What air treatment does a PSA oxygen plant need?

WHO, Foundations of medical oxygen systems (2023), lists these components for a typical PSA plant:

• Air compressor
• Water trap
• Refrigeration or adsorption dryer
• Filtration assembly: pre-filter (> 5 micron), coalescing filter (0.1 micron) and a coal filter or coal tower, or alternatively an activated carbon filter
• Compressed air tank
• PSA oxygen generator, control panel, product (buffer) tank, bacteria or sterile filter and oxygen analyser

WHO notes that after compression the air passes through a filtration assembly to remove particulate matter and remaining moisture before the molecular sieve. The Global Fund briefing note on PSA, VPSA and VSA (4 March 2025) adds that an air dryer is needed because moisture in the air condenses at high pressures, and that an air storage tank sits before the oxygen generator. PATH and CHAI (July 2021) describe the air as dried, with impurities such as carbon dioxide, hydrocarbons and water removed.

AIGA 113/20, adopted from EIGA Doc 195, requires a drying and filtration system downstream of the air compressors to deliver dry and oil-free air at the inlet of the adsorber vessels, with gas temperature and pressure monitored at each stage of compression and alarms and automatic shutdown at the thresholds. It names trace oil, heavy hydrocarbons, acid gases and engine exhaust in the compressed air as directly affecting the safe operation of oxygen 93 units.

Oil and the compressor type

The 2020 WHO interim specification for PSA plants (superseded by WHO's technical specifications of 31 December 2024), from its COVID-19 (2019-nCoV) series, accepted an oil-free compressor or a filtered oil-injected or oil-lubricated rotary screw compressor. It asked for an air compressor with air dryer and pre-filters with automatic drains, the same three filters as applicable, an external air dryer sized to the compressor output, and an alarm when the dryer pressure dew point rose above 3 °C. With an oil-injected screw compressor, the Global Fund (2025) notes that more filters are needed and need replacing; AIGA asks for food grade lubricants and oils wherever possible.

NHS England guidance, HTM 02-01 (2026), says desiccant air dryers are usually integrated within the molecular sieves, that refrigerant dryers may also be included where environmental conditions require them, and that the filters and molecular sieves of a PSA plant should be duplexed. It also says the medical air plant should not be used for the PSA plant, and AIGA asks for an air compressor dedicated to the oxygen 93 system.

After the generator

AIGA places a filtration system downstream of the adsorber vessels to prevent the release of molecular sieve dust, which a double filter can achieve, with its pressure differential monitored.

In ULTRAOX plants

The ULTRAOX datasheet (2022) includes a compressed air treatment package which, it says, ensures proper functioning and low degradation of the PSA: a water separator with electronic drain, high efficiency micro, sub-micro and activated carbon filters, and an air quality control system. The same datasheet lists the ULTRASEC refrigeration dryer, with a digital dew point indicator, automatic condensate evacuation and a dry contact alarm, for inlet air up to 70 °C, as well as a compressed air tank and a high efficiency particulate filter for medical oxygen.

The ULTRAOX SC and ST basic configurations specify one, two or more compressed air treatment chains and one or more air tanks. According to its datasheet, the ULTRACON concentrator itself uses adsorption to remove contaminants such as H2O and CO2 from the compressed air. The compressors differ: ULTRAOX SC uses oil-less ULTRACECO Class 0 compressors and ULTRAOX ST uses oil-injected ULTRASCREW screw compressors; the ULTRAOX datasheet states that oil-free technology extends the filter element replacement period.