R744, LMTD, LMED
Studies show that the performance of transcritical R744 systems is highly sensitive to the high side pressure in the gas cooler. Hence use of appropriate strategy to control the high side pressure is essential to extract the best performance from these systems. In the present study, a R744 based air conditioning cycle with two expansion valves is considered for proper control of high side pressure and quality of refrigerant at evaporator exit. A system simulation model is developed for this cycle considering detailed model for each component. Both the gas cooler and evaporator considered here are crossflow heat exchangers with spiral fin and tube geometry. A discretized approach is considered for developing the models for gas cooler as well as evaporator. Elemental Log Mean Temperature Difference (LMTD) and Log Mean Enthalpy Difference (LMED) approaches are used to model the gas cooler and evaporator, respectively (Yin et al., 2001a; Threlkeld, 1970a). An empirical model proposed by Brown et al. (2002a) is considered for modelling the reciprocating compressor. Expansion process in the valves is assumed to be isenthalpic. Using the developed model, the effects of variation of various important environmental parameters on system performance is analyzed. From the results obtained, optimum operating conditions are identified for which the system attains maximum COP. It is expected that this study will be beneficial in the design and development of suitable control strategies for small air conditioners based on transcritical CO2 cycles. References Brown, J. S., Yana-Motta, S. F., Domanski, P. A. (2002). Comparitive analysis of an automotive air conditioning systems operating with CO2 and R134a. Int. J. Refrig., 25, 19â€“32. Threlkeld, J. L. (1970). Thermal Environmental Engineering, Prentice-Hall, Inc., New York, NY. Yin, J. M., Bullard, C. B., Hrnjak, P. S. (2001). R-744 gas cooler model development and validation,Â Int. J. Refrig., 24(7), 692-701.