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Energy consumption during Refractance Window® evaporation of selected berry juices

The Refractance Window® evaporator represents a novel concept in the design of evaporation systems for small food processing plants. In this system thermal energy from circulating hot water is transmitted through a plastic sheet to evaporate water from a liquid product flowing concurrently on the top surface of the plastic. The objectives of this study were to investigate the heat transfer characteristics of this evaporator, determine its energy consumption, and capacity at different tilt angles and product flow rates. The system performance was evaluated with tap water, raspberry juice, and blueberry juice and puree as feed. With a direct steam injection heating method, the steam economy ranged from 0.64 to 0.84, while the overall heat transfer coefficient (U) was 666 W m−2 °C−1. Under this condition, the highest evaporation capacity was 27.1 kg h−1 m−2 for blueberry juice and 31.8 kg h−1 m−2 for blueberry puree. The energy consumption was 2492–2719 kJ kg−1 of water evaporated. Installation of a shell and tube heat exchanger with better temperature control minimized incidences of boiling and frequent discharge of condensate. The steam economy, highest evaporation rate and overall heat transfer coefficient increased to 0.99, 36.0 kg h−1 m−2 and 733 W m−2 °C−1, respectively. Copyright © 2004 John Wiley & Sons, Ltd.