Page 246 - Mechanical Engineers' Handbook (Volume 4)

8 Heat Transfer 235

Figure 17 View factors for spaced cylinders with backup wall. 1

coke oven gas and fuel oils. Emissivities for producer gas and blast furnace gas will be
lower, because of dilution of radiating gases by nitrogen.
The emissivity of a layer of combustion gases does not increase directly with thickness
or density, because of partial absorption during transmission through the depth of the layer.
The chart provides several curves for a range of values of L, the effective beam length in
feet, at a total pressure of 1 atm. For other pressures, the effective beam length will vary
directly with gas density.
Beam lengths for average gas densities will be somewhat less than for very low density
because of partial absorption. For some geometrical configurations, average beam lengths
are:
Between two large parallel planes, 1.8 spacing
Inside long cylinder, about 0.85 diameter in feet
For rectangular combustion chambers, 3.4V/A where V is volume in cubic feet and A
is total wall area in square feet
Transverse radiation to tube banks, with tubes of D outside diameter spaced at x centers:
L/D ranges from 1.48 for staggered tubes at x/D 1.5 to 10.46 for tubes in line
and x/D 3 in both directions