Development of a Refrigerant Mixture Package for Dynamic Simulation of Auto-Cascade Refrigeration: A Case Study with R23/R134a

Abstract

The auto-cascade refrigeration cycle offers higherreliability and lower manufacturing costs compared to thecascade refrigeration cycle, making it particularlyattractive for applications involving significanttemperature differences between the heat source and sink.However, this advantage comes with increased complexity inits operation. This article aims to develop a media packagein Modelica to simulate auto-cascade refrigeration cycles,with the goal of enhancing understanding of their operationand control. The method is demonstrated using an R23/R134amixture, a commonly used refrigerant combination in suchapplications. The media package is compatible with theModelica Standard Library and is constructed using curvefits based on the REFPROP dataset. Unlike typicalrefrigeration systems, the media package for auto-cascadecomponents includes the independent mixture composition(Xi) as an additional variable in function calls. Acombination of polynomial and Chebyshev polynomialcurve-fits for the refrigerant properties has been shown toprovide an optimal balance between accuracy andcomputational efficiency. The article presents examplesimulations performed to demonstrate use of the mediapackage with component models from the Modelica.Fluidpackage. A lumped model for the phase separator isdeveloped and simulated with the R23/R134a media todemonstrate ideal phase separation.