ABSTRACT
The Refractance Window (RW™) is an innovative thin-fi lm method developed by MCD Technologies for drying liquid and pasty materials and converting them into fl akes or granules (Magoon, 1986; Anonymous, 2008). According to the developers of this concept for drying, the term Refractance Window relates to the design principles. The dryer, shown schematically in Figure 23.1, is built as an endless strip made of infrared-transparent polyester such as Mylar® to form a conveyor belt carrying the product that is deposited on the belt as a layer that is 0.2-1 mm thick. The relatively thin belt (<0.2 mm) fl oats on the surface of hot water held in a shallow trough. Thus, the sensible heat of water is transferred through the belt to the drying material, which is transported along the dryer with a belt velocity ranging from 0.6 to 3 m/min. When the material on the belt is dry, heat is transferred from hot water only by conduction. However, when the material is wet, heat is transferred by conduction and radiation because the liquid water held in the drying material absorbs the electromagnetic energy in the wavelength range spanning from 3.0 to 15.3 μm. Thus, when the water-laden material is placed on the belt conveyor, a window for the passage of infrared energy is created at all the surface areas where the material covers the belt, because refraction at the belt-material interface is minimized. As the moisture content falls in the course of drying, the infrared window is gradually closed, leaving conduction as the predominant heat-transfer mode. Since plastic is a poor conductor of heat, little heat is transferred to the nearly dry material, so the risk of overheating is greatly reduced.
