Comparison of refrigeration cycles with natural refrigerants for very rapid food freezing

In the industrial refrigeration natural refrigerants are gaining more and more importance, mainly because of increasing environmental concerns. In this work food processing applications are considered, with particular reference to very rapid freezing refrigeration, for which a temperature of -70°C is needed. Two alternative technologies are presented: (i) an open air cycle freezer; (ii) a cascade cycle using CO2 or blends of CO2 and natural refrigerants as low temperature working fluid. A comparison between the two technologies is performed. The purpose of the paper is to outline the main features of each system, comparing them on a technical point of view on the basis of theoretical models and experimental results on existing prototype units.