Heat transfer and energy consumption in the freezing of guava pulp in large containers

The heat transfer process, including the convective transfer coefficients (h), freezing times and power consumption, was evaluated during the air blast freezing of 600 kg of guava pulp. Three packaging configurations were tested: plastic boxes (34 L), buckets (20 L) and metal drums (200 L). The air velocity inside the freezer tunnel was measured at several points, and sensors were installed to monitor the temperature. The heat infiltration from external ambient air was also verified. Correlations of the Nusselt number versus the Reynolds and Prandtl numbers were used to estimate the convective heat transfer coefficients according to the configuration of the systems. The coefficients were applied to freezing-time prediction models. The h values and consequent freezing time prediction were found to be more precise for the packaging in the bucket configuration. For the drum configuration, the correlation that considered the turbulence factor effects was found to be satisfactory. In the box configuration, the correlation produced good results only for the boxes located in the central area of the stack. The results indicate that the correlations may be applicable to the analysis and the differences explained by airflow and the difficulty of maintaining constant heat flow on the surfaces.