Convective section damage

Mixed with fresh air flowing through convective section leaks, the flue gases may reignite. This phenomenon, called afterburn, results in damage to the convective section finned tubing. Afterburn is promoted by insufficient oxygen in the firebox, excessive draft, and leaks in the convective section exterior walls. 

On one particular unit, the fins on the convective section tubes had been reduced to a brittle metal scale. In this condition the fins retarded rather than enhanced heat transfer. The cause of this fin damage was secondary combustion in the convective section. This is an extremely common but frequently unrecognized problem. 

Draft gauge plugged CO2 in flue gas Fouled convective section Leaks in furnace skin Stuck stack damper Damage to furnace structure Smoke leaks out of convective section... 

When the combustion air was cut back this time, fire started to come out of the stack. Now operator A was forced to admit that they would have to increase the combustion air again. The problem they then faced was that the amount of draft in the heater seemed to be less than before and they were not quite able to re-establish the same air flow. The reason for the restricted air flow was that they had caused afterburn in the convective section and damaged the convective-section tubes, which now restricted the flow of flue gas and hence the flow of combustion air to the burners. 

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