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OIL AEROSOLS OIL VAPOUR

Figure 16.OIL REMOVAL FILTER WITH GENERAL PURPOSE PRE-FILTER These particulate oil droplets exist in theairstream and the most troublesome are in the size range 0,01 to 1 micron (approx 90%), the rest may be slightly larger (see figure 12 particle size chart).Most standard air line filters achievewater removal by centrifugal action but due to their small particle size these aerosols are unaffected and require special coalescing filters.In addition to removing the oildroplets these filters will also remove minute water droplets, but they must be protectedagainst gross dirt or water contamination bymeans of standard air line filters mounted immediately upstream (figure 16). It is normally advisable that these filters are capable of removing particles down to 5 microns or less otherwise the coalescing filter may quickly become choked and blocked with dirt, requiring a filter element replacement.Coalescing filters are normally ratedby the amount of air which they can ‘process’to achieve a given oil removal performance, normally a maximum remaining oil content in the exit air of 0,01 mg/m For most processes the removal of oil vapour is unnecessary since unlike water vapour, oil vapour exists only in minute quantities and is not objectionable except in circumstances where its odour is unacceptable eg. in foodprocessing, pharmaceutical and beverageindustries and breathing air applications.The most common method ofremoval is to pass the air through an adsorbing bed, usually of activated carbon, although other materials can be used.Such vapour removal filters willnormally reduce the total remaining oilcontent when used in conjunction with a pre-filter (general purpose filter) and a coalescing filter to 0,003mg/m
3 .A common misconception of thesefilters is that they will remove carbon monoxide or carbon dioxide - they will not.As with oil removal (coalescing)filters the vapour removal filters should onlybe employed where their function is needed,the maximum flow rating is not exceeded and they are preceded by a general purpose and a coalescing filter. This will minimise the size of the filters required and therefore the cost of the installation.Norgren offers an integratedcoalescing and vapour removal filter in the Olympian Plus range. (See figure 17). This includes a colour change service indicator as standard.The location of the compressorintake may also have an effect on the level of filtration required, if for example the intake is situated by a source of hydrocarbon vapours etc. Clean air intake will reduce the cost of producing clean compressed air.
3 (or 0,01 ppm).To try to overflow these units will not only result in a greater pressure drop across the unit and therefore extra energy cost but more importantly the remaining oil content will increase. This may be acceptable for someapplications where oil removal down to theorder of 0,5mg/m
Figure 17. ‘ULTRAIRE’ OIL VAPOUR REMOVAL FILTER 3 is quite adequate to givea degree of protection to a system particu- larly prone to gross oil contamination.Figure 19 shows Norgren coalescingfilters flow capacities to achieve their given performance.
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