Solid particles floating in air are described as dust. In general, dust particles display widely varying sizes. Floating solid particles also include pollen, while bacteria and viruses must also be counted as solids. Depending on particle size, dusts are subdivided into 3 classes:
Coarse dust grain size >10 µm
Fine dust grain size 1 - 10 µm
Finest dust grain size <1µm
From amongst these three groups, it is essentially only the finest dusts which have to be counted among the floating substances. Finest dust very often forms by far the largest share of the solids floating in air. These often form 80 - 90% (by weight) of the contaminants in air. The majority of these particles is smaller than 1 mm.
The sinking speed of dust depends on the grain size. If the air is at rest, the distance fallen may amount to fractions of a millimetre for very fine dust whereas, for very coarse dust, falling may take place at more than 1 m/s.
The dust content of air is influenced largely by the geographical situation and the location of the point of measurement, as well as by weather conditions such as wind, rain and snow. The airborne dust concentration in large cities often amounts to more than 0.2 mg/m3 of air. This content can be many times higher in industrial areas. Meanwhile, dust contents of 5 - 10 mg/m3 and more have been noted in relatively clean areas. This can be caused by high industrial chimneys and low precipitation speed of the released dust particles. In dusty workplaces, particularly on building sites or in quarries, the dust content may amount to some 100 mg/m3 and even reach values of up about 250 mg/m3. F
urther solid particles not originally forming part of air consist of oil soot, scale, rust as well as products of metal abrasion. These solid particles originate from the compressor or can be released from the internal walls of the pipework. Solid particles and lubricating oil or grease act on pneumatic installations like an abrasive paste, causing particular trouble such as wear at sensitive points. Dust particles from castings, formed when cutting metal with machine tools, are particularly abrasive in their effect. In addition, there are types of dust presenting a risk of explosion, particularly dust from aluminium and magnesium. Other dusts are injurious to health such as quartz or radioactive dust, others again have a toxic effect, are chemically aggressive or have pyrophoric (readily flammable) qualities. Some types of composite dust form encrustations in conjunction with water or moisture. Solids of any kind and quantity in the intake area of the compressor frequently cause far reaching malfunctions in compressed air systems during or after compression.
Depending on where the compressor is located, suitable steps have always to be taken at the intake end of the compressor as far as the suction filters are concerned. The following examples of types of solids describe the possible sources of compressed air pollution: Lignite, coal or anthracite mining result in increased coal dust content in the ambient air. To reduce wear and avoid explosion risk, special protective measures in the compressor system against these dusts are necessary. Chemical production which results in aggressive substances such as sulphur compounds, calls for measures which also aim to reduce corrosion. In the paper and wood industry, fine particles arising from sawing and cutting, often form the cause of.breakdowns in various manufacturing sections. This fine dust in the compressed air blocks the tools and machines installed downstream
