# Rotary screw compressor

A rotary screw compressor compresses air between two meshing helical rotors turning in a casing; in the oil-injected type, oil seals and cools the compression, so the air needs oil separation and filtration downstream.

Screw compressors are used for hospital technical air and as feed compressors for PSA oxygen plants. The WHO interim technical specifications for PSA oxygen plants (2020, superseded by WHO's technical specifications of 31 December 2024) accepted a feed compressor that is oil-free, or a filtered oil-injected or oil-lubricated rotary screw type, of at least 7.5 bar. For medical air, an oil-lubricated compressor depends on separators and filters to keep its oil out of the network, and the EDQM history of Ph. Eur. monograph 1238 notes that its oil test for medical air is relevant only when an oil-lubricated compressor is used.

ULTRASCREW is the oil-injected rotary screw compressor range from Ultra Controlo, with IE3 motors and star delta starting; its variable speed version saves up to 35% energy and operates between 5 and 14 bar, and its flow figures are measured to ISO 1217:2009, Annex C. ULTRASCREW compressors feed the ULTRAOX ST oxygen plant, so the ST is not an oil-free plant, while the ULTRAOX SC uses oil-less ULTRACECO compressors. The ACIH hospital industrial air system and ULTRAAR ME medical air plants use Gardner Denver rotary screw compressors, and ULTRAAR ME adds an oil and water separator.

## Sources

- [WHO, Technical specifications for PSA oxygen plants, interim guidance (2020)](https://www.who.int/publications/i/item/WHO-2019-nCoV-PSA_Specifications-2020.1)
- [EDQM, Ph. Eur. monograph 1238 Air, medicinal: history (oil test)](https://extranet.edqm.eu/4DLink1/Web_mono_hist?1238)
- [ULTRASCREW datasheet, p. 2 and p. 3](https://www.ultracontrolo.com/products/ultrascrew)
- [ULTRAOX ST datasheet, p. 2; ULTRAOX SC datasheet, p. 2](https://www.ultracontrolo.com/products/ultraox-st)
- [ACIH datasheet, p. 2](https://www.ultracontrolo.com/products/acih)
- [ULTRAAR ME datasheet, p. 2](https://www.ultracontrolo.com/products/ultraar-me)
- [Ultra Controlo, Medical gas answers: oil-free vs oil-lubricated medical air](https://www.ultracontrolo.com/medical-gas-answers)
- [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/glossary/rotary-screw-compressor/