Sometimes a certain quantity of oil in the compressed air is desired or even prescribed. Pneumatic machines and tools receive their lubrication through a mixture of finely distributed oil added to the compressed air as an oil mist. However, there is other equipment which must remain totally free from oil as, otherwise, ducts could be blocked or deposits could form at critical points, which would endanger correct functioning. In automatic conveying plants, oil content may cause caking of (powdery) bulk materials.
Some substances such as food and beverage, but also pharmaceutical products which enter into contact with compressed air, must be kept free from even the smallest mixture of oil. However, if oil mist is used for lubrication, the correct type of oil and the state of this oil are critical. Dirty, residue rich or resinous oil exerts a harmful effect on pneumatic components. The same applies to oil types which oxidise easily or form oil sludges. Oils of the type and characteristics just described must be filtered out of the compressed air. This applies particularly to ordinary lubricating oils or heavy motor oils which have an undesirable effect on the properties of compressed air. Being mineral oil products, the usual lubricating oils contain sulphur, albeit in small quantities. Added to this is a certain hydrogen content which will settle on metal surfaces. Under the influence of condensed water, this can lead to acid mixtures which attack seals or even cause corrosion damage on metal surfaces.
At the relatively high temperatures of compression, particularly with piston compressors, the effect of hydrogen and oxygen on oils susceptible to this, easily leads to the formation of solid residues which settle on the surfaces of sliding components of pneumatic devices leading to jamming. The presence of undesirable oils in compressed air is frequently the result of two causes: 1. The air drawn in by the compressor already contains some oil. This introduction of oil can be avoided or diminished by locating the intake filter into the open air, however protected against rain and dust, or finding an optimum location for the compressor itself. 2. The compressor passes oil into the compressed air, such as with piston compressors where the lubricating oil helps to form a seal between piston and cylinder wall. This film of oil is subjected to a strong shearing effect, causing oil particles to be detached from the interface between piston and cylinder wall and to enter the compression space.
On the other hand, with oil flooded rotary and screw compressors with oil injection cooling, it is the content of oil not caught by the oil separator which is thus withdrawn from oil circulation within the compressor but passed into the compressed air piping system. This second cause does not arise with dry running compressors with oil free compression, i.e. with piston and screw compressors without oil lubrication as well as with diaphragm compressors. The same applies to blowers and ventilators which generate air of low pressure only. In addition, and in most cases only small, oil or grease contamination arises from directional control valves, cylinders and regulators, which may have been greased or oiled before despatch.
By far the largest share of the oil contained in compressed air, sometimes 99% or more, occurs as floating droplets. Their diameter is usually less than 0.8 mmand even as low as 0.1 µm. Whereas larger droplets occur only to a small extent and represent an aerosol, the smaller microdroplets form an oil mist or oil haze. These microdroplets can be removed from the compressed air only by means of high performance filters. Coarse filters or separators do not achieve this.
