Why Is Compressed Air Wet? The Science of Moisture in Air Systems

We all know the importance of drying compressed air. It’s a crucial step in the compressed air process that, if skipped, can lead to gallons of water building up in your system. But why is compressed air wet?

The simple answer is that air always contains moisture. You may notice it on a humid day in Florida, or you may not on a temperate day in Arizona. Whether or not you detect it, moisture is there. The moisture is in the form of water vapor molecules interspersed among the gas particles (mainly nitrogen and oxygen) in the ambient air.

The air’s temperature, pressure and relative humidity play interdependent roles in determining how much water is in vapor or liquid form. Let’s take a look at 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.

When purchasing a compressor system, make sure you also purchase a dryer. Depending on your facility location and size, you could end up with gallons of water in your system. Have a compressor system, but don’t have a dryer? Contact one of our experts to discuss getting your system fitted with the right dryer for your application.