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The good, the bad and the wet

Why compressed air is wet and what to do about it

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If you’ve ever worked with compressed air, you’ll have noticed already: it is wet. Soaking wet, in fact! And the more you compress it, the wetter it gets.

If you’re wondering how that comes about, why it might be a problem and what you can do to resolve it, this blog post will not leave you high and dry. Read on!

2020년 4월 2일 (목)

Why is compressed air wet?

Short answer?

Because gasses (like air) are compressible and liquids (like water) are not. So, even though it’s perfectly possible for us to squeeze 100m3 of air at regular atmospheric pressure into a space just 1/8 or even 1/100 that size, the water it contains will remain at the same volume.

Long answer?

Before we talk about the effects of compressing air, we need to take a closer look at what we’re actually compressing. In addition to the nitrogen, oxygen, argon, carbon dioxide and other gases which make up the stuff we breathe, a significant percentage is in fact water vapour. But there’s only so much water vapour that a given volume of air can hold before, like a sponge, it becomes saturated. And when you compress that air, the ratio of water to air (by volume) typically goes well beyond that saturation point. The result? Precipitation! Or, in other words, it gets wet!

How wet is wet?

How wet depends on multiple factors.

Chief among them being how much water was in the air to begin with. Relative humidity typically changes with local weather patterns. And the amount of moisture it can hold varies with temperature and pressure. At sea level and at 30°C (86°F), for instance, the ambient air can contain anywhere up to 2.5% water vapour by weight. So for every 100m3, there can be as much as 3L of water vapour.

Let’s see what happens to that water when we apply a modest compression rate of 8:1 (i.e. when we compress that 100m3 of air into 12.5m3).

As we’ve said above, even though we’ve increased the density of the resultant air, the maximum content of water remains constant by volume. In other words the now reduced volume of 12.5m3 can hold 1/8 of the initial 3L (i.e. 375mL) of water at the same temperature (with the remaining 2.6L condensing into liquid water). Compress the volume of air inside the average house and you could generate enough water for your morning tea!

But what’s wrong with a little water?

Water, in the right quantities, might be good for you. But even in the minutest concentrations, it can be disastrous for your infrastructure and/or your product. Excess moisture in your systems’ air can cause:

  • Corrosion in processing piping, tanks and other metal components and equipment
  • Malfunctions in your instrumentation and controls
  • Ice build-up in colder climates or operating conditions, resulting in blockages in your piping and other systems (a potentially serious safety risk!) 
  • Staining, dilution, contamination and other quality issues with your final product
  • Microbial contamination, since wherever there is water there will typically be (potentially harmful) micro-organisms. 

Needless to say, there are enough reasons to be concerned about the presence of water in the compressed air you use, no matter what the application.

So what can we do about it?

Luckily, for as many problems as there are caused by the wetness of compressed air, there are even more technologies at our disposal to prevent them from ever arising. And just as the best way to fight fire is often with fire, many of these techniques fight condensation with, well, condensation.

From after cooling and (centrifugal) separation to refrigeration, desiccant drying and/or adsorption drying, … they all work by turning the problem - the increased dew point of compressed air – into the solution. Forcing the moisture in the air to precipitate into liquid water so we can get rid of it. Kind of like squeezing that sponge we talked about in the beginning.

So if you’re worried about the fact that compressed air is wet, don’t get mad. Our next article on Air Drying 101 will go into more detail about some of the ways you can get even!

Want to know more about our solutions for dry, 100% oil-free compressed air?

Tom Wouters

Product Marketing Manager - Atlas Copco Specialty Rental Division