Heating Values

When a fuel is burned in oxygen and water is one of the combustion products, the adiabatic flame temperature is high enough that the water is in the vapor phase. In many heat exchangers the final temperature of the combustion products (their exit temperature from the heat exchanger) is still higher than the boiling point of water and the heat of transformation of the vapor is lost to the atmosphere. This reduces the "heating value" of the fuel to its "Low Heating Value".

If the water vapor created in the combustion reaction is condensed, the heat of transformation (condensation) can be recovered and the energy obtained from the combustion process is increased. These conditions yield the "High Heating Value" of the fuel