How do you calculate and improve fired heater efficiency?

Efficiency = heat absorbed by process / heat released by fuel. Losses include: stack losses (largest - sensible heat in flue gas), unburned fuel, radiation/convection from casing, and blowdown. Stack loss depends on excess air and stack temperature - measured via O2 and temperature. Improvement methods: reduce excess air to 10-15% (O2 analyzer control), lower stack temperature with convection section/air preheater (limit to acid dew point), insulate casing, and use heat recovery from flue gas. Well-designed heaters achieve 85-93% efficiency.