SFP for a given fan system and operating point (combination of flow rate and pressure rise) is defined as: where: There are various sub-definitions of SFP for different specific applications, including SFPe (building energy performance calculations), SFPv (for performance verification tests), SFPi (individual fan), SFPAHU (air handling unit), SFPFCU (fan coil unit), and SFPBLDG (whole building).
[2] Reference 1 also describes how account for intermittently operated fans, e.g. kitchen hoods, and part-load performance in variable air volume (VAV) systems.
SFP can be expressed in the following equivalent SI units: As you can see above, SFP can be expressed in units of pressure, since pressure is a measure of energy per m³ air.
); it is lowest for small fans or inefficient operating points (e.g. throttled flow or free-flow).
For more insight into how to maximise energy efficiency and minimize noise in fan systems, see ref.1