I agree. All tubular heat exchanger failures I have seen on any brand are in the first bend after the burners. Usually from an overfired condition, moisture or other contaminants in the combustion air etc.
Originally posted by Modman
So, my answer is there are too many things to consider. Tubes will fail on an over fired condition, Transitions will fail on bad airflow conditions, water will rust them out on under fired conditions.
And you want to know what will fail or is the most likely to have the highest stress? under what conditions? Thermal stresses are compensated for in the design.