Design And Research Of Two-stage Low Temperature Flue Gas Heat Recovery Device

Many combustion devices discharge flue gas in their working process. For example, a large number of gas boilers are used to heat water in large and medium-sized cities in northern China. The temperature of boilers’flue gas is about 100℃ (called low-temperature flue gas). The direct discharging of the flue gas will cause great heat loss. If this waste heat is recovered and utilized effectively, it will help to improve the overall energy efficiency of the combustion devices (such as boilers), which has important application value.Firstly, a two-stage scheme is designed to recover the waste heat of low-temperature flue gas, and it uses fin-and-tube heat exchanger and the flue gas-water heat pump working in tandem. The low temperature waste heat (including the sensible heat and latent heat) is deeply recovered to heat the heating backwater. The fin-and-tube heat exchanger is used for primary recovery, and the heat pump is used for secondary recovery. In the present design the flue gas temperature is 130℃ and the heat load is 70 kW. The performance experiment of primary fin-and-tube heat exchanger is carried out and useful heat transfer and pressure drop data are obtained, which agree to calculating results.Secondly, the dynamic model of flue gas-water heat pump system is established, and the corresponding simulation software is developed. The response characteristics of the device under typical dynamic conditions (start condition and disturbance condition) can be obtained from the simulation. The results show that, the start condition is strong dynamic process. Evaporating pressure and condensing pressure approach stable in about 100s (the fast response time is 45s), evaporation temperature and condensation temperature take about 480s to reach the steady state (the fast response time is 200s). The disturbance conditions are generally weak dynamic process. Improving flue gas’s inlet temperature and mass flow rate or improving the cooling water inlet temperature can improve the heating water temperature. The response time is about 300s. The development of dynamic simulation software can also be used to analyze the performance under different steady conditions.Finally, in order to reduce pressure drop in the flue gas side of the fin-and-tube heat exchanger, the author uses elliptical tube-based fin-and-tube heat exchanger instead of round tube-based fin-and-tube heat exchanger. The flow and heat transfer characteristics at the gas side of elliptical tube-based and round tube-based fin-and-tube heat transfer units are simulated. Standard k-ε turbulence model is used and the frontal air velocity is in the range of 1-10m/s. Comparative analysis of the results show that the heat transfer rate difference between elliptical tube-based and round tube-based fin-and-tube heat transfer units is insignificant (the former is slightly greater), but the air side resistance of elliptical tube-based heat transfer unit is significantly lower than that of round tube-based fin-and-tube heat transfer unit. The higher the frontal air velocity is, the more obvious the advantage is. The effects of fin pitch and tube spacing along the direction perpendicular to air flow on the heat transfer and pressure drop for elliptical tube-based heat transfer units are also documented.