Heat Transfer And Flow Dynamics Of Condensing Heat Exchanger Applied In Gas-fired Boiler

With the adjustment of energy structure in China and the rising of social concern about the atmospheric haze, gas-fired boiler is gradually replacing the wide ues of coal-fired boiler in heat supply at urban areas. Broad utilization of gas and great reduction of heat loss and emissions due to exhaust gas become a major topic in the study of energy conservation and environmental protection work. Generally, the flue gas of gas-fired boiler contains as much as 15 to 20 percent of water vapor, and the temperature of flue gas is always above 150 ℃. The exhaust gas contains huge amounts of sensible heat and latent heat. Global energy efficiency could be effectively improved through the waste heat recovery.Condensing heat exchanger is the essential equipment for waste heat recovery of gas-fired boiler. Thermal coefficient and pressure drop characteristic of gas side directly impact the energy utilization of the boiler system. Investigations on the gas side thermal coefficient and resistance characteristic is of vital significance in improving the performance of the condensing heat exchanger and the efficiency of heat exchanging.This study aims at the condensing heat recovery of the gas-fired boiler of a factory in zhuzhou. Firstly, the flue gas composition of natural gas was analyzed and the condensing heat recovery potential for exhaust of the gas boiler was calculated. Following that, the size of heat exchanger was designed depending on thermal load of heat exchanger. ANSYS FLUENT is adopted in the numerical simulation for its relatively good performance. According to the flue gas flow characteristics, a mixture model is chosen for multiphase flow simulation. The heat and mass transfer of condensed phase transition is simulated by compiling quality of the source term and energy source term in UDF. Heat exchanger and flow resistance under dry conditions and wet conditions are respectively analyzed and the effects on heat transfer coefficient and pressure drop of the flue gas of the condensing heat exchanger at different inlet flue gas parameters are compared.By analyzing the numerical simulation results, it could be found that thermal coefficient under condensation condition is higher than that under on-condensation condition. The former can be 107.66 percent higher than the later when the velocity of flue gas is 3m/s. As the inlet water temperature rises, the gas side thermal coefficient decreases obviously, and the pressure drop is unconspicuous. As the velocity of the flue gas increases, the gas side pressure drop increases parabolically, and the thermal coefficient increases linearly. The increase rate of pressure drop is evidently faster than the rate of thermal coefficient. There is an optimal value for flue gas velocity when improving the comprehensive performance of the condensation heat exchanger, which is confirmed with the value of 3m/s in the present work. The increase of water vapor content causes the promotion of water vapor partial pressure, which causes condensation. More condensation water and latent heat could be obtained and the thermal recovery efficiency is improved.This study could provide a valuable reference for the optimization and improvement of condensing heat exchanger.