Compressed air systems inherently suffer from performance and reliability issues and almost all the problems associated with the compressed air system and many manufacturing related quality issues can be directly attributed to contamination found in the compressed air.
Unknown to many compressed air users, the compressed air system contains a large array of both visible and invisible contamination which originate from four different sources.
The primary source of contamination found in a compressed air system is the ambient air surrounding the compressor. In simple terms, the air compressor is just a large air mover. When operating, it pulls in large volumes of air around it, squeezes it and pushes it out down a pipe. However, when doing so it also acts as a large vacuum cleaner, pulling in invisible contaminants. So, when the ambient air is compressed, the compressor is also concentrating the contamination at the same time.
The four contaminants highlighted are the main contaminants of interest for general applications, however, there are also other contaminants entering the compressor intake, which depending upon how the compressed air is used, may also require focus. The additional contaminants include:
During the compression and cooling process, the air compressor also changes the phase of the invisible gaseous contaminants it has ingested, and with a change of phase, many invisible contaminants now become visible. In addition to changing the phase of the ambient contaminants, the air compressor is also responsible for adding contamination of its own, making it contamination source number two.
So, by the time the air exits the compressor aftercooler and no matter what type of compressor is used (oil-lubricated or oil-free), the following contaminants will be present in the compressed air:
Contamination source number three is the air receiver. Installed in a compressed air system to store compressed air and increase the efficiency and reliability of the compressor, the air receiver also stores large quantities of contamination. These contaminants also lead to chemical reactions and oxidation which in turn, lead to additional contaminants being added to the compressed air system.
The wet air receiver (a receiver installed before a dryer) can reduce the compressed air temperature by up to 5°C. This cooling will cause further condensation of oil and water vapours into liquid oil and water. A wet air receiver is often chosen for this purpose as it can provide additional cooling of the compressed air at times where the ambient and compressed air temperatures are higher than expected. Unfortunately, it also provides the ideal environment for the rapid growth of micro organisms, especially in the compressor condensate.
In a typical compressed air system, the final source of contamination is the distribution piping which takes the compressed air from the compressor and distributes it around the manufacturing facility. Just like the air receiver, the distribution piping not only stores contamination, it also adds to the contaminant problem, through chemical reactions and oxidation, again adding rust and pipe scale to the compressed air and allowing the growth of micro-organisms.
As with the air receiver, the distribution piping will also cool compressed air causing further condensation of oil and water vapours into liquid oil and water which in turn form aerosols of oil and water as the air pulls the liquid along the piping.
To protect equipment and processes that use compressed air or products that have direct or indirect contact with compressed air, there are a minimum of ten contaminants originating from four different sources that must be treated.
If the compressed air is used for breathing air, medical air or other critical applications, then additional, potentially life-threatening contaminants in the ambient air must also be considered.
Therefore, for breathing air / medical air / critical applications, there are a minimum of 15 contaminants that must be treated.
To many, the ambient levels of contaminants may be considered “negligible”, however, when we talk about compressed air contamination, we must also consider the effect that compressing the air has on the ambient contamination, the amount of air flowing into the compressed air system and the time the compressor is operating.
When operating, the air compressor is constantly pulling in large volumes of ambient air and as operating pressure and or flow rate increases, the greater the volume of ambient air is required. The greater the volume of ambient air, the greater the amount of contamination.
For example: In simplistic terms, to generate 1 cubic meter of air at a pressure of 7 bar g (8 bar A) requires 8 cubic meters of ambient air.
When the stored compressed air is used, it expanded back to ambient pressure and there are those that believe that when this expansion takes place, the levels of contamination return to the ambient levels and are, therefore negligible, This, unfortunately, isn’t true.
When the air is compressed, the heat of compression makes the compressed air too hot to use, so it must be cooled to a usable temperature. Intercoolers and aftercoolers are installed for the reduction of the compressed air temperature (either integrated into the compressor or fitted externally). As the air is cooled, condensation of the gaseous contaminants into liquids and their subsequent conversion into aerosols (very fine droplets) takes place in the coolers and unfortunately, the liquid separators supplied with the coolers are unable to remove 100% of the liquids and are completely ineffective on aerosol reduction. Therefore, untreated compressed air is heavily contaminated by the time it reaches the point of use.
The table below provides an example of just how contaminated 1 cubic metre of compressed air can be at a typical operating pressure of 7 barg (102 psi g).
It is often believed that oil introduced by the compressor causes the most problems in a compressed air system. However, oil is not the major problem everyone thinks it is.
The most problematic contaminants are water and microorganisms. The presence of one directly impacts the growth of the other.
Oil is often perceived to be the most prolific contaminant as it is can be seen emanating from open drain points and exhausting valves. Usually, it is oily condensate (oil mixed with water) that is being observed.
The table below provides an example of how much water can enter a compressed air system per hour and per year.
The example used above is based upon a single compressor. If a compressed air system has larger compressors installed, runs for longer periods of time, is installed in a country with high ambient temperatures and/or relative humidity or a combination of all the above. then the volume of condensate in the system would increase significantly.
This article was contributed by Mark White, compressed air treatment applications manager, Parker Gas Separation and Filtration Division EMEA