Oil fogs or mists comprise the very finest droplets in a system, and are visible as aerosols. As oil is a hydrocarbon compound, oil can also exist in the gaseous state. The gaseous or vapour like state refers in principle only to the hydrocarbons which are condensed from the liquid oil, depending on the temperature. The relative share of condensed hydrocarbons, oil fog or spray in the compressed air depends on the type of compressor oil used and the temperature generated by the compressor. Modern compressors are usually fitted with air or water cooled aftercoolers. By means of the aftercoolers, the temperature of compression is reduced to a low operating temperature after the compressor. In the course of this cooling process, hydrocarbons are, condensed. The residual oil content varies as a rule between 5 - 20 mg/m3, depending upon compressor design. With compressed air filters, a distinction is made between two functions of the filter element : Filtering out and screening. The screen or sieve elements mainly retain coarse particles exclusively on the surface of the element (surface filtration). When filtering, also fine or oil particles are separated by the filter element (depth filtration), on the surface as well as in the course of flow through the filter element. Filter materials made from fine fibres separate fine particles in accordance with their fibre thickness. Modern technologies are capable of producing fibres with a diameter of less than 2mm. For the majority of compressed air users, the most important characteristic when assessing compressed air filters, is the retention capacity of these filters (separation efficiency) as far as the oil contained in the compressed air is concerned. The radial design of filter elements, with its flow always from inside to outside, has proved its value. Compactness, large throughflow surfaces at optimum layer thickness and favourable dynamic behaviour form outstanding attributes of these filter elements. Fig. 4.8.2.1 shows the separation of oil mist. The compressed air initially enters through the inner layer of the filter element (item 4), which provides support for the filter medium and, moreover, has the effect of an integrated pre-filter. Larger dust and liquid particles are retained, ensure a high separation efficiency. The smaller liquid particles are caught by the filter media (item 5) and form larger droplets after coalescing. These droplets, found within compressed air as well as larger droplets, impinge against the outer layer of lower density (item 6) which gathers droplets when these become detached from the fibre. Within the cellular structure, these now larger drops gravitate down towards the lower end cap (item 1) and form the so-called wet band (item 9) in the lower region of the filter element.
Figure 4.8.2.1
The purified air passes above the wet band as the resistance to flow is less. This causes a quiet zone without air circulation in the lower region of the filter element. The separated drops fall from the lower part of the filter element through the quiet zone without being re-entrained by the air current. This condensate is collected in the bottom of the filter housing and discharged through a drain.
