Why Does Compressed Air Require a Dryer?

"Why does the compression process always produce moisture?"

"Moisture contamination in my compressed air is fouling processes, products and equipment downstream."

These are commonly heard complaints when organizations utilize compressed air and fail to support downstream processes and equipment with moisture removal systems.

Thus, the question arises “Why does compressed air require a dryer?

When compressed air is produced, moisture is a natural byproduct along with the compressed air itself. Not only is this moisture an unwelcome reality, but it can also contaminate processes and products to the point where downtime and lost production are an inevitable result.

During the compression process, we know that to produce one cubic foot of compressed air at 100psi we need roughly eight cubic feet of atmospheric air at the inlet of the compressor. Each of these initial cubes of air contains grains of moisture that are compressed into one cubic foot.

This compression process causes a great deal of heat and concentrates the moisture in this very small area. Fortunately, moisture can be held in a gas state at higher temperatures, but unfortunately, the air eventually cools.

The chart below depicts the number of grains that can be held in a gas state at various temperatures. At higher temperatures more grains of moisture can be held in a gas state. At lower temperatures the grains of moisture fall from a gas state to a liquid state.

Let’s look back at our compressor.

As the fully saturated air leaves the compressor at extremely high temperatures, the moisture that is contained can be held in the form of a harmless gas. However, as this compressed air travels downstream and cools, it reaches its point of saturation or dewpoint. (Dewpoint is the temperature where moisture can no longer be held in the form of a gas and takes the form of liquid.)

Traveling further downstream, our compressor loop’s natural cooling process begins to condense the compressed grains of moisture into physical water. This moisture, if not treated by a proper compressed air dryer system, will drop out in various parts of the compressed air system.

The illustration below represents a typical compressor loop. In this case, the compressor is followed by a receiver tank and a refrigerated air dryer. This dryer provides enough cooling for the compressed air to lower the temperature where the grains of moisture in the compressed air are minimal.

Refrigerated dryers typically produce a +39F PDP, and this dewpoint typically suffices for many indoor conditions. As the compressed air continues downstream and through the loop, no visible moisture will condense if the temperature around the compressed air does not experience temperatures lower than +39F.

Importance of Cooling

It’s apparent that the most logical way to remove moisture from a compressed air system is to cool the air to the point where the grains of moisture are minimal, and condensation is negated.

Aftercoolers are of course the first line of attack when reducing moisture content and setting up an ideal situation for further downstream treatment. Aftercoolers are simple heat exchangers that use atmospheric cooling to lower the heat of compression while removing bulk amounts of moisture.

The only cost associated with an aftercooler is the electricity that powers the fan. This fan pushes air over the hot compressed air as it flows through. This exchange of heat for cooling eliminates vast amounts of moisture while decreasing the workload on additional downstream dryer equipment. An aftercooler is regarded as the most cost-efficient component of a compressed air system and should seldom be omitted.

It is well known in the industry that every 20F increase in temperature of compressed air nearly doubles the water capacity held. By utilizing a properly sized aftercooler, you can effectively reduce the workload on a downstream dryer, and possibly prolong the life of the dryer.

It is important to note that even with a properly-sized aftercooler, your compressed air system is still fully saturated as it exits. The only way to ensure that no moisture will condense downstream is to lower the pressure dewpoint below the atmospheric temperature. This is achieved by following the aftercooler with a compressed air dryer.

There are three different types of compressed air dryers, all of which function by lowering the dewpoint below atmospheric temperatures. The trick is to find the right dryer for the situation.

Refrigerated dryers lower the dewpoint of the compressed air by simply chilling the compressed air as it passes through. The lowest dewpoint a refrigerated dryer can achieve is around +39F. They are not normally recommended for outside protection in winter months due to this limitation. However, they are popular products for indoor applications.

For critical situations where the strictest tolerances are required and the lowest dewpoints are needed, a regenerative system is often selected. These dryer systems utilize very porous Activated Alumina media to attract and hold (adsorb) moisture until the alumina is regenerated in a dual tower system. Dewpoints of -40F to -100 F are achievable with these systems and they are ideal for cold temperatures or conditions where absolutely no moisture can be tolerated.

A Deliquescent system such as the Van Air D series dryer utilizes a patented tablet that absorbs moisture from the compressed air as it travels through a specially designed pressure vessel. As the compressed air passes through the tablet filled dryer, moisture is adsorbed and falls to a sump area where it is drained. The tablets dissolve and are periodically replenished to maintain the moisture removal process. This system requires no electricity and has no moving parts associated with its operation.

The deliquescent tablets do all the work. Deliquescent dryers provide a fluctuating dewpoint protection that changes daily with the temperature. This system ensures that the compressed air never reaches its saturation/dewpoint as it travels downstream through the compressor loop. Impressively low dewpoints are possible by utilizing various Van Air deliquescent desiccants.

Teaming a water or air cooler with two successive Van Air desiccants, such as Dry-O-Lite and new Super Dry-O-Lite, can provide results that nearly rival regenerative systems. This unique system removes moisture from compressed air without the expense of purge loss or electricity.

In conclusion, it’s important to recognize that moisture is a naturally occurring part of our world. Compressing air concentrates this moisture, where it will eventually re-emerge downstream in your compressor loop. With a proper air treatment system, moisture and the pitfalls of condensation are greatly reduced.