The Research Of Spray Heat Exchanger In Treated Sewage-Raw Sewage Heat Pump

Fouling occurs when sewage goes through the conventional submerged-tube heat exchanger and the shell and tube heat exchanger. This in turn, can lead to a decrease in the overall heat transfer coefficient and efficiency of heat exchanger. Due to the deficiencies, a spray heat exchanger was developed for Sewage Source Heat Pump System (SSHPS). The optimal structure of heat exchanger was discussed in consideration of the abundant effects caused by the impurities and the fouling of the sewage. Then, the SSHPS with the spray heat exchanger was used in a pharmaceutical manufacturer in Harbin. The heat of the treated sewage was taken to increase the temperature of the raw sewage for the biological treatment in low-temperature environment. It was considered as an effective and practical technique for the sewage treatment in cold region and in winter.In this paper, the spray heat exchanger and the treated sewage - raw sewage heat pump system with the spray heat exchanger were discussed. Firstly, a numerical model to predict the gravity-driven flow of liquid films in horizontal tube and the thermal characters were developed in the surface-fitted coordinate systems. The methods of the variable transform and the dimensionless function were used. The irregular solving domain, resulted from the changes of the liquid film thickness, was transformed to the regular rectangle solving domain by the methods. The two-dimension model of the exchanger was solved numerically through MATLAB codes. Moreover, the characters of the flow and heat transfer of the falling film were discussed.Secondly, the impact of the flow patterns and the tube's placement and spacing on the liquid film thickness distribution were studied. The film-distribution was the main state in the liquid film flow. The thicknesses of the liquid film reached the maximum on the top and bottom of the tube and reached the minimum on the middle region of tube surface. Furthermore, the results shows that the horizontal spacing maked less impact on the thickness of the liquid film. The improved predictor on dimensionless thickness of the liquid film was derived. The results by correlating on data value showed in close agreement with the experimental data of the experiment on the falling film of the horizontal tube obtained by Tianjin University and another reported experimental data.Thirdly, the flow velocity and temperature distribution and the Nusselt number variation of the liquid film were obtained. Moreover, the parameters impacted on the flow velocity, the temperature and the heat transfer were discussed. The spray density and viscosity were the key factors associated with the flow velocity of the liquid film. With a constant spray density of 0.148kg/(m.s), the mean velocity of the liquid film can reach the optimal value of 0.3m/s as the spray heat exchanger used the high-viscosity fluid without the external power pumping. It was an important technique for reducing the energy consumption and cost on pumping in contrast to the shell and tube heat exchanger. The diameter of the tube had less impact on the mean velocity of the liquid film. However, the reduction of the diameter of the tube resulted in decrease of the flow stability and the fluctuation of the liquid film. The temperature of the liquid film varied violently with the circular angle near the surface of the tube, and the temperature tended to be accordance near the free surface of the liquid film. The dimensionless Nusselt Number and the improved correlating equation were used to predict the heat transfer of the liquid film. Moreover, the stability, the fluctuation and the fracture character of the liquid film on the tube were discussed. At the same contact angle, the larger the flux of the sewage was, the harder the liquid film was fractured. At the same heat transfer, the bigger contract angle was, the easier the liquid film was fractured. The material of the tube also related to the stability of the liquid film: The tube made of the titanium was superior in stability to the stainless steel and the red copper. This in turn, the methods to improve the stability for the liquid film on the tube were derived.Fourthly, the distributed parameter model for spray heat exchanger and heat pump system were developed to predict the performance of the SSHPS of the pharmaceutical manufacturer in Harbin. The thermal performance of the three-level spray heat exchanger was predicted and studied. Moreover, the impact of the thermal resistance of fouling occurred on the thermal performance of SSHPS was discussed. As less fouling occurred, the overall heat transfer coefficient of three-level shell and tube heat exchanger was even larger than the three-level spray heat exchanger. However, Effects of fouling were more abundant in the shell and tube heat exchanger, than in the spray heat exchanger. The increment of thermal resistance caused much more decrease of the overall heat transfer coefficient of the shell and tube heat exchanger than the spray heat exchanger. So did the speed of the reduction. When compared versus each other, the spray heat exchanger was generally more efficient than the shell and tube heat exchanger etc using heat from the sewage, especially the raw sewage with the impurities.Fifthly, the circular tube was replaced with the elliptical tube to improving the heat transfer coefficient in the spray heat exchangers. The flow velocity distribution, the temperature distribution, the dimensionless thickness of the liquid film and the dimensionless Nusselt Number based correlating equation for the elliptical tube were obtained. The results showed that the increment of shape factor of a/b resulted in the decrease of mean thickness of the liquid film and the increment of the Nusselt Number. The decrease of the thickness of the liquid film caused the increment of the Nusselt Number trended to slow down smoothly. At the condition of a/b>1.8, there were less variation of the Nusselt Number with the value of a/b. The area with high film heat transfer coefficient in the elliptical tube was 65% of the half-perimeter of the elliptical tube at a/b=2. However, the area with high film heat transfer coefficient in the circular tube was only 48% of the half-perimeter of the circular tube. With the same perimeter, the more area with high film heat transfer coefficient in a tube leaded to the better thermal performance for the improvement of heat transfer. Therefore, it was recommended for improving the heat transfer of the exchanger that the optimal shape factor was from 1 to 1.8 and the form of the tube was ellipse.Our research was one of the sub-projects of Study on the key technique and demonstration of high-efficiency water source heat pump system, supported by National Eleventh-Five Year Technological Supporting Projects from China Ministry of Science and Technology (MOST) and coded as 2006BAJ01A06. The technique is also useful for analyzing and improving the heat transfer of falling film on the spray heat exchanger, for optimizing the performance of the SSHPS and for using the heat of the sewage in beer factory, hospital, natatorium etc.