Research On Heat Transfer Characteristics And Application Of Thermosyphon Heat Recovery Devices

For energy recovery in an air-conditioning system, a thermosyphon heat exchanger is a kind of good heat recovery device with the advantages of high effectiveness , no leakage and cross-contamination between the exhaust and fresh air flows, low cost but high reliability, no moving parts, flexible installation position, and no auxiliary facilities. It has a good prospect for heat recovery to apply in hospitals, shopping mall and natatorium etc, where cross-contamination is not allowed or a high ventilation requirement is necessary. Based on the study of heat transfer performance in atmospheric temperature, the paper is aimed to develop and high effectiveness thermosyphon heat recovery technology suitable to application in air-conditioning system. By theoretical analysis and experimental research, the heat transfer characteristics of thermosyphon and its heat exchanger and their structure parameters optimization have been studied, and the thermophysical properties of the selected thermosyphon refrigerants with zero ozone depeletion potential and the performance of the thermosyphon with different refrigerant have been calculated. Also, the optimal operation mode and the annual running benefit have been analysed for thermosyphon heat recovery unit. The research work mainly includes as follows:(1) The thermal resistance model for a thermosyphon was established, and the heat transfer characteristics of thermosyphon were analyzed based on the model. The influences of the geometric construction, filling ratio, inclination of the thermosyphon on its performance were discussed. The heat transfer process of the thermosyphon heat exchanger was numerically calculated by the the discrete model. The influences of the filling ratio, inclination and tube rows of the thermosyphon on the heat transfer of the heat exchanger were presented. The thermosyphon and its heat exchanger were experimented in detail and the measured data were verified by the simulated ones. The favorable parameters of thermosyphon heat exchanger, such as size, filling ratio, inclination, tube rows and face velocity etc, were obtained for the temperature range of air conditioning.(2) The optimized principle of the working fluid for the thermosyphon was presented. According to the principle of complementary advantages, the designed projects of the component ratio for binary and ternary mixtures of low boiling point organic refrigerant have been proposed. Through heat transfer simulation of the heat transfer coefficient and temperature effectiveness are favorable for the thermosyphon heat exchanger charged the binary mixture of R32 and R245fa with weight component ratio of 50% and 50% and the ternary mixture of R32, R290 and R245fa with the ratio of 30%, 50% and 20% both in the winter and summer conditions, respectively.(3) A prototype of thermosyphon heat recovery unit was developed. By monitoring the operation parameters of the prototype in practical application, the matching relationship between the inclination of the prototype and its outdoor amient temperature were investigated, and the optimal incline angle of the prototype with higher temperature effectiveness was found out under different working conditions. A control system to regulate the inclination mechanism was designed with PLC, makes the prototype run automatically in the reasonable operation mode under different ambient conditions and achieves a maximal energy-saving effect.(4) The seasonal temperature effectiveness was proposed in order to evaluate the annual or seasonal heat recovery effect of a thermosyphon heat exchanger, which referred to the calculation method of SEER for the air-conditioner in GB/T7725, Chinese national standard. Its calculation formula was derived. The annual heat recovery effect of the prototype was analyzed by the proposed method of the seasonal temperature effectiveness, while static economic evaluation method is applied in economic benefit analysis of the prototype. The analyzed results demonstrate that the seasonal temperature effectiveness of the prototype is 66.08% in winter, and the seasonal temperature effectiveness is 55.43% in summer. And the payback period of the prototype is about two and a half years.