Shell boilers are often operated with feedwater temperatures lower than 100°C. Consequently the boiler is required to supply enthalpy to bring the water up to boiling point. Most boilers operate at pressures higher than atmospheric because steam at an elevated pressure carries more heat
There are only three numbers that go into the heat rate calculation so it should be simple right? But when you look at those three numbers a little closer several questions can come up. First the calculation: Heat Rate = Fuel Flow * Fuel Heating Value / Power Output. The first question is: What are the engineering units on these values? In the US Heat Rate is most often shown in Btu/kWh.