Back to basics: Everything you need to know about desiccant dryers

August 6, 2020

5 minutes

Need clean, dry air? A compressed air dryer can do the trick! Like their name would suggest, dryers are very common pieces of compressed air equipment that play an essential role in drying your compressed air. Designed to reduce the amount of moisture in the compressed air, desiccant dryers in particular are essential for applications and processes that require very high air quality - or with applications requiring an ultra-low dew point, typically around -40°C /-40°F. Having air at a reliable, predictable dew point can be important for demanding applications in industries such as pharmaceuticals and food processing. Read more about how desiccant dryers work, as well as the types of desiccant dryer technologies, below.

What is a desiccant dryer?

A desiccant air dryer is a type of compressed air drying equipment that uses a hygroscopic material called desiccant to remove moisture from compressed air through a process called adsorption. Unlike refrigerated air dryers, which cool the air to condense and drain away moisture, a desiccant air dryer works by passing the compressed air over or through the desiccant material, which chemically attracts and holds on to water vapor molecules, pulling them out of the air stream.

The result is extremely dry compressed air with pressure dew points that can reach as low as -40°C / -40°F or even lower on certain models, making desiccant air dryers the preferred choice for applications where refrigerated drying alone is not sufficient. Because the desiccant material gradually becomes saturated with the moisture it captures, it must be regularly regenerated, meaning the collected moisture is purged from the desiccant to restore its drying capacity. The method used to regenerate the desiccant is what distinguishes the different types of desiccant air dryers from one another.

In practical terms, a desiccant air dryer is the go-to solution whenever your process, product, or equipment cannot tolerate any trace of moisture in the compressed air supply. It is a critical piece of infrastructure in industries where air quality is non-negotiable.

What is desiccant?

Open a box containing new electronics, medicines, or clothing, and often you find a small mesh packet with words such as “Desiccant, Do Not Eat.” Inside those packets are hygroscopic beads (typically silica or activated alumina) that attract moisture, protecting the merchandise during shipment and storage. Desiccant also has industrial uses, notably removing moisture from a compressed air stream. Water vapor is exchanged from the moist compressed air into the desiccant, drying the air and causing the desiccant to gradually be saturated with adsorbed water. The desiccant must be regenerated (the collected moisture is purged) to regain its drying capacity.

How does a desiccant dryer work?

The general working principle of desiccant air dryers is simple: moist air flows over hygroscopic material (desiccant) and is thereby dried. The exchange of water vapor from the moist compressed air into the desiccant causes the desiccant to gradually be saturated with adsorbed water. Therefore, the desiccant needs to be regenerated regularly to regain its drying capacity.

Is there only one type of desiccant dryer on the market?

Actually, there are four types of desiccant dryers available - each with a specific method for regenerating the desiccant.

Purge regenerated adsorption dryers ("heatless-type dryers") use expanded compressed air to purge moisture from the desiccant. They are best suited for lower air flow applications.
Heated purge regenerated dryers heat the expanded purge air to improve purge efficiency and reduce energy consumption by 25% compared to heatless-type dryers.
Blower regenerated dryers blow heated ambient air to regenerate wet desiccant. Since no compressed air is used, energy consumption is 40% lower than for heatless-type dryers.
Heat of compression dryers regenerate desiccant with the heat naturally given off by the compressor, without consuming additional energy.

Are there any new developments coming in desiccant dryers?

Yes, Atlas Copco developed and patented a revolutionary new solid desiccant called Cerades. Compared to granular desiccants, Cerades delivers higher air quality, lower energy and service costs, and health and environmental benefits. Compressed air flows straight through the Cerades structure, reducing pressure drop (up to 70%) in the dryer to save energy. It handles higher air flow that granular desiccant, so the dryer can be much smaller. Cerades is vibration resistant and can be mounted horizontally, so it works in applications that could not previously use a desiccant dryer. It also lasts longer than granular desiccant, doesn’t decay and break down into dust like granular desiccant, and delivers longer cycle times to improve energy efficiency and process productivity.

Want to learn more about desiccant dryers – or dryers in general? Contact an expert today!

Frequently asked questions

What is a desiccant air dryer used for?

A desiccant air dryer is used to remove moisture from compressed air to very low levels, achieving pressure dew points as low as -40°C / -40°F or below. It is used in applications and industries where even trace amounts of moisture in the compressed air supply can damage equipment, compromise product quality, or create safety risks. Common uses include pharmaceutical manufacturing, food and beverage processing, electronics production, medical air systems, and any outdoor or cold-climate application where condensation or freezing in the pipework is a concern.

What is the difference between a desiccant air dryer and a refrigerated air dryer?

A refrigerated air dryer removes moisture from compressed air by cooling it until the water vapor condenses into liquid, which is then drained away. It is effective for most general industrial applications and achieves pressure dew points of around 37°F / 3°C. A desiccant air dryer removes moisture through adsorption, using a hygroscopic material to chemically capture water vapor from the air stream. Desiccant air dryers can achieve significantly lower dew points, as low as -40°C / -40°F or below, making them the right choice for applications that require ultra-dry air beyond what a refrigerated dryer can deliver.

What desiccant materials are used in a desiccant air dryer?

The most common desiccant materials used in industrial desiccant air dryers are silica gel and activated alumina, both of which are highly porous hygroscopic materials that attract and hold water vapor molecules through adsorption. Atlas Copco has also developed Cerades, a patented solid desiccant material that offers significant advantages over traditional granular desiccants, including lower pressure drop, higher air quality, longer service life, and the ability to be mounted horizontally for greater installation flexibility.

How often does the desiccant in a desiccant air dryer need to be replaced?

The service life of desiccant in a desiccant air dryer depends on the type of desiccant used, the operating conditions, and how well the dryer is maintained. Traditional granular desiccants such as silica gel and activated alumina typically need to be replaced every two to five years under normal operating conditions, though contamination from oil carryover or other impurities can shorten their effective life significantly. Atlas Copco's Cerades desiccant lasts longer than conventional granular materials, does not break down into dust, and maintains its performance over extended service intervals, reducing both maintenance frequency and cost.

Can a desiccant air dryer handle oil-contaminated compressed air?

Desiccant air dryers are sensitive to oil contamination. Oil carried over from the compressor can coat the surface of the desiccant beads, blocking their pores and dramatically reducing their moisture adsorption capacity. To protect the desiccant and maintain dryer performance, it is important to install appropriate coalescing pre-filters upstream of the desiccant air dryer to remove oil aerosols and particles from the compressed air before it enters the dryer. Regular maintenance and filter element replacement are essential to keeping oil contamination levels within acceptable limits.

Is a desiccant air dryer the right choice for my application?

A desiccant air dryer is the right choice if your application requires ultra-dry compressed air with a pressure dew point below what a refrigerated dryer can achieve, typically below 37°F / 3°C. It is also the preferred option for outdoor installations in cold climates where the compressed air pipework could be exposed to freezing temperatures, for processes involving temperature-sensitive or moisture-sensitive products, and for industries with strict air quality regulations such as pharmaceuticals and food processing. If your application only requires a moderate dew point for general industrial use, a refrigerated air dryer may be a simpler and more cost-effective solution. An Atlas Copco compressed air expert can help you assess your specific air quality requirements and recommend the right dryer technology for your needs.