To develop an excellent heat transfer element under the vacuum condition, experiments about the heat transfer performance of horizontal tube bundles of different materials under various vacuum conditions were carried out, including the stainless steel tube, the brass tube, the Ni-based implanted steel tube and the ion implanted brass tube. The relative trends show that the condensation heat transfer coefficient and the overall heat transfer coefficient of bundles of four materials all increase with the vacuum degree, especially, those of the Ni-based implanted steel tube and the ion implanted brass tube. Under a high vacuum condition (0.07 MPa), the condensation heat transfer coefficient of the Ni-based implanted steel tube bundle is about 1.4 times of that of the stainless steel tube bundle, the condensation heat transfer coefficient of the ion implanted brass tube bundle is found to be about 1.3 times of that of the common brass tube bundle. Therefore, according to the condensation heat transfer characteristics studied under high vacuum conditions, it is believed that a dropwise condensation is partly achieved on the surface of these two implanted tube bundles, and the ion implantation is shown to be an effective method to achieve the dropwise condensation. Based on this study, it is believed that the Ni-based steel tube may replace the brass tube, which is more expensive as a heat transfer component.
In order to study how deep we can reduce the temperature of exhaust flue gas, an experiment focusing on the combined influence of ash deposition and acid condensation on the heat transfer characteristics of the heat exchanger was carried out in a 300-MW boiler unit. An annular tube was inserted into the flue gas duct between the air preheater and the electrostatic precipitator. The water with given temperature firstly entered the inner tube and then flowed into the outer tube at the end of the inner tube. It is found that heat transfer performance drops sharply at fixed temperature range under given coal quality. The turning temperature is defined as the engineering acid dew temperature(EADT). When the inlet water temperature is under the EADT, ash sticks to the tube and is difficult to blow off, and thus the flow resistance rises. Furthermore, the corrosion can be observed obviously. From the point of economy and reliability, EADT is the limitation for flue gas heat recovery. The EADT is at least 30 °C lower than ADT calculated by traditional empirical and theoretical equations.
Acid condensation rate is an important factor denoting the acid corrosion, and the reduction of the acid condensation can significantly relieve the acid corrosive effect on the wall surface and improve the security of the equipments. In this study, the characteristics of both heat transfer and acid condensation of the finned tube in heat exchanger were numerically studied. In the numerical model, we simulated the acid condensation by considering the vapor–liquid equilibrium effect and multi-component diffusion effect. Based on the H-type finned oval tube, we proposed three novel types of fins to both enhance the heat transfer and reduce the acid condensation. The parametric effects of gas temperature, acid vapor concentration, water vapor concentration, and Reynolds number were investigated on different fin structures. The results show that the tube bank with the new structured fins can improve the performance on both heat transfer and acid anticondensation.
