Dew Point

Dew Point; the dew point of air has significant ramifications for compressed air users. Here’s what it’s 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's imagine 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.

The film of air around the mug in the picture cools rapidly due to the cold temperature of the surface of the glass, and the air that is touching the glass gives up moisture just as quickly.

The air temperature of the air touching the glass falls below its dew point, and liquid water results.

That water then trickles down your mug and onto the front of your chest. Hmmmmm, delicious!

Note that in order for the water in the air to remain in a vapor form, the air must be warm.

Now think of your compressed air supply. Through the compressing process, the air in the compressor tank or receiver gets very, very warm. And as a result it contains huge amount of water in vapor form.

As air cools, it can hold less water vapor.

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 condenses, and you now have free water flowing through your air lines.

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

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.

One purpose of "air-preparation", is to lower the dew point of the compressed air before it gets into your air lines, taking the temperature of the compressed air to below the ambient air temperature in your plant, thereby ensuring that as much of the "condensable" water vapor is gone, and ensuring as much as possible that water won't condense from your compressed air as it traverses your air mains.

Cooling the air does that. Drying it does that. More on these two subjects here .







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