Can a PSA plant run efficiently at part load?

WHO's Foundations of medical oxygen systems (2023) says PSA plants are typically manufactured to operate non-stop, and that combining more than one plant (single, duplex or multiplex) can reach the expected output or alternate usage and scheduled maintenance, at the cost of higher maintenance resources. One of its example configurations is a single system connected to the pipeline that alternates to filling cylinders when oxygen demand is low.

How plants are run at lower demand

• One side of a duplex: the Global Fund briefing note on oxygen plant staffing (11 April 2025) describes duplex plants that operate only one side under normal conditions and turn the second side on in times of high demand, such as COVID-19 surges.
• Surplus to cylinders: some plants use their whole capacity to pipe oxygen during the day and fill cylinders at night, when pipeline demand is lower (same note).
• Running hours to match: plants that only fill cylinders operate for as many hours as it takes to fill the cylinders needed, and some plants are turned off at night while cylinders feed the pipeline. A pipeline that relies on the plant at all hours needs it running 24 hours a day, 7 days a week.
• Compressor control: WHO's interim PSA specifications (8 June 2020, superseded by WHO's technical specifications of 31 December 2024) listed a soft start or variable speed drive (VSD) compressor as an option. WHO (2023) says the energy efficiency of PSA plants is largely determined by the feed air compressor and that some brands offer VSD feed compressors to minimise peak power, after a careful assessment of power supply reliability.

What to watch

• Maintenance continues: WHO (2023) says some maintenance tasks should be performed periodically even when the plant is not running continuously, for example because of lower oxygen demand.
• Underused plants: Global Fund sustainability guidance (22 August 2024) says need should not be estimated from current demand, asks for production to be compared with consumption every year and, if a plant is underused, suggests extending the pipeline or the cylinder distribution to peripheral facilities.
• The opposite limit: a USAID RISE operating procedure, written for hospitals in India, warns against drawing more than the plant's rated flow, which drops purity and pressure and can cause malfunction or permanent failure of valves, sensors and gauges.

ULTRAOX plants run fully automatically, starting and stopping oxygen production in direct response to demand from the downstream distribution system, according to the 2022 datasheet. The ULTRAOX SC is built with a modular design that increases flow capacity progressively, and it switches between compressors and starts its generators in cascade to keep the oxygen supply continuous (ULTRAOX SC datasheet, p. 2). For the variable speed version of its ULTRASCREW compressor, Ultra Controlo states energy savings of up to 35%.