Performance study of thermostatic and electronic expansion valves of a water-cooled scroll chiller

Electronic expansion valves are progressively substituting conventional expansion devices such as capillary tubes and thermostatic expansion valves. Despite of higher initial cost, electronic expansion valve offers greater benefits that include wide range of capacity coverage, precise superheat control and optimum refrigerant flow rate. In this paper, the performance characteristics for both thermostatic and electronic expansion valve were evaluated. An externally equalized thermostatic expansion valve and a pulse width modulating electronic expansion were installed in parallel on a 15.5 kW water cooled scroll chiller. The performance ratings of the expansion valves were investigated at various set points that cover a wide range of operating conditions. A total of 27 different operating steady state set points were studied by manipulating the leaving chilled water temperatures, T_out,evap (6.7°C, 8.0°C, and 9.5°C), entering cooling water temperatures, T_in,cond (28°C, 29.4°C, and 31.0°C) and the compressor frequencies (50 Hz, 42.5 Hz and 35 Hz). The required water temperatures were achieved using mixing chambers and regulated to within 0.15°C of the desired set points with the aid of PID controlled modulating valves. As for the compressor frequency modulation, a variable speed drive was retrofitted to the system to study the part load performances. The performances of the valves were reviewed from the perspective of superheat control, cooling load, heat exchangers' effectiveness and internal entropy generation.