In industry, compressed air represents a form of energy whose spectrum of applications is unrivalled by any other. Without compressed air, the level of automation typical of modern factories nowadays would be unthinkable. Particularly stringent quality requirements apply to compressed air in the food and pharmaceuticals industries. Compressed air must therefore be properly conditioned for use and its quality monitored by means of suitable measurement methods.
Karl-Heinz Pannwitz
Depending on where the compressor is situated, the ambient air it sucks in may contain impurities such as mineral oil aerosols and hydrocarbon vapours, water vapour, particles and harmful substances in gaseous form, such as carbon monoxide. During the compression process, lubricant oil and tiny abrasion particles from the compressor may additionally find their way into the compressed air. Dust and other particles increase the extent of abrasion within the compressed air network and prematurely wear pneumatic systems. Resinous oil in the pipeline system may reduce pipe diameters and cause blockages within the system. If the water vapour content of the compressed air is too high, the pneumatic system may corrode, electrical elements may form and, at low temperatures, ice may additionally form within the compressed air network, resulting once again in reduced diameters and blockages. Particularly stringent quality requirements apply to compressed air in the food and pharmaceuticals industries if there is a chance that the compressed air will come into contact with products. The compressed air must therefore be properly conditioned for use and its quality monitored by means of suitable measurement methods.
The ISO 8573 series of standards, entitled “Compressed air“, sets out the quality requirements for compressed air and the test procedures which need to be applied. Part 1 explains the contaminants and purity classes (Table 1), while Parts 2 to 7 describe the measurement procedures for oil aerosols, humidity, articles, oil and solvent vapours, gaseous contaminants, and microbiological contaminants.
Test tube measurements
To test the quality of compressed air, either ready-for-use analytical methods can be applied – e. g. test tubes which can be used to conduct testing on site – or more complicated laboratory measurements which involve a sample being taken on site and subsequently analysed in the laboratory. Test tubes can be used to determine the concentrations of inorganic gases as well as oil contents of 0.1 mg/m³ or higher. Using the Dräger MultiTest medical gases, for example, the compressed air is fed through the Dräger tubes:
- Carbon dioxide 100/a-P
- Carbon monoxide 5/a-P
- Nitrous gases 0.5/a
- Oil 10/a-P
- Sulphur dioxide 0.5/a
- Hydrogen sulphide 1/d
- Water vapour 20/a-P
- Water vapour 5/a-P
The technical data is summarised in Table 2. If the type of oil used in the compressor is known, a specific threshold value of, for example, 0.1 or 0.5 mg/m³ can be selected when conducting a measurement with the tube oil 10/a-P. If the result of the measurement is negative, it can be assumed that the oil concentration is below the chosen threshold value. In the case of a positive reading, the oil concentration should be verified by a laboratory test method. Residual oil concentrations below 0.1 mg/m³, such as are required by ISO 8573–1 for compressed air in general applications, especially in the food and pharmaceuticals industries, cannot be checked using this method.
Checking the oil content
The residual oil content in compressed air in the 0.01 mg/m³ range is determined using a laboratory measurement procedure which was developed on the basis of ISO 8573–2 and 8573–5. The sampling system comprises a glass fibre filter inside a filter cartridge to enrich the oil aerosols and a downstream activated charcoal tube for the adsorption of the oil and solvent vapours. The air is decompressed at the outlet point using a flow control unit and routed through the sampling system to enrich the oil aerosols and vapours. Depending on the quality class required, the following volumes are routed through the sampling system:
- Quality class 1 (maximum oil content 0.01 mg/m³) at least 2500 l of air
- Quality class 2 (maximum oil content 0.1 mg/m³) at least 250 l of air
The oil aerosols are determined by extracting the glass fibre filter using a solvent and subsequently analysing them by means of a Fourier transform infrared spectrometer (FT-IR) in the wave number range from 2800 to 3100 cm-1. The oil vapour and solvent concentration is determined by extracting the activated charcoal and then separating it by gas chromatography on a capillary column using a flame ionisation detector. The oil concentration (aerosol and vapour) is calculated as the sum of the two analysis results. Additional analyses can be performed to answer specific questions. For example, by conducting comparative IR analyses of the air sample and the oil inside the compressor, it is possible to determine whether the oil found at the outlet point is identical to the oil type inside the compressor.
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