Ambient air is naturally contaminated, and so is the compressed air that powers entire industries. Invisible threats like moisture, particles, and oil are always present. Others, such as lubricant residue or corrosion particles, are introduced during compression and distribution. Each of these can seriously affect your tools, piping, products, and processes.
Let’s take a closer look at these hidden enemies so we can better protect our compressed air systems.
We often think of air as invisible, quietly surrounding us at all times. And usually, that’s true, we don’t see it. But there are moments when air becomes visible, and those moments reveal something important.
Think of fog, smog, or smoke. What we’re really seeing are tiny particles or moisture suspended in the air. And these are more than just visual phenomena, they’re signs of the very same contaminants that can pose serious challenges inside a compressed air system.
Moisture, dust particles, oil and micro-organisms are the most common threats to compressed air piping. If not properly managed, they can compromise performance, efficiency, and reliability. That’s why understanding what’s in the air isn’t just useful but essential.
The first contaminant that we can face 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.
A compressor that works at 7 bar(e) compresses air to 8 times its volume. This also reduces the air's ability to hold water vapor by a factor of 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:
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.
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.
There are the so-called “distribution system contaminants.” These could include rust particles from distribution pipes that get into the compressed air stream.
More than 80% of the particles that contaminate compressed air are smaller than 2 µm. These tiny particles easily pass through the compressor’s inlet filter, spreading through the piping where they mix with moisture, oil residues, and pipe deposits. This creates ideal conditions for the growth of micro-organisms.
These organisms, including bacteria, viruses, and bacteriophages, may be invisible but they are a real threat. Bacteria range from 0.2 to 4 µm, while viruses can be as small as 0.04 µm. Anything smaller than 1 µm can pass through standard inlet filters. As living organisms, they multiply rapidly under the right conditions. High humidity, especially in systems without drying air, accelerates their growth.
Placing a high-efficiency filter right after the compressor helps, but filtration alone is not enough. Micro-organisms and aerosols can still penetrate or regenerate on both sides of a filter.
The most effective defense includes two steps:
Drying air to keep relative humidity below 40%.
Installing a sterile filter that allows regular steam sterilization or easy cleaning.
This approach protects your equipment, products, and processes, ensuring clean, reliable compressed air every step of the way.
When oil and moisture are present in compressed air, they provide the ideal environment for micro-organisms such as bacteria, viruses and bacteriophages to grow.
Due to they are living organisms, they multiply when conditions are right, especially in non-dried compressed air with high humidity. This is particularly critical in industries like food and beverage, medical or pharmaceutical, where contamination can lead to serious consequences.
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.
This guide covers everything you need to know about air treatment, from types of contaminants to air quality requirements.
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