For over 100 years, compressed air has been recognised as a safe and reliable power source that is widely used throughout industry. Known as the fourth utility, approximately 90% of all industrial manufacturing companies use compressed air in some aspect of their operations. Unlike gas, water and electricity which is supplied to the site by a utility supplier to strict tolerances and quality specifications, compressed air is generated on-site by the user. The quality of the compressed air and the cost of producing this powerful utility is, therefore, the responsibility of the user.
The problem with compressed air
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.
Compressed air contamination sources
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 four sources of compressed air contamination:
- The ambient air
- The air compressor
- The air receiver
- The distribution piping
1. Ambient air
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.
Main contaminants found in ambient air:
- Water vapour
- Micro-organisms
- Atmospheric particulate
- Oil vapour
Additional contaminants found in ambient air:
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:
- Sulphur dioxide (SO2)
- Nitrogen oxides (NO + NO2)
- Carbon monoxide (CO)
- Carbon dioxide (CO2)
- Vapours from caustic cleaning agents, bleach, etc.
2. Air compressor
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.
Contaminants added by the air compressor:
- Liquid oil
- Oil aerosols
- Oil vapour (from compressor oil)
- Wear particles
Ambient contaminants converted by the air compressor:
- Oil vapour
- Water vapour
Both gaseous contaminants cool and condense to form:
- Liquid oil
- Oil aerosols
- Liquid water
- Water aerosols
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:
- Liquid water
- Water aerosols
- Water vapour (100% saturation)
- Liquid oil
- Oil aerosols
- Oil vapour
- Micro-organisms
- Atmospheric particulate and compressor wear particles
3. Air receiver
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.
Contaminants added by the air receiver:
- Rust
- Pipe scale
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.
4. Distribution piping
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.
Contaminants added by the distribution piping:
- Rust
- Pipe scale
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.
Contaminant summary
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.
Contamination entering the compressed air system
Breathing air / medical air
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.
Contaminants of concern for breathing air / medical air applications
- Sulphur dioxide (SO2)
- Nitrogen oxides (NO + NO2)
- Carbon monoxide (CO)
- Carbon dioxide (CO2)
Therefore, for breathing air / medical air / critical applications, there are a minimum of 15 contaminants that must be treated.
Contamination entering the compressed air system (breathing air):
Compressing air – the problem increases
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.
The concentrating effects of compression
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.
Myth
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.
Fact
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).
Which contaminant causes the most problems?
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.
Water causes:
- The growth of micro-organisms
- The formation of rust and pipe scale
- The production of oily, acidic compressor condensate
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.
So how big is the water problem?
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.
Summary
- There are many contaminants found in a compressed air system.
- Generally, for most compressed air applications, there are 10 contaminants that must be treated.
- Other contaminants present may also cause issues (application dependent).
- Contamination comes from 4 different sources, not just the compressor.
- Compressing air concentrates contamination found in the ambient air.
- Water is the most prolific and problematic contaminant, not oil.
- Liquid contamination will increase significantly in warmer, more humid environments.
This article was contributed by Mark White, compressed air treatment applications manager, Parker Gas Separation and Filtration Division EMEA
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