Atmospheric air contains water vapour which is generally known as humidity. Water is therefore always present in factory compressed air systems due to the fact that the air compressor draws air from the surrounding atmosphere. The amount of water in a compressed air system varies with the humidity and temperature of the outside air.
After passing through the air compressor, factory compressed are is normally about 1/8th of its original volume but the water content in the air is incompressible and therefore remains at its original volume. This means that the water content in the compressed air is approximately 8 times what it was in the outside air and it is much more likely to condense at the higher pressure.
Moisture that is present as a vapor in the compressed air begins to condense into liquid water as the compressed air moves through the system and cools. The result is more noticeable in summer because of higher humidity in the atmospheric air.
A build-up of water in a compressed air system can be a serious problem when you are using the air for spray painting due to the effect of the atomized water on the finish. Other uses where moisture is detrimental include sand blasting/soda blasting, pneumatic tools, CNC machines, robotics, air cylinders, valves, and many other applications.
While it is impossible to prevent moisture from being drawn into the air compressor, the resultant water vapour / condensed water in the compressed air system can be removed by careful design of the system and the use of various filtering components.
Following is a list of means by which moisture can be eliminated from compressed air systems:
- The first point of attack is in the use of a compressed air receiver tank. Compressed air emerging from the air compressor is hot and contains water in its vapor state. Whilst the air is held in the receiver tank it cools and the water vapour condenses into droplets which fall to the bottom of the tank. The water which collects at this point can be drained out either manually or by means of an automatic condensate drain.
- The next recommended solution for moisture removal is a water separator, which is an in-line water collection bowl fitted with a manual or automatic drain. This unit removes bulk condensate from the air supply by cyclonic separation. It effectively eliminates 40 to 60 percent of the moisture in the air making it dry enough for general factory applications such as blowing down.
- If further moisture removal is needed, the next step is a refrigerated air dryer. Temperature, pressure and moisture content are correlated, and by chilling the air these dryers force any remaining water vapour to condense taking out more water. Refrigerated air dryers typically bring the compressed air to between a 1 and 5 degree dew-point which is sufficient for most applications.
- Desiccant dryers can be another further step that gets air very dry and are helpful in painting, printing and instrument applications, or when compressed air meets ambient temperatures of less than 1° C. Desiccant dryers can dry compressed air to a dew-point range of between -40° C and -75° C depending on the selected model or combination of models.
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