Page 248 - Mechanical Engineers' Handbook (Volume 4)

8 Heat Transfer 237

Figure 20 Gas-emissivity for products of combustion of methane burned with 110% air. Approximate
for fuel oils and coke oven gas. 1

8.6 Evaluation of Mean Emissivity–Absorptivity
For a gas with emissivity e radiating to a solid surface at a temperature of T F, the
s
g
absorptivity a will be less than e at T because the density of the gas is still determined
g
s
g
by T . The effective PL becomes T /T PL at T . Accurate calculation of the combined
g
s
g
s
absorptivity for carbon dioxide and water vapor requires a determination of a for either gas
g
and a correction factor for the total. For the range of temperatures and PL factors encountered
in industrial heat transfer, the net heat transfer can be approximated by using a factor e gm
somewhat less than e at T in the formula:
g
g
4
4
Q/A 0.1713e gm F(T T )
s
g
where T is an average of gas temperatures in various parts of the combustion chamber; the
g
effective emissivity will be about e gm 0.9e at T and can be used with the chart in Fig.
g
g
20 to approximate net values.
8.7 Combined Radiation Factors
For a complete calculation of heat transfer from combustion gases to furnace loads, the
following factors will need to be evaluated in terms of the equivalent fraction of blackbody
radiation per unit area of the exposed receiving surface:
F Coefficient for gas direct to load, plus radiation reflected from walls to load.
gs
F gw Coefficient for gas radiation absorbed by walls.
F ws Coefficient for solid-state radiation from walls to load.