Heat Reactivated Desiccant Air Dryers

Heat Reactivated Type Desiccant Air Dryers

There are two main types of heat reactivated type desiccant air dryers of which I will talk about each separately, the Internal Type and more modern External Type.

Internal Heat Re-activated Type

Heat Reactivated dryers are used when one needs compressed air of very low Dew point of (-)60˚C or (-)80˚C. In this design, the desiccant is regenerated at higher temperature, along with a quantity of Dry air purge. Approximately 12-15% of the air that passes though 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 dewpoints but the downside is the high requirement of the dryer itself for the purging process.

Characteristics

• 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 drying cycle for 4 hours, while other vessel is simultaneously regenerated at atmospheric pressure. Around 12-15% flow of dryer capacity is used as purge for regeneration. Electrical heaters are provided in a central finned stainless steel pipe in both drying vessels. Heating cycle is for 2 hours and in this time hot purge air increases desiccant temperature to over 100˚C. At this temperature regeneration is complete (due to dry air purge). Dewpoint achieved is around (-)40˚C. If a lower Dew point is required more air is purged.

Filters

A Pre-filter with 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 size. Thus, you get absolutely “Clean & dry compressed air”.

Features

• A fully Automatic unit requiring no attention of the operator. Heaters switch ON/ off and all valves operate automatically.
• Pressure is equalized to line pressure before changeover of drying vessels. This prevents pressure and flow surge, and, desiccant dusting
• Lower purge air requirement in comparison to Heatless type dryer.
• Desiccant life is around 5 years.
• Internal heaters provide maximum heat transfer efficiency resulting in lower power requirement.
• 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, Dew point of (-) 80˚C can be achieved.
• Heaters are mounted internally making servicing of the heaters very difficult.

External Heat Re-activated Type

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

Characteristics

• 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. Time cycle is 6+6 hours with drying towers filled with desiccant. For regeneration, a centrifugal air blower and an electrical heater is provided. Desiccant is heated upto 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 dewpoint.

Comparison with Internal Heat Reactivated Type Dryers

• Investment cost of these Air dryers is greater than Internal Heat Re-activated Type Dryers but they are more economical in operation so the life time running costs are lower.
• Work well with dewpoint control switching so the dryer will go into standby mode if the dewpoint 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 of the operator. Heaters and fans switch on/off and all valves operate automatically.
• Pressure is equalized to line pressure before changeover of drying vessels. This prevents pressure and flow surge, and, desiccant dusting

Enquiry

Here at Glaston Compressor Services, we can supply bespoke compressed air dryer solutions. For more information or for a quote please contact us.