Dew Point

The dew point of air has significant ramifications for compressed air users. Here's what dewpoint is all about.

Let our minds wander to where we are sitting on the back deck, and the hot summer sun is beaming down. A bead of sweat trickles down our chest, and we are thirsty! We reach into the cooler for another frosty bottle of our favourite malted beverage.

Let us also imagine that we put this ice-cold mug of suds on the railing of the deck right beside us. Very quickly we'll see beads of water forming on the outside of the cold glass.

Why? The dew point!

The dew point is the temperature at which the water vapor in the air can no longer stay in a vapor form, and condenses from a vapor into a liquid on a surface.
mug of beer

The film of air around the mug in the picture cools rapidly due to the cold temperature of the surface of the glass. The air that is actually touching the glass quickly gives up the moisture it is carrying.

The temperature of the air that is touching the glass quickly falls below its dew point, and water condenses out of that air.

That water then trickles down your mug and onto the front of your chest as you swallow the frosty beverage. Mmmm, delicious! The beer of course, not the air dew point.

In order for the water in the air to remain in a vapor form, whether it is the air on your back deck or the compressed air in your plant lines, the air must be warmer than the surface it is touching.

Compressed Air Is Often Hot

Now think of your compressed air supply. Through the compressing process, the air in the compressor tank or receiver can get very warm. And as a result that warm air can contain a significant amount of water in vapor form.

As air cools, it can hold less water vapor.

Water Condenses In The Air Lines

What happens when the hot, vapor-laden air from your compressor tank reaches the plants air lines? The compressed air cools rapidly, the water vapor in the air condenses out, and you now have free water flowing through your air lines.

However, if the air entering your plant from your compressor had a dew-point that was below the temperature existing throughout your plant, then no water vapor should condense from the compressed air into free water in your air lines.

Air Dryers Lower The DewPoint

That is why industrial compressed air users use air dryers.

Please see these pages for more information about the problems that compressing air can generate relating to water generation.

Air dryers help to take water and water vapor out of the compressed air stream. They lower the dew point of the compressed air stream to try and make sure that water won't condense from your compressed air as it traverses your air mains.

Cooling the air helps. Drying it helps. More on the cooling and drying of compressed air here.