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Our ambient air is inherently dirty. As a result, so is the compressed air that keeps entire industries running. It is under constant assault from barely visible enemies. Who are these tiny assailants? They are different contaminants that are present in our ambient air at all times, such as moisture and dust, traces of oil. Some of them are also added during the compression process, such as oil used as lubricant or corrosion particles that could come loose while the air is distributed.
Each of them can do serious harm to your production processes, compressed air piping system, tools, and products.
Let’s get to know each of these threats, which will allow us to effectively protect our compressed air systems.
We think of air as something that is invisible and all around us. And, most of the time, we cannot see it. However, there are circumstances in which we can and they illustrate the contaminants a compressed air system faces. For example, we see air in the form of fog, smog or smoke.
In those cases, either moisture or tiny particles make it visible. And these are two of the main threats a compressed air piping system faces.
Then there is the moisture contained in our ambient air. It enters the compressed air piping system through the intake in the form of water vapor. This water vapor is the most prominent contaminant in compressed air in total volume terms and it forms most of the liquid contamination that can be found in the air system.
The water content is measured in terms of the dewpoint. It is the temperature at which the compressed air is still able to handle its water vapor content before the moisture forms condensate.
If the moisture is not removed, it can reduce the service lives of pneumatic equipment through corrosion. In addition, it could lead to bacterial growth, which could adversely impact the quality of final products. This is especially problematic in applications in the food and beverage and pharmaceutical sectors.
While air quality is not as important for some processes, it is crucial to applications in the food & beverage and pharmaceutical sectors, where bacterial growth would be particularly harmful.
A compressor that works with 7 bar(e) overpressure compresses air to 7/8 of its volume. This also reduces the air's ability to hold water vapor by 7/8. The quantity of water that is released is considerable. For example, a 100 kW air compressor that draws in air at 20°C and 60% relative humidity will give off approximately 85 liters of water during an 8 hour shift. Consequently, the amount of water that will be separated depends on the compressed air's application area. This, in turn, determines which combination of coolers and dryers are suitable.
The quantity of oil in compressed air depends on several factors, including the type of machine, design, age and condition. There are two main types of compressor design in this respect: those that function with lubricant in the compression chamber and those that function without lubricant. In lubricated compressors, oil is involved in the compression process and also is included in the (fully or partially) compressed air. However, in modern, lubricated piston and screw compressors the quantity of oil is very limited.
In this case, it is known as a compression contaminant.
For example, in an oil-injected screw compressor, the oil content in the air is less than 3 mg/m3 at 20°C. The oil content can be reduced further by using multi-stage filters. If this solution is chosen, it is important to consider the quality limitations, risks, and energy costs involved.
There are the so-called “distribution system contaminants.” These could include rust particles from distribution pipes that get into the compressed air stream.
It all starts with the ambient air that has to be compressed. In a typical industrial environment, it can contain more than 140 million dirt particles per cubic meter. When it is compressed, these contaminants are concentrated in line with the air pressure increase.
That means that compressed air can contain many times as many dirt particles. Unfortunately, most of them are so small (under two microns) that an inlet filter only removes 20% of them.
More than 80% of the particles that contaminate compressed air are smaller than 2 µm in size and can therefore easily pass through the compressor's inlet filter. From that point, the particles spread throughout the pipe system and mix with the water and oil residue and pipe deposits. This can result in the growth of micro-organisms. A filter positioned directly after the compressor can eliminate these risks.
Nevertheless, to have pure compressed air, bacteria growth after the filter must be kept fully under control. The situation is complicated further as gases and aerosol can be concentrated into droplets (through concentration or electrical charging) even after passing several filters. Micro-organisms can germinate through the filter walls and therefore exist in the same concentrations on the inlet as well as the outlet sides of the filter.
Micro-organisms are extremely small and include bacteria, viruses and bacteriophages. Typically, bacteria can be as small as 0.2 µm to 4 µm and viruses from 0.3 µm to as small as 0.04 µm. Contamination smaller than 1 µm diameter and, consequently, micro-organisms can pass easily through the compressor inlet filter. Despite their size, these micro-organisms are a serious problem in many industries, because as 'living' organisms they are able to multiply freely under the right conditions. Investigations have established that micro-organisms thrive in compressed air systems with non-dried air at high humidity (100%).
Oil and other contamination act as nutrients and allow micro-organisms to flourish. The most effective treatment involves drying air to a relative humidity of <40% (this can be achieved by using any type of dryer) and fitting a sterile filter in the system. The sterile filter must be fitted in a filter housing that allows in situ steam sterilization or that can be easily opened. Sterilization must be performed frequently to maintain good air quality.
The contaminants can harm your production in three main ways.
Specifically, they can
reduce the performance of your compressed air system itself
negatively impact your air-powered equipment
impact the integrity and quality of your end products
Individually, each of them can negatively impact your system. In combination, however, they can be even worse. For example, the oil and moisture in your compressed air can allow the microorganisms contained in the intake air to grow and thrive.
This poses a serious challenge. After all, one cubic meter of ambient air can contain more than 140 million particles – from dust to microorganisms like bacteria, viruses and bacteriophages. In fact, they are so small (microorganisms range from 0.0.4 µm to 4 µm) that they can’t be caught by an inlet filter.
Because they are living organisms, they would multiply if the conditions are right, such as in non-dried compressed air.
This is particularly bad in the food & beverage, medical and pharmaceutical industries – it could have catastrophic consequences if microorganisms like bacteria and fungi were to contaminate food and pharmaceuticals.
Protecting your compressed air system
Fortunately, there is good news: With the right treatment, such as filters and dryers, your compressed air system can be protected from all of these contaminants.
If you want to find out how, then the first step is to determine the air quality that is required for your application, e.g. if you have to meet a certain ISO class.
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