A Quantitative Method to Determine Oil Carryover from High Efficiency Coalescing Filters in Compressed Gases Under Varying Dynamic Conditions and the Interpretation of Results to Assist Filter System Design

Discharge air/gas from compressors is often not of the right quality for certain uses unless it is suitably treated. The various contaminants present in compressed air/gases include atmospheric pollution, compressor wear particles, bulk lubricating oil and aerosols, watervapour and condensed water, oil/ water emulsion, rust and carbon. Suitable treatment of the air/gases involves the removal of the contaminants using aftercoolers, refrigerant or desiccant dryers and filters. Coolers and dryers are used to removewater vapour and reduce the dew point of the air, thus preventing further condensation of water downstream. High efficiency coalescing filters are used to remove the other contaminants to extremely low remaining levels. Measurement techniques to determine the dew point of compressed air/gases are well known, but the important measurement of remaining oil content after filtration has not been so well established. A new test method has now been developed which is capable of giving very accurate measurements of remaining oil content down to 0.001 mg/m'. The test method will be explained in detail and an outline of results from the method obtained over a period of several years will be discussed, including data collected over a varying range of operating parameters, i.e. temperature, velocity, oil loading and the effect of these on filter efficiency and filter system design. CONTAMINANTS Compressed air is one of the essential services in any industry. It possesses many advantages over other types of energy and an increasing number of applications for it are being discovered. Unfortunately, compressed air/gases can contain a considerable amount of contamination which can often have drastic effects on the equipment using it. 398 Contamination can enter a compressed air/gas system from three sources. These are namely, the atmosphere, the compressor and the piping system. Taking the atmosphere first; in a ty~ical.metropolitag environment there are somethlng l~ke 140 x 10 dirt particles per m' (4 x 10 /cu.ft.) Approximately 80% of these are less than 2 microns in size (a micron is one millionth part of a metre or approximately 0.04 thousandths of an inch) and are not removed by the compressor intake filter. In addition to this, there are contaminants such as hydrocarbon vapours (from unburnt fuels and industrial processes) and water vapour. All of these are drawn into the compressor and their concentration increased by the compression process; Inside the compressor, wear particles and lubricating oil are added to the air/gas. The amount ofoil contamination depends on the type of compressor and its condition but can typically vary between 5 and 50 mg/m'. To put this into context, a 286 1/s (600 scfm) A.N.R. compressor carrying over 30 mg/m' (25 ppm) of oil during an operational period of 6000 h will deposit approximately 212 litres (47 gallons) of oil into the system. It is never an advantage to have oil from a compressor in the system as this oil is inevitably degraded and oxidized after the heat of compression. Usually it is acidic and can appear as a varnish like substance possessing properties completely opposed to lubrication. If synthetic oils are being used these can cause chemical attack on downstream components such as seals and polycarbonate parts. Carbonization of the lubricating oil in the compressor is not uncommon due to the temperature of compression and the presence of 'hot spots' results in deposits of carbon which are again carried over. It is worth taking note at this point that even using an 'oil free' compressor does not necessarily generate 'oil free' air as quite large amounts of hydrocarbons can be drawn in from the atmosphere. After compression the air/gas then passes into the piping system. Normally, aftercooling reduces the amount of water vapour contained in the air/gas but further cooling in the system causes more water to condense. Pipe-scale and rust are also present and when these contaminants mix together with the oil they form an abrasive, creamy sludge. This combination of contaminants causes costly problems with both the equipment and the product. These can be eliminated by suitably treating the compressed air using an effective filtration system. Water vapour itself is sometimes a particular problem and cannot be tolerated in any amount, in which case a dryer is also necessary. Oil aerosol is present in a very fine submicron form similar in size to tobacco smoke (Fig. 1) and has the ability to travel many miles down pipework systems without settling due to gravity and/or other effects. FIG.1-Atmospheric pollution size cornpari.on chart High efficiency coalescing filters are used to remove from the air/gas stream the variety of contaminants present. High removal efficiency is achieved by the use of filter systems employing borosilicate microfibre media, this media being constructed into filter elements of suitable design for specific applications. The main purpose of this paper is to discuss the behaviour of high efficiency coalescing filters to ensure that high proven quality compressed air/ gas can be made available for industrial applications.