Compressed Air Treatment

An overview of the key steps to ensure clean, reliable compressed air

Air is contaminated

Hardly identifiable with the naked eye: pollen, germs, particles, fibres, aerosols as well as heavy metals such as lead and cadmium. Contamination present in ambient air can impair the function of the compressed gas system, the quality of the products and even consumers’ health.

The compressor compresses the aspirated ambient air to the required pressure level. By compressing the ambient air, contamination is also compressed. Without the corresponding processing technology, the contamination would easily ingress into the compressed gas system and get into the final product, depending on the production process involved. This means compressed air treatment is also a process safety issue.

Why compressed air quality defines the treatment process

The energy medium air is available in almost unlimited quantities all over the world, but the quality is not the same everywhere. There are major differences in the air in the different climatic zones, due to different pollutant emissions caused by traffic, industry, agriculture and other influences. This has to be taken into account accordingly during compressed air treatment. The right compressed air treatment can significantly reduce machine breakdowns and standstill states. Not only the machines need clean and dry compressed air. As a process medium, compressed air must fulfil technical conditions and be conditioned accordingly. For this reason, the quality of compressed air is based on the respective requirements.

Standard ISO 8573-1 stipulates the permissible quantity of contaminant per cubic metre of compressed air. Solid particles, water and oil are named as the 3 primary contaminants.

3 codes designate the respective quality class according to ISO 8573-1.

Accordingly, various processes engage with each other during compressed air treatment. Special note must be taken of the pollution caused by germs and bacteria. However, DIN ISO standard 8573-1 does not define any limit values for germs or micro-organisms.

You will find more information about the subject of compressed air quality according to ISO 8573.1 here …

Particle concentration

When compressed air is generated, ambient air is aspirated and compressed by a compressor. This means that dust, humidity, oils, aerosols as well as micro-biological components get into the compressed air in correspondingly shares.

The correct particle concentration can be achieved gradually via suitable filters.

Residual humidity

Water is the most problematic of all types of contamination in compressed air. It not only causes damage in the form of corrosion, it also promotes the growth of micro-organisms which can harm users as well as contaminate products and processes.

The required degree of residual humidity can be generated using suitable dryers.

Residual oil content

Ambient air contains oil in a gaseous state (oil vapour). Typical pollutant values are between 0.05 mg/m³ and 0.5 mg/m³. In heavily built-up areas in cities or on industrial estates, however, this level can be higher.

The residual oil content can be reduced and monitored using suitable procedures.

Germ-free

Compressed air systems contain large quantities of micro-biological contamination and a warm, humid compressed air system offers the ideal environment for growth.

Yet germs and micro-organisms can also be removed permanently from the compressed air using compressed air treatment technology like sterile filters or catalysis technology.

Compressed air treatment technologies

In the context of compressed air filters you will come across a wide range of different terms. Some of these names are manufacturer-specific, some have simply become part of the vernacular or are specialist terms. Some of the frequently used terms are explained below.

Water separator / cyclone separator

The water separator is not a filter in the classical meaning, rather it is a separator, as the name already indicates. It blocks the path of slow-moving fluid quantities in compressed air and separates water by means of gravity and centrifugal force.

In other words, water separators get large quantities of fluids out of a system and thus relieve the pressure on downstream dryers etc. This fluid is known as condensate and has to be treated accordingly (condensate technology).

Wet filtration / coalescence filter

The most widespread coalescence filters combine various filtration techniques in order to achieve optimum results. Non-woven filter materials with different material properties and suitable manufacturing processes (pleating, winding, ...) are often used for these. The individual manufacturers try to separate both particles as well as fluid and oil drops and aerosols in one housing. The following illustrations show just how this works.

Surface filtration

Particles that are larger than the distance between the fibres of the non-woven filter material are retained directly between these fibres, in other words trapped like in a sieve. Since this mostly takes place on the surface, the term surface filtration is often used.

Depth filtration

In the case of depth filtration, or so-called collision separation, solid particles and aerosols enter the filter bed. There, they collide with many fine fibres, lose kinetic energy, become slower and slower, and finally stick to the fibres

Brownian motion

Tiniest aerosols collide with the fibres through Brownian motion, move along the outer fibre wall in the direction of compressed air flow. They form larger drops at the branches and flow into a collecting container from which they must be discharged as compressed air condensate.

Limits of coalescence filtration

Coalescence filters effectively remove liquid oil and aerosols by combining small droplets into larger ones.

However, oil vapour cannot be captured this way and passes through the filter. As ISO 8573-1 considers all oil components, additional technologies such as adsorption or catalytic treatment are required to achieve the highest air quality classes.

Multi-stage filtration

To improve the result after filtration, several particle filters can be used in succession. The coarser filter must always be installed upstream of the finer filter. This procedure is known as multi-stage filtration since the desired quality is filtered stage by stage. BEKO TECHNOLOGIES supplies coalescence filters in 3 stages:

	Oil aerosol			-	Particle
Filtration rate	Oil aerosol deposition rate	Inlet concentration	Outlet concentration	-	Particle deposition	Particle size	Class according to ISO 8573-1
Coarse Filter C	84,00 %	30 mg/m³	<=5 mg/m³	-	99,00 %	2,0 -5,0 µm	4. - 4.
Fine Filter F	99,50 %	10 mg/m³	0,05 mg/m³	-	99,83 %	0,5 -2,0 µm	2. - 2.
Ultra-fine filter S	99,95 %	10 mg/m³	0,005 mg/m³	-	99,98 %	0,1 -0,5 µm	1. - 2.*

* In order to achieve Class 1.-.1, an additional active carbon filter and dust filter are generally required as the coalescence filter cannot retain the oil vapour.

Drying (residual humidity control)

Water vapour in compressed air can lead to corrosion, process disruptions and product quality issues.

Compressed air dryers are used to reduce the residual humidity and achieve a defined pressure dew point. Depending on the application, different drying technologies are used to ensure reliable operation and consistent air quality.

Procedure	Refrigeration dryer	Membrane dryer	Adsorption dryer
Pressure dew point	Pressure dew points are between +3 and +10°C	Pressure dew points are between +10 and -40 °C	Pressure dew points are between -20 and -70 °C
Volume flow	approx. 20 to 17,600 m³/h	approx. 20 to 2,250 l/min	approx. 10 to 100,000 m³/h
Working principle	Compressor and heat exchanger using condensation principle	The humidity is discharged to the surroundings via the purge air	The humidity is adsorbed by an adsorbent
Note	Not suitable for ambient temperatures below freezing	Terminal dryers, can also be used for compressed air distributors subject to frost	Very large range of possible pressure dew points and high volume flows

Refrigeration dryers are usually used at the start of a compressed air system, downstream from suitable water separation and condensate discharge. Membrane dryers are frequently placed near the application, in other words near the terminal point. They are often used to supplement refrigeration dryers, as so-called terminal dryers for smaller volume flows. Adsorption dryers are used either at the start of the compressed air system or near the application, depending on what it is, and are suitable for very large volume flows.

Residual oil content and germs in compressed air

In many industries, compressed air comes into direct contact with products and packaging. Even minimal contamination from oil or microorganisms can put product quality, safety, and brand reputation at risk.

Reliable protection depends on one thing: a perfectly coordinated treatment system—yet contamination risks are often underestimated.

Find out more

Your way to becoming a compressed air expert

During compressed air treatment, different processes engage with each other. The correct order, design and dimensioning of the components involved is decisive for the quality, and efficient treatment makes a significant contribution towards reducing the operating costs.

Would you like to find out more about the basics of compressed air and compressed air treatment? We recommend our comprehensive range of training events to you.