VMAC Air Compressor Blog

Water in my compressed air

Why is there water in my air lines?

People have been living with water in their compressed air lines for so long, many look upon the problem as a necessary nuisance. It is certainly not necessary.  To find out why, we need to answer a number of questions.

Where does the water in the air come from?

Let’s take one standard cubic foot of atmospheric pressure air (See Figure 1.) and see what happens to it when it becomes usable air.  It begins as 1 cubic foot waiting to be taken into the air compressor.

one standard cubic foot

Figure One

This cubic foot of atmospheric air contains:


  1. Oils and other vapours
  2. Water vapour
  3. Dust and dirt

Now it is compressed to a pressure of 145 PSI and in fact, becomes 1/10th of its previous volume.

We end up with something that looks like this: (See Figure 2)

volume of the air

Figure Two

The volume of the air definitely has changed, but one thing is also for sure, the amount of water, other vapours and dust and dirt have not changed in volume. These contaminants have just become more concentrated.

None of these contaminants will compress so each will be present in the compressed air. However the water and other vapours will act differently than the other contaminants. To explain this, we need to understand that when air is heated it will hold more vapour than when it was at the original temperature and that the act of compression generates heat.

Let’s look again at our cubic example of the compression which took place in the compressor.

As the air was forced into a smaller area by the compressor and was therefore compressed, the contaminants did not compress. What happened to them?

Well, the dirt travels within the system which is why we have filters. The water and other vapour is acted on by the two laws of physics. As the air is compressed, it becomes warmer and this warmer air holds more water vapour. The water vapour and the contaminants travel in the system.

If we could depend on this air remaining hot we could use it and very little water would condense from it.

However, that is not what really happens.  As soon as the air leaves the compressor, the air begins to cool and water droplets begin to condense.  By the time the air has cooled to the temperature range we are considering, about 120º F. and below, the air is 100% saturated.  In practical terms, for every bit of a degree the saturated air cools, more condensation takes place.

So, when you find water in your air lines, it’s a natural occurrence in compressed air.  It’s not something you can prevent but there are measures you can take to remove the water and other concentrated contaminants before they reach your air tools or other applications.  We’ve written about the most common air drying methods here.

If you have any questions about this article or anything mobile compressor related, please contact us.