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When to Use Heated Desiccant Air Dryers and the Types of Dryers

In this Glaston blog, we go into the specifics of industrial air dryers and explore their various types and functionalities. In today’s discussion, we will focus specifically on heated desiccant air dryers, which are an essential component in numerous industries where moisture control is paramount.

Whether you are involved in manufacturing, pharmaceuticals, food processing or any other moisture-sensitive industry, understanding the distinctions between these dryer types can help you make an informed decision about which one best suits your specific requirements.

We will discuss two main types of heated desiccant air dryers: the internal type and the more modern external type.

Internal Heat Reactivated Type

Heat Reactivated dryers are used when one needs compressed air with a very low dew point of (-)60˚C or (-)80˚C. In this design, the desiccant is regenerated at a higher temperature along with a quantity of Dry air purge. Approximately 12-15% of the air that passes through is used as purge air.

Due to purging with dry air and thermal regeneration, residual moisture loading on desiccant becomes low and this gives very low dew points but the downside is the high requirement of the dryer itself for the purging process.


Air capacity: 5 to 5000 NM3/hr
Dew point: (-)40˚C to (-)80˚C
Operating pressure: 2 to 16 barg

Operating principle

The drying unit has 2 vessels filled with activated alumina or molecular sieves desiccant. One vessel remains in the drying cycle for 4 hours, while the other vessel is simultaneously regenerated at atmospheric pressure. Around 12-15% flow of dryer capacity is used as a purge for regeneration. Electrical heaters are provided in a central finned stainless steel pipe in both drying vessels.

The heating cycle is for 2 hours and during this time, the hot purge air increases desiccant temperature to over 100˚C. At this temperature, the regeneration is complete (due to dry air purge). The dew point achieved is around (-)40˚C. If a lower dew point is required, more air is purged.


A pre-filter with an automatic drain valve is provided to remove any physical moisture from compressed air before entering the air dryer. A micronic filter is also provided on the dryer outlet to arrest any desiccant dust particles, up to 1 micron in size. Thus, you get fully “clean and dry compressed air”.


A fully automatic unit requires no attention from the operator. Heaters switch on/off and all valves operate automatically.

Pressure is equalized to line pressure before the changeover of drying vessels. This prevents pressure, flow surge and desiccant dusting

Lower purge air requirement in comparison to a Heatless type dryer.

Desiccant life is around 5 years.

Internal heaters provide maximum heat transfer efficiency resulting in lower power requirements.

Counter current heated dry purge air provides maximum removal of moisture with minimum gas purge loss.

Standard units are designed to deliver dry gas/air of (-) 40˚C dew point. By using more purge airflow, and using molecular sieves, a dew point of (-) 80˚C can be achieved.

Heaters are mounted internally, making servicing the heaters very difficult.

External Heat Reactivated Type

Externally heat-reactivated air dryers are bulkier in size. In capacities less than 500 NM3/hr these may be uneconomical.


Air capacity: 500 to 6000 NM3/hr
Dew point: Up to (-)40˚C
Operating pressure: 2 to 16 barg

Operating principle

In this design, there is NO LOSS or MINIMAL LOSS of expensive compressed air. The time cycle is 6+6 hours with drying towers filled with desiccant. For regeneration, a centrifugal air blower and an electrical heater are provided. Desiccant is heated up to 100˚C by hot air for complete regeneration and a small amount of purge air is used to polish the desiccant beds to give the required dew point.

Comparison with Internal Heat Reactivated Type Dryers

The investment cost of these air dryers is greater than Internal Heat Reactivated Type Dryers, but they are more economical in operation so the lifetime running costs are lower.

Work well with dew point control switching so the dryer will go into standby mode if the dew point doesn’t deteriorate thus saving money.

Maintenance on the dryers is simpler as the heaters are externally mounted and not mounted inside the internally heated dryer.

If steam is available then this can be used instead of the heater to further reduce electrical consumption costs.

Lower purge air requirement in comparison to Heatless and Internally heated type dryer.

A fully automatic unit requiring no attention from the operator. Heaters and fans switch on/off and all valves operate automatically.

Pressure is equalized to line pressure before the changeover of drying vessels. This prevents pressure and flow surge, and, desiccant dusting.

Get in touch about our heated desiccant air dryers

Here at Glaston Compressor Services, we can supply bespoke compressed air dryers, compressed air filtration and air compressor systems that can help to enhance your application and save energy while doing so. For more information on how our services can benefit your business or a quote, please contact us.

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