Oxygen and PSA
Why does a PSA oxygen plant need an air dryer?
Why does the compressed air feeding a PSA generator have to be dried first?
Short answer
A PSA plant dries its feed air because moisture in the air condenses at high pressures, as a 2025 Global Fund note puts it; AIGA 113/20 (EIGA Doc 195) adds that rising moisture accelerates damage to the adsorbers, and a USAID procedure for Indian hospitals says dried air avoids problems in sieves and solenoid valves.
In a PSA plant, the air compressor raises incoming air to 4.5 to 7 bar before it reaches the two sieve-bed tanks of the oxygen generator, according to the Global Fund briefing note on PSA, VPSA and VSA (4 March 2025), which says an air dryer is needed because moisture in the air condenses at high pressures. WHO's Foundations of medical oxygen systems (2023) describes the assembly as an air compressor always followed by a dryer, filters and a compressed air tank, and says the air passes through a filtration assembly that removes particulate matter and remaining moisture before it reaches the zeolite molecular sieve.
What moisture does to the plant
- Adsorbers: AIGA 113/20 notes that an increase in moisture content would lead to accelerated damage of the oxygen 93 concentrator adsorbers. It asks for moisture analysers, together with carbon monoxide and carbon dioxide analysers, connected to the control and alarm systems, so that the main shut-off valve may close automatically if impurity levels exceed the pharmacopoeia specification.
- Sieves and valves: a USAID RISE operating procedure for PSA plants says the air dryer removes moisture from compressed air, that the dried air eliminates operational issues in the molecular sieves and solenoid valve, and that refrigeration dryers are typically used.
- Air treatment: the ULTRACON concentrator datasheet describes adsorption removing contaminants such as H2O and CO2 from the compressed air, and the ULTRAOX 2022 datasheet says its air filtration and treatment package is included to ensure proper functioning and low degradation of the PSA.
How dryness is checked
| Check | Value | Source |
|---|---|---|
| Alarm on air dryer pressure dew point | Above 3 °C | WHO interim PSA specifications (8 June 2020, superseded by WHO's technical specifications of 31 December 2024) |
| Pressure dew point at commissioning | Under 5 °C | Global Fund commissioning guidelines (28 August 2024) |
| Daily pressure dew point of a refrigeration dryer | Normally 3 to 5 °C; up to 6 °C is acceptable, anything higher calls for immediate inspection; values to be confirmed against the manufacturer's specifications | Global Fund expected values for the daily checklist (5 June 2025) |
| Dryer dew point before the compressor is started | Set value, usually less than 7 °C | USAID RISE procedure, for plants where the dryer comes before the air receiver |
WHO's 2020 interim specification also asked for an external air dryer sized to manage the compressor output. During start-up in plants where the dryer comes before the air receiver, the USAID procedure has the operator start the dryer before the compressor and check the air receiver and filter drains for water, and not let air into the generator if water appears. It warns never to run the plant if the automatic drain valves are malfunctioning or not draining properly.
In ULTRAOX plants
The ULTRAOX 2022 datasheet includes the ULTRASEC high efficiency refrigeration dryer, with a digital dew point indicator, automatic condensate evacuation, a dry contact alarm and a hot gas bypass designed to prevent freezing and give a constant dew point; it is designed for inlet air temperatures up to 70 °C. The air treatment chain adds a water separator with electronic drain, high efficiency micro and sub-micro filters and an activated carbon filter. The ULTRAOX SC and ULTRAOX ST datasheets give a working dew point of -46 °C for the oxygen, as a residual content in initial operation conditions.
More questions
Refrigeration or adsorption dryer?
WHO's typical PSA configuration (2023) shows a refrigeration or adsorption dryer, and the USAID RISE procedure says refrigeration dryers are typically used in PSA plants.
What happens if the dew point alarm sounds?
The Global Fund expected values say anything above 6 °C on a refrigeration dryer calls for immediate inspection, after checking the manufacturer's specifications, and AIGA 113/20 links moisture analysers to the automatic closure of the main shut-off valve.
Sources
- Global Fund / Build Health International, Briefing Note: PSA, VPSA and VSA (4 March 2025)
- WHO, Foundations of medical oxygen systems (17 February 2023), section 2.1 and Fig. 2.2
- AIGA 113/20 (adopted from EIGA Doc 195), On site generation of oxygen 93% for medicinal use, sections 4.3.1 and 7.6
- USAID RISE, Standard Operating Procedures: PSA Plant Operations and Preventive Maintenance by the Facility Management
- WHO, Technical specifications for PSA oxygen plants, interim guidance (8 June 2020)
- Global Fund / Build Health International, Overview: PSA Oxygen Plant Commissioning Guidelines (28 August 2024)
- Global Fund / Build Health International, Valeurs attendues de la liste de vérification quotidienne (5 June 2025, French)
- ULTRACON datasheet, p. 2
- ULTRAOX datasheet 2022, p. 12 and p. 16
- ULTRAOX SC datasheet, p. 3
- ULTRAOX ST datasheet, p. 3
- WHO, Technical specifications for health facility based medical oxygen systems (31 December 2024), note on the oxygen generator plant specification (supersedes the 2020 interim PSA specifications)
Ultra Controlo figures come from its technical datasheets. Third-party facts were read at the source on the date shown.