The F-factor is also known as the emissivity factor and is basically the fraction of heat generated by the flare flame that is radiated. This figure is calculated for each case and is primarily based upon the gas composition and flare type. High pressure sonic flare tips have good air / gas mixing characteristics and their corresponding F-factors are relatively low, in the range 0.07 to 0.15. Low pressure sub-sonic flare tips don’t have such naturally good air / gas mixing characteristics and their corresponding F-factors are relatively high, in the range 0.20 to 0.35. Generally, the lower the F-factor the smaller the flare stack and shorter the flare boom.