Emissivity Coefficient or F factor for flare tip
Emissivity coefficient for a flare tip is the fraction of heat generated at the flare tip that is radiated to surroundings. Not all the heat that is generated by burning hydrocarbon gases at the tip of a flare is radiated. Major fraction of the heat generated by a flare is carried to the surroundings in the form of hot gases by convection and the fraction of this heat is simply radiated to surroundings.
The emissivity coefficient depends on the nature of gas being burned, design of the flare tip, liquid content in the gas in flare stack and gas flow rate coming out of the flare tip.
Emissivity coefficient as per nature of gas
Fraction of heat generated being radiated depends on the hydrocarbon gases which are being burned. For light hydrocarbons and subsonic gas flow rates, the emissivity coefficient values lie in the range 0.2-0.3.
Liquid content in the flare stack
Although a flare knock out drum might be present at the base of a flare stack to remove the entrained liquids, still some small liquid droplets escape the knock out drum and are burned in the flare stack. Higher content of such liquid droplets in the flare stack results in increased emissivity coefficient and higher fraction of heat radiated.
Escape velocity of gas at the flare tip
For higher gas escape velocities at the flare tip, emissivity coefficients are typically low. For sonic flares, having gas velocity close to speed of sound, the emissivity coefficients occur to be around 0.15. For low gas velocities, such as continuous purge gas flow in a flare, the gas velocities of the heated burnt out gas is not high enough to ensure high convection. Hence low gas velocities result in higher emissivity coefficients.
Design of the flare tip
The flare tip manufacturer has the best knowledge of flare tip design and can predict the flare tip exit velocity for different pressure values in the flare stack. Based on this knowledge of exit velocity, the flare tip maker can very well estimate the emissivity coefficient values for different designs.