and high efficiency filters (oilremoval/coalescing filters) must be employed. Standard air line filters should still be employed as pre-filters to these high efficiency filters. High efficiency filters willremove these extremely fine particles and ifexposed also to the coarser particles they will simply clog and become congested with dirt extremely quickly.All elements will become blocked inuse. The level to which the blocking is acceptable is dependent upon the application and the energy consciousness of the plantoperation. Standard filters can be cleaned and reused but in today’s environment withlabour costs high and spare parts inexpen- sive it is normally better to replace elements. This will also ensure minimum pressure drop on reinstallation as cleaning at very best will only remove 70% of accumulated particles. High efficiency filter elements cannot be cleaned and must be replaced before theybecome blocked with dirt.Under normal usage conditionsgeneral purpose filter elements are usually changed before their pressure drop is greater than 0,5 bar, or in routine annual mainte- nance. The period can always be adjusted by monitoring for critical applications using a service indicator (figure15).High efficiency filters should havetheir elements replaced when a pressure drop of 0,7 bar is achieved. Again a low costservice indicator is often employed. Thisdevice has a scale of two colours, usually green/red. The elements should be changed when or before all red is achieved. Electrical service indicators are also available from Norgren, to provide remote signalling. Maintenance schedules can be produced to ensure this ‘last chance’ situation is not achieved, indeed some applications cannot tolerate even this much pressure drop,especially if this is at the generation point ofa large compressed air distribution main as the cost of extra energy alone would be very large.

Clean Compressed Air Brochure - 14694 OIL

The principle source of oil contamination within a compressed air system is from the compressor. An oil lubricated compressor of 50dm

/s capacity may introduce as much as0,16 litres of oil per week into the system.Oil is used for lubrication of thecompressor but when it emerges with the compressed air prior to distribution the oil is now in a totally unusable state. Having been subjected to high temperatures during air compression it becomes oxidized and acidic and can be considered as an aggressive contaminant rather than a lubricant and so must be removed.Normal air line filters will removesufficient liquid oil (along with water) toleave the air in a suitable condition to supplymost pneumatic tools and cylinders, but certain processes demand completely oil-free air.One solution is to use oil-freecompressors. These will still produce air contaminated with dirt and water and it is often more economical to use lubricated compressors in conjunction with after coolers and standard air line filters, only fitting high efficiency oil removal filters at the points in the system which demand oil-free air. This ensures that the amount of air needing special treatment is kept to a minimum by allowing a smaller specialised filter in the affected area and not a large specialised filter for the whole plant.Oil in a compressed air system canexist in three forms, oil/water emulsions, aerosols (small particles suspended in the air) and oil vapours.Emulsions can be removed bystandard air line filters but the aerosols areour next concern.