The Research Of Spray Heat Exchanger Based On The Direct Sewage-source Heat Pump

Urban sewage, as an ideal kind of cold and heat source, has excellent temperature characteristic, large flow-rate, high heat capacity and other advantages. In the sewage-source heat pump application, on the one hand, direct sewage-source heat pump, which has great advantages in comparison with the indirect system, is the key point and direction in the development of sewage-source heat pump; on the other hand, sewage heat exchanger affects the performance of sewage-source heat pump heavily. In numerous sewage heat exchangers, spray heat exchanger has an open and simple structure, high film thermal efficiency, good operability in the case of serious scaling and other advantages. Therefore, the research of spray type sewage heat exchanger in the direct sewage-source heat pump has great significance.The application of spray type sewage heat exchange when used in the direct sewage-source heat pump is discussed from theory and experiment in this paper. The theory mainly introduces model of falling film outside tubes, analysis of heat and mass transfer during heat transfer of spray type sewage heat exchanger which also refers to its mathematical model, analysis of fouling formation and method of monitoring fouling thermal resistance. In experiment part, the test bench the spray sewage heat exchanger as evaporator of the direct sewage-source heat pump system is built. This paper defines the k as spray percentage. The experiment mainly researched the main influencing factors of the system can effect comparing(spray density, air inlet temperature and sewage temperature), the spray type sewage heat exchanger heat transfer main influencing factors(spray density, spray method and sewage temperature), the change law of fouling thermal resistance outside tubes with time changes. And the conclusion as follows:①Energy efficiency ratio increases with the increase of spray density. What’s more, there exists an optimized spray density that makes energy efficiency ratio will reach the maximum. And energy efficiency ratio will decrease when the spray density is high than the optimized spray density. Energy efficiency ratio decreases with the increase of inlet air temperature, and increases with the increase of sewage temperature.②Spray density mainly affected flow state outside heat exchanges tubes when the structure of spray type sewage heat exchanger is fixed. And flow state outside tubes will change from droplet state to columnar state with the increase of spray density. When the film formation device is uninstalled, it’s difficult to form complete droplet flow state. The droplet state will become most apparent when Γ is 0.053 kg/(m·s) and the k is about 155%. And the mixture of columnar and film state is the main state.③Heat transfer coefficient and heat exchange amount of spray type sewage heat exchanger increase with the increase of spray density before spray density reach the optimized spray density. Heat transfer coefficient and heat exchange amount of spray type sewage heat exchanger increase slowly even decreases with the increase of spray density after spray density reach the optimized spray density. In this experiment, the optimized spray density is about 0.055 kg/(m·s)and the k is about 160%.④The improvement of spray methods is helpful for enhancing the heat transfer. This experiment in spray type sewage heat exchanger set up in the middle of a spray tube can improve water spray uniformity, mainly to enhance the heat exchanger outside the heat exchange tube at the bottom of the water spray effect to enhance heat exchanger heat exchanger effect, this method has certain application value in engineering.⑤It froze outside tubes to varying degrees, which made heat resistance increased and influenced heat transfer, under different sewage temperature and spray density. In engineering application, in order to reduce freeze, when the water temperature is below 10 ℃, the spray percentage must be at least more than 120%, when the water temperature is 10 ℃, the spray percentage must be at least more than 90%, when the water temperature is more than 10 ℃, the spray percentage must be at least more than 60%, low temperature region for sewage should be appropriate increase the spray density.⑥Thermal resistance of fouling outside tubes showed a zigzag growth trend with the change of time. It barely exists the induction period. The system running stable after 70 h in a certain range, and its stable fouling resistance with the increase of water temperature increase⑦Based on the experimental data this paper convective heat-transfer coefficient of outside tubes, which play a guiding role for its engineering applications.