Analytical and experimental study on operating characteristics of a closed-cycle turbo-refrigerator for deep freezing

Abstract

Compared to open-cycle turbo-refrigerators that have been extensively studied, very limited research has been done on the operating characteristics of closed-cycle turbo-refrigerators for deep freezing. Particularly, working fluid charge, as a critical operating condition, has not been considered in existing studies, and therefore a more comprehensive study on the operating characteristics of the closed-cycle turbo-refrigerator is needed. In this work, a system performance model was developed for the first time considering the working fluid charge in turbo-refrigerators, providing a great advantage in fully characterizing the system off-design operations, and a newly designed prototype for deep freezing at −120 °C was tested for model validation. Furthermore, a novel optimized operating strategy combining variable speed and working fluid charge was proposed to improve system performance. The tests demonstrated the prototype’s adaptability to a refrigeration temperature range from −125.8 °C to −62.9 °C and showed a system efficiency of 0.079 corresponding to the design point. Test data also validated the proposed model well with deviations below 15 % throughout the operating range. Further analysis shows that active speed control significantly expands the system operating range, while system efficiency penalties are imposed when dealing with reduced cooling capacities (i.e., partial-load demands) at given refrigeration temperatures by lowering the speed. Reducing the working fluid charge was found to increase system efficiency, and reason for the system efficiency improvement was revealed to be an increase in recuperator effectiveness. Combined increasing speed with reducing working fluid charge is suggested as an optimized operational strategy, especially for partial-load applications with reduced cooling capacity. By using the optimized strategy, a 30.5 % increase in system efficiency can be achieved for 15 %-reduced cooling at −120 °C.