Optimum performance of external intercooling two stage cycles with real compressor curves.

Two stage cycles have been used traditionally in the case of high temperature difference between heat sink and heat source in order to overcome the high pressure ratios that deteriorate compressor volumetric and isentropic efficiency thus needing huge swept volumes for giving the required cooling capacity and increasing the power consumption. Normally, intermediate cycle pressure is determined using geometric mean between evaporation and condensation pressure which gives optimum COP in the case of perfect gas assumption and same inlet vapor temperature to both compressors. These assumptions are not fulfilled in real conditions, so the objective of the paper is to study optimum intermediate pressures in two stage cycles with external subcooling. A model for two stage cycles with external subcooling is developed that takes into account volumetric and isentropic efficiency of compressors dependent on the pressure ratio. Compressor curves are taken from general expressions provided by Pierre and are tested against actual compressor performance from manufacturer catalog. First, COP dependence on intermediate pressure is calculated for standard low temperature conditions (-35°C evaporation temperature, 40°C condensation temperature) working with refrigerants R-404A and R-717 (Ammonia). Results show that optimum intermediate pressure is close to the arithmetic mean in the case of R-404A but there is a significant difference in the case of Ammonia. Also, it is compared the case of using constant compressor efficiencies vs. pressure ratio dependent compressor efficiencies. Results show that there are higher differences in the determination of optimum intermediate pressure, Next, the model is used in order to calculate optimum intermediate pressure dependent on evaporation and condensation temperature with a constant superheat and subcooling of 10 K and 5K respectively. Contour surface maps and analytical expressions giving optimum intermediate pressure dependent on evaporation and condensation temperature are presented. Results show that there is less than 1K equivalent saturation temperature difference between optimum intermediate pressure and geometric mean pressure for R-404A in all the map; for ammonia there are significant differences that depends mainly on the evaporation temperature. However, comparison between COP calculated at optimum intermediate pressure and at geometric mean pressure shows very low differences (less than 2%) for both refrigerants and therefore validating the use of the simple geometric mean equation for this type of cycles. Second main conclusion of this paper is that significant errors can be made when calculating optimum conditions in two stage cycles when using constant compressor efficiencies as used by many authors in the literature.