Experiences from Experimental Investigation of an R744 Dual Evaporator Automotive A/C System

This paper presents experimental data from an investigation of an R744 dual evaporator system for automobile applications in a breadboard laboratory. Several expansion devices combinations were investigated, focusing on comparing fixed area and controlled area expansion devices. The role of an accumulator in an R744 dual evaporator system is demonstrated by experiments conducted with a prototype accumulator in which the outflow of liquid R744 and oil is controllable. Contrary to a conventional single R744 evaporator system, the refrigerant outlet qualities in a dual evaporator system are not fixed by the accumulator inlet/exit quality in steady state. An accumulator in an R744 dual evaporator system only guarantees that the quality of the mixed flow downstream of the evaporators has the same quality as the inlet/exit quality of the accumulator. Therefore, to have equal refrigerant exit qualities at each evaporator it is necessary to adjust the refrigerant mass flow rate distribution through the evaporators by changing the expansion valve openings. From the conclusions of the expansion devices and accumulator investigations, a control strategy is presented based on two control objectives. The primary objective is to adjust the expansion valves to achieve the required high side pressure, while the secondary objective is to adjust the expansion valves to avoid superheat or match superheats at the evaporators. Transient experiments with reduced airflow rates at one evaporator are analyzed. The results show improved performance can be achieved by changing the expansion valve openings based on the control objectives mentioned above.