cool air does, a dryer should be used in a compressed air system to remove that water vapor. Without a dryer in place, the water vapor entrained in the compressed air will condense inside the cooler compressed air piping, and turn rapidly from vapor to liquid when the pressure in the compressed air is released (an endothermic – or heat-losing) reaction. Compressor systems without dryers will foul tools, impede media flow, contaminate systems and result in vastly increased maintenance costs and poor product quality (where that compressed air has direct contact with the end product).