Page 263 - Mechanical Engineers' Handbook (Volume 4)

252 Furnaces

Figure 30 Configuration factors for convection heat transfer, air or flue gas through tube banks. 1

diameter of 0.75 ft, the net thickness will be 0.125 ft. The mean area at midthickness is
2
(0.5 0.75)/2, or 1.964 ft per ft of length. Outer surface area of insulation is 0.75 ,or
2
2.36 ft per linear foot. Conductivity of insulation is k 0.20. The effective radiation factor
from gas to surface is assumed at 0.5 including reradiation from walls. For the two resistances
in series,
0.20
0.1713 0.5 2.36(29.6 T ) 1.964(T 150)
4
4
s
s
0.125
By trial, the receiver surface temperature is found to be about 2465 F. Heat transfer is
about 7250 Btu/hr linear ft or 9063 Btu/hr ft water-cooled tube surface.
2
If the insulated tube in the preceding example is heated primarily by convection, a
similar treatment can be used to find receiver surface temperature and overall heat transfer.
2
For radiation through furnace wall openings, heat transfer in Btu/hr ft F is reduced
by wall thickness, and the result can be calculated similarly to the problem of two parallel
planes of equal size connected by reradiating walls, as shown in Fig. 19.
Heat transfer in internally fired combustion tubes (‘‘radiant tubes’’) is a combination of
convection and gas radiation from combustion gases to tube wall. External heat transfer from
tubes to load will be direct radiation and reradiation from furnace walls, as illustrated in Fig.
19. The overall factor for internal heat transfer can be estimated from Fig. 31, calculated for
6 in. and 8 in. inside diameter tubes. The convection coefficient increases with firing rate