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Why is there water in my compressed air?


For many, the presence of water in their compressed air lines has become a common annoyance. Some have come to terms with the fact that their equipment may leak or spray hot water, or that they need to frequently empty the water from their air compressor receiver tank. Although this might be a common occurrence with some air compressor brands, it isn't necessary.

Why Is There Water In My Air Lines?

The air that surrounds us is not composed solely of oxygen—it also includes water vapor, oils and other vapors, as well as dust and dirt. The water in our atmosphere plays a crucial role in our weather systems, influencing temperature, humidity, rainfall, climate patterns, radiation, and more.

When air gets compressed, the water is separated from it. If not properly filtered out, this water can find its way into air lines, receiver tanks, and pneumatic tools.

A small amount of water in the compressed air is acceptable for many applications. However, if there is noticeable dripping or your application requires clean, dry air, it might be time to improve your air compressor system with filters, aftercoolers, or air dryers.

How Does Compressing Air Produce Water?

Let's delve into how compressing air results in water. We'll use an example of one standard cubic foot of atmospheric pressure air and observe what happens when it is converted into usable, compressed air.

Before compression, this cubic foot of air is invisible to the naked eye and resides near the air compressor’s intake valve, appearing as ordinary air. But what does it actually contain? It consists of:

Air / Oxygen
Water Vapor
Oils and other Vapors
Dust and Dirt

The volume of the air changes during compression, but the amount of water, vapors, dust, and dirt stays the same. Instead, these contaminants become more concentrated because we now have the same amount of water and contaminants in a much smaller volume of air.

Interestingly, our compressed air can hold even more water vapor moisture now. To understand how compressed air can contain more water, we need to consider two principles of physics:

Hot air can hold more water vapor than cold air
Compression generates heat

When air is compressed, it heats up, allowing it to retain more water vapor. In our example, we still have a tenth of the original volume of air, but the same amount of water, resulting in higher humidity. But how does this water vapor transform into liquid water?

Compressed air is warmer than the surrounding air, so it cools down quickly once it leaves the compressor. (Many air compressor systems are designed to facilitate this cooling). As the compressed air cools, it can't hold as much water vapor, forcing the excess moisture to condense into liquid form.

This process is known as "condensation," the same natural phenomenon that causes dew on grass in the early morning or water droplets on a cold beer glass. By the time compressed air has cooled to the typical operating temperature for pneumatic tools (around 120°F and below), a significant amount of water has condensed.

What Happens To Contaminants During Compression?
Now let's revisit our example of compressed atmospheric air. We know that atmospheric air turns into compressed air when forced into a smaller volume and that water vapor becomes water droplets when heated air cools. But what happens to other contaminants like oil, dust, and dirt that don't compress?

The answer is straightforward: they get filtered out or expelled from the air compressor. Many contaminants get caught in an intake filter, preventing dirt, dust, and other airborne particles from entering the system initially.

Those contaminants that do make it into the air compressor system will either be caught by another filtration system, such as an oil filter, or leave the system with the compressed air.

Every air compressor system is configured differently, so if you have queries about a specific air compressor's filtration system, you should contact the manufacturer. You can also find more information about compressed air treatment methods here.

How Is Water Extracted From Air Compressor Systems?
Water is a natural byproduct in compressed air systems. While it can't be prevented, there are ways to remove water droplets and other concentrated contaminants before they reach your air tools or other applications.

It's crucial not to overlook the presence of water in your compressed air. Water can lead to rust and damage to air compressor systems and tools, and hot discharged water can pose a risk to operators. Plus, this water can cause tools to freeze in winter.

When your application necessitates less water in the compressed air, several options are available to remove water from air compressor systems.

One such option is an aftercooler, which reduces the air temperature from 200-350°F to a usable range around 15-20°F above the ambient temperature, removing most of the moisture created during cooling. For most applications, an air aftercooler will eliminate enough water to safely operate pneumatic tools.


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