A Beginner’s Guide to Desiccant Air Dryers

A desiccant air dryer is a device that lowers the dew point to between -20°C and -70°C by removing water vapour from compressed air through adsorption. 

When a compressed air system requires extremely dry air to prevent pipes from freezing or to adhere to stringent hygienic requirements, these systems are required. 

Even when temperatures drop, an adsorption dryer keeps pulling moisture out of the air – right down to the point where it simply can’t turn into liquid water at all.

That’s a very different process from conventional drying. Traditional methods rely on cooling air until water condenses. Adsorption doesn’t. It removes the moisture itself – regardless of how cold the surroundings are.

How does a desiccant dryer work?

Wet air moves through a tower packed with hygroscopic desiccant beads – that’s the drying stage. Moisture doesn’t condense or drain away. It simply clings to the bead surfaces as the air passes through.

It doesn’t stop there. These dryers use two towers, not one. They work as a pair, swapping roles on a continuous cycle.

While one tower is drying the compressed air, the other is offline – being regenerated. Heat or purge air drives the trapped moisture back out of the desiccant, readying it for the next cycle.

In practice, it means dry, compressed air is always there – exactly when manufacturing needs it.

What is the difference between refrigerant and desiccant dryers?

The main difference comes down to pressure dew point. 

Refrigerant dryers typically cool the air to around +3 °C, which is usually fine for general indoor factory use. Desiccant dryers go much further, reducing the dew point to –40 °C or lower. 

In the UK, outdoor air lines are exposed to winter temperatures. If moisture remains in the system, it can freeze. This level of drying is used to avoid that.

What are the main types of desiccant air dryers?

Desiccant air dryers are generally grouped into four types. The sorting of these groups tends to be based on how the desiccant is regenerated.

• Heatless Dryers (CD): These units use a portion of the dried air to regenerate the offline tower. They consume 15–20% of the compressed air capacity as purge waste. They are simple and robust but have high operating costs.
• Heated Purge Dryers: These incorporate an electric heater to warm the purge air. Heating the air increases its moisture-carrying capacity and reduces the purge requirement to approximately 7–8%.
• Blower Purge Dryers (BD+): These systems use an external blower and heater to regenerate the desiccant using ambient air. Blower purge dryers operate with zero or near-zero purge loss (0–2%) and offer a rapid return on investment.
• Heat of Compression (XD+): These dryers use the hot waste heat from an oil-free compressor to regenerate the desiccant. They operate with zero purge loss and consume minimal electricity.

Which desiccant material is best?

The necessary dew point, energy objectives, and the dust sensitivity of the application all influence the material selection.

• Activated Alumina: This is the standard bead material for -40°C PDP. It is physically robust and resistant to liquid water.
• Molecular Sieve: This material achieves -70°C PDP. It is required for critical applications but is highly sensitive to oil contamination.
• Silica Gel: This bead type has high capacity at high humidity but shatters if exposed to liquid water.
• Cerades™: This is a solid, structured ceramic desiccant used in Atlas Copco CD+ dryers. It has straight channels that lower pressure drop and it generates zero dust.

Why is pressure drop important for energy efficiency?

High pressure drop forces the air compressor to run at a higher set pressure to maintain downstream flow.

Every 1 bar of pressure drop increases energy consumption by approximately 7%. Traditional loose-bead dryers create turbulence which increases resistance. Structured desiccant (Cerades™) allows straight airflow, significantly reducing pressure drop and energy use. 

A compressed air energy audit identifies these inefficiencies.

What are the UK compliance requirements for air purity?

ISO 8573-1 applies to compressed air used in food, drink, and pharmaceutical production.

Moisture levels are limited. Excess water creates a contamination risk and allows microbes to grow.

ISO 8573-1 Water Purity Classes:

According to Guideline 102 from the British Compressed Air Society (BCAS), air that comes into direct contact with food should be Class 2 (-40°C). 

This dew point makes it impossible for microorganisms to reproduce, which is called a bacteriostatic environment. Design Air does ISO 8573 air quality testing to make sure that these standards are being met.

How should a desiccant dryer be installed?

To protect the desiccant, the order of filtration matters:

• Pre-Filtration: A coalescing filter is fitted before the dryer – keeping oil aerosols away from the desiccant bed.
• Water Separation: Bulk water is removed immediately after the compressor using a water separator.
• Post-Filtration: A particulate filter is fitted downstream of bead-based dryers to capture desiccant dust.
• Bypass: A three-valve bypass is included so the dryer can be isolated for maintenance without shutting down the compressed air system.

What maintenance is required?

Maintenance intervals range hugely. You can start from something as simple as daily checks, and go all the way to major overhauls every few years.

• Daily: Check the dew point reading – and confirm the condensate drains are operating properly.
• Weekly: Check tower switching cycles and inspect purge mufflers for clogging.
• 4,000 Hours: Replace pre-filter and post-filter elements.
• 8,000 Hours: Service solenoid valves and shuttle valves to prevent switching failures.
• 40,000 Hours: Replace the desiccant media.

If this maintenance is missed, moisture builds up in the compressed air – leading to corrosion and avoidable production delays. 

What is the cost of operation?

The most expensive part of running a desiccant dryer is the energy it takes to regenerate.

A normal heatless dryer on a 1000 cfm system loses about £54,000 worth of electricity each year because of purge air loss. Upgrading to a zero-purge blower dryer eliminates this waste. 

In the UK, companies can use full expensing tax relief to claim a 100% first-year deduction on new capital equipment.

When should you contact Design Air?

You should contact Design Air if you need to upgrade your compressed air treatment, require ISO compliance testing, or need repairs on existing dryers. 

Design Air is a Premier Distributor for Atlas Copco serving Glasgow, Edinburgh, Dundee, Fife, Stirling, and the Central Belt. We supply new and refurbished dryers with a 6-month guarantee. We also perform leak detection and system checks to ensure air used in various applications remains clean and dry.

Next Steps

To discuss sizing a desiccant dryer or to arrange a site survey, contact the Design Air team.