What does 'normal conditions' mean in plant capacity?

WHO, Foundations of medical oxygen systems (2023), explains that a plant capacity in Nm³/h refers to a reference condition of temperature and pressure, that some capacities are expressed in Sm³/h, based on standard reference conditions, and that the units used determine the actual gas volume produced.

In EU law, Commission Implementing Regulation (EU) 2018/2066 on the monitoring of greenhouse gas emissions defines standard conditions as a temperature of 273.15 K and a pressure of 101,325 Pa, the conditions defining normal cubic metres (Nm³).

Compressor datasheets can use another basis. The ULTRASCREW datasheet measures air flow in accordance with ISO 1217:2009, Annex C, with values referring to 1 bar absolute, 0% relative humidity and 20 °C inlet air, and the ULTRACECO SC datasheet gives reference conditions of 1 bar absolute inlet pressure and 20 °C suction temperature.

Capacity and purity

The Global Fund briefing note on PSA plant oxygen purity (3 April 2024) gives an example: a 20 Nm³/h PSA plant set to 93 ± 3% could produce 20 Nm³/h, while the same equipment set to 95 ± 1% would produce 18 Nm³/h. The note adds that the purity decreases as more oxygen is demanded from the plant.

The ULTRACON concentrator datasheet gives each model's oxygen flow at 93% and at 95%, in Nm³/h, Nl/min and SCFH, together with the compressed air it consumes in Nm³/h; the 14D, the largest model in the table, is listed at 111.4 Nm³/h at 93% and 90.4 Nm³/h at 95%. The ULTRAOX SC and ST model tables give each plant's oxygen capacity in l/min and m³/h.