Aftercoolers

The compressor types described in section 3.2 share oil lubrication as a common characteristic. For many users of compressed air, oil was not only undesirable but also dangerous under certain circumstances. While the number of fires and explosions which occurred in conjunction with air compression remained relatively small in relation to the number of compressors operated, consequences and previous accidents caused adequate safety measures to be introduced. ISO 5388 pays heed to this safety requirement.

For this reason, the compressor assembly is supplemented by an aftercooler in order to eliminate any negative consequences of the operation of the compressor and to ensure a safe application of the compressed air. Uncooled compressed air in the piping system would, through the naturally occurring drop in temperature, form an aggressive and dirty mixture containing condensed water emulsified with oil and solid particles. Corrosion, malfunction of applications, together with breakdown of production, would be the ultimate consequence.

The aftercooler integrated with the compressor therefore forms the first important step of compressed air treatment, as it significantly cools the hot compressed air flowing out of the compressor. This alone leads to the condensation of up to two thirds of the oil, water and other vapours contained in the compressed air. Effective water separation can be achieved only through cooling the compressed air, as this causes the precipitation of the water contained in vapour form in the compressed air. Oil/Water Separators alone cannot totally achieve effective water separation. Aftercoolers of water and air cooled designs reduce the temperature of the compressed air theoretically to about 25 - 40°C outlet temperature at the pressure outlet of the compressors, depending on the cooling medium. Compressed air is, in.principle, saturated at the pressure outlet of the compressor. This compressed air, at a pressure of 7 bar and an assumed temperature of 35°C, has a moisture content of about 40 g/m³.

In practice, less favourable values, i.e. higher compressed air output temperatures are frequently observed, because, on one hand, the coolers are dirty and, on the other, of too small a size. Here, the so-called heat of condensation plays a decisive role. If a cooler is designed for an average condition of Tamb. = 15°C at a relative humidity of 60%, but is used during the summer at a temperature of Tamb = 25°C accompanied by the then higher relative humidity of e.g. 75%, one has to count on a compressed air outlet temperature augmented by 5°C, bearing in mind the changed heat content.which increases from 9.6 kgcal/m³ to 18.7 kgcal/m³ .