Formula & Engineering Notes
Boiler duty (heat added to the water):
hg is the saturated-vapour enthalpy at the steam pressure; hfw is the feedwater enthalpy, taken as hf at the feedwater temperature. Fuel energy = duty ÷ boiler efficiency.
All steam properties are interpolated from the shared IAPWS-IF97 steam-table datasets — see the full steam tables and the Steam Property Calculator.
Boiler duty is just an energy balance: you take water in at the feed temperature and hand back dry steam, and the duty is whatever energy bridges that gap, times the flow. The trap is forgetting the feedwater already carries heat — sizing on hg alone over-states the duty, sometimes badly, when the feed is hot.
The duty is the heat into the water. What you pay for is fuel, and the burner is never perfect, so the fuel energy is always larger — that's why the efficiency divisor matters. Quote duty to the mechanical team, fuel energy to whoever signs the gas bill.