15
Appendix
15.1
Condensate formation in compressed air plants
If air is compressed, then a part of the environment moisture contained in the intake air
precipitates as condensate, since compressed air cannot hold so much water as the air at
atmospheric pressure. Higher the pressure rises, lower the water which the compressed air
can hold and hence the relative humidity further increases. As soon as the relative humidity
value reaches 100% rH, it exceeds the dew point and the surplus amount of water vapour
precipitates as condensate.
The compressed gas after the compressor is first hot due to the compression heat. However,
hot air can hold more water than cold air. If hot air further cools down in the air chamber,
then again condensate precipitates. Since normally water surplus in the compressed air is
always after the compressor, the dew point and also the temperature at which water
condenses corresponds to the temperature of the air chamber.
This is also the reason for condensation of water in the pipeline network: A part of the water
always remains in gaseous state in compressed air and it is transported together in the
compressed air network towards the point of consumption. Now if the temperature in a part
of the pipeline falls further, again humidity precipitates and accumulates in the pipes and
then swept away away by the flow. The liquid water causes damages to the machines and
pneumatic equipment.
Remedy comes from installation of a cold dryer: With the help of a cooling system, the
compressed air can be cooled down to approx. 1 °C. The water precipitates at the cooler,
and it is removed from the plant with a water separator and condensate trap. The dew point
temperature of compressed air corresponds to the surface temperature of coolers, even after
the air is warmed up again. As long as it is ensured that at no position in the distribution
network, the temperature is colder than that of the evaporator of the cold dryer, no more
water can condense out.
Besides cold dryers, there are other type of compressed air dryers, which work over
adsorption drying media or by means of membranes and in such drying, the dew point
temperature after the dryer is also a criterion for compressed air quality.
Apart from the problem of condensate formation, there are many other reasons for drying the
compressed air. For example, in semiconductor production or in pharmaceutical industry,
many technical processes can be carried out only with dry air of defined dew point.
As shown with the above example, the dew point is the temperature, at which gaseous water
begins to condense out as liquid. Hence, in a compressed air system, it is the critical
temperature, falling below which results into condensate formation and is likely to cause
damage to the system.
15.2
Pressure dependence of Dew point
In a compressed air distribution network, the pressure is flow dependent and is lower at the
point of usage. Hence, the dew point value to be measured also changes: The value falls
and air becomes "Drier".
This also applies, if compressed air is expanded to atmospheric pressure. After expansion,
the air is drier and can assume very deep dew point values with higher differential pressures.