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Compressed Air

Everything you need to know about compressed air:
summarized by netinform and our partner BAUER KOMPRESSOREN GmbH München

Compressed Air
Compressor Types
Compressor Installation
Laws, Regulations, Technical rules
Definition
Definition

What is compressed air ?

Compressed air is atmospheric air held under pressure greater than that of the atmosphere. Compressed air is an energy carrier. It is able to transmit this energy over long distances and convert it into work as the pressure is removed. In contrast to steam and electrical energy, compressed-air is a relatively safe energy medium for people. For time immemorial, compressed air has been used by people to perform work. Blowpipes, for example, were used by hunters to kill game with poison arrows. Manually operated bellows, too, were already used in ancient Egypt to melt metals such as gold and copper etc. Compressed air finally made its breakthrough in the late 19th century, when the French installed a compressed-air supply system in Paris that provided large areas of the city with energy. An over 1000 kW compressor plant pumped high-pressure air into the several miles of piping that had been installed in buildings. Over the course of the last century, the advance of compressed air proved unstoppable and still remains so. Countless applications both in the low and high-pressure sector, became possible with the help of increasingly refined compressors and processing systems.

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Fields of application
Fields of application

Where is compressed air used ?

Owing to its relatively simple handling, compressed air is used as control and process air in many low-pressure (8-15 bar) applications, and has become indispensable in many work and production processes. Typical areas in which compressed air is used include:
     
  • automotive industry
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  • power-station engineering
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  • heavy industry
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  • breweries
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  • car paintshops
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  • sand and shot blasting
  • hospitals, nursing homes and emergency medical centers

The use of compressed air, however, is not limited to the low-pressure sector. On the contrary, a very wide range of applications has emerged in the high-pressure sector. This is a result, in particular, of the development and construction of multi-stage compressors that deliver up to a maximum of 500 bar. Membrane compressors nowadays even reach pressures as high as 3000 bar. High-pressure applications are now very important for

     
  • diving
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  • industrial respiratory protection
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  • power-station engineering
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  • ship building
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  • military use, including night-vision technology (infrared technology)
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  • aviation
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  • electrical engineering
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  • seismology
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  • fertilizer production
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  • automotive supply industry, etc.

For all these users of compressed air, the high-pressure air must be supplied in a processed, i.e. filtered, and dried state to the work process.

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Processing
Processing

Compressed-air processing

a) Separation of water and oil
The increasing and intensified use of compressed air has meant that the quality requirements for compressed air have become stricter year after year. Compressed air must be free of particles, moisture, other gases and oil. Preliminary separation of liquids, such as water and oil, is already effected after each stage in the compressor by mechanical separators, such as spin or cyclone collectors. The more stages a compressor has, the more efficiently liquids will be separated out of the compressed air. Surface or depth filters (coalescing filters) in the compressor filter out dust, dirt particles, sediments, liquids and spray mist. Oil vapor in the compressed air cannot be separated by mechanical filters, since the oil is present in molecular form. The amount of oil vapor in the compressed air depends on temperature. The higher the temperature, the more oil vapor passes through a filter. If the compressed air is then cooled down again sufficiently, the oil vapor condenses and can then be filtered out easily via a depth filter. The remaining oil vapor can be further reduced by means of activated carbon adsorbers or carbon filters located downstream. Thanks to these adsorbers, residual oil contents of < 0.1-0.2 mg/m3 are reached. Since the water content influences adsorption capability, pre-drying of the compressed air is an absolute must. When the following criteria are observed, the residual oil content can be reduced to the aforementioned level:
     
  • low filtration input temperature of the compressed air
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  • maximum relative humidity of approx. 60%
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  • pre-filtration with depth filter

If these criteria are met, these activated carbon adsorbers have a service life of approx. 8,000 - 10,000 hours, the smaller activated carbon filters of 300 - 400 hours. In practice, monitoring the service life of activated carbon filters or activated carbon adsorbers is simple. Oil-test indicators that show the oil concentration by changing color have proved especially useful here.

b) Drying
To dry the compressed air up to a pressure dew point of -20°C and more, a molecular sieve is used as adsorbent. For high-pressure Bauer compressors, the following distinction is made depending on whether drying is effected by:

     
  • cartridges (BAUER P systems), or
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  • adsorption - cold regenerated (BAUER Seccant III und IV).

In both systems, a pressure dew point of approx. -20°C (200 - 500 bar) is achieved. In the pressure range from 6 to 60 bar, cold dryers are used for post-drying. These cold dryers have a far lower pressure dew point, namely between 2°C and 5°C. Cold dryers are mostly used for preliminary air drying (moisture separation only). In the case of downstream filter systems, this may increase the degree of purity or the filter's service life. A signaling system monitors cartridge saturation by measuring the capacity of the molecular sieve. The higher the humidity, the faster the cartridge will become saturated. The control system then emits a preliminary warning. Unless the filter is changed, it will shut down the compressor and indicate the malfunction on the visual display unit.

c) Monitoring of oil content and compressed-air dryness
In practice, the residual oil content in the high-pressure air under operating conditions is hardly ever measured, since measuring methods are inaccurate and do not supply reproducible results. The residual oil content in the air can be demonstrated in the lab via spectral analysis of a sample taken from the compressor. The dryness of the compressed air (in mg/m3 or ppm) can be continuously displayed by means of established pressure dew point measuring equipment. These devices must be installed in the pressure line between the drier and the high-pressure storage tank

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Quality classes
Quality classes

Compressed-air quality classes

Operators of a high-pressure compressor plant need to know which filters and driers are necessary for optimum processing and testing of the manufacturing product. The answer is: "As many as necessary, as few as possible." Assistance with this difficult subject, especially for industrial applications (each one can be different) is provided in the relevant regulations:

     
  • EN 12021 for breathing air (hitherto DIN 8188)
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  • ISO 8573-1 Compressed air for general use - Part 1 Contaminants and quality classes
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  • High-pressure air for medical applications as per the "European Pharmacopoeia" (Ph. Eur.)
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  • Specification "Defence Standard 58-96/1" or "MIL-R-81202 D for military night-vision application (infrared technology)"

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Storage
Storage

Compressed-air storage

Compressed air is stored in stationary pressure vessels made of high-strength steel. These vessels are designed for cyclic loads. In this context, the pressure may fluctuate between atmospheric pressure and the maximum permissible operating pressure. The wider the operating pressure range (pressure fluctuation), the shorter the vessel's service life. After the defined number of cycles has been reached (2 load changes, i.e. pressurization and depressurization, are referred to as a cycle), the vessel must be replaced by a new one. Pressure vessels are manufactured in line with the regulations on accident prevention, AD Codes of Practice and are subject to acceptance by TÜV. Periodic visual inspections from the outside and the inside as well as water pressure tests at given intervals are mandatory.

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