Airborne dirt. The amount of dirt within the air of a typical city or industrial area is staggering. For each cubic meter of air, there are 140 million particles of dirt. This bodes particularly badly for the process of air compression, because 80% of those airborne dirt particles are too small to be caught by the intake filter of a given machine, and therefore slip into the compressed air.
Water vapor. With the changes in temperature that air is put through during the compression process, moisture within an air compressor is inevitable. If the air is hot, it will retain vapor; if the pressure is high, the air will release condensed water.
Condensed water and aerosols. During the latter stages of the compression cycle, the air is cooled to a more reasonable temperature. This turns the vapor in the air to water, which is then sent to the condensate drain. However, condensate remains a factor throughout the process, because vapors are present as the air changes temperature. Condensation is not only bad for the internal mechanisms of a compressor, it can be even more damaging if the drain is never emptied.
Corrosion and pipe scale. As condensation takes its toll within the piping and air receivers, rust can form on the inner lining. Gradually, the rust can cause the piping to clog and break. If the problem is left untreated, internal corrosion can lead to outright system failure.
Oil. Compressed air should always be oil free. Nonetheless, oil serves as a necessary evil in the process of air compression. As such, the process has its share of potential consequences. As the air is compressed, oil is used for the purposes of cooling, lubrication, and sealing. Unfortunately, up to half of the degraded oil can pass through the system in vaporized form, especially when temperatures are high. The system itself can also draw unburned hydrocarbons, which condense once cooled. When acidic oil vapors mesh with moisture in the compressed air, corrosive buildup forms along the air receivers and valve cylinders.
It must be noted that oil-free compressors are not contaminant-resistant compressors. In other words, the inlet valve of an oil-free compressor cannot magically filter out airborne contaminants from ambient air. Just as with an oil-lubricated system, a dental air compressor oil free needs filtration to keep water, dirt, unburned hydrocarbons, and other impurities from the compression process.