Determination of heat transfer coefficient in cooling-freezing tunnels using experimental time-temperature data

Abstract

In industrial freezing tunnels, knowledge of the spatial distribution of convective heat transfer (h) is crucial in predicting process times. The objective of this study was to develop an appropriate and simple methodology to determine the spatial distribution of h values inside tunnels. A dedicated device, consisting of a rectangular metal tank filled with an alcoholic solution, with a temperature sensor inserted in its interior, was used as a model system. The transient temperatures of the system (tank solution) were used to calculate h values at different positions inside the tunnels. The main result of this paper is the experimental method itself, which is a reproducible and reliable procedure for determining heat transfer coefficients inside industrial tunnels. The method is sensitive enough to determine spatial heterogeneities and is versatile with respect to the system geometry, which makes it a useful and simple tool to carry out diagnostic tests in industrial plants. In cooling/freezing tunnels, it is important to have convective heat transfer conditions that are spatially homogeneous, leading to homogeneous cooling/freezing processes. This is not obvious in industrial tunnels because of the arrangement of packages and fan positions. Tunnel operational conditions and layout lead to a specific airflow pattern, creating zones of high and low air flux that leads to zones of high and low convective heat transfer (h) values. This is the case for industrial tunnels used to freeze pack poultry cuts. The method proposed in this work is a useful tool to detect h value heterogeneities and to determine the average h value of preexisting tunnels. Such diagnoses can support decisions regarding layout changes or changes in the amount and positions of the fans in a tunnel. Computational fluid dynamics can also be used, but this provides more realistic data if actual h values are used. © 2007, Blackwell Publishing.