FACTOR ANALYSIS OF INTERNAL EXPANSION RATIO FOR SINGLE SCREW EXPANDERS

asme-orc2015 Tracking Number 23

For low temperature waste heat recovery, Organic Rankine Cycle (ORC) is generally considered the most promising choice in varieties of potential technologies, and become a hotspot of research and development in international academic and industrial fields. However, reviewing the research results related ORC system in recent years, the actual situation was not optimistic. There were many technical bottlenecks hindered the application of ORC, especially for small scale system. Among those problems, the performance of expander was the key issue, and how to improve it was two aspects. First one was improving the shaft efficiency of expander, and it was the common sense of the researchers in this field. Second one was controlling appropriate expansion ratio, and it was special requirement of small scale ORC system. Due to relative small expansion ratio, the thermal efficiency of ORC was low even if high efficiency of expanders. In actual conditions, the high efficiency and expansion ratio of expanders was hardly obtained simultaneously. So, it is necessary to carry out the special discussion about expansion ratio. Single screw expander was a type of volumetric prime mover. Due to special configuration, it had the potential of realized relative high expansion ratio. So, it makes possible getting high thermal efficiency of ORC. In this paper, we tried to analyze the influence factors of internal expansion ratio for single screw expanders. Firstly, the thermodynamic model of ORC was described, and the analysis of expansion ratio influenced cycle thermal efficiency was carried out. From the calculation results, it was found that thermal efficiency was increased with expansion ratio, and the increment speed was to increase rapidly first and then slow down gradually. Considering the actual efficiencies of expander and pumps, appropriate expansion ratio should be existed. Secondly, the analysis of the influence factors of internal expansion ratio for single screw expanders was carried out, from the aspects of configuration, process and condition. From the results, high expansion theoretically could be obtained by changing the configuration of screw and gaterotor, inlet and outlet structure. The maximum volumetric ratio could above 20, and it could completely cover the temperature range of low temperature waste heat recovery. However, in actual condition, different meshing and fit clearances would influence leakage and cause expansion ratio reduced. So, configuration design and clearance control were key issues to improve the expansion ratio of single screw expanders.