Free air delivery (FAD)

The sourcebook (third edition) also defines cfm of free air as air delivered to a point and converted back to ambient conditions, and standard cfm as free air converted to reference conditions of 14.5 psia, 68 °F and 0% relative humidity. It notes that CAGI and PNEUROP have adopted the ISO definition of standard air, 1 bar, 20 °C and dry, and that their simplified performance testing standards have been incorporated as addenda in ISO 1217 for displacement compressors.

NHS England guidance, HTM 02-01 (2026), uses free air in several places: terminal unit flows are expressed in free air, the minimum water capacity of the receivers should be 50% of the compressor output in one minute, stated in terms of free air delivered at normal working pressure, and plant efficiency, expressed as the volume of free air delivered to the pipeline after drying and filtration losses per kWh, should be stated by the supplier of the system.

The ULTRACECO SC, SCA and ESW datasheets from Ultra Controlo give capacities measured to ISO 1217, Annex C, with reference conditions of 1 bar absolute inlet pressure and 20 °C suction temperature; the ULTRASCREW datasheet gives values at 1 bar absolute, 0% relative humidity, 20 °C inlet air temperature and a 71 °C thermostatic valve value, with air flow measured to ISO 1217:2009, Annex C.