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In traditional boilers, the flue gas temperature typically ranges from 160°C to 250°C. At this temperature, the water vapor present in the exhaust remains in a superheated state and cannot condense into liquid form. As a result, the latent heat of vaporization is not recovered, leading to significant energy loss. It is well understood that the thermal efficiency of conventional boilers is calculated using the lower heating value (LHV) of the fuel, which does not account for the heat lost due to the evaporation of water vapor. This is why the efficiency of traditional boilers usually only reaches between 87% and 91%.
In contrast, condensing boilers are designed to lower the flue gas temperature to around 50°C to 70°C, allowing for the full recovery of both the sensible heat and the latent heat of condensation from the water vapor. The volume of water vapor in flue gas varies depending on the fuel type. For natural gas-fired condensing boilers, the water vapor content is generally between 15% and 19%, while for oil-fired boilers, it ranges from 10% to 12%. These levels are significantly higher than those found in coal-fired boiler exhaust gases.
Currently, boiler efficiency is still primarily calculated based on the lower heating value. Although the nominal efficiency may appear high, the actual energy utilization is limited by the difference between the higher heating value (HHV) and LHV, especially with natural gas, where the gap can be as much as 10%. To maximize energy use and reduce waste, it is essential to lower the flue gas temperature and recover the physical heat contained in the exhaust. When the surface temperature of the heat exchanger drops below the dew point of the flue gas, the water vapor begins to condense, releasing its latent heat. This process allows for the effective utilization of the 10% difference between HHV and LHV, greatly improving overall system efficiency.