Estudo experimental da performance termodinâmica de um ultrafreezer

Abstract

This work presents an experimental study of the thermodynamic performance of a Coldlab ultrafreezer, designed to operate at ultra-low temperatures down to -86 °C. To achieve this, a cascade refrigeration system was implemented in the equipment, consisting of two interdependent circuits that utilize R-404A (high-stage cycle) and R-23 (low-stage cycle) refrigerants. The general objective of this study was to evaluate the thermal performance of the system by monitoring variables such as pressure, temperature, and electrical power. For this purpose, the equipment was instrumented with thermocouples connected to a National Instruments data acquisition system, programmed using LabVIEW software. Operating pressures were measured with the aid of two manifold sets (analog and digital), allowing the reading of evaporation and condensation pressures for both cycles. The electrical powers of the compressors were recorded with a Minipa digital clamp wattmeter. Based on temperature and pressure data, and calculations of superheating and subcooling, pressure-enthalpy (P×h) diagrams were constructed using Coolpack software, enabling the determination of the coefficient of performance (COP) for each stage over time. The system reached temperatures around -55 °C during the determined test period; it is important to note that analysis of high simultaneous superheating and subcooling degrees indicated a low refrigerant charge. The analysis of COPs, correlated with electrical powers, demonstrated operational stability and satisfactory performance of the cascade cycle. Among other conclusions, the study reinforces the importance of replacing fluids with high global warming potential (GWP) with more sustainable alternatives, in compliance with environmental regulations and good engineering practices. As a contribution, this work proposes an experimental analysis methodology applicable to cascade refrigeration systems, focusing on energy efficiency and sustainability.