Ambient air always contains water vapour to varying degrees. This water content mixes with the compressed air in the form of invisible water vapour. More often than not, such air is unsaturated i.e. the temperature of the air is higher than the dewpoint in question at which condensation would take place. Upon compression followed by cooling to approximate ambient temperature, also after isothermal compression, the dewpoint limit rises. This leads to the condensation of the surplus water from the compressed air, until the new saturation point is reached. The condensed water flows together and is collected at a suitable point at the aftercooler to be removed via automatically operating condensate drains.
Compression also causes finest fog droplets to form in the air. These can be retained in suitable filters (see Chapter 4), giving them the opportunity to coalesce. However, filters can solely remove droplets from the air current, not gaseous water vapour. In order to remove this water vapour as well, specially designed compressed air dryers have to be fitted.
The harmful effect of water on pneumatic installations is not limited solely to the formation of rust. In addition, the lubricating film is washed off rotating or linear drives, sliding parts jam or get eaten away, and a malfunction of the plant remains solely a question of time. Cost intensive maintenance work forms the logical consequence. Furthermore, there is the danger of freezing of equipment installed outside. At certain points of contact of differing types of metal, electro chemical micro elements may be formed, leading to surface corrosion.
