Water vapour is always contained in the air and will, depending on the temperature, be drawn in, in larger or smaller quantities, whenever air is compressed. If the air is oversaturated with water vapour, the excess water vapour will condense. Oversaturation is possible when unsaturated air is cooled or when air is compressed while the temperature remains roughly the same. Heating due to compression initially prevents oversaturation. If this warm, moist air impinges upon a cold surface, the excess quantity of water vapour will again condense from the air.
Diagram 2.5.3.1
Evaporation and condensation hold each other in balance, if the vapour pressure of the liquid is of the same magnitude as that part of the external pressure which is generated by the water vapour contained in the air. The dewpoint is that air temperature at which gaseous water vapour begins to condense in cooling air and is visibly precipitated as dew. Diagram 2.5.3.1 establishes how much water is being condensed within the saturation limit, or how much water can still be picked up by the unsaturated compressed air, depending on the temperature. This can be demonstrated by means of an example: Referred to an ambient temperature Tamb = 20°C and a relative humidity of 60%, the moisture content in the ambient air amounts to 10.4 g/m3. A compressor with 100m3/h intake capacity carries a total of 100m3/h x 10.4g/m3 = 1040 g/h of moisture. When compressing to p = 7 bar,.100m3/h intake air thus become 12.5m3/h of compressed air. However, this compressed air is no longer saturated to 60% but, due to compression, saturated with water vapour to 100%
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Diagram 2.5.3.2
With an ultimate compressed air temperature of Tult = 30°C, this compressed quantity of air hourly feeds a temperature dependent saturated water load of 12.5m3/h x 30.3 g/m3 = 378 g/h into the compressed air system. The difference in moisture loading between intake and output quantity of 1040 g/h - 378 g/h = 824 g/h water load is separated out in the compressor’s after cooler. The compressed air temperature following the aftercooler are very often much higher, so that the quantities of water load in the compressed air system will be higher than the values indicated by the example. Diagram 2.5.3.2 shows the influence of temperature on the moisture content of compressed air. Pneumatic systems without air drying equipment, therefore, need not only water traps but also moisture separators. Separators and filters suitable for this task should, however, be fitted only just before the point of take-off, so there is little opportunity for the air downstream of the filter to cool further and thus precipitate further condensate. The expansion of compressed air in a pneumatic device is accompanied by cooling. However, there is little danger of this causing the formation of condensed water because, with expansion, the dewpoint is lowered in proportion to the degree of expansion.
