Shift in composition of zeotropic mixtures in two phase regions of refrigerating circuits

A calculation procedure has been proposed for finding the shift from the composition of the charged refrigerant to the flow composition of the same fluid during the operation of the refrigerating circuit; this composition shift arises with zeotropic mixtures as an effect of the slip velocity between liquid and vapour during condensation and evaporation. Some diagrams are presented showing the effect of the more significant operational variables on the composition shift as well as the impact of the correlation chosen for predicting the void fraction. Experimental data concerning the composition shift, previously measured by the present authors, are compared with theoretical data drawn from simulation models of the condenser and the evaporator used in the testing rig. The results of this comparison seem to confirm the general analysis proposed in the paper: the Hughmark correlation for the void fraction appears as the most reliable; the composition shift occurring with the zeotropic mixtures on the market does not seem to be a problem with the current technology of refrigerating installations.