Water, oil, dust particles and micro-organisms must all be eliminated from delivered compressed air before it is used within the process. Paul O’Neill, manager of Atlas Copco Compressors Oil-free Air division, explains how this can be achieved, but also recommends the use of oil-free units
The quality of compressed air has a direct bearing on production efficiency and end product quality, and when contaminated air comes into contact with production processes, the resulting equipment downtime and rejection costs can be high. To prevent this happening, it is essential that water, oil, dust particles and micro-organisms are eliminated from the delivered air by adopting a suitable air treatment system.
Air in the atmosphere contains moisture in the form of water vapour, and when compressed, a lot of water is released. For example, a 100kW compressor drawing in air at 20°C and 60% relative humidity will produce approximately 85l of water during an 8h shift.
That amount of water in compressed air can cause maintenance problems, shortened service life and impaired production performance. It can lead to increased leakage, disturbances in the control system and instrumentation, and a shorter service life for the pipework due to corrosion. However, the water content can be separated out by using aftercoolers, condensation separators and either refrigerant, adsorption or membrane dryers.
Although modern fibre filters are efficient at removing oil, it is difficult to precisely control the quantity of oil remaining in the air after filtration. This is because temperature impacts on the separation process.
Filter efficiency is also affected by the oil concentration in the compressed air, as well as the amount of free water present. The quantity of oil present depends, among other factors, upon the type and design of the compressor, its age and condition. In modern, lubricated piston and screw compressors the quantity of oil is limited. For example, in an oil-injected, screw compressor, the oil content in the air is less than 3mg/m3 at 20°C. Although this volume may seem insignificant, and can be reduced further using multi-stage filters, its presence still poses potential quality limitations, increased risks and energy costs.
Oil vapour must be removed using an activated carbon filter. A correctly installed fibre filter, together with a suitable pre-filter, can reduce the quantity of oil in the compressed air to approximately 0.01mg/m3, but an activated carbon filter can reduce the concentration to 0.003mg/m3.
Filters with activated carbon only remove air contamination in the form of vapour and should be preceded by other, appropriate filters. They should be placed as close as possible to the application site, and be checked and replaced regularly.
More than 80% of the particles that contaminate compressed air are smaller than 2µm in size and can therefore easily pass through the compressor's inlet filter, spreading throughout the pipework system and mixing with the water/oil residue and pipe deposits, resulting in micro-organism growth.
Micro-organisms can germinate through the filter walls and therefore can exist on the inlet as well as the outlet sides of the filter. A filter positioned directly after the compressor can eliminate these risks but, to ensure pure compressed air quality, any bacterial growth after the filter must be kept under control.
The most effective treatment involves drying air to a relative humidity of <40%, achieved by using an appropriate dryer and including a sterile filter within the system. The filter must be located in a housing that allows in-situ steam sterilisation or that can be easily accessed. Sterilisation has to be performed frequently.
Oil in the form of droplets is separated partly in the aftercooler, condensation separator or a condensation trap and flows through the system with the condensation water. This oil/water emulsion is classed as waste oil and therefore must not be drained off into the sewage system or directly into the environment.
Condensate disposal is subject to stringent laws with regard to the handling of environmentally hazardous waste. An easy and cost-effective solution involves installing an oil/water separator with a diaphragm filter to produce clean drainage water and to separate off the oil into a receiver.
Overriding all of the preceding considerations, an oil-free compressor eliminates the need for an oil filter and guarantees freedom from contamination. These compressors can operate at a lower discharge pressure, reducing energy consumption. In many cases, oil-free compressors offer the best solution from an economical standpoint and for the quality of air delivered.
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