Experimental evaluation of a cascade refrigeration system prototype with CO2 and NH3 for freezing process applications

A prototype of a cascade refrigeration system using NH3 and CO2 as refrigerants has been designed and built. The prototype is used to supply a 9 kW refrigeration capacity horizontal plate freezer at an evaporating temperature of −50 °C as design conditions. The prototype includes a specific control system and a data acq*uisition system. The experimental evaluation started with the real conditions within the design operating parameters. Subsequently, several tests were performed fixing four CO2 evaporating temperatures (−50, −45, −40 and −35 °C). At each one of the evaporating temperatures evaluated, the CO2 condensing temperature was varied from −17.5 to −7.5 °C and an experimental optimum value of CO2 condensing temperature was determined. The discussions on the experimental results include the influence of the operating parameters on the cascade system’s performance. In addition, the experimental results are compared with two common double stage refrigeration systems using NH3 as refrigerant.