Quantitative understanding of Refractance Window™ drying

Abstract

Refractance Window™ (RW™) drying is a novel drying technique in which the material to be dried is placed on a thin plastic sheet and, the plastic sheet is heated from below via circulating hot water. Current understanding suggests that the use of a thin plastic sheet that is transparent to infrared radiation (IR) creates a “window” for thermal radiation from hot water to the wet material. This “window” gradually closes as the material dries out cutting off thermal radiation and prevents the sample to reach water bath temperatures. It is suggested that radiation heat transfer is one of the dominant modes of heat transfer that promotes faster drying. However, no mathematical model-based mechanistic understanding of this process is available to support the above hypotheses, which is the objective of the present work. A conjugate heat and mass transfer model is developed to simulate the drying of pumpkin slices and investigate in particular the effect of the optical properties of a commonly used plastic sheet (Mylar®) on the radiative component of heat transfer. Computed results indicate that there is only a 5% increase in transmission of IR radiation through Mylar between a dry and wet product. A major portion of thermal energy is transferred via conduction (99%) through the plastic sheet. The relatively low sample temperature observed for RW™ drying of thin samples is attributed to the development of a dried, thermally resistive layer at the base that prevents heat transfer from the plastic sheet during the later portions of the drying process. However, for thick-sized samples, the low sample temperature is a result of development of air spaces between the product and the plastic sheet, which reduces the heat flux from the hot water. As a consequence, quality of the final product is preserved when compared with other drying techniques.