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Making water disappear into thin air

The science behind air drying

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As you will have learned from our blog post on the Good, the bad and the wet, the increased concentration of moisture and condensate in (compressed) air can be a real problem. Among other things, it can compromise the safety, performance, efficiency and longevity of your infrastructure but also the quality of your end-product.

Which is why this post talks about one of the most common methods of making that water “disappear”. As you’ll see, it’s no magic trick but 100% science.

April 2, 2020

Relative humidity: where it all begins

When it comes to removing water from the equation – and from your system air – everything depends on how wet that air is to begin with, how dry you want to get it and how quickly.

First things first, let’s talk about the moisture that’s already in your air. This is typically expressed in terms of “relative humidity”.

The relative humidity (RH%) is a percentage of the maximum amount of moisture the air can contain (at a given temperature and pressure) before it condenses. In other words, how full the glass is before it starts overflowing. But relative humidity is not just an indication of how much water is in the air, but how much water the air can still absorb, i.e. its capacity for drying! For example, if the RH of the ambient air in your factory is 20%, that means that there is still 80% that can be used to absorb moisture from wherever you don’t want it to be. So from our perspective, the glass is never half-full, it’s half-empty! Or in this case, 4/5 empty. And assuming we can generate a continuous flow of air, that 4/5 can be used to take water out of the equation and out of your system.

Dew point: where you want to get

The level of dryness you want to achieve is typically expressed in terms of dew point, i.e. the temperature at which the water in that particular air will condense – like dew on your morning lawn, fog on your bathroom window or ice on the walls of your freezer.

The wetter the air, the higher the temperature at which dew will form. And the drier it is, the colder you have to make it before it does so. This is handy to bear in mind because if you have a certain target dew point in mind, the air you inject for the purposes of drying must have the same value or (preferably) lower.

Let’s take that last example: defrosting the freezer. Typically, removing the frost (which is simply frozen dew) means melting it, soaking it up with a sponge and wiping it off with a cloth. But just when you think your freezer is perfectly dry, you plug it back in and before you know it, new ice begins to form. Why is that? Because the air we “injected” into the freezer had a dew point that’s higher than the operating temperature of the freezer. Once the temperature drops below that dew point, even the small amount of moisture inside the air will condense and subsequently freeze. And while that’s a bit of a nuisance at home, in a process piping environment, it can be downright dangerous! Which is why using industrial dryers that achieve dew points of -40°C and lower are standard practice at Atlas Copco Rental.

Temperature: speeding up the process

When it comes to influencing how fast the desired level of dryness is achieved, the most important weapon in our arsenal is temperature.

If you’ve ever wondered why hair dryers blow hot air, this is why! By increasing the temperature of the air, the hair dryer reduces the relative humidity by increasing the capacity of the air to absorb moisture. At the same time, it adds energy to the system, causing more water to evaporate. 

There is just one small hitch, however: while adding heat speeds up the process of drying, it does not help you achieve a higher level of dryness. No matter how hot the air gets, it will never be able to achieve a dew point lower than the dew point of the air you use to dry.

Purity: the secret ingredient

We've talked about how wet, dry, hot and cold your drying air should be. But there is one more very important element in the equation and that is how clean!

Studies show that the presence of particles or aerosols in the air can act as a catalyst, causing saturation and thus condensation at far higher temperatures than normal. In other words, dirty or impure air will have a much lower capacity to hold water. And just as cleaner diesel engines have resulted in less smog, the cleaner the air, the greater its capacity will be for absorbing moisture. That is why Atlas Copco manufacturers “Class 0” oil-free blowers and compressors that remove virtually all dust, oil aerosols and oil vapor from the drying air, for extremely efficient drying. 

So the next time you use air drying to remove moisture or water from your systems and equipment, remember that the level of dryness achieved will depend on the dew point of the air you use, and the amount of time the whole operation takes will depend on its temperature and purity. And we’re sure we don’t need to remind you that time equals money.

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

Tom Wouters

Product Marketing Manager - Atlas Copco Specialty Rental Division