Fireboxes with Rapidly and Widely Varying Heat Releases
A typical firing control system for these fireboxes is shown in Figure 1-6 of API RP 550, Part III—Fired Heaters and Inert Gas Generators.
This control technique is used only on heaters equipped with forced-draft controls. It is similar to the control system used on boilers.
When using the fuel/air flow control on large fireboxes, consider using an oxygen analyzer to adjust the fuel/air ratio, with the carbon monoxide measurement making only very fine adjustments to correct for the quality of combustion. The control action of both analyzers should be limited so that false information from either analyzer will not take the firebox into an unsafe operating region.