Evaluation of pressure drop effect on COP of single-stage vapor compression refrigeration cycles

Abstract

This paper addresses a theoretical investigation about the effect of pressure drop along the heat exchangers on the coefficient of performance, heat transfer area and compressor capacity of a single-stage vapor compression refrigeration system. This analysis is performed based on a model of the complete system with one-dimensional heat exchangers, compressor and expansion device modelled as concentrated parameters. The fluid thermodynamic state along the heat exchangers is evaluated based on the energy and momentum balance considering predictive methods available in the open literature for heat transfer coefficient, pressure drop and void fraction. The compressor is modelled based on single isentropic efficiencies and the expansion device is assumed as adiabatic. Uniform wall temperatures are imposed in both heat exchangers to mimic heat removal from a refrigerated space and rejection to external ambient. Commercially available tube diameters were considered for the channel dimensions. Based on the analysis, it can be concluded that the pressure drop presents significant impact at the efficiency and sizing of the main components in a refrigeration system depending on the operational conditions. COP reductions of more than 15% for R600a and R134a were observed, as well as up to 29.2% increase of the heat exchanger area for the condenser. Correlations were proposed to prompt estimative of the impact of pressure drop on the COP and heat exchangers area.