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Convection bank performance

Q: We have a 50 MMKcl/hr fuel oil-fired, natural draft, box-type furnace. It has a convection bank with a super-heater steam coil sandwiched in between process lines. It is also equipped with soot blowers and normally operates once every 24 hours. Suddenly, stack temperature went up and the furnace is firing is very hard. Please let me know how to confirm whether the convection bank is fouled up or not?

A: The condition described can be due to two reasons:

1. Incomplete combustion
2. Reduced heat transfer

To check for incomplete combustion, check the stack for O₂ and CO emissions. If the problem is not from incomplete combustion, then the problem is due to reduced heat transfer. Reduced heat transfer can be due to fouling either inside or outside the tubes. If the assumption can be made that fouling and has not occurred inside the tubes, it would be safe to assume that fouling has occurred on the outside of the tubes.