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 …
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.
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.
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.
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.
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.
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.
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.
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
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:
|Filtration rate||Oil aerosol deposition rate||Inlet concentration||Outlet concentration||-||Particle deposition||Particle size||
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.
So-called coalescence filters are meant when talking about compressed air filters in general. As the name implies, small droplets “coalesce” into larger drops in such a filter. This improves the separating capacity of a filter.
Oil vapour, for example, is present in such a “small” form that no coalescence can take place. The oil vapour flows freely through the filter. ISO 8573-1 takes all oil components, i.e. fluid, aerosols and vapour, into account. Quality Class 1 cannot be achieved using only a coalescence filter.
A suitable method is adsorption or, alternately, catalytic treatment. See oil-free compressed air…
The term air humidity is used to describe the share of water vapour in the air. Air humidity can vary despite the ambient temperature being the same (desert climate, tropical climate). At a certain temperature, a volume of air can only contain a limited quantity of water vapour. A distinction is made between maximum, absolute and relative humidity.
The term pressure dew point describes the temperature at which an operating cubic metre under a corresponding pressure is saturated to 100 percent with water vapour. If the compressed air then cools to below this temperature, condensate is formed. Although it is expressed as a temperature value, the dew point does not correspond to the actual air temperature. Compressed air with a temperature of 35°C can have a dew point of -40°C, for example.
For applications where residual humidity in the compressed air is a quality criterion, the pressure dew point is an important parameter
- Excessively high residual humidity in the compressed air has a negative impact on the quality of painting e.g. by entrapping bubbles.
- Water-attracting, hygroscopic products such as powder, spices, salt and sugar stick together during the production process.
- In unprotected, cold environments, for example, humidity in the compressed air leads to control valves icing up and corrosion in pneumatic devices.
Many users do not realise how much water can be generated by using a compressed air system. Large quantities of humid, atmospheric air are compressed in the compressor, leave the compressor and enter the compressed gas system as compressed air saturated to 100 percent with water vapour. Since the compressed air is stored in the compressed air tank and then moves in the pipe network, it cools and condenses the water vapour to liquid water, which in turns also forms aerosols or water mist. The installation of water separators reduces the liquid water in the compressed air flow and coalescence filters reduce the aerosols in the water, but filtration alone is not sufficient for water reduction. If only water separators are installed, the best possible classification in accordance with ISO 8573-1 is Class 6 for water.
A dryer is used for reducing the water vapour. Three processes have become established for this – refrigeration dryers, membrane dryers and adsorption dryers The selection criteria are the PDP, volume flow, application and the required compressed air quality in accordance with the required class as well as the economic efficiency of the system and the related costs.
|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.
The pressure dew point (PDP) is an important parameter and a quality criterion for compressed air treatment. Normal ambient air with an atmospheric pressure of 1 bar can absorb significantly more humidity than compressed air. The pressure dew point rises with increasing system pressure, and vice versa.
If your compressed air system is leaking, the PDP will worsen. The quality established by the complex drying processes will then be reduced again. In this context, localising and eliminating leaks is important not only for economic reasons but also in terms of process reliability.
In many production facilities, compressed air comes directly or indirectly into contact with production lines, products or packaging materials (particularly in the food, beverages, pharmaceutical and electronics industries). Contamination through residual oil content, micro-organisms and germs then has significant consequences for product quality, consumer safety and market reputation.
Effective protection against oil entering the compressed air system and process reliability in use with sensitive products – with interaction playing a very important role. Only the perfectly matched interplay between different treatment components guarantees the required compressed air quality. The possible sources of contamination are often underestimated .
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.
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