Page 258 - Mechanical Engineers' Handbook (Volume 4)

8 Heat Transfer 247

Figure 27 Furnace temperature profiles.

with a maximum temperature and firing rate consistent with furnace maintenance require-
ments and limits imposed by the need to avoid overheating of the load during operating
delays, and a final or soak zone to balance furnace temperature with maximum and minimum
load temperature specifications. In some designs, the preheat zone is unheated except by flue
gases from the firing zone, with the resulting loss of furnace capacity offset by operating
the firing zone at the maximum practical limit.

8.13 Equivalent Furnace Temperature Profiles
Furnace heating capacities are readily calculated on the assumption that furnace temperature,
either combustion gases or radiating walls, is constant as a function of position or time.
Neither condition is realized in practice; and to secure improved capacity with reduced fuel