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In traditional boilers, the exhaust gas temperature typically ranges from 160°C to 250°C, and the water vapor present in the flue gas remains in a superheated state. As a result, it cannot condense into liquid form, meaning the latent heat of vaporization is not recovered. It's widely understood that boiler efficiency is calculated using the lower heating value (LHV) of the fuel, which means the heat lost due to the evaporation of water vapor—part of the higher heating value (HHV)—is not accounted for. This is why conventional boilers usually achieve an efficiency of only 87% to 91%.
In contrast, condensing boilers are designed to reduce the exhaust gas temperature to between 50°C and 70°C. This allows them to capture both the sensible heat in the flue gas and the latent heat released when water vapor condenses. For natural gas-fired condensing boilers, the volume of water vapor in the flue gas is typically 15% to 19%, while oil-fired boilers have around 10% to 12%. These levels are significantly higher than those found in coal-fired boilers, where the water vapor content is generally much lower.
Currently, boiler efficiency is still primarily calculated based on the LHV. Even though the nominal efficiency may appear higher, the actual energy utilization can be improved by considering the difference between HHV and LHV, which is approximately 10% for natural gas. To maximize energy use, it’s essential to lower the flue gas temperature and recover the physical heat contained within it. When the surface temperature of the heat exchanger drops below the dew point of the flue gas, the water vapor condenses, releasing its latent heat. This process effectively captures the additional 10% of energy that would otherwise be lost, making condensing technology a more efficient and sustainable option.