Why Is There Water In My Compressed Air?

September 5, 2018

People have been living with water in their compressed air lines for so long, many look upon the problem as a necessary nuisance. Some operators have just accepted that their tools drip or spew hot water, or that they need to drain the water out of their air compressor receiver tank on a regular basis. While this may be normal to some air compressor brands, it is certainly not necessary.

Why Is There Water In My Air Lines?

The air around you is made up of more than just oxygen—atmospheric air also includes water vapor, oils and other vapors, and dust and dirt. The water in our air plays an important role in our climate systems, impacting temperatures, humidity, rain and weather patterns, radiation, and more.

When air is compressed, water gets separated from the compressed air, and this water can end up in air lines, receiver tanks, or pneumatic tools, if it isn’t properly filtered out.

For many applications, having a small amount of water in the compressed air is okay. However, if you see dripping or flowing water in your air lines or if your application requires clean, dry air, you may need to upgrade your filtration system or your air compressor system.

How Water Is Created When Compressing Air

one standard cubic foot

Figure 1

Let’s talk about how compressing air creates water. We’ll start with an example using one standard cubic foot of atmospheric pressure air—Figure 1—and see what happens when we make it into usable, compressed air.

Before compression, our cubic foot of air exists completely invisible to the naked eye, hanging out near the air compressor’s intake valve, just being ordinary air. …Or is it? This cubic foot of atmospheric air actually contains:

  • Air / Oxygen
  • Water vapor
  • Oils and other vapors
  • Dust and dirt


volume of the air

Figure 2

Our air isn’t that pure and contains a lot of contaminants. Now, let’s say we compress our cubic foot of atmospheric air to a pressure of 145 PSI and, as a result, it becomes 1/10th of its previous volume. We end up with something that looks like Figure 2.

The volume of the air has changed but the amount of water, other vapors, and dust and dirt have not changed in volume. Instead, these contaminants have become more concentrated, as we end up with the same amount of water and contaminants in a much smaller amount of air.

In fact, our compressed air can hold even more water vapor moisture now. To explain how the amount of water can increase in compressed air, we need to recognize two fun physics facts:

  1. Hot air will hold more water vapor than cool air
  2. The act of compression generates heat

Air gets really hot when it’s compressed, which allows it to hold more water vapor than it otherwise would. In our example, we still have a tenth of the amount of air, but the same amount of water, making the air that is left a lot more humid. But how does this water vapor condense to become liquid water?

Remember that the air we just compressed is hotter than the air surrounding the air compressor. That means that the air will begin to rapidly cool as soon as it leaves the compressor. (Many air compressor systems are also designed to help this air cool even faster.) As the compressed air cools down, it can no longer hold the same amount of water vapor, so the extra water gets forced out of the air in liquid form.

By the time the air has cooled to the temperature range typical for pneumatic tools, which is about 120° F. and below, quite a bit of water has formed. You likely already know this process as “condensation” and it’s the same natural process that causes dew on early morning grass or water droplets on an ice-cold beer glass. It’s condensation that results in water in air compressor systems.

Before we wrap this up, let’s take a moment to look again at our example of atmospheric air that was compressed. We know the atmospheric air became compressed air when it was forced into a smaller volume, and that water vapor transformed into water droplets when the heated air was cooled.

What happened to the other contaminates, like oil, dust and dirt, that didn’t compress? It’s simple: they got filtered out or left the air compressor. Many contaminants will get trapped in an intake filter, preventing the dirt, dust and other airborne contaminants from entering the system in the first place.

Contaminants that do make it into the air compressor system will either be filtered out through an additional filtration system, such as a coalescing filter, or the contaminants will leave the system as part of the compressed air. Each air compressor system will be set up a bit differently, so you should talk to the manufacturer if you’re curious about a specific air compressor’s filtration system. You may also learn more about compressed air treatment methods here.

At the end of the day, water is a natural occurrence in compressed air. It’s not something you can prevent but there are measures you can take to remove water droplets and other concentrated contaminants before they reach your air tools or other applications.

How Is Water Removed From Air Compressor Systems?

Water is a byproduct of compressing air but that doesn’t mean it can be ignored. Water can cause rust and damage to air compressor systems and tools, and hot discharged water can be dangerous to operators. In addition, this water can cause tools to freeze up in the winter.

If your application requires less water in the compressed air, the water can be removed from air compressor systems via several options. One option is an aftercooler, which cools air from 200-350°F to a usable temperature around 15-20°F above the ambient temperature, and removes most of the moisture that is created during this cooling. For most applications, an air aftercooler will eliminate enough water to safely operate pneumatic tools.

A Filter Regulator Lubricator, or FRL, can also help remove water. Although an FRL isn’t as effective in water removal as an aftercooler, it has several additional benefits: FRLs filter solid contaminants, remove oil vapors, regulate pressure, and lubricate the compressed air with tool oil (if required).

Some air compressor systems that require an air receiver tank, like a reciprocating air compressor, will also rely on the receiver tank to collect and trap water before it reaches your air tools. This trapped water is then released through a drainage valve.

Air compressor applications that need exceptionally dry air may also use a refrigerated air dryer, an absorption air dryer, or both. You can learn more about these common air drying methods here.

Do I Need To Drain The Water In My Air Compressors?

It depends on your air compressor. If you have an air compressor with an air receiver tank that collects water, you will need to follow the manufacturers guidelines to determine how often the air receiver tank should be drained. Many air tank manufacturers recommend draining the tank daily and before long-term storage.