In recent years, filter manufacturers have been incorporating new filter materials to improve flow capacity and capture even smaller particles, down to 0.01 µm, which improves efficiencies to 99.999%, in some cases.
One of the most attractive reasons for using compressed air is its reliability. When properly filtered, compressed air systems can work uninterrupted for millions of hours. But even the tiniest particles can plug small orifices in the system and cause excessive wear or system failure. Proper selection of air filters can prevent probable short- and long-term issues in a pneumatic system.
The air/oil separator is basically a coalescing filter. It follows the compression, and comprises primary and secondary stages, with the objective of reclaiming the lubricating oil prior to the air being discharged at the required pressure. The primary stage utilizes gravity settling assisted by a reduction in gas velocity; downstream from it, the typical oil loading is 5–50 g/m3 of polydispersed aerosols.
The second stage is normally a multi-layer cartridge, the media used depending on whether the flow through it is out-to-in or in-to-out. With the latter, the first, prefiltration layer can be a choice of several high particulate loading fibrous fabrics, such as a 0.3–0.7 mm thick, 100 g/m2 viscose rayon bonded with regenerated cellulose. There is then an overlapping support layer, typically a 1 mm thick, 120 g/m2 50% mixture of polyester/nylon bonded with synthetic rubber; the function of this is to contain the multiple layers of high-efficiency media wherein the fine oil mist droplets coalesce into much larger droplets.
These high-efficiency layers are of borosilicate glass fibres of various characteristics. They include a thin felt of coarser fibres bonded with phenolic resin and also microfibres bonded with an acrylic binder; integral support layers of spunbonded nylon provide intimate support for the fragile glass media to help the separator survive the rigours of frequent changes in pressure and the resultant cyclic loading of the media. Following the coalescing action of the glass fibre media, the large oil droplets are prevented from re-entrainment by a barrier comprising a 3–5 mm thick, 250 g/m2 nylon or polyester non-woven acrylic bonded fabric; this ensures rapid drainage of the coalesced liquid to the base of the separator, for subsequent pressurized expulsion back to the air intake.
The problem of oil removal is complicated by the fact that oil present in compressed air can exist in three forms: liquid oil, oil–water emulsions and oil vapour. Special filters are required to remove oil vapour and oil aerosol. Modern oil removal filters are of the coalescing depth type and commonly use glass fibre elements. Oil particles of varying sizes impinge on and adhere to the fibres resulting in a gradual build-up of coalesced drops. These drops are driven to the outside of the filter by the airstream. When the oil comes to the outside of the medium, it is stopped in a porous sock covering the element. The oil then flows by the force of gravity down to the bottom of the sock, where it drops to the filter bowl. The oil is then automatically drained from the filter. These filters are capable of removing the oil content in a compressed airflow down to a level of 0.1 mg/m3 or less.
Glass microfibres are considered to be ideal filter media for the coalescing of liquid aerosols. Inherently, this material is neither adsorptive nor absorptive to liquids, and consequently is superior to natural fibre media insofar as retention of its original properties is concerned. Glass microfibres are quite hydrophobic (i.e. water repellent), so that water forms on such fibres as droplets rather than as a film, a condition that is favourable to continuing filtration efficiency. Unfortunately, neither glass nor any other material is oleophobic (oil repellent), and so oil will form as a film on glass microfibres, increasing their effective diameter. Allowance for this diameter increase, which is relatively minimal, can be made and this film of oil will not appreciably detract from filtration efficiency once the filter medium has been wetted.