Control Strategy of Vapor Injection Cycle

Vapor refrigerant injection cycle with a flash tank has proven to be effective in improving system performance significantly at extremely low and high ambient temperatures. However, the control strategy of operating the system is still unclear. There is no open publication on the comprehensive study of the vapor injection cycle control strategy to date. This paper presents the experimental investigation on the control strategy of the vapor injection system using R410A as its working fluid. A prototype flash tank equipped with a flow visualization window was utilized to monitor and investigate the liquid-vapor separation, which guided the development of the system control strategy. The control of three valves was found to be critical to the reliable system operation: the injection port valve, the upper-stage expansion valve and the lower-stage expansion valve. The system was tested with different vapor refrigerant injection ratios under ASHRAE Standard Rating and Performance test conditions. Severe conditions of -17.8oC and 46.1oC were also used to evaluate the system behavior. The system was found to be operating steady with the liquid refrigerant in the flash tank maintaining a level of 40% to 60% of the tank height to ensure the reliable system operation.