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On a refinery crude unit the furnace inlet temperature can typically degrade at the rate of 5°C (9°F) per year owing to progressive fouling of the exchangers in the preheat train. In a 660 m3/h (100,000 bbl/day) refinery that represents an increased furnace duty of 2500 kW (8.5 MM BTU/hr) which, at today’s prices, would costs in the region of $200,000 per year.

Fouling rates can vary widely – one refinery reported a furnace inlet temperature drop of 8°C (15°F) over a single month. Unchecked that would have cost $3.6M per year.