Infrared thermography as a complementary tool for the evaluation of heat transfer in the freezing of fruit juice model solutions

Abstract

The heat transfer process during the freezing of 600 kg of fruit juice model solutions in common containers (boxes, buckets and metallic drums), and different settings in a freezing tunnel, was studied. The air velocity was measured at several points in the entire tunnel. Thermocouples were installed to monitor the temperature profiles within the solution, at the packaging surface and cooling air. To measure the experimental effective heat transfer coefficients conventional temperature measurements with thermocouples and infrared thermography technology were used to map the distribution of the coefficients throughout the surface. Energy consumption involved in each configuration was evaluated. The higher velocities occurred at greater height (above the stacking and drums), being possible to verify the existence of preferential airflow pathways near the door and at the tunnel bottom. The highest air velocities observed were 2.85 m s−1; 2.72 m s−1, and 2.62 m s−1 for drums, boxes and buckets, respectively. The movement of the freezing front has begun from the outermost containers toward those located in the center of the stacks and the average freezing time was 51 h (plastic boxes), 55 h (plastic buckets) and 102 h for metal drums. The energy consumption for drums has been almost the double when comparing with buckets and boxes. The distribution of the local convective heat coefficients throughout the freezing process was not constant. Variations and different intensities of scatter were observed for the different packaging configurations and for the different periods during the freezing process (precooling, phase change and tempering). Thermal imaging technology proved useful in the study of heat transfer coefficients, allowing their complete mapping on the surface of the packaging, without the necessity of direct contact with the product.