A tunnel dryer is basically a group of truck-and-tray batch dryers, operated in a programmed series so as to be quasi-continuous. Truckloads of freshly prepared material are moved at intervals into one end of the long, closely fitting enclosure, the whole string of trucks is periodically advanced a step, and the dried truckloads are removed at the other end of the tunnel. The hot drying of air is supplied to the tunnel in any of several different ways, known as the counter flow, concurrent or parallel-flow, centre exhaust, multistage, and compartment Arrangements.
In operation, thin layers of food dried on trays, which are stacked on trucks programmed to move semi-continuously through an insulated tunnel. The trays are fabricated of wood or light metal, with thin slat or open-mesh bottoms, and are designed so that when they are properly stacked a clear air passage is left between trays. The loaded trucks are pushed either manually or mechanically, one at a time, into the “wet end” of the drier. The passageway, which constitutes the tunnel, is just large enough to accommodate the loaded trucks; clearances are kept small so that hot air will not circulate uselessly around the stacks of trays but will be forced to flow mainly between trays. Typically a 20 m tunnel contains 12-15 trucks with a total capacity of 5000 kg of food. This ability to dry large quantities of food in a relatively short time (5-6 h) made tunnel drying widely used. However, the method has now been largely superseded by conveyor drying and fluidised-bed drying, as a result of their higher energy efficiency, reduced labour costs and better product quality. The drying characteristics of a tunnel are strongly influenced by its general design andarrangement, especially the direction of progression of the trucks relative to the direction ofthe main airflow. In the far more common types of tunnel design, the main airflow is parallelto the direction of truck movement (Table 1). It may be same direction (concurrent, or parallelflow), or the opposite direction (counter flow), or it may be partly the other, as in various types of multistage dryer. Figure13 illustrates a simple concurrent tunnel and counter flowtunnel. Figure14 shows a more complex counter flow tunnel, designed to allow a portion ofthe drying air to be re-circulated. The essential differences between the effects of the twoarrangements is that in the counter current tunnel very rapid initial drying of the material takesplace, causing a high moisture gradient within each piece, rapid setting of the outer layers
after only a little shrinkage, and formation of internal splits or porosity as the internal fleshfinally dries and shrinks; final stages of drying are very slow because not only is the materialapproaches dryness. Unless the evaporative load is very light, initial stages of drying takeplace in much cooler and more humid air, internal moisture gradients are not so steep, andmore nearly unhindered and complete volume shrinkage can take place. For many years,counter flow drying was standard practice for dehydration of fruits such as prunes, which
amount to about 90 % of the fruit so processed. However, more than half of the current prunecrop is dried in parallel-flow tunnels. Air recirculation is common to reduce heat costs.