Why is Compressed Air Wet?APUSA
Simple answer being that air always contains moisture. However, you may notice it more in Florida on a humid day versus Arizona where it’s a dry heat. Regardless of the region, moisture is in the air. It in the form of water vapor molecules among the gas particles in the ambient air.
Factors such as temperature, pressure, and relative humidity from the air all play a role in determining how much water vapor is present.
Here’s an example.
Say for instance there is a volume of air that is 1 m3 prior to compression. If that air is at 77 degrees Fahrenheit with a relative humidity of 60 percent under 1 bar(a) pressure, the water holding capacity is 23g and that actual water content is 13.8g. Because the capacity is larger than the water content, there is not condensation in the system.
When that air is compressed, the pressure and the temperature increase. After it’s compressed at 7 bar(g), the air is 95 degrees Fahrenheit with relative humidity of 100 percent. The saturated air can only hold 4.95g of water at this pressure and temperature, so 8.85g of moisture turned into condensation.