Estimate gas emissivities for equipment and process design

The major contribution to heat transfer from flames and gases produced by conventional fuels is thermal radiation from water vapor, carbon dioxide, carbon monoxide, and soot. At the high temperatures typically encountered in furnaces and process equipment, radiative heat transfer is important mostly for heteropolar gases, the most common of which are CO{sub 2}, H{sub 2}O, CO, CH{sub 4}, SO{sub 2}, and NH{sub 3}. On the other hand, gases with symmetrical molecules, such as H{sub 2}, O{sub 2}, and N{sub 2}, do not show absorption bands at the wavelengths encountered in most industrial equipment; hence, these are transparent to radiation and have negligible emissivities. This article presents a simple design equation for estimating the total emissivity of six nontransparent gray gases, namely CO{sub 2}, H{sub 2}O, CO, CH{sub 4}, SO{sub 2}, and NH{sub 3}, based on the data plotted in published emissivity charts. Correlations are also provided for three emissivity correction factors, namely the pressure correction factors for CO{sub 2} and H{sub 2}O and the spectral overlap correction in CO{sub 2}-H{sub 2}O mixtures. Finally, the use of the design equation is illustrated by estimating the emissivity of the gas mixture in the waste heat boiler of a Claus plant.