When compressing air liquid is also brought into the air stream. As the liquid cools, it condenses with the compressed air that is delivered directly to your tool or application.

But liquid water in pneumatic systems creates problems. Bacteria growth, rust and corrosion affect how the machine operates, resulting in sluggish performance and component breakdowns. To mitigate these issues and prevent the system shutting down, the wet air needs to be treated using drying equipment.

Typically, this is done by cooling the air as it comes out of the system. Cold air is incapable of holding vaporised water. This is why the dewpoint (air temperature) is the most important factor used in drying processes.

There are multiple ways to dry compressed air. To better understand which air dryer is suitable for any particular air compressor and application, you need to understand the different types.

Refrigerated dryers

These dryers remove water by cooling the compressed air temperature and causing condensation. Once the vapour turns to liquid form it’s collected into a separator and drained away.

• Most cost-effective solution (low operating/maintenance costs and initial outlay)
• Most commonly used
• Removes largest amount of water the least amount of energy use

• Doesn’t completely remove moisture, making it unsuitable for water-sensitive applications
• Limited dewpoint capability

Recommended use for: Applications that require high-performing air dryers in extreme working conditions, such as extremely low- or high-pressure environments. Refrigerated dyers are the best choice for general air compressor use.

Non-cycling dryers

Refrigerated air dryers come as non-cycling or cycling systems.
In non-cycling, the refrigerant circulates constantly within the system regardless of airflow or moisture content.

• Allows for quick responses to load changes
• Flow is regulated by a hot gas bypass valve, to accommodate variable conditions
• Low initial cost
• Consistent dew point

• No energy savings during partial flow and low moisture conditions, despite the bypass valve regulating loads

Recommended use for: Improving the quality of compressed air on a budget. Because they don’t save energy at partial and zero flow, they’re best for systems that regularly.

Cycling dryers

On the other hand, cycling dryers are the most commonly used refrigerated air dryers.

They use a three-stage heat-exchanger design, which allows the unit to switch on/off depending on demand. When thermal mass has been chilled to its low point, the system can shut down and save energy.

• Best for energy management and consumption

• High initial cost
• Dew point fluctuations
• Bulkier design to accommodate thermal mass
• Shorter lifespan because of stop-start operation

Recommended use for: When you’re frequently using less than full loads. Once these units reach the desired temperature, they switch off and continue to adjust accordingly.

Desiccant dryers

Desiccant dryers move compressed air through a dual tower design. One container is filled with materials that absorb moisture from the air stream in a reversible process to refrigerated dryers.

Two separate compartments are used to alternate between heating and drying. As one tower dries air from the compressor, the second tower is being regenerated after its pressure is reduced.

• Highly effective for drying compressed air, removing 99.99% of moisture
• Low dewpoints (as low as 73°C)
• Reasonable operating costs


• Heated desiccant dryers are expensive to run as they use large amounts of power, but savings can be made if partially loaded and by using dewpoint controls
• High setup costs
• Desiccant materials must be replaced every three to five years
• Without proper filtration, airborne oil particles can degrade the desiccant agent

Recommended use for: Critical and sensitive applications or work in freezing cold conditions.

Membrane dryers

Membrane dryers rely on specially formulated membrane materials and pressure differences inside the dryer.

They work by shifting pressurised air through fine membrane fibres, which allows small water molecules to pass and exit the system as dry air; larger nitrogen and oxygen molecules cannot.

• Compact and lightweight
• Designed with no moving parts and no need for power supply
• Silent and simple operation
• Minimal maintenance

• Expensive
• Will eventually need a full replacement

Recommended use for: Remote areas, explosive or sub-zero environments and applications requiring a consistent dewpoint.

Air dryers are the most efficient way to prevent moisture in your system. To choose the best for your application, make sure you know your required dewpoint and whether the project can tolerate some water moisture or not.

Jayde Walker writes for Express Compressors – Australia’s high quality, energy efficient air compressor and air dryer products. Catch her on LinkedIn.


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