Transferência de calor no congelamento de polpa de goiaba

• Autor(a) principal: Reno, Marciu José
• Data de Publicação: 2010
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: Repositório Institucional da UFLA
• Texto Completo: http://repositorio.ufla.br/jspui/handle/1/3259
• topic: Goiaba; Transferência de calor; Consumo energético; Energy consumption; Calor - Convecção; CNPQ_NÃO_INFORMADO

This study evaluated the heat transfer by forced convection flow inside a circular tube during the freezing process of guava pulp in batches. The convection heat-transfer coefficients, the freezing time and the energy requirements were also evaluated. For the experiment, 600 kg of guava pulp were frozen, using three common packaging configurations all of them stacked on pallets. In the freezing tunnel, one the configuration settings was made of 40 plastic boxes (HDPE), each containing 15kg of guava pulp filled in polyethylene bags The second configuration had 40 plastic buckets (HDPE) containing 15kg of guava pulp each and in the third configuration, 3steel drums were lined up laterally, each of them with 200 kg capacity. The temperature within all packaging was monitored though sensors placed in the samples, in strategic points in order to determine the heat transfers. It were used the thermocouples and resistive thermal devices (RTDs) sensors connected to a data acquisition system to monitor the temperature. The air velocity in the tunnel was measured with a hot wire anemometer. The energy consumed throughout the procedures was also measured. The establishment of coordinate systems designed to specify the position of both packaging and sensors inside the tunnel allowed the airflow parameters and heat transfers analyzes through the contour plots using the least square regression. The turbulence and the Nusselt number correlation as a function of the Reynolds and Prandlt numbers were used to determine the convective heat transfer coefficients according to the configuration of the systems. The coefficients were applied to the freezing time prediction using models found in the literature review. In all configurations, the samples froze more rapidly in the periphery of the arrangements, with the highest temperatures located in the central region of the stacking. It was possible to observe the formation of air flow preferential channels in peripheral regions. Estimates for the h values and, consequently, the freezing time prediction were more efficient for the configuration that used plastic buckets (HDPE). For the steel drums the correlation that considers the effects of a turbulence factor was satisfactory. As for the plastic boxes (HDPE), the correlation yielded encouraging results only for those boxes located in the central area of the stacking. On average, the freezing times were of 96 hours for the guava pulp in steel drums, of 45.8 hours, in polyethylene bags packed and placed in the plastic boxes (HDPE) and of 50.9 hours, in plastic buckets (HDPE). When compared to process using steel drums and under the same operational conditions of the equipment, the results show that it is possible to process the same amount of product, with higher quality (faster cooling), and about 50% reduction in power consumption with settings of plastic boxes (HDPE) and plastic buckets (HDPE). The decision making over a given configuration should be based on its handling easiness and on the initial cost. Regarding these tow late considerations, the guava pulp processing in plastic buckets (HDPE) have shown more advantageous.