Conceptual global optimal bounds for COP of Cascaded-Flashed R134a irreversible refrigeration system

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.

Present study of the flash-mixed two-stage cascade R134a refrigeration system has considered the optimal COP cycle characteristics over whole ranges of all cycle controlling parameters, with consideration of pressure and thermal losses in all units and connecting lines of cycle. Application of Monte Carlo Method (MCM) has proved to be quite effective in optimizing cycle performance. COP, cooling power Qin, total input power Wt and second law efficiency ŋII are used as cycle performance parameters. Present COP and ŋII results have been shown to compare well with other published results. COP hyperbolically decreases with Wt. Higher COP values at lower Wt values exemplify the favorable use of this cycle in small-scale units. The study shows a parabolic relationship between COP and Qin. Higher COP values at higher loads favor the use of this cycle for cooling purposes. Values of COP increase with ηII, justifying that reductions in cycle irreversibilities result in enhancement of COP. Systems having intermediate pressures that are about equal to geometric mean of terminal pressures as well as systems with about equal mass flow rates in upper and lower cycles result in optimum values of COP. This concludes that better systems have their units of comparable sizes. MCM analysis shows that both evaporative temperature t8 and inlet superheated temperatures to LP compressor tsup have monotonic effects on conceptual optimal upper-enveloped COP bounds, with their optimum normalized values are both in the range 0.4 to 0.8 of their variation ranges of 265-284.4°K, 265.5-293.9°K respectively. COP is not sensitive to the cycle highest temperature T4 as long as it is below about 332.5°K, beyond which COP suffers very sharp decrease. This is in favor of using reasonable pressure ratio of HP compressor. Fitted equations are developed to relate upper bounds of COP with these three temperatures.