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