A A C C   2 0 0 0   A n n u a l   M e e t i n g

As an extruded product is dried experimentally, data are obtained relating moisture content to time. When these data are plotted graphically, it results in what is known as a drying curve. Three distinct regions exist on any given drying curve. The first region is the portion of the curve where internal product heat causes evaporation. This period ends when the product surface temperature reaches the wet-bulb temperature of the air. The second portion of the curve is called the constant-rate period. During the constant-rate period, drying proceeds by diffusion of vapor from the saturated surface of the material through a stagnant-film into the moving air stream. Moisture movement within the sample is rapid enough to maintain a saturated condition at the product surface. The rate of drying is controlled by the rate of heat transfer to the drying surface. Since the rates of heat and mass transfer remain balanced, the product temperature remains close to the wet bulb temperature of the air. The practical result of these phenomena is that this portion of the dryer can safely use high temperatures that would otherwise damage the product. The final portion of the curve is known as the falling-rate period. During this period the evaporating surface can no longer remain saturated by the moisture movement within the product. Thus, the drying rate begins to decrease. This period usually predominates in determining the overall drying time. As this period continues, the product surface temperature begins to rise and approaches the dry-bulb temperature of the process air. Typically the drying temperature must be reduced to prevent product damage. This drying theory can be applied to both conveyor and vertical dryers. Many extruded products such as salmon feed and 3G snack pellets are temperature sensitive. During the constant-rate period the evaporative cooling effects keep the product temperature low. However, once the product enters the falling-rate period the air temperature must be reduced to prevent product damage. Thus, it is often advantageous to stage the drying temperatures. The first method uses an independent zone - independent pass conveyor dryer. This dryer design allows floors to be placed between the conveyors. By ducting alternate top plenum sections (fans and heating devices) to separate top and bottom passes, it is possible for each pass to have temperatures and air flows completely isolated from each other. A vertical dryer consists of modular drying decks that are stacked on top of each other. Stacking the decks allows high air temperatures at the inlet (top) of the dryer and low air temperatures near the discharge (bottom) of the dryer. A vertical dryer also offers notable advantages over a conveyor dryer in terms of energy efficiency, moisture uniformity, floor space requirements, fines removal, and quick product changeover.