# How does a desiccant air dryer regenerate?

**How does an adsorption (desiccant) air dryer regenerate, and how much purge air does it use?**

According to the US Department of Energy sourcebook (2016), an adsorption (desiccant) dryer normally has two towers: one dries the compressed air while the other is regenerated by lowering its pressure and passing dried purge air through the desiccant; a Compressed Air Challenge fact sheet (May 2026) says the purge air requirement depends on the type of dryer.

Desiccants such as silica gel, activated alumina and molecular sieves adsorb water vapour from compressed air. The third edition (2016) of the US Department of Energy sourcebook Improving Compressed Air System Performance describes the usual twin-tower arrangement: compressed air flows through one tower while the desiccant in the other is regenerated, by reducing the pressure in that tower and passing previously dried purge air through the bed. The regeneration cycle is normally built in and can be based on time, dew point or a combination of the two.

HTM 02-01 (2026), the NHS England guidance, describes hospital medical air dryers this way: each dryer assembly has two dryer columns, and while one column dries the compressed air, the other is dried with the dry air produced by the active column. The full assembly is duplicated by a second dryer assembly to ensure continuity of supply, and each assembly should have its own dew-point sensor and test point after the bacteria filter.

## What regeneration costs

The Compressed Air Challenge Fact Sheet #11 (May 2026) compares desiccant dryer types by how they regenerate:

- Twin-tower desiccant dryers: the purge air requirement depends on the type of dryer, and this energy loss must be considered in addition to the pressure drop across the dryer.
- Heated dryers: less purge air for regeneration, because heaters heat the desiccant bed or the purge air; the heater energy must be considered against the reduction in purge air, in addition to the pressure drop.
- Heat-of-compression dryers: they may be used where the lubricant-free compressor discharge temperature is high enough to regenerate the desiccant, and there is no reduction of air capacity, because hot, unsaturated air from the compressor discharge is used for regeneration.

The 2016 sourcebook adds that drying compressed air beyond the required pressure dew point results in unnecessary energy and costs.

## Ultra Controlo adsorption dryers

The ULTRADRY is the Ultra Controlo adsorption dryer for compressed air treatment. Its datasheet gives a pressure dew point range of −40 °C to −70 °C, a coalescing filter at the inlet, a dust filter at the outlet and desiccant in replaceable cartridges. A control of the actual water load adjusts the adsorption cycles, which the datasheet says reduces the costs associated with regeneration. The table below lists, for each size, the inlet volume flow and the compressed air consumption at a pressure dew point of −40 °C, with inlet air at +35 °C and 7 bar g.

| ULTRADRY model | Inlet volume flow (Nm³/h) | Compressed air consumption (Nm³/h) |
|---|---|---|
| ULTRADRY 1 | 5 | 0.85 |
| ULTRADRY 2 | 10 | 1.70 |
| ULTRADRY 3 | 15 | 2.55 |
| ULTRADRY 4 | 25 | 4.25 |
| ULTRADRY 5 | 35 | 5.95 |
| ULTRADRY 6 | 50 | 8.50 |
| ULTRADRY 7 | 65 | 11.05 |
| ULTRADRY 8 | 80 | 13.60 |
| ULTRADRY 9 | 100 | 17.00 |

The ULTRAMED air treatment station uses two adsorption columns with dew-point-dependent changeover, which its datasheet describes as giving minimum purge air use; in the ULTRAMED DS version, dew point monitoring stops column changeover whenever the ideal dew point is reached. The [ULTRAAR ME](https://library.ultracontrolo.com/products/ultraar-me/) medical air plant can be fitted with ULTRADRY adsorption dryers as an option.

### Which desiccants do adsorption dryers use?

The US Department of Energy sourcebook (2016) lists silica gel, activated alumina and molecular sieves, with molecular sieve added as the final drying agent where very low dew points are required.

### Why does a hospital medical air plant have two dryer assemblies?

HTM 02-01 (2026) duplicates the dryer assembly to ensure continuity of supply, and says each assembly should have its own dew-point sensor and test point after the bacteria filter.

## Sources

- [US Department of Energy and Compressed Air Challenge, Improving Compressed Air System Performance: A Sourcebook for Industry, third edition (2016, DOE/EE-1340), p. 12 to 13](https://www.energy.gov/sites/prod/files/2016/03/f30/Improving%20Compressed%20Air%20Sourcebook%20version%203.pdf)
- [Compressed Air Challenge, Compressed Air Systems Fact Sheet #11: Proven Opportunities at the Component Level (May 2026, Rev. 01)](https://www.compressedairchallenge.org/data/sites/1/media/library/factsheets2026/fact-sheet-11-proven-opportunities-at-the-component-level_updated2026.pdf)
- [NHS England, HTM 02-01 Medical gas pipeline systems (2026), Part A, 7.66 to 7.68](https://www.england.nhs.uk/wp-content/uploads/2016/05/health-technical-memorandum-02-01-medical-gas-pipeline-systems-part-a-design-installation-validation-verification-aug-2026.pdf)
- [ULTRADRY datasheet, p. 2 and p. 3](https://www.ultracontrolo.com/products/ultradry)
- [ULTRAMED datasheet, p. 2 and p. 3](https://www.ultracontrolo.com/products/ultramed)
- [ULTRAAR ME datasheet, p. 4](https://www.ultracontrolo.com/products/ultraar-me)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/how-does-a-desiccant-dryer-regenerate/