Characterization of Refrigeration System Compressor Performance

The overall objective of this research is to identify a physics-based method to characterize compressor performance in refrigeration systems with limited experimental data.The focus of this project is on positive displacement compressors, i.e., reciprocating, rotary, scroll, and screw types, configured as either semi-hermetic or open drive.Compressor performance data for these types of compressors with different sizes were obtained from various manufacturers.One data set consisted of raw experimental data, while the others datasets were based on published catalog data.Mass flow estimates are based on the polytropic compression process with a clearance volume that leads to a volumetric efficiency expression.The overall performance of the model was acceptable with maximum average mean weighted errors of 3.7%, 2.3%, and 0.6% for reciprocating, scroll, and screw compressors, respectively.Furthermore, it was found that the mass flow rate model with parameters estimated using data for one refrigerant accurately predicted data for a different refrigerant.The compressor power requirement is also based on the polytropic model with the introduction of a combined efficiency to account for frictional effects, leakage, and motor performance in hermetic units.Comparisons of the predicted power requirements with data showed that the model fell short of predicting compressor power performance within acceptable accuracy.Average mean weighted errors over a range of operating conditions were 8% for the screw, 7.6% for the scroll, 6.4% for the open-drive reciprocating, and 5% for the semi-hermetic reciprocating compressors.Errors of as much as 40% were observed for some operating conditions.NOMENCLATURE C: clearance volume fraction P discharge : discharge pressure d, e, f: parameters in Eqn (5) P suction : suction pressure k: isentropic index RPM: compressor speed m & : refrigerant mass flow rate v suction : specific volume at suction conditions n: polytropic coefficient V: compressor displacement volume N: number of data points w: compressor power per unit mass flow OF: objective function defined in Eqn (1) ∆p: parameter defined in Eqn (3) P evaporation : evaporator pressure η combined : efficiency factor defined in Eqn (4)