# How is the air compressor for a PSA plant sized?

**How is the feed air compressor of a PSA oxygen plant sized relative to oxygen output?**

A PSA plant's feed air compressor is sized to supply the compressed air its oxygen generator consumes at the chosen purity, at the pressure the generator needs. The ULTRACON datasheet lists that air consumption for every generator size at 93% and 95% oxygen, and ULTRAOX model tables give 7.5 bar at the inlet.

## What sets the compressor size

- Air consumption of the generator. The ULTRACON generator produces oxygen from the compressed air fed to it, and its datasheet table gives, for each size, the compressed air consumed at 93% and at 95% purity, which is what the compressors must supply. The ULTRACON 10, for example, delivers 18.2 Nm³/h of oxygen at 93% and consumes 206 Nm³/h of compressed air; see [how much compressed air a PSA generator needs](https://library.ultracontrolo.com/answers/how-much-compressed-air-does-a-psa-generator-need/).
- Purity setting. At 95% the same ULTRACON 10 delivers 14.8 Nm³/h of oxygen and consumes 185 Nm³/h of air. A Global Fund briefing note (3 April 2024) gives a similar example: equipment producing 20 Nm³/h at 93% (±3%) would produce 18 Nm³/h set to 95% (±1%).
- Pressure. The ULTRAOX SC and ST model tables list an inlet pressure of 7.5 bar for the compressed air and 6 bar for the oxygen outlet on every model, and the ULTRACON has a low inlet pressure shutdown among its safety functions. For comparison, the 2020 WHO interim specification for PSA plants (superseded by WHO's technical specifications of 31 December 2024) asked for a feed air compressor of at least 750 kPa (7.5 bar), and a Global Fund briefing note (4 March 2025) describes the PSA air compressor raising incoming air to 4.5 to 7 bar.
- Air treatment. The same WHO specification asked for an external air dryer sized to manage the compressor output.
- The plant's own use. HTM 02-01 (2026), the NHS England guidance, notes that regeneration of the molecular sieves uses a proportion of the product gas, and that this consumption should be taken into account when sizing the plant.

The WHO specification was issued on 8 June 2020 as interim COVID-19 guidance and stated that it would expire two years after publication; WHO still hosts it.

## Energy and redundancy

WHO's Foundations of medical oxygen systems (2023) states that the energy efficiency of a PSA plant is largely determined by the feed air compressor, and that peak power demand is determined by the inflow current and voltage of the feed air compressors. The ULTRAOX datasheet (2022) states that each extra bar of compressor pressure adds 7% to total electricity consumption, and that if the system is not calculated correctly, more than 20% of the installed power is also consumed when the compressor switches out of its load cycle.

For redundancy, HTM 02-01 (2026) asks for a duplex compressor on PSA plants and, where more than two compressors are installed, a system that can deliver the design flow with one compressor out of service. It adds that compressors for PSA systems should comply with the medical air requirements, but that the medical air plant should not be used for the PSA plant.

## ULTRAOX compressors by model

ULTRAOX plants combine one or more compressors with one or more ULTRACON concentrators; the compressors switch and the generators start in cascade. The ULTRAOX SC uses oil-less ULTRACECO compressors, described in its datasheet as Class 0; the ULTRAOX ST uses ULTRASCREW compressors, which are oil-injected rotary screw compressors. The ULTRAOX ST datasheet gives the compressor power for each model and notes that compressor power and pressure can vary with the service conditions required for the facility. PATH and CHAI (2021) note that, depending on the manufacturer, an air compressor, a filter or dryer and tanks may be required, and that it is common for PSA manufacturers to provide recommended models or specifications for this equipment as a package.

| ULTRAOX ST model | Oxygen output (m³/h) | Compressor power (kW) |
|---|---|---|
| 3 | 0.32 | 2.2 |
| 30 | 3.1 | 5.5 |
| 120 | 12.4 | 15 |
| 370 | 36.5 | 37 |
| 560 | 55.7 | 75 |
| 1120 | 111.4 | 150 |
| 1640 | 164 | 220 |

### Can the medical air plant also feed the PSA plant?

HTM 02-01 (2026), the NHS England guidance, says the medical air plant should not be used for the PSA plant, and that compressors for PSA systems should comply with the medical air requirements.

### Does a 95% purity setting need a bigger compressor?

For the same oxygen flow, the Global Fund purity note (2024) says a 95% (±1%) plant needs larger equipment than a 93% (±3%) plant, with higher power consumption.

### Are ULTRAOX compressors oil-free?

In the ULTRAOX SC, yes: it uses oil-less ULTRACECO compressors, described in its datasheet as Class 0. The ULTRAOX ST uses ULTRASCREW compressors, which are oil-injected rotary screw compressors.

## Sources

- [ULTRACON datasheet, p. 2 and p. 3 (model table)](https://www.ultracontrolo.com/products/ultracon)
- [ULTRAOX SC datasheet, p. 2 and p. 5 (model table)](https://www.ultracontrolo.com/products/ultraox)
- [ULTRAOX ST datasheet, p. 2 and p. 5 (model table)](https://www.ultracontrolo.com/products/ultraox-st)
- [ULTRASCREW datasheet, p. 2](https://www.ultracontrolo.com/products/ultrascrew)
- [ULTRAOX datasheet 2022, p. 5](https://www.ultracontrolo.com/ultraox)
- [Global Fund / Build Health International, Briefing Note: PSA Plant Oxygen Purity (3 April 2024)](https://resources.theglobalfund.org/media/lenpqaoa/cr_c19rm-psa-oxygen-purity_technical-briefing-note_en.pdf)
- [Global Fund / Build Health International, Briefing Note: PSA, VPSA and VSA (4 March 2025)](https://resources.theglobalfund.org/media/dp3clsrr/cr_c19rm-psa-vpsa-vsa_technical-briefing-note_en.pdf)
- [WHO, Technical specifications for PSA oxygen plants, interim guidance (8 June 2020)](https://www.who.int/publications/i/item/WHO-2019-nCoV-PSA_Specifications-2020.1)
- [WHO, Foundations of medical oxygen systems (17 February 2023)](https://iris.who.int/server/api/core/bitstreams/4ad6f5fd-089e-4142-8439-b64e98d816d8/content)
- [NHS England, HTM 02-01 Part A (2026), paragraphs 6.110, 6.116 and 6.117](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)
- [PATH and CHAI, Oxygen Generation and Storage: Pressure/Vacuum Swing Adsorption Plant (July 2021)](https://media.path.org/documents/O2_generation_and_storage_PSA_VSA_v1.pdf)
- [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)](https://iris.who.int/server/api/core/bitstreams/ff437601-add3-44c8-8149-b85f0cdb2b52/content)

Reviewed: 2026-10-22. Ultra Controlo engineering. https://library.ultracontrolo.com/answers/how-is-the-air-compressor-for-a-psa-plant-sized/