In traditional boilers, the exhaust gas temperature is generally 160 ~ 250 C, and the steam in the flue gas is still in an overheated state, so it is impossible to condense into liquid water and emit latent heat of vaporization. As we all know, the thermal efficiency of boiler is calculated by the low calorific value of fuel, without considering the heat loss of the latent heat of vaporization in the high calorific value of fuel. So the thermal efficiency of traditional boilers can only reach 87%~91%. The condensing waste heat recovery boiler reduces the exhaust gas temperature to 50 - 70 C, fully recovering the sensible heat in the flue gas and the latent heat of condensation of water vapor. The volume composition of water vapor in flue gas of condensation waste heat recovery boilers fueled with natural gas is generally 15%~19%, and that of oil-fired boilers is 10%~12%, which is higher than that of coal-fired boilers below 6%. At present, the thermal efficiency of boilers is calculated by low calorific value. Although the nominal thermal efficiency is higher, the actual energy utilization rate needs to be improved because of the difference of about 10% between the high calorific value and the low calorific value of natural gas. In order to make full use of energy, reduce the exhaust temperature and recover the physical heat energy of flue gas, when the wall temperature of heat exchanger is lower than the dew point temperature of flue gas, the water vapor in flue gas will be condensed, release latent heat, and 10% of the difference between high and low calorific value can be effectively utilized.